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Recent Advances in Transition Metal-Catalyzed Synthetic Transformations of Organosilicon Reagents Takeshi Komiyama, Yasunori Minami, and Tamejiro Hiyama ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02374 • Publication Date (Web): 02 Dec 2016 Downloaded from http://pubs.acs.org on December 2, 2016

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ACS Catalysis

Recent Advances in Transition Metal-Catalyzed Synthetic Transformations of Organosilicon Reagents Takeshi Komiyama,† Yasunori Minami,‡,* and Tamejiro Hiyama‡,* †

Department of Applied Chemistry, Chuo University, Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan

‡

Research and Development Initiative, Chuo University, Kasuga, Bunkyo-ku, Tokyo 112-8551,

Japan

ABSTRACT

Organosilicon compounds act as a nucleophile upon activation by an appropriate base and behave in a manner similar to main group organometallic reagents. In the last decades, structurally divergent organosilicon reagents are available and have become more employed for synthetic transformation with the aid of transition metal complexes, because organosilicon compounds are in general superior to other organometallic compounds in view of stability, solubility, non-toxicity, and easy-handling. Particularly, cross-coupling of organosilicon reagents with organic halides or pseudohalides has been considered to be a useful tool for constructing the carbon frameworks of various target molecules such as pharmaceuticals and π-conjugated functional materials. Perfluoroalkylsilicon compounds such as CF3SiEt3 have found the use as reagents for the metal-catalyzed introduction of perfluoroalkyl groups into many substrates. In addition, functionalized organosilicon reagents are readily accessible by catalytic approach starting with appropriate hydrocarbons such as alkenes, alkynes, alkanes, and arenes. This article reviews recent advances in transition metal-catalyzed transformations of organosilicon reagents according to the type of synthetic transformation and metal catalyst.

Graphical Abstract

ACS Paragon Plus Environment

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Keywords

C–C bond formation, C–heteroatom bond formation, C–Si bond activation, metal catalysis, silylation, organic synthesis

1. INTRODUCTION In view that silicon is an element second-rich in the natural resources and organosilicon compounds are characterized by stability, nontoxicity, and easy-handling, organosilicon chemistry is considered to contribute to the green chemistry. Nowadays, numerous synthetic methods are available for a wide variety of organosilicon chemicals such as alkyl-, alkenyl-, alkynyl-, aryl-, and perfluoroalkylsilanes. Thus, many organosilicon compounds have been shown to be applicable to various organic reactions: a typical example is the cross-coupling reaction,1 the importance of which is growing continuously and tremendoulsly. Organosilicon compounds are used as nucleophiles like organometallic reagents of main group elements, although the C–Si bond need to be activated by an appropriate base such as a fluoride ion or alkali alkoxides. In the absence of such a base, the silicon compounds can in general be stored for long period without any deterioration. At present, a wide variety of organoboron reagents such as boronic acids and its esters are commercially available and employed for industrial synthesis. However, toxicological aspects of several boronic acids and esters were suggested in 2011.2 In addition, problems of the boron-based coupling are


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described to be solved, e.g. homo-coupling of organoborane reagents and contamination of products by a phosphorus residue derived from phosphine ligands.3 These defects do not undermine the value of organoborane chemistry but seem to indicate a need for an alternative which uses silicon. Recently, straightforward silylation of C–H bonds catalyzed by a Rh, Ir, or Pt catalyst using a hydrosilane is extensively studied in parallel to the similar transformation with boron reagents, making an access to novel organosilicon compounds much easier.4 Studies on activation of stable C–Si bonds have made the use of silicon reagents much easy and convenient. For example, many smooth and selective synthetic transformations under mild conditions are invented. One of such achievements starts with tetraorganosilanes as a source of carbanion equivalents and a weak base. An example uses organo{2(hydroxymethyl)phenyl}dimethylsilanes (organoHOMSi) which deliver nucleophilic species for transition metal-catalyzed synthetic reactions.5,6 After the transformation is completed, the silicon residue, cyclic silyl ether, can be recovered and reconverted back to the same or other organoHOMSi reagents. Moreover, the reagents are versatile to generate conveniently various nucleophiles which exhibit reactivity similar to organoborane reagents and can be activated by a weak base. Recently, perfluoroalkyl(trialkyl)silanes such as Et3SiCF3 and Me3SiCF2C(O)R are employed to generate CF3 and CF2C(O)R nucleophiles for organic synthesis. The present review article highlights briefly recently published transition metalcatalyzed preparative methods for organosilicon reagents and synthetic transformations of organosilicon compounds mainly as a source of carbonaceous nucleophile.

2. PREPARATION OF ORGANOSILICON REAGENTS


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The most representative and useful method for synthesis of organosilicon reagents is the reaction of organolithium and -magnesium reagents with silicon electrophiles such as chlorosilanes and alkoxysilanes to provide with various alkenyl-, alkynyl-, alkyl-, and arylsilanes. Normally, such organic alkaline and alkaline earth metal reagents are too reactive to attack many electrophilic centers such as ester, nitrile, and ketone. To avoid the chemoselectivity

problem,

turbo

Grignard

reagent,

or

a

diisopropylmagnesium

chloride/lithium chloride complex as a metalation reagent precursor, allows to prepare Grignard reagents under low temperature by the halogen-magnesium exchange reaction with aryl halides. This preparative method made an access to a range of Grignard reagents easy particularly when electrophilic centers are present in substrates.7 An alternative chemoselective method is the transition metal-catalyzed hydrosilylation of alkenes and alkynes to give alkyl- and vinylsilanes. Hereby platinum is the representative catalyst, which gives syn-adduct of H and SiR3 to unsaturated carbon-carbon bonds, the Chalk-Harrod mechanism being suggested, namely, (1) oxidative addition of H–Si bond to Pt(0) to give H–Pt(II)–Si, (2) insertion of alkenes into the H–Pt(II) bond, followed by (3) reductive elimination to give the final adduct and regenerate Pt(0). Representative catalysts are

the

Speier

catalyst

(H2PtCl6)

and

the

alkene-stabilized

Karsted’s

catalyst

{Pt2[(Me2SiCH=CH2)2O]3}.8 Rhodium and palladium, were later introduced to the catalyst. Recently ruthenium, cobalt, and iron is added to the member.9 In particular, the rutheniumcatalyzed hydrosilylation of alkynes is characterized by anti-addition to carbon-carbon triple bonds.10 The mechanistic pathway is suggested as follows: (1) syn-addition of Ru–Si to alkynes, (2) isomerization to anti-adducts, and (3) reductive elimination.11 The stereochemical outcome combined with regioselectivity in the propargylic system12a is immediately applied to synthesis of many natural products.12b Various hydrosilanes are applied also to 1,2- or 1,4-reduction of polar functional groups like carbonyls and imines with


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the aid of Brønsted (or Lewis) acid or Rh catalysts. Thus, hydrosilanes are considered to behave in a manner similar to molecular hydrogen.13 Transfer hydrosilylation is also achieved, hereby hydrosilanes being in situ provided from silylated dihydrobenzenes.14 Compounds containing a Si–Si bond are also susceptible to oxidative addition of transition metal complexes to achieve silylation of acid chlorides to give silyl ketones15 and disilylation of alkenes and alkynes,16 the typical catalyst being palladium. Thus, various novel organosilicon compounds are conveniently prepared. In addition to disilanes, silylboranes, and silylstannanes also add across carbon-carbon double and triple bonds to give multi-functionalized silicon reagents.17 Silylstannylation of alkynes or allenes is achieved using a combination of silylboranes and alkoxystannanes in the presence of a copper catalyst.18 Catalytic conversion of organic halides to the corresponding organosilanes is a promising synthetic method for functionalized organosilicon reagents.19 To this end, hydrosilanes, disilanes, and silylborans are applicable. Recently, catalytic straightforward silylation of various aryl, vinyl, alkynyl, and alkyl C–H bonds is studied using commercially available hydrosilanes, and a wide variety of organosilanes are now accessible.4 For example, dehydrogenative silylation of arenes is conveniently achieved by Hartwig, who used a rhodium or an iridium catalyst and HSiMe(OSiMe3)2 (Eq. 1).20 The rhodium catalyst allows for silylation of simple aromatic substrates as a limiting reagent under mild conditions, but heterocycles and various functional groups do not tolerate the catalytic conditions. In contrast, heteroarenes and electron-deficient arenes are silylated by Ir/2,4,7-trimethylphenanthroline (2,4,7-Me3phen).20b The improved direct C–H silylation system is also effective for late-stage functionalization of pharmaceuticals like aripiprazole (Eq 2). Very recently, aryl C–H silylation is achieved with B(C6F5)3 catalyst in lieu of transition metal catalyst. For instance,


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electron-rich aniline derivatives are readily silylated.21 Five-membered heterocycles such as thiophene are silylated in the presence of KO(t-Bu) as a catalyst.22 H–SiMe(OSiMe3)2 (2 eq) 1 mol% [Rh(OH)(coe)2]2 2.2 mol% L1

(1)

cyclohexene (2 eq) THF, 45 °C

H

SiMe(OSiMe3)2 83%

OMe OMe MeO MeO

P P

OMe OMe

2

OMe OMe

2

L1

H

SiMe(OSiMe3)2

N N

H–SiMe(OSiMe3)2 (5 eq) 3 mol% [Ir(OMe)cod]2 6 mol% 2,4,7-Me3phen

Cl Cl

N N

Cl Cl

(2)

cyclohexene (2 eq) THF, 100 °C O

N H

aripiprazole

O

O

N H

O

75%

The Mizoroki-Heck type reaction using silyl iodides or triflates and terminal alkenes proceeds in the presence of a palladium catalyst and a base to give silylated alkenes.23 Equation 3 is an example of dimethylsilylene ditriflate.23c This reaction is considered to be triggered by oxidative addition of an Si–OTf bond to the palladium center to form a silylpalladium complex.24 Trimethylvinylsilane effects direct silylation of alkenes (Eq 4).25 Herein, hydridoruthenium and -rhodium complexes are usually employed as the catalyst for extraction of the silyl group from vinylsilane, the reaction proceeding via addition of a metal hydride, β-Si elimination, followed by insertion of alkenes as a target reactant into Si–Ru (or Rh) bond, and β-H elimination to give desired silylated alkenes. Formal C–H silylation of terminal alkenes using a hydrosilane to produce (Z)-alkenylsilanes is made possible with [Ir(OMe)cod]2 and 4,4’-di-tert-butyl-2,2’-bipyridyl (dtbpy) (Eq 5).26 Use of 3,4,7,8tetramethylphenanthroline (Me4phen) or 2-methylphenanthroline (2-Mephen) as the ligand is shown to be a better ligand to give (Z)- or (E)-alkenylsilanes as a major product (Eq 6 and Eq 7).27 The reaction is considered to proceed according to the Mizoroki-Heck mechanism,


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namely, tandem insertion and β-hydrogen elimination rather than direct C–H activation. Terminal alkynes also undergo dehydrogenative silylation in the presence of a catalytic amount of Zn(OTf)2 and pyridine to give silylalkynes (Eq 8).28 1.25 mol% Pd2(dba)3 5.5 mol% Pt-BuPPh2 5 mol% NaI Et3N (3 eq) toluene, 35 °C, 24 h

t-Bu + Me2Si(OTf)2

then EtOH (1 eq)

Me2 Si OEt 92%

(1.1 eq)

2 mol% RuHCl(CO)(PCy3)2

Ph +

toluene, 100 °C, 24 h

4

+ H SiEt3 (3 eq)

(1 eq)

SiMe3

Ph

SiMe3 (1.2 eq)

EtOCO2

5 mol% [Ir(OMe)cod]2 10 mol% dtbpy

EtO2C

O 4

SiEt3

norbornene (3 eq ) THF, 40 °C

SiMe(OSiMe3 )2

0.5 mol% [Ir(OH)cod]2 1.5 mol% Me4 phen

(6) 93% (Z/E = 90:10)

SiMe(OSiMe3)2

(7)

73% (Z/E = 7:93)

H−SiMePh2 (1.5 eq) 5 mol% Zn(OTf)2 20 mol% pyridine EtCN, 100 °C

(5)

96% (Z/E = 8:1)

0.5 mol% [Ir(OMe)cod]2 (2 eq) 1.7 mol% 2-Mephen + H SiMe(OSiMe3 )2 THF, 80 °C (1 eq)

+ H SiMePh2 (1.5 eq)

(4)

89% (EE/EZ = 93:7))

+ H SiMe(OSiMe3)2 2-norbornene (3 eq) THF, 50 °C (3 eq)

S

(3)

t-Bu

SiMePh2

S

(8)

98%

Direct C–H silylation of terminal alkanes is reported in 2016 (Eq 9).29 The net transformation is considered to proceed in the following way. First, dehydrogenation of alkanes gives a mixture of terminal and internal alkenes using an iridium catalyst Ir1 with an (P-C-P) pincer ligand, tert-butylethene as a hydrogen acceptor, and a strong base. Then, sequential hydrosilylation proceeds to form terminal silylalkanes solely by an iron catalyst Fe1 catalyst bearing an (N-N-N)-pincer ligand.


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H–SiMe(OSiMe3)2 (1 eq) 1 mol% Ir1 t-BuCHCH2 (1 eq) 10 mol% Fe1 5

O

S

SiMe(OSiMe3)2 (9)

20 mol% NaHBEt3 rt One-pot

NaOt-Bu (1.2 eq) neat, 200 °C

5

N P(i-Pr)2 H Ir Cl P(i-Pr)2

Ar Br

N

Fe N

Br Ar

Ar = 2,6-Et2C6H3 Ir1

Fe1

3. C–C CROSS-COUPLING REACTION 3.1. Pd-Catalyzed C–C Cross-coupling Reaction The first example of transition metal-catalyzed carbon–carbon bond-forming reactions using organosilicon reagents was reported in 1978 by Kumada and Tamao, who employed organo(pentafluoro)silicates for cross-coupling with allyl chlorides using a catalytic amount of Pd(OAc)2 (Eq 10).30 The hexacoordinated silicates were later shown to react also with activated olefins and aryl halides in the presence of a catalytic or stoichiometric amount of palladium acetate (Eq 11 and Eq 12).31 Due to poor reactivity toward C–C bond formation, the coupling reaction with hexacoordinated silicates was not used widely. However, the observations clearly show that highly coordinated silicates undergo transmetalation of alkenyl and aryl groups from silicon to palladium.

K2 n-Bu

10 mol% Pd(OAc)2

SiF5 + Cl

n-Bu

(2.0 eq)

K2 n-Bu

SiF5 +

60%

Pd(OAc)2 (1 eq)

Ph

SiF5

+

CO2Et

CO2Et THF, rt, 20 h (10 eq)

K2 Ph

(10)

Et2O, rt, 24 h

I Ph (1.3 eq)

(11)

48% 5 mol% Pd(OAc)2 10 mol% PPh3

Ph

Ph

Ph +

Et3N, 135 °C, 20 h 51%

(12) Ph 8%

In 1988, we demonstrated that simple tetracoordinate organosilicon reagents like vinyland alkynyl(trimethyl)silanes underwent the Pd-catalyzed cross-coupling with aryl iodides in


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the presence of tris(dimethylamino)sulfonium difluoro(trimethyl)silicate (TASF) to form C– C bond-forming products (Eq 13).32 The key of the discovery is attributed to pentacoordinate silicates generated in situ from the reaction of tetragonal silicon reagents with a fluoride ion derived from TASF to induce transmetalation smoothly without deactivation of the palladium catalyst. 2.5 mol% [Pd(allyl)Cl]2 TASF (1.3 eq)

I SiMe3

+ Me

(13)

HMPA, 50 °C

Me

(1.3 eq)

89%

Mechanism of the silicon-based cross-coupling is proposed as shown in Figure 1. First, oxidative addition of organic halides (R2–X) to palladium(0) proceeds to give R2–Pd(II)–X. Following transmetalation of hypervalent silicon species, generated in situ from R1SiY3 and a fluoride ion, with R2–Pd–X gives R2–Pd–R1. Finally, reductive elimination from the palladium(II) complex results in the formation of cross-coupled products and the palladium(0). Some experimental results are consistent with transmetalation through transition state TS-A. Recent theoretical and electrochemical studies33 suggest an alternative, TS-B, which involves an interaction of R1–SiY3 with R2–Pd–F from the reaction of R2–Pd–X with a fluoride ion. In both pathways, it can be concluded that Lewis acidity of the silicon center is a key for successful transmetalation. F R2 X

R2 Pd X

Y Si 1

Y

F

R1 SiY3

Y

R

F Y Y Si X 1 Y R Pd R2 TS-A

Pd

or F R2 R1

R2 Pd R1

X Si Y

Pd Y Y

F

R2 Si Y

Y Y

TS-B

Figure 1. Plausible mechanism of Pd-catalyzed cross-coupling of organosilicon reagents


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Thereafter a great number of C–C bond-forming cross-coupling reactions have been reported1 with organsilanes substituted by electron withdrawing groups such as fluorine, chlorine, alkoxy or siloxy group on silicon for enhancement of Lewis acidity at the silicon center in order to accelerate the pentacoordinate silicate formation, subsequent transmetalation, and total cross-coupling reaction. Metal alkoxides and alkaline hydroxides can also activate silicon reagents by forming pentacoordinated silicates in a manner like a fluoride ion. This silicon-based cross-coupling is applied to tetraorganosilanes of type organo-trimethylsilanes,

-triallylsilanes,

-silacyclobutanes,

and

-aryldimethylsilanes.

However, reagents with a trialkylsilyl group hardly participate in the cross-coupling because tetraorganosilicon reagents are remarkably stable under the typical reaction conditions. On the other hand, 2-trimethylsilylpyridine is easy to handle as a cross-coupling nucleophile,34 whereas 2-pyridylboronic acid and its esters encounter notorious instability.35,36 Alkynylsilanes, which can be easily prepared and used as a cross-coupling reagent, are a common building block, which can be converted to various reactive organosilanes by a variety of addition reactions. It is noteworthy that aryl or vinyl triflates, tosylates, mesylates, and chlorides are applicable coupling partners like other cross-coupling reactions. Organosilanols are effective silicon nucleophiles to cross-couple with various (pseudo)haloarenes with the aid of various activators. Since the synthetic reactions with organosilanoles are reviewed in detail recently,1d the present paper briefly refers to the crosscoupling of organosilanols. Hiyama and Denmark independently reported Pd-catalyzed crosscoupling of aryl- and alkenylsilanols using Ag2O and tetrabutylammonium fluoride (TBAF) respectively as the activator.37,38 Later, Denmark improved the silanol-based cross-coupling and demonstrated that the cross-coupling reactions took place using a less nucleophilic base KOSiMe3 (Eq 14).39 Notably, tert-butyldimethylsilyl (TBS) ethers tolerate the conditions. Furthermore, external-base-free cross-coupling proceeds when potassium silanolates are


10

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employed as the coupling reagents (Eq 15).40 A reaction mechanism for the cross-coupling of a potassium alkenylsilanolate is suggested to involve intramolecular transmetalation of siloxypalladium(II).41 Possibility of an alternative transmetalation through pentacoordinate silicates also is suggested. n-Pent SiMe2OH (1.1 eq) +

5 mol% Pd(dba)2 KOSiMe3 (2 eq)

OTBS

DME, rt

OTBS R I

76%

Me Me Si O Pd

SiMe2OK TBSO (1.5 eq) + OMe

(14)

n-Pent

OMe

5 mol% Pd(Pt-Bu3)2

(15)

toluene, 90 °C TBSO

Br Me Me Si Ar O Pd

Ar or Pd

62%

OSiMe2Ar Me Me

Si O

R2

Silicon-based cross-coupling usually needs the presence of a fluoride ion or a base to activate organosilicon reagents. On the other hand, in 2005 we invented organo{2(hydroxymethyl)phenyl}dimethylsilanes (organo-HOMSi) for universal cross-coupling reagents.5a The ortho-hydroxymethylphenyl group is the key element for activation of the silicon center intramolecularly with a weak base like K2CO3. Palladium and in some cases a copper cocatalyst are applied.5 For example, (E)-octenyl-HOMSi 1a reacts with 3TBSOCH2-C6H4-I with a PdCl2/P(2-furyl)3 catalyst and K2CO3 to give a cross-coupled product without desilylation (Eq 16). It should be noted that the silicon residue, cyclic silyl ether 2, can be recovered after cross-coupling and converted back to organo-HOMSi. Furthermore, the cross-coupling with bifunctional organo-HOMSi is applicable to polyarylene synthesis.5j The reaction of fluorenylene-2,8-bisHOMSi 3 with 4,7dibromobenzothiadiazole results in the formation of the corresponding polymer, poly(9,9-


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dioctylfluorene-co-benzothiadiazole) (F8BT), which is attracting more interest as a photovoltaic and light-emitting materials (Eq 17).42 OH

n-Hex

1 mol% PdCl2 2 mol% P(2-furyl)3

TBSO I

+

TBSO n-Hex

1a (1.1 eq)

n-Oct

N

S

5 mol% Pd[P(o-tolyl)3]2 5.3 mol% DPPF 7.5 mol% CuBrSMe2

N

Si + Br

Br

3 (1.00 eq) (1.00 eq) HO

Cs2CO3 (4 eq) MS 3A toluene/DME (3:1) 85 °C, 24 h

Si =

n-Oct

(16)

Si Me2 2 Recyclable

98%

n-Oct

Si

O

+

K2CO3 (2.2 eq) DMSO, 35 °C, 23 h

Si Me2

n-Oct

N

S

N (17) n

>99%, n = 44 Mw = 23000, Mw/Mn = 2.97

Si Me2

Similar cross-coupling strategy using 1,2-oxasilabenzocyclopentene 4 is suggested in 2006 by Tamao and his coworkers.43 The cyclic silyl ether, upon treatment with aryllithium and CuI, produces cross-coupling active silicon reagents, which react with aryl iodides in the presence of a palladium catalyst to provide coupled products (Eq 18). 1) PhLi (1.1 eq) THF 0 °C, 10 min

O Me2Si

2) CuI (1.1 eq ) 0 °C, 10 min 4 (1.1 eq) CuO PhMe2Si

I−C6H4-p-Ac (1 eq) 2 mol% PdCl2(PPh3)2 DMF, 80 °C, 2 h one-pot

Ac (18) Ph 94%

A series of organo[(2-lithioxymethyl)phenyl]dimethylsilanes, generated in situ by the reaction of the cyclic silyl ethers with various aryl- or vinyllithium reagents, undergo the cross-coupling reaction with organic halides, as disclosed by Smith III to produce a wide range of coupled products.44 This reaction may be deemed as a modified Murahashi crosscoupling which uses organolithium reagents and Pd catalyst. Several cyclic silyl ethers are fixed on polymer supports to easily recover and recycle the silicon mediator for another coupling reaction. In 2016, 3,3-bis(trifluoromethyl)-2,1-oxasilabenzocyclopentene 5 was introduced as a reusable, bench-stable transfer agents with high reactivity. Treatment of this


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cyclic silyl ether with aryllithium generates lithiated aryl-HOMSi which effects crosscoupling with chloroarenes, and finally gives rise to biaryls at room temperature (Eq 19).44f F3C CF3 PhLi (1.3 eq) O Si i-Pr2 5 (1.4 eq)

THF -78 °C ~ rt, 2 h

LiO

Cl−C6H4-p-OMe (1.0 eq) 3 mol% Pd1 3 mol% XPhos

Ph Si i-Pr2

THF, rt, 24 h one-pot

F3C

CF3

OMe (19) Ph 98%

Cy2P i-Pr i-Pr MsO

Pd NHMe XPhos Pd1

i-Pr XPhos

Tetrafluoroethylene (TFE) is a unique building block for functionalized substituted polyfluoroethenes

and

their

polymers.

In

2014,

cross-coupling

of

TFE

with

aryl(trimethoxy)silanes was achieved by Ogoshi who isolated aryltrifluoroethenes (Eq 20).45 Noteworthy is that this reaction does not need any nucleophilic silicon activator. Instead, a catalytic amount of fluoro(triethoxy)silane promotes the reaction, although the role of the additive is not well clarified yet. In any event, the reaction mechanism is considered to proceed by oxidative addition of a C–F bond to Pd(0) followed by transmetalation between Pd–F and aryl(trimethoxy)silanes possibly in a manner similar to TS-B in Figure 1. F

Si(OMe)3 +

F

F

F (>3.0 eq)

2.5 mol% Pd(dba)2(C6H6) 5 mol% PCy3 10 mol% FSi(OEt)3 THF, 100 °C, 23 h

F F

(20)

F 94%

F Cy3P F Pd F Cy3P F via

As discussed at the beginning, dehydrogenative silylation of arenes is a straightforward preparative method for arylsilanes. Subsequent cross-coupling results in a formal C–H coupling of arenes with organic halides in two steps. In accord with this schematic concept, Ph–SiMe(OSiMe3)2 was prepared by platinum-catalyzed silylation46 of benzene with H– SiMe(OSiMe3)2. Following cross-coupling with aryl iodides proceeds smoothly to give


13

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Page 14 of 69

biaryls (Eq 21).47 Furthermore Ir- or Rh-catalyzed silylation of a wide range of arenes is achieved using H–SiMe(OSiMe3)2, and the resulting silylarenes are successfully employed for the cross-coupling. For example, silylation of palonosteron catalyzed by an iridium produces regioselectively a silicon reagent, which undergoes palladium-catalyzed crosscoupling with m-tolyl iodide to give arylated palonosteron (Eq 22).48 H-SiMe(OSiMe3)2 (1 eq) 1 mol% PdCl2 1 mol% TPMe2K 200 °C solvent (10 eq)

Ar H B Ar Ar = Ar TPMe2

N N Br−C6H4-4-CF3 (0.5 eq) 3 mol% PdCl2(PPh3)2 TBAF (3 eq)

SiMe(OSiMe3)2

(21)

THF/H2O (5:1) 100 °C, 16 h

CF3 83%

N O

N H

H-SiMe(OSiMe3)2 (1.5 eq) 1.5 mol% [Ir(OMe)cod]2 3.1 mol% 2,4,7-Me3phen cyclohexene 80-100 °C, 1-2d

palonosetron (anti-cancer agent) N O

N H

(Me3SiO)2MeSi 87%

N I−(m-tolyl) (1.0 eq) 5 mol% Pd(OAc)2 5 mol% dcpe

O

N H

KOSiMe3 (3 eq) toluene 80 °C, 16 h

(22)

70%

Although halo- and oxysilanes are reactive enough for cross-coupling, their air- and moisture-instability is troublesome. In contrast, as seen in organo-HOMSi, tetraorganosilanes are characterized by high stability, high solubility, and easy-handling. Although most of them except organo-HOMSi are, however, inert toward the cross-coupling reaction, functional groups like 2-pyridyl, 2-thienyl, and allyl introduced on silicon are readily removed in situ upon exposure to a fluoride ion to make silanes reactive for cross-coupling. In view that silylated alkenes, alkynes, and arenes have a potential as versatile building blocks for synthesis of poly-substituted alkenes and arenes, Yoshida and Itami disclosed a protocol leading to triarylethenes using dimethylpyridylsilylethene as a synthetic platform. The


14

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ACS Catalysis

procedure starts with one-pot double arylation of the silylethene proceeds stereoselectively by repeating the Mizoroki-Heck reaction using two different or same aryl iodides to form 2,2diaryl-1-silyl-ethenes (Eq 23).49a The pyridyl group acts as a key factor for stereoselective double arylation by ligation through nitrogen. The diaryl-substituted silylethenes undergo cross-coupling with aryl iodides by a palladium catalyst and TBAF to give triarylethenes. They applied the sequence to synthesis of Tamoxifen.49b Gevorgyan used diisopropyl(2pyrimidyl)silylbenzene (Eq 24) for similar palladium-catalyzed sequential double functionalization at both ortho-C–H bonds.50 Tandem ortho-bromination/pivaloxylation of silylarenes proceed in the presence of palladium acetate as a catalyst. This process is achieved by a pyrimidyl group as a directing group for C–H bond cleavage. After orthofunctionalization of arenes, the silyl group is employed in the cross-coupling leading to biaryls by treatment of HF followed by palladium-catalyzed Ag2O-mediated reaction with aryl iodides. I−2-thienyl (1.0 eq) 2.5 mol% Pd2(dba)3CHCl3 5 mol% P(2-furyl)3 Et3N (3 eq) I-Ph (1.4 eq)

N Si Me2

60 °C

MS4A THF, 60 °C

I-C6H4-p-CO2Et Ph (0.67 eq) 3.3 mol% PdCl2(MeCN)2

N Ph

one-pot

Si Me2

OEt (23)

S

TBAF (1.0 eq) THF, 60 °C

S

O quant.

N Si N i-Pr2 Br

NBS (1.2 eq) 10 mol% Pd(OAc)2 10 mol% PhI(OAc)2

PhI(OPiv)2 (1.25 eq) 5 mol% Pd(OAc)2 30 mol% LiOAc

DCE, rt

DCE, 80 °C

N

Si N i-Pr2 OPiv 72% (2 steps)

HF

I-Ph (1.5 eq) 5 mol% Pd(PPh3)4

THF rt

Ag2O (1.1 eq) THF, 60 °C

Br (24) OPiv 68% (2 steps)

In 2015, we reported a straightforward synthesis of heteroaryl-HOMSi by the Ir/ Me4phen

catalyzed

C–H

silylation

of

heteroarenes

using

hydrido[(2-

THPoxymethyl)phenyl]dimethylsilane (H-HOMSi) under the modified Falck’s conditions.51 The net transformation opened an entry to systematic synthesis of oligothiophenes by


15

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Page 16 of 69

silylation of brominated thiophenes followed by cross-coupling with benzodithienylenebisHOMSi reagents. The reagents are readily prepared by double silylation of benzobithiophene with HO-protected H-HOMSi reagent followed by deprotection (Scheme 1).52 H-SiTHP (1.5 eq) 5 mol% [Ir(OMe)cod]2 10 mol% Me4phen

n-Dodec Br

S

norbornene (1.5 eq) (i-Pr)2O, 80 °C

H

5 mol% Pd[P(o-tolyl)3]2 5.3 mol% DPPF 7.5 mol% CuBrSMe2

n-Dodec Br

Cs2CO3 (2 eq) MS 3A toluene/glyme (3:1) 50 °C, 20 h

SiTHP

S 93%

S H

TsOH•H2O MeOH/CH2Cl2 85%

H S H-SiTHP (3 eq) 5 mol% [Ir(OMe)cod]2 10 mol% Me4phen norbornene (3 eq) (i-Pr)2O, 80 °C

SiTHP

n-Dodec S

n-Dodec

S

S

S

SiTHP

54% S SiTHP

SiTHP S 86%

(THP)O SiTHP =

Si Me2

Scheme 1. Linear synthesis of oligothiophenes by C–H silylation and cross-coupling

Organosilicon reagents can be used for synthesis of unsymmetrical ketones. Aryl(trimethoxy)silanes cross-couple with thioesters in the presence of a palladium(0) catalyst, CuI, and TBAF to give the corresponding ketones (Eq 25).53 Similarly, acid chlorides react with aryl(trifluoro)silanes in the presence of Pd(OAc)2, a bulky ligand tri-tertbutylphosphine, and CsF (Eq 26).54 Silyl ketones as acyl anion equivalents and thus give aryl ketones upon reaction with aryl bromides using Pd2 catalyst, H2O, and K3PO4 (Eq 27).55


16

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ACS Catalysis

5 mol% Pd(PPh3)4 CuI (1.0 eq) TBAF (2.0 eq)

O Ph Si(OMe)3 + PhS

O (25)

THF, 60 °C, 0.5 h

Ph

Ph Ph 88%

(1.1-1.2 eq) 5 mol% Pd(OAc)2 15 mol% Pt-Bu3 CsF (3.5 eq)

O Ph SiF3

+

Cl

O Ph

(3 eq)

75% 2 mol% Pd2 H2O (6 eq) K3PO4 (2.5 eq)

Br

O + Ph

(26)

xylene, 140 °C, 24 h

SiMe3

OMe

O (27)

Ph

2-MeTHF 60 °C, 20-48 h

78%

(1.5 eq)

OMe

O

O O

Pd P N Cl H Ph 2 Pd2

Alkynyl(trimethyl)silanes undergo the Sonogashira-type reaction with aryl tosylates directly with the aid of a catalyst consisting of PdCl2(bpy), a bulky biarylphosphine BrettPhos, and a CuCl cocatalyst to provide internal alkynes (Eq 28).56 5 mol% PdCl2(bpy) 15 mol% BrettPhos 10 mol% CuCl Ph

SiMe3 + (1.5 eq)

TsO Ph

Ph

DMI, 120 °C, 24 h

Ph

(28)

84%

MeO Cy2P i-Pr iPr MeO i-Pr BrettPhos

In contrast to the sp2C–sp2C and spC–sp2C bond forming reactions, cross-coupling of alkyl halides or alkylsilanes with aromatic electrophiles, namely, sp3C–sp2C bond formation, remained less studied possibly because the same transformation was conveniently achieved with copper catalysis. In 1994, we reported the cross-coupling of alkyl(trifluoro)silanes with aryl bromides to give alkylarenes in good to moderate yields (Eq 29).57 Later, Fu disclosed that cross-coupling of alkyl bromides with aryl(trimethoxy)silanes successfully took place in the presence of a palladium catalyst and a bulky electron-donating phosphine, which was assumed to prevent β-hydride elimination (Eq 30).58


17

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5 mol% Pd(PPh3)4 TBAF (3 eq)

Br n-Hex SiF3

+ Ac

(2 eq)

Me Si(OMe)3 +

Br

CN 4

Page 18 of 69

n-Hex (29)

THF, 100 °C, 37 h

Ac

63%

4 mol% PdBr2 10 mol% P(t-Bu)2Me

Me CN

TBAF (2.4 eq) THF, rt

4

(1.2 eq)

(30)

72%

Tetraorganosilanes are difficult to participate in the cross-coupling reaction due to low tendency of transmetalation. In addition, selective activation of one alkyl group over other similar carbonaceous groups on silicon remained uncleared for long time. We invented in 2010 HOMSi-type reagents applicable to sp3C–sp2C bond-forming cross-coupling.5g For example, pentenylHOMSi 6a was shown to cross-couple with p-bromobenzonitrile in the presence of a palladium/copper catalyst and K3PO4 to give 4-penten-1-ylarene (Eq 31). This protocol is applicable to the coupling of secondary alkyl groups with aryl bromides (Eq 32).

HO Si i-Pr2 6a (1.3 eq)

K3PO3 (2.5 eq) THF, 100 °C, 1 h

+ Br

1 mol% Pd(OAc)2 4.2 mol% DPPF 3 mol% Cu(hfacac)2

CN

(31) CN 93%

HO Si Cyp2 6b (1.3 eq) + Br

CN

1 mol% Pd(OAc)2 4.2 mol% DPPF 3 mol% Cu(hfacac)2 K3PO3 (2.5 eq) t-BuOH, 100 °C, 3 h

CN

(32)

84%

As trifluoromethyl-substituted agents have received much attention in pharmaceutical and agrochemical science, trifluoromethylmetals have been utilized in various nucleophilic trifluoromethylation through carbonyl addition or substitution reactions.59 Moreover, trifluoromethyl(trimethyl)silane, known as the Ruppert reagent, has been often employed as a precursor of various trifluoromethylmetal reagents or directly used for nucleophilic trifluoromethylation.60 In 2010, Buchwald succeeded in trifluoromethylation of aryl chlorides with trifluoro(triethyl)silane using a Pd/Brettphos catalyst and KF (Eq 33).61 A stoichiometric


18

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ACS Catalysis

study suggests that the BrettPhos ligand is effective in promotion of reductive elimination from intermediates Aryl–Pd–CF3 to give coupled products Aryl–CF3.

CF3 SiEt3

+ Cl

n-Bu

(2.0 eq)

3 mol% [Pd(allyl)Cl]2 9 mol% BrettPhos KF (2 eq) 1,4-dioxane, 120 °C

CF3

n-Bu

(33)

80%

An α,α-difluorobenzyl group is an important structural motif in medicinal chemistry. DifluoroTMSacetamide 7a was allowed to react with aryl bromides in the presence of a PCy(t-Bu)2-containing palladacycle complex and KF to give aryldifluoroacetamides, as disclosed by Hartwig (Eq 34).62 The resulting substituted amide can be converted to various functional molecules such as amines, aldehydes, ketones, alcohols, esters, and acids. O Me 3Si

NEt2 F F 7a (2.0 eq) +

t-Bu

1 mol%

H2N Pd PCy(t-Bu)2 Cl

t-Bu

O NEt2

KF (3 eq) 1,4-dioxane/toluene 100 °C, 30 h

(34)

F F 89%

Br

3.2. Ni-Catalyzed C–C Cross-coupling Reaction Nickel catalyst often shows reactivity quite different from palladium, because nickel complex can participate in the catalysis via single-electron-transfer. In 2004, Fu was the first to

report

the

Ni-catalyzed

cross-coupling

of

arylsilanes

with

cis-1-bromo-4-

chlorocyclohexane. The reaction is assumed to involve a selective radical process of alkyl–Br bond to give racemic coupled products (Eq 35).63 Stereochemical outcome clearly shows the evidence of radical process. The Ni-catalyzed cross-coupling was applied to an enantioselective arylation of racemic α-bromo carbonyl compounds using a chiral diamine ligand, leaving the terminal –CH2Br intact (Eq 36).64 Asymmetric induction is effectively put forth apparently at the C–C bond forming step.


19

ACS Catalysis

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Ph SiF3

+

(1.5 eq)

Br

Cl

cis/trans = 95/5

6.5 mol% NiBr2diglyme 7.5 mol% 4,7-Ph2phen CsF (3.8 eq) DMSO, 60 °C

Ph

O

Br

Br

Cl

(35)

60% (cis/trans = 55/45)

10 mol% NiCl2diglyme Ph Ph 12 mol% MeHN NHMe

Ph Si(OMe)3 + ArO (1.3 eq)

Page 20 of 69

TBAT (2.0 eq) 1,4-dioxane, rt

Ar = 2,6-di-t-Bu-4-Me-C6H2

O (36)

ArO Ph Br 70% (86% ee)

A cyclohexane analogue 8 of Aryl-HOMSi was shown in 2011 by us to undergo crosscoupling with aryl tosylates and chlorides in the presence of a nickel catalyst, PPh3 and PCy3 ligands and Cs2CO3 as a weak base to give biaryls (Eq 37).65 The reaction keeps protecting group TBS intact, as TBS tolerates the weak base used here.

TBSO

HO

+

Ph Si Me2 8 (1.3 eq)

TsO

5 mol% Ni(PPh3)2Cl2 10 mol% Zn 5 mol% PPh3 15 mol% PCy3 Cs2CO3 (2 eq) acetone/DMF (2 : 1) 80 °C, 24 h

TBSO (37)

Ph 62%

Alkyl(bis-catecholato)silicates 9 were demonstrated in 2015 by Goddard, Ollivier, and Fensterbank to generate alkyl radicals upon irradiation with blue LED in the presence of a photocatalyst, Ir[(dF(CF3)ppy)2(bpy)]PF6 (Ir2). The resulting alkyl radical reacts with an activated allyl sulfone to give an allylated product (Eq 38).66 The silicate contains a K+ counter ion complexed by 18-crown-6 to enhance both stability and solubility. The alkyl silicates alkylate aryl bromides by use of an Ni/dtbpy and Ir2 dual catalyst under the blue LED irradiation (Eq 39). Independently, Molander demonstrated that aryl bromides crosscoupled with similar alkyl(bis-catecholato)silicates with an ammonium counter ion in the presence of Ni/dtbpy and Ru(bpy)3(PF6)2 dual catalyst under the blue LED irradiation (Eq 40).67 On the basis of similar photo-induced cross-coupling,68 a plausible catalytic cycle of both reactions is shown, taking an iridium catalyst as an example (Figure 2). First, Ir(III) is excited by irradiation with blue LED; photoexcited species Ir*(III) oxidizes the alkyl silicate to generate an alkyl radical and is reduced to Ir(II) by single electron transfer (SET). The


20

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ACS Catalysis

alkyl radical couples with Ni(0) to form an alkyl–Ni(I) complex, which undergoes oxidative addition by Ar–Br to give an aryl(aryl)Ni(III)–Br complex. Finally, reductive elimination produces the coupled product and Ni(I)–Br, which interacts with Ir(II) through SET to regenerate Ir(III) and Ni(0), respectively. CO2Et Ts (4 eq)

CO2Et

2 mol% Ir2 n-Hex

blue LED DMF, rt, 24 h

M = K(18C6) R = n-Hex

(38)

75%

Br O (0.8 eq)

R Si O O O

3 mol% Ni(cod)2 CN 3 mol% dtbpy 2 mol% Ir2

(39)

blue LED DMF, rt, 24 h

M = K(18C6) R = n-Hex

M

n-Hex CN 92%

9 Br

5 mol% NiCl2(dme) 5 mol% dtbpy OMe (0.8 eq) 2 mol% Ru(bpy)3(PF6)2

M = NH2 (i-Pr)2 R = c-Hex

CF3

F

OMe

94%

PF6

2PF6

N

N N

F F

(40)

hν, DMF, rt, 16 h

N

Ir

N Ru

N

N

N

N

N

F CF3 Ir2 = [Ir(dF(CF3)ppy)2(bpy)](PF6)

Ru(bpy)3(PF6)2

alkyl light

Ir(III) Ni(I)

Ar

R

Ni(0)

X

alkyl

Ir*(III)

SET

Ni

Ni(I)X

X

SET Ir(II)

Ar alkyl

Ar

O alkyl Si O O O

alkyl

Figure 2. Ni/Ir dual catalytic cycle

3.3. Cu-Catalyzed C–C Cross-coupling Reaction


21

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Page 22 of 69

Copper salts, often used as a co-catalyst for transition metal-catalyzed reactions with organosilicon reagents, have high potential as a major catalyst metal like palladium or nickel. Most studied is the cross-coupling. With a CuI/PPh2(2-NMe2C6H4) catalyst system, aryl(triethoxy)silanes couple with iodoarenes to form biaryls (Eq 41).69a The proposed reaction mechanism consists of (1) arylcopper(I) formation by transmetalation of arylsilicates with CuI or alkoxysilanes with CuF, (2) oxidative addition of iodoarenes to arylcopper(I) to form diaryliodocopper(III), and (3) reductive elimination to produce biaryls and regenerates CuI. When iodoheteroarenes are employed, the cross-coupling proceeds with a ligand-free copper catalyst (Eq 42).69b 10 mol% CuI 10 mol% PPh2(2-NMe2 C6H4)

I Ph Si(OEt)3 + (1.0 eq)

Ph (41)

CsF (1.5 eq) DMF, 120 °C, 24 h 55%

MeO

MeO

I

Cl 10 mol% CuI

+ Si(OEt)3

N (1.0 eq)

Cl

CsF (1.5 eq ) DMF, 120 °C

(42)

N 54%

Vinyl(triethoxy)silanes are converted with complete retention of stereochemistry to enynes by the cross-coupling with bromoalkynes in the presence of a Cu cationic catalyst and tetrabutylammonium difluorotriphenylsilicate (TBAT) (Eq 43).70 A combination of CuCl and PPh3 effects the Sonogashira-type reaction between alkynyl(trimethyl)silanes and aryl iodides to give the corresponding internal alkynes (Eq 44).71 Homo-coupling of silylalkynes takes place in the presence of a stoichiometric amount of CuCl, allowing polymerization of a bis(silylethynyl) monomer 10 to give poly(butadiynilidene-p-phenylene) (Eq 45).72


22

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ACS Catalysis

Br

5 mol% Cu(MeCN)4PF6 2.5 mol% TBAT

Si(OEt)3 +

(43)

MeCN, 40 °C, 18 h Br Br

(1.5 eq)

Ph

SiMe3 + I

OMe

96% 10 mol% CuCl 10 mol% PPh3 PhCO2K (1 eq)

OMe (44)

Ph

DMI, 120 °C, 12 h

(1.2 eq)

89%

O n-C9H19

O n-C9H19 CuCl (1 eq)

Me3Si

SiMe3

(45)

DMF, 80 °C, 3 h

n

n-H19C9 O

n-H19C9 O 74% Mn 8900, Mw/Mn 1.78

10

A (Z)-alkenyl(triethoxy)silane [isomeric ratio (i.r.), 80:20] is allylated by allyl bromide in the presence of a CuI catalyst with a high degree of retention of configuration (Eq 46).73 Noteworthy is that the allyl coupling proceeds in preference to 1,4-addition.74 Si(OEt)3 +

20 mol% CuI TBAT (2.4 eq)

Br

(46)

MeCN, rt, 16 h

O

O i.r. (80:20)

73% (i.r. 83:17)

(2.0 eq)

Aryl-HOMSi reagents undergo the copper-catalyzed cross-coupling with allyl, benzyl, or alkyl halides. For example, treatment of aryl-HOMSi 11a with butyllithium followed by reaction with 1-iodobutane in the presence of a catalytic amount of CuI and P(OEt)3 gives a coupled product, butylated arenes (Eq 47).75 Moreover, (Z)-alkenyl(benzyl)dimethylsilanes 12 undergo coupling with alkyl iodides in the presence of stoichiometric amounts of CuI and P(OEt)3 to give 1,2-disubstituted alkenes with perfect retention of configuration (Eq 48).76 OH Ph2N

11a +

Si Me2

I

1) BuLi (1.1 eq) THF, 0 °C

Ph2N (47)

2) 15 mol% CuI 15 mol% P(OEt)3 DMF, 50 °C, 16 h

91%

(1.5 eq)

Ph SiMe2Bn

12 + I

Ph

(48)

DMF, 25 °C OH

(3 eq)

CuI (1.5 eq) P(OEt)3 (1.5 eq) Bu4NFt-BuOH (2.4 eq)

OH 42%


23

ACS Catalysis

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Page 24 of 69

Amii demonstrated in 2009 that trifluoromethyl(triethyl)silane highly efficiently underwent trifluoromethylation of aryl iodides using CuI and a phenanthroline ligand.77 For example, reaction of the trifluoromethylsilane with ethyl p-iodobenzoate proceeds by the catalyst and KF under mild conditions to provide ethyl p-trifluoromethylbenzoate (Eq 49). 10 mol% CuI 10 mol% phen CF3 SiEt3

+

I

CO2Et

CF3

KF (2 eq) NMP/DMI (1:1) 60 °C, 24 h

(2.0 eq)

CO2Et

(49)

89%

Hartwig observed perfluoroalkylCu(phen) complexes were produced upon treatment of perfluoroalkyl(trimethyl)silanes including the Ruppert-Prakash reagent (CF3–SiMe3) with in situ generated a t-BuOCu(phen) complex (Eq 50).78 This method is similar to the synthesis of CF3Cu(NHC) complexes reported by Vicic.79 The perfluoroalkylCu(phen) complexes reacted with aryl iodides smoothly, giving trifluoromethyl- or heptafluoropropylarenes (Eq 51). Hartwig disclosed also that the CF3Cu(phen) reacts with aryl-SiMe(OSiMe3)2 in the presence of stoichiometric amounts of AgF and benzoquinone (BQ) in the open air to provide trifluoromethylarenes (Eq 52).80 Gooßen reported that trifluoromethylcopper reagent, in situ generated by the reaction of CF3–SiMe3 with CuSCN, reacted with aryldiazonium salts to give trifluoromethylated arenes (Eq 53).81

1/4 [CuOt-Bu]4

1,10-phen (1.0 eq)

RFSiMe3 (1.1 eq)

N

benzene, rt, 0.5 h

rt, 18 h

N

Cu RF

(50)

96% (RF = CF3) 97% (RF = CF2CF2CF3) N Ph

I

Cu RF

+ N

Ph

DMF 50 °C, 18 h

RF

83% (RF = CF3) 79% (RF = CF2CF2CF3)

(1.2-1.5 eq) Cl

Cl

AgF (2.0 eq) BQ (3.0 eq)

N SiMe(OSiMe3)2 +

Cu CF3 N

Cl

CF3 (52)

air, DMF 50 °C, 16 h

Cl 70%

(1.2 eq)

R CF3 SiMe3

CuSCN (0.6 eq) Cs2CO3 (1.5 eq) MeCN, rt, 10 min

(51)

CF3 Cu

N2 (1 eq)

BF4 R

CF3 (53)

95%, R = NMe2 68%, R = CN


24

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ACS Catalysis

Hartwig further demonstrated a gram-scale synthesis of the difluoromethyl analogue, HF2C–SiMe3, by reduction of CF3–SiMe3 with NaBH4 (Eq 54).82 The difluoromethylsilane can be used as a difluoromethylation reagent, which reacts with iodoarenes in the presence of stoichiometric amounts of CuI and CsF to produce difluoromethylarenes (Eq 55).

CF3 SiMe3

NaBH4

(54)

CHF2 SiMe3

diglyme, rt

70%

CuI (1.0 eq) CsF (3.0 eq) CHF2 SiMe3 + I

n-Bu

NMP 120 °C, 24 h

(5.0 eq)

n-Bu (55)

CHF2

quant. (90%)

He also observed that difluoro(trimethylsilyl)acetamide 7b underwent cross-coupling with various haloarenes using a copper catalyst in a manner similar to palladium-catalyzed reaction in Eq 34. For example, 7b reacted with 2-bromo-5-ethylpyridine by CuOAc catalyst to form the corresponding difluoro(2-pyridyl)acetamide which is converted to difluoro analogue of pioglitazone (Scheme 2).83 Et

O Me3Si

Et +

N F F

O

20 mol% CuOAc N

O

Br

KF (1.2 eq) NMP, 100 °C

N

N F

F

O

68%

7b (2.0 eq) Et

S N

O F

F

O

O

NH

difluoro pioglitazone analogue

Scheme 2. Cu-catalyzed cross-coupling of 2-bromopyridine with difluoro(trimethylsilyl)acetamides

4. C–N, C–O, and C–S CROSS-COUPLING REACTION 4.1. C–N Cross-Coupling Reaction


25

ACS Catalysis

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Page 26 of 69

The silicon-based cross-coupling is applicable not only to carbon–carbon bond formation but also carbon–nitrogen, –oxygen, and –sulfur bond formations. This particular coupling reaction, named the Buchwald-Hartwig amination after the respective principal investigators, uses unprotected amines and usually needs strong bases, silicon-based C–N bond-forming reaction using N-TMS-amines proceeds with a weak base or fluoride ion. Holmes reported that N-TMS-amine coupled with aryl bromides in the presence of palladium acetate, JohnPhos, and a stoichiometric amount of Cs2CO3 in supercritical CO2 (Eq 56).84 Very recently we disclosed that the C–N bond-forming cross-coupling between aryl bromides or chlorides and N-TMS-amines including N-TMS-aniline proceeded smoothly under Pd(dba)2/XPhos catalysts and CsF in N,N-dimethylimidazolidinone (DMI).85 In the amination with N-TMS-amines, unsubstituted N–H bonds remain intact. This protocol is applied to poly(triaryl)amine synthesis using N-phenylbis(trimethylsilyl)amine and dibromofluorene (Eq 57). Study on reactivity of silylamines shows that N-TMS-diarylamines are more reactive than N-TMS-aryl(alkyl)amines (Eq 58). This reactivity order contrasts sharply to that of the Buchwald-Hartwig amination.86 Me N SiMe3 Ph (1.2 eq) + Br

1 mol% Pd(OAc)2 2 mol% XPhos

Me

Cs2CO3 (1.4 eq) scCO2, 100 °C, 17 h

Ph

(56)

CO2Me

N 90%

CO2Me

Ph N Me3Si SiMe3 (1.00 eq)

Oct Oct 1 mol% Pd(dba)2 2 mol% XPhos

+ Oct Oct Br

Ph N

(57)

CsF (3 eq) DMI, 100 °C, 21 h

n

85%, Mn = 4400, Mw = 9900 Mw/Mn = 2.2

Br (1.00 eq)

Ph

Me N SiMe3 +

Ph (1.1 eq)

N SiMe3 + Br

Me

Ph (1.1 eq)

1 mol% Pd(dba)2 2 mol% XPhos CsF (1.5 eq) DMI, 100 °C, 3 h Me

Ph Me +

N

N

Me

(58)

Ph

Ph 70%

17%


26

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ACS Catalysis

Barluenga reported that N-TMS-imines serve as an imination reagent of aryl bromides in the presence of a palladium catalyst (Eq 59).87 p-An N SiMe3 (1.2 eq) +

4 mol% Pd2(dba)3 8 mol% BINAP

MeO N

NaOt-Bu (1.4 eq) OMe toluene, 90 °C, 14 h

Br

O-Benzoyl-N,N-dibenzylhydroxylamine

(59)

OMe

97%

(Bn2N-OBz)

electrophilically

aminates

nucleophilic reactants 11b in the presence of a CuI/JohnPhos catalyst system and LiOtBu to give an amioarene (Eq 60).88 Smith also reported that in situ generated lithiated aryl-HOMSi (see Eq 19) reacted with Bn2N-OBz under a CuI/JohnPhos catalyst system to form an aniline derivative.44d OH Br + BzO NBn2 Si Me2

(1.2 eq)

5 mol% CuI 10 mol% JohnPhos Br LiOt-Bu (2.0 eq)

(60)

1,4-dioxane, rt, 4 h

NBn2 77%

Pt-Bu2

11b

JohnPhos

4.2. C–O Cross-Coupling Reaction Vinyl(trialkoxy)silanes can behave as an alkoxylation reagent of aryl halides. Chlorine in 4-chloroacetophenone is substituted by a methoxy group with Pd(OAc)2/1-di-tertbutylphosphino-1’-diphenylphosphinoferrocene (dtbdppf) and NaOH under microwave heating (Eq 61).89 Aryl bromides are good electrophiles for this particular alkoxylation reaction.

Ac Si(OR)3 + X (1.5 eq)

1 mol% Pd(OAc)2 2 mol% dtbdppf 1.25 eq NaOH xylene (MW) 120 °C, 20 min

Ac (61) RO 74% (X = Cl, R = Me) 77% (X = Br, R = Et)


27

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Page 28 of 69

Combination of silver(I) and Cu(OAc)2 catalysts under an aerobic atmosphere enables tetraalkoxysilanes to act as an alkoxylation reagent for ortho-functionalized potassium benzoates, producing alkoxyarenes ipso-selectively via decarboxylation (Eq 62).90 The silver catalyst induces the decarboxylation of potassium benzoates to form an arylsilver which reacts with tetraalkoxysilanes and Cu(OAc)2 to provide the alkoxylated products. KO 2C

25 mol% Ag 2CO 3 Cu(OAc) 2 (1.0 eq)

MeO

O 2N

O2 DMF, 145 °C, 18 h

O 2N 84%

Si(OMe)4 + (1.2 eq)

(62)

In contrast to the above alkoxylations, aryl(trimethyl)silanes can be converted oxidatively to acetoxyarenes as shown by Kitamura who reported that 4-fluoro(1trimethylsilyl)benzene reacted with PhI(OCOCF3)2 in acetic acid in the presence of a Pd(OAc)2 catalyst to give 4-acetoxy(1-trimethylsilyl)benzene via a palladium(IV) complex which smoothly transmetalated with arylsilanes (Eq 63).91

SiMe3

5 mol% Pd(OAc)2 PhI(OCOCF3) (1.5 eq) AcOH 80 °C, 17 h

F

OAc (63) F 90%

4.3. C–S Cross-Coupling Reaction Hartwig showed that triisopropylsilyl phenyl sulfide, prepared by palladium-catalyzed silylation of aryl bromides using triisopropylsilylthiol, reacts with bromoarenes in the presence of Pd(OAc)2/CyPF-t-Bu, and diaryl sulfides are obtained (Eq 64).92 H-S-Sii-Pr3 (1 eq) 0.1 mol% Pd(OAc)2 0.1 mol% CyPF-t-Bu Ph Br

LiHMDS (1.1 eq) toluene, 90 °C, 2 h Pt-Bu2 Fe

Ph

S

Sii-Pr3

91%

Br-C6H4-p-Me (1 eq) 1 mol% Pd(OAc)2 S 1 mol% CyPF-t-Bu Ph CsF (3 eq) DME, 70 °C, 12h

(64) 88%

PCy2 CyPF-t-Bu


28

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ACS Catalysis

Asymmetric sulfoxides are accessible by use of aryl or alkyl 2-(trimethylsilyl)ethyl sulfoxides as a sulfenate anion equivalent.93 Treatment of 2-(trimethylsilyl)ethyl sulfoxide with CsF and then with alkyl or aryl halides with the aid of a palladium catalyst under mild conditions give a variety of dialkyl, alkyl aryl, and diaryl sulfoxides. For example, phenyl 2(trimethylsilyl)ethyl sulfoxide is allowed to react with 1-bromo-4-chlorobenzene in the presence of Pd(dba)2, SSphos, and CsF as a base to form 4-chlorophenyl phenyl sulfoxide (Eq 65).93b

Ph

2 mol% Pd(dba)2 4 mol% SSPhos CsF (3 eq)

Br

O S

+ SiMe3

Cl

Ph

O S

(65)

2-MeTHF 80 °C, 24 h

Cl

(2 eq)

85% Cy2P OMe

MeO

SO3Na

S

SPhos

Trifluoromethylthiolato Cu(bpy) complex is readily prepared by reaction of CF3–SiMe3, CuF2, bpy, and S8, in a manner similar to synthesis of perfluoroalkylato Cu(phen) reagents. The trifluoromethylcopper reagent reacts with iodoarenes or electron-deficient bromoarenes to give trifluoromethylthioarenes (Eq 66).94 Formation of a monomeric bpy-ligated copper complex is thermodynamically more stable than a dimeric one, thus being active for this coupling reaction. CF3 SiMe3 + CuF2

I t-Bu

+ S8

R

N

+ bpy

Cu SCF3

(1 eq)

CF3S (66) R

N t-Bu

76% (R = Me) 84% (R = Ac)

5. ADDITION REACTION 5.1. Conjugate Addition


29

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Page 30 of 69

Like organoboron reagents, organosilicon reagents can be employed for enantioselective 1,4-addition to enones by a suitable palladium catalyst with a chiral ligand. Miyaura demonstrated for the first time such reaction using aryl(trifluoro)silanes with a S,S-chiraphosligated palladium dication catalyst.95 For example, cyclohexenone undergoes the Michael addition of phenyl group by phenyl(trifluoro)silane to give (S)-3-phenylcyclohexanone (Eq 67). O Ph SiF3 + (2.0 eq)

3 mol% [Pd(S,S-chiraphos)(MeCN)2](PF6)2 ZnF2 (1 eq)

O (67)

MeOH/H2O, rt Ph Me Ph2P

Me

83% (94% ee)

PPh2

S,S-chiraphos

Organorhodium complexes are employed effectively as intermediates for 1,4-addition to enones, producing a wide variety of β-functionalized ketones. Combination of Rh metal and a chiral ligand allows for asymmetric 1,4-addition to form optically active ketones.96 Organosilicon reagents are equally applicable to the asymmetric 1,4-addition reaction. In 2003, Oi demonstrated that aryl(trimethoxy)silanes proceed asymmetric 1,4-addition to enones, α,β-unsaturated esters and acrylamides, by a rhodium cationic complex and (S)BINAP ligand without any base in 1,4-dioxane/H2O (Eq 68).97 Since rhodium catalysts are effective for hydrosilylation of alkynes to result in the regio- and stereoselective formation of vinylsilanes, terminal alkynes are available as alkenylating reagents for the 1,4-addition. Hayashi disclosed that one-pot sequential hydrosilylation of terminal alkynes with triethoxysilane and asymmetric 1,4-addition to enones gave chiral β-alkenylated ketones via formation of terminal vinylsilanes (Eq 69).98 Both reactions are catalyzed by the same rhodium complex. Although organosilicon reagents can also be used as 1,4-addition partners, common silicon reagents are less reactive in a transmetalation from silicon to rhodium. We found that organo-HOMSi reagents undergo transmetalation with rhodium hydroxide


30

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ACS Catalysis

smoothly without any base and conjugate addition of the organic group to enones under mild conditions.6a The reaction proceeds in a rate comparable with the reaction of phenylboronic acid/a base activator.99 Namely, a p-pinacolatoborylphenyl group could be introduced into enones via selective C–Si bond activation under base-free catalytic conditions, leaving the boryl group intact (Eq 70). Moreover, we observed that organo-HOMSi reagents can react rhodium(I) chloride complexes smoothly using a catalytic amount of KOH and demonstrated that aryl- and ethenyl-HOMSi reagents underwent enantioselective arylation or alkenylation using a [RhCl(ethylene)2]2/(R,R)-Bn-bod* catalyst system (Eq 71). This is advantageous in view that the conjugated addition using corresponding vinylboronic acids often face instability problems.100 Aryl(triallyl)silanes also are shown to undergo conjugate addition to enones smoothly by a [RhCl(cod)]2 catalyst (Eq 72).101 Ph Si(OMe)3 4 mol% [Rh(cod)(MeCN)2 ](BF4) 6 mol% (S)-BINAP

(2.0 eq) + O

Ph

NH2 75% (81% ee)

NH2

Ph

H

O (68)

1,4-dioxane/H2O (10:1) 90 °C, 20 h

H−Si(OEt)3 (1.5 eq) 1.2 mol% [Rh(S)-BINAP(MeCN)2]BF4 0.4 mol% (S)-binap 1,4-dioxane, rt, 1 h

Ph

Si(OEt)3

(E/Z = 74:1)

2-cyclohexenone (0.4 eq)

Ph

O (69)

H2O 90 °C, 20 h rt, 0.5 h one pot

89% (93% ee)

OH (pin)B

11c +

Si Me2

(nip)B O

0.5 mol% [RhOH(cod)]2 O

(70)

THF, 35 °C, 18 h 94% (97% ee)

(1.0 eq) HO Si Me2 1b +

1.5 mol% [RhCl(C2H4)2]2 3.3 mol% (R,R)-Bn-bod* O

O (71)

15 mol% 1 M KOH aq. THF, 40 °C,10 h 70% (94% ee) Ph

(1.0 eq) Ph (R,R)-Bn-bod* p-tolyl

Si(allyl)3 2 mol% [RhCl(cod)]2 TBAF3H2O (0.5 eq)

(2.0 eq) + O Ph

1,4-dioxane/H2O (10:1) 50 °C, 20 h

p-tolyl

O (72)

Ph

OMe 84%

OMe


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Page 32 of 69

Aminomethyl radicals are readily generated by photosensitized oxidation of (silylmethyl)amines.102 This elemental process is applicable to enantioselective 1,4-addition. As shown in Eq 73, aminomethyl radicals generated by a photoredox activation of (silylmethyl)amines by Ru(bpy)3Cl2 catalyst are allowed to add to a crotonylimide 13 in the presence of a Lewis acid catalyst Sc(OTf)3, a chiral t-BuPybox ligand, and tetrabutylammonium chloride (TBAC) to give the corresponding adduct with high ee.103

Ph

N

SiMe3 + O

O

N EtN

15 mol% Sc(OTf)3 20 mol% (S,S)-i-BuPybox N 30 mol% TBAC Ph 2 mol% Ru(bpy)3Cl2 visible light MeCN

O

O

N EtN

(73)

87% (93% ee)

13 (2.0 eq) O

O

N N

N

i-Bu

i-Bu (S,S)-i-BuPybox

5.2. Addition to Aldehydes and Ketones Shibasaki disclosed highly enantioselective 1,2-addition of vinyl(trimethoxy)silanes to aldehyde employing copper-catalyst and (R)-DTBM-SEGPHOS as a chiral bidentate bisphosphine ligand (Eq 74).104 Before this paper, there were few reports on copper-catalyzed transformation of organosilicon reagents. The reaction shown here is assumed to involve a vinylcopper species generated by transmetalation from silicon to copper. Si(OMe)3 (2.0 eq) +

1) 3 mol% CuF22H2O 3 mol% (R)-DTBM-SEGPHOS DMF, 40 °C

OH (74)

2) TBAF

O

Cu

H

99% (94% ee)

via t-Bu OMe

O O O

P P

t-Bu t-Bu

O

2

OMe t-Bu

2

(R)-DTBM-SEGPHOS


32

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ACS Catalysis

Allylsilanes are used for allylation of aldehydes by a Lewis acid catalyst such as TiCl4, known as the Hosomi-Sakurai reaction. Yamamoto disclosed that AgF and a chiral bisphosphine ligand catalyst system allow for enantioselective addition of allylsilanes to aldehydes with γ and anti selectivities (Eq 75).105 The reaction is assumed to proceed via a 6membered cyclic transition-state between aldehydes and allylsilver, generated in situ from transmetalation of allylsilanes to AgF. Ketones also are applicable to the Ag-catalyzed asymmetric allylation (Eq 76).106 Si(OMe)3 (2.0 eq, E/Z = 45:55) 10 mol% AgF 6 mol% (R)-p-Tol-BINAP + MeOH, -20 °C, 7 h to rt, 17 h O

OH (75) 99% {anti (% ee)/syn (% ee) = 93 (94)/7 (60)}

H

Si(OMe)3 (2.0 eq) + O

5 mol% AgF 5 mol% (R)-DIFLUOROPHOS

OH (76)

MeOH (1 eq) THF, -78 °C, 12 h 98% (95% ee) F F

O

F F

O

O

PPh2 PPh2

O

(R)-DIFLUOROPHOS

In 2014, Ni/NHC complex was shown by Ogoshi107a to catalyze intramolecular addition of arylsilanes to aldehydes to give 3-aryl-2,1-benzoxasiloles 14. The key intermediate of this reaction was shown to be a η2-aldehyde nickel complex which definitely stimulated the carbonyl oxygen to interact with the silicon center and promote the migration of phenyl. Asymmetric formation of benzoxasiloles was also achieved using chiral NHC, (R,R)-L2• HBF4, to obtain the corresponding cyclic silyl ether of high ee (Eq 77).107b The product crosscouples

with

an

aryl

iodide

in

the

presence

of

a

palladium/1,2-

bis(dicyclohexylphosphino)ethane (DCPE) catalyst to form 2-(α-hydroxybenzyl)biaryl without loss of ee.


33

ACS Catalysis

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H

2 mol% Ni(cod)2 2.2 mol% (R,R)-L2HBF4 2 mol% NaOt-Bu

O Ph

toluene, rt, 1 h

Si Me2 (5 mmol) Ph N

Page 34 of 69

H

Ph

Ph O Si Me2

N

BF4 (R,R)-L2 HBF4

I-C6H4-p-OMe (1.2 eq) 8 mol% Pd(OAc)2 16 mol% DCPE 2 M aq. NaOH 1,4-dioxane 65 °C, 16 h

Ph OH (77) OMe 74%, 98% ee

14, 99%, 98% ee

5.3. Radical Addition to Alkenes Oxidative trifluoromethylation of terminal alkenes with CF3–SiMe3 takes place in the presence of copper thiophene-2-carboxylate (CuTC) catalyst and PhI(OAc)2 and K2CO3 to give corresponding allyl trifluoromethylsilanes (Eq 78).108 The combination of Cu catalyst with PhI(OAc)2 is proposed to oxidize first terminal alkenes to a radical cation intermediate, which is attacked by a trifluoromethyl anion to give trifluoromethylated alkyl radicals. This is further one-electron oxidized and deprotonated to give final products. CF3 SiMe3 (4.0 eq) +

O O

30 mol% CuTc PhI(OAc)2 (2.0 eq) K2CO3 (4.0 eq) NMP 80 °C, 24 h

CF3 (78) O 72% (E:Z = 7.7:1)

O

Hydrotrifluoromethylation of terminal alkenes proceeds with a silver catalyst and PhI(OAc)2, NaOAc, and 1,4-cyclohexadiene (1,4-CHD), giving rise to trifluoromethylated alkanes (Eq 79).109 The selectivity leading to hydrotrifluoromethylation is enhanced by the presence of 1,4-CHD, a sufficient H donor. To increase yield of products and complete the reaction, CF3–SiMe3, PhI(OAc)2, NaOAc, and 1,4-CHD are added in two portions (4 h interval).


34

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ACS Catalysis

CF3 SiMe3

CF3

(8.0 eq) +

O

O

20 mol% AgNO3 H PhI(OAc)2 (4.0 eq) NaOAc (4.0 eq) 1,4-CHD (2.0 eq) NMP, rt

(79) O 72%

O

6. C–H ACTIVATION AND CROSS-COUPLING REACTION Direct transformation of a C–H bond to a C–C bond is a synthetically potential tool in view of atom- and step-economy. Organosilicon reagents are applicable to this kind of C–H activation/cross-coupling sequence as evidenced by Shi, who reported that ortho-C–H bond cleaving arylation in acetanilide proceeded by use of aryl(trimethoxy)silanes in the presence of Pd(OAc)2, Cu(OTf)2 as an oxidant for the palladium catalyst, and AgF to form orthoarylated products (Eq 80).110 Similar C–H arylation reaction was reported by Loh, who used Pd(OAc)2 and AgF (Eq 81).111 Yu demonstrated that a methyl C–H bond in α-branched alkanamides could be selectively converted into an sp3C–aryl bond by aryl(triethoxy)silanes due to a C(O)NHC6F4-p-CF3 directing group and a catalyst consisting of Pd(OAc)2 and an oxyquinoline ligand L3, forming methyl-arylated products (Eq 82).112

Ph Si(OMe)3 +

HN

5 mol% Pd(OAc)2 Cu(OTf)2 (2.0 eq)

O H

HN

O Ph

AgF (2.0 eq) 1,4-dioxane, 110 °C

(2.0 eq)

(80)

74%

Ph Si(OMe)3 +

HN

O

10 mol% Pd(OAc)2 AgF (3 eq)

HN

O Ph

(81)

NHArF

(82)

1,4-dioxane, 80 °C

(3 eq) 73% H

Ph Si(OEt)3 (2.0 eq)

+

10 mol% Pd(OAc)2 NHArF 20 mol% L3 AgF (3.0 eq) O KHCO3 (2.0 eq) ArF = 4-CF3C6F4 1,4-dioxane, 110 °C

Ph

O 67%

t-Bu N L3

O

Although the C–H bond functionalization is a straightforward method for construction of complex molecules, this approach often needs the presence of an appropriate directing group such as carbonyl, whereas Zhang showed direct C2 arylation in N-methylindole using


35

ACS Catalysis

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Page 36 of 69

aryl(trimethoxy)silanes, Pd(OAc)2 catalyst, Ag2O, and TBAF (Eq 83).113 Georg was successful in C5 arylation of 2,3-dihydropyridin-4(1H)-ones with aryl(triethoxy)silanes (Eq 84).114 Selective C3-arylation of thiophenes was demonstrated by Oi (Eq 85),115 who found that C–H arylation of such π-extended aromatics as naphthalene and phenanthrene took place by use of aryl(trimethyl)silanes under PdCl2 catalyst and stoichiometric CuCl2.116 For example,

the

C9

position

of

phenanthrene

is

selectively

arylated

by

p-

bromophenyl(trimethyl)silane with the bromine being intact (Eq 86). Similarly, 9-silafluorene was applied by Itami to double arylation of phenanthrene, pyrene, and their analogues. An example is a K-region selective annulation of 2,7-di-tert-butylpyrene with 9,9-dimethyl-9silafluorene in the presence of Pd(MeCN)4(SbF6)2 as a catalyst and o-chloranil (Eq 87).117 10 mol% Pd(OAc)2 Ag2O (3 eq) TBAF (3 eq) Ph Si(OMe)3

+

N

Ph

(83)

N

AcOH/EtOH (5:2), rt, 18 h

(3.0 eq)

82%

Ph Si(OEt)3 + Bn

N

25 mol% Pd(OAc)2 O CuF (2.5 eq ) 2

Ph

t-BuOH/AcOH (4:1) 65 °C, 3 h

Bn

(2.5 eq)

O N

(84) 85%

5 mol% PdCl2(MeCN)2 CuCl (2 eq) Ph SiMe3

+ S

DCE, 80 °C, 16 h

Ph (85) S 80% (93% β)

(2.0 eq)

Br Br

5 mol% PdCl2 CuCl2 (2 eq)

SiMe3 +

(86)

DCE, 80 °C, 16 h 72%

(2.0 eq)

Si Me2 (3.0 eq)

5 mol% Pd(MeCN)(SbF6)2 o-chloranil (10 eq)

+

DCE, 80 °C, 1 h

t-Bu (87)

t-Bu

Cl t-Bu

t-Bu

Cl

O

Cl Cl

O o-chloranil

83%

Nickel also is an effective catalyst for C–H bond-cleaving arylation. Miura disclosed that aryltrimethoxysilanes reacted with oxazoles and their analogues in the presence of a nickel catalyst, bpy ligand, CsF, and CuF2 to form 2-arylated products (Eq 88).118


36

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ACS Catalysis

10 mol% NiBr2(diglyme) 10 mol% bpy CsF (3.0 eq) CuF2 (2.0 eq)

N

H O Ph Si(OMe)3 +

N

Ph O

(88)

DMAc, 150 °C, 2.5 h

(2.0 eq) CF3

73%

CF3

Like palladium and nickel, rhodium is an effective catalyst for aryl C–H bond cleavage and thus straightforward arylation by organosilicon reagents is readily attained. For example, aryl(trialkoxy)silanes react with 1-(2-pyrimidyl)indole at C2 to give 2-arylindoles (Eq 89).119 The pyrimidyl group on nitrogen acts as a directing group, which definitely participates in C2–H bond cleavage by a rhodium catalyst. The resulting five-membered rhodacycle reacts with arylsilicates generated from arylsilanes and a fluoride ion. Not only 1-(2pyrimidyl)indoles but also 2-pyridyl-, pyrazolyl-, and 2-pyrimidylbenzenes, and 1azaphenanthrene are good substrates of the ortho-arylation.

2 mol% [Cp*RhCl2]2 H AgF (2 eq)

N

p-An Si(OMe3)3 + N

N

N

Cu(OAc)2 (2 eq) THF/H2O (1:1) 80 °C, 24 h

N

OMe (89) N 92%

(2 eq)

Trifluoromethylsilver species can be generated in situ from the reaction of a Ag(I) source such as AgF and AgOTf with CF3–SiMe3 and a base. The AgCF3 species is applied to various trifluoromethylation reaction of aryldiazonium salts120 and alcohols121,122 to produce trifluoromethylarenes

and

trifluoromethyl

ethers,

respectively.

Moreover,

ortho-

trifluoromethylation of aryltriazenes proceeds with high functional group tolerance (Eq 90).123 The fact that experiments with TEMPO as a radical scavenger decreased the yield of products suggests that this ortho-trifluoromethylation takes place via the generation of the CF3 radicals as reactive species. CF3 SiMe3 (2.0 eq) + N I

H

N

AgF (4 eq) i-Pr N i-Pr

C6F14 100 °C, 4 h

N I

N

i-Pr N i-Pr (90)

CF3 64%


37

ACS Catalysis

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Page 38 of 69

(Trimethylsilyl)difluoromethyl esters react with N-phenylmethacrylamides in the presence of stoichiometric amounts of Ag(I) and PhI(OAc)2 to give 1 : 1 adducts, 3,3disubstituted oxindoles via ortho-C–H bond cleavage.124 An Ag-catalytic version of this reaction

was

reported

afterwards.125

soon

As

shown

in

Eq

91,

difluoro(trimethylsilyl)acetamides starts the reaction by an AgOAc catalyst and PhI(OAc)2 oxidant to give a corresponding oxindole. The reaction is suggested to generate first a difluoroacetamidated silver by the reaction of 7 with AgOAc. The silver species is oxidized by PhI(OAc)2 to form PhI(OAc){CF2C(O)NEt2}, which releases a difluoroacetamidated radical, which adds to the terminal methylene carbon of methacrylamide to produce an αalkyl radical, cyclization of which followed by oxidation give the final cyclic product. O Me3Si

NEt2 F F 15 mol% AgOAc 7a (5.0 eq) PhI(OAc)2 (3.0 eq) + MeCN, 80 °C, 24 h N

F O

F CONEt2 (91)

N 83%

O

Russell disclosed that electron-deficient aryl(trimethyl)silanes underwent direct C–H arylation of electron-rich arenes with high regioselectivity via C–Si bond cleavage by Ph3PAuOTs catalyst, PhI(OAc)2, and CSA (Eq 92).126a Iodine and bromine did not interfere with the reaction. This method was applied to rapid access to synthesis of various biaryls such as a nonsteroidal anti-inflammatory agent, diflunisal. Mechanism of the reaction is considered to involve ipso-substitution of arylsilanes with gold(III) followed by SEAr reaction of electron-rich arenes. Details of this C–H bond cleaving arylation reaction are discussed based on experimental evidences.126b

SiMe3

OMe +

I

H (1.0 eq)

Br

2 mol% Ph3PAuOTs PhI(OAc)2 (1.3 eq) CSA (1.5 eq) CHCl3/MeOH (50:1) rt

OMe Br

(92)

I 67%


38

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ACS Catalysis

Formal double C–H bond-cleaving cross-coupling is feasible.127 First, Ir-catalyzed reduction of phenyl acetate with diethylsilane followed by Rh-catalyzed ortho-silylation of phenyl acetate using diethylsilane128 gives benzodioxasiline 15, which reacts with MeLi to give ortho-diethylmethylsilyl phenol. After tosylation of the phenolic hydroxy group, Aucatalyzed oxidative coupling with methyl thiophene-2-carboxylate results in formation of 2thienylphenyl tosylate (Scheme 3).

OAc

1) H2SiEt2 (2 eq) 0.1 mol% [IrCl(coe)2]2 THF, rt to 60 °C 2) 0.4 mol% [RhCl(nbd)]2 2.4 mol% P(p-MeO-C6H4)3 norbornene (2 eq) THF, 12 °C

OX

Et2 Si Me

O

O SiEt2

MeLi (3 eq) THF, -78 °C to rt

15, 95%

CO2Me (2 eq) S 1 mol% Ph3PAuOTs PhI(OAc)2 (1.5 eq) CSA (1.3 eq)

OTs S

CO2Me

CHCl3/MeOH (25:1) 70 °C 83% (X = H) 96% (X = Ts)

Ts2O pyridine CH2Cl2

92%

Scheme 3. Sequential ortho-silylation and Au-catalyzed direct arylation

7. SMALL RING SILACYCLES FOR ORGANIC SYNTHESIS Small ring molecules such as cyclopropane undergo ring-opening reaction with various electrophilic reactants smoothly by relieving the ring-strain. Actually, oxidative addition of a divalent platinum complex into cyclopropane forms platina(IV)cyclobutanes.129 Silacyclopropanes having two tert-butyl groups on silicon are photochemically or thermally converted smoothly to di-tert-butylsilylene, which reacts with alkenes to give different silacyclopropanes.130 This silylene transfer reaction is smoothly accelerated by metal catalysts such as silver.131,132 On the other hand, combination of silacyclopropanes with a transition metal catalyst effects a variety of transformations. Woerpel reported a series of


39

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Page 40 of 69

reactions using silacyclopropanes catalyzed by metal complexes. For example, 2,3-transdimethyl-1,1-di-tert-butylsilacyclopropane reacts with phenylacetylene in the presence of PdCl2(PPh3)2 catalyst at 23 °C to give a 3,4-diphenylsilole regioselectively due to steric repulsion by tert-butyl groups (Eq 93),133a whereas its cis-isomer reacts with diphenylacetylene

to

form

2,3-diphenylsilacyclopropene

(Eq

94).133b

Methylenesilacyclopropane undergoes cycloaddition with terminal acetylenes to give 4methylenesilacyclopentenes (Eq 95).133d The starting material is accessible by Ag-catalyzed silacyclopropanation of allenes using cyclohexenesilacyclopropane.134 The ring-expansion reaction is applicable to internal alkynes, terminal alkenes, and allenes. The silacycle formation from silacyclopropanes is triggerred by oxidative addition of C–Si bonds to the palladium(0) catalysts to generate palladasilacyclobutane intermediates. Following insertion of alkynes finally gives rise to corresponding products.133c

t-Bu

0.2-3.0 mol% PdCl2(PPh3)2

t-Bu Si

Me

+ Ph

Me

benzene 23 °C, 48 h

t-Bu

(93) Ph

(2.0-3.5 eq)

t-Bu

Ph

t-Bu Si

Me

+ Me

t-Bu

Ph (1.0 eq)

t-Bu + Ph n-Hex (2 eq)

Ph 83%

0.5 mol% PdCl2(PPh3)2 benzene 117 °C, 17 h

t-Bu

t-Bu Si

(94)

Ph

Ph 84% t-Bu

5 mol% Pd(PPh3)4

Si

t-Bu Si

toluene 22 °C

t-Bu Si (95)

n-Hex

Ph 89%

In 1975, Sakurai reported that 1,1-dimethyl-1-silacyclobutane undergoes an addition reaction to alkynes via C–Si bond cleavage to give a silacyclohexne (Eq 96).135 The reaction with terminal alkynes such as methyl propiolate proceeds regioselectively. A mechanism is proposed which involves formation of a palladasilacyclopentane by oxidative addition of C– Si bond of the silacyclobutane, followed by insertion and reductive elimination.136 Various four-membered silacycles such as benzosilacyclobutenes and methylenesilacyclobutenes also react with alkynes by a palladium catalyst to give the corresponding silacyclohexenes.137


40

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ACS Catalysis

Instead of alkynes, the reaction with enones formed eight-membered cyclic silyl enolates (Eq 97).138 Substrates 16 having a cyclobutanone and a silacyclobutane undergoes skeletal exchange via C–C and C–Si bond cleavage (Eq 98).139 A bulky ligand, P(1-Ad)2(n-Bu), selectively gives eight-membered silacycles 17 as a single cis-diastereomer, whereas a small ligand PMe3 prefers silahydroindane 18 selectively as a single diastereomer. CO2Me Si Me2

CO2Me 1 mol% PdCl2(PPh3)2

+

benzene, reflux, 2 h R

(96)

Si R Me2 R = CO2Me, 95% R = H, 35%

n-Pr Si Me2

7.5 mol% Pd(OAc)2 15 mol% PCy3

+

n-Pr (97)

THF, reflux, 20-24 h O

Ph

Si O Me2 80%

Ph

O

10 mol% Pd(allyl)Cp 20 mol% P(Ad)2(n-Bu) xylene, 150 °C, 24 h O Si 16

Si 17, 76%

5 mol% Pd(allyl)Cp 20 mol% PMe3 xylene, 130 °C, 3 h

(98) O

Si 18, 87%

The reaction of silacyclobutanes is applicable to asymmetric synthesis of a variety of 6membered silacycles with a chiral silicon center. Shintani and Hayashi achieved in 2011 asymmetric intramolecular cycloaddition in the presence of an optically active phosphoramidite ligand (S,S,S)-L4 to give the corresponding polycycles having optically active silicon center at the bridge-head (Eq 99).140 This catalytic conditions using (S,S,S)-L4 were applied to the intermolecular version to form chiral 1-silacyclohex-2-enes (Eq 100).141


41

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p-An Si

5 mol% Pd(allyl)Cp 10 mol% (S,S,S)-L4

Page 42 of 69

p-An Si (99)

toluene, 30 °C, 48 h

Ph

Ph 93% (92% ee) Me Ph Me

O P N O

Me

Ph Me (S,S,S)-L4

p-An Si

CO2Me 5 mol% Pd(allyl)Cp p-An 5.5 mol% (S,S,S)-L4 Si + toluene, 10 °C, 14 h CO2Me (1.2 eq)

CO2Me (100) CO2Me

95% (92% ee)

In a manner similar to palladium-catalyzed reaction, C–Si bonds in silacyclobutanes are cleaved by a nickel catalyst and react with unsaturated bond compounds take place. Yorimitsu and Oshima reported that silacyclobutanes reacted with aldehydes in the presence of

an

Ni(cod)2/PPh2Me

catalyst

to

give

alkoxyallylsilanes

(Eq

101).142

When

benzosilacyclobutenes are used for the reaction with aldehydes, six-menbered oxasilacycles are produced (Eq 102). The total transformation is understood in the following way. η2Coordinated nickel complexes, or its resonance form nickelaoxacycles, is generated by interaction of nickel(0) with the formyl group. Subsequent transmetalation with the fourmembered silacycles forms 7-membered cyclic intermediates. In the case of silacyclobutanes, β-H elimination is followed to provide an oxyalkyl nickel hydrides whose reductive elimination produces alkoxyallylsilanes. On the other hand, benzosilacyclobutene-based intermediates, where β-H elimination is impossible, undergo reductive elimination to produce six-membered oxasilacycles.


42

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ACS Catalysis

Si Me2

5 mol% Ni(cod)2 10 mol% PPh2Me

+

R

O

toluene 100 °C, 20 h

O Si Me2

(101)

R = Ph, 88% R = (CH2)2Ph, 73% R = c-C6H11, 69% R = trans-CH=CHPh, 51%

5 mol% Ni(cod)2 10 mol% PPh2Me

+ SiMe2 O

R

SiMe2 O

Ph toluene 100 °C, 20 h

(102)

Ph 65%

Ni

O R

Ni

SiMe2 O

SiMe2

SiMe2 O

Ni

O

R

R

R SiMe2 O

O Si Me2

Ni H

R

R

Silylation of terminal alkenes proceed using silacyclobutanes under Ni(cod)2/PCy3 catalytic conditions to give the corresponding vinylsilanes regio- and stereoselectively (Eq 103).143 A reaction mechanism is proposed which involves a nickelasilacyclobutane by oxidative addition, alkene insertion to form seven-membered nickelasilacycloheptanes, finally β-H elimination and reductive elimination to produce vinylsilanes. In case of the reaction of benzosilacyclobutenes with styrene, the Ar–Si bond is activated to form mainly benzylvinylsilanes (Eq 104).

+ Si Me2

R

5 mol% Ni(cod)2 10 mol% PCy3 toluene 100 °C, 12 h

R Ni SiMe2

SiMe2 + Ph

R

Si Me2

(103)

R = CO2CH2Ph, 95% R = Ph, 98% R = n-C12H25, 93% R H Ni

Ni SiMe2

5 mol% Ni(cod)2 10 mol% PCy3 toluene 100 °C, 12 h

R

SiMe2

Si Me2

Ph (104)

64%

Simultaneous cleavage of C–Si and C–H bonds in silacyclobutanes takes place upon exposure to a rhodium catalyst and is applied to intramolecular annulation reaction. In the presence of a Rh/TMS-segphos catalyst, 2-silacyclobutan-1-ylbiphenyl 19 is transformed to 9-methyl-9-propyl-9-silafluorenes 20 (Eq 105).144 This reaction is initiated by C–Si bond


43

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Page 44 of 69

cleavage to form silarhodacycle I, which is converted to allylsilylrhodium II via β-H elimination. The 2’-C–H bond in II oxidatively adds to Rh to give a 6-membered rhodacycle III. Final reductive elimination leads to silafluorenes. When a 2’,6’-dimethylphenylsubstituted substrate is applied, the benzylic methyl C–H bond is cleaved to give 9siladihydrophenanthrene (Eq 106). 5 mol% [RhCl(cod)]2 10 mol% TMS-segphos

Si Me

Me Si

(105)

toluene, 80 °C, 12 h 20, 84%

19

H Ph

Si Rh

Si Rh Cl β-H elimination - HCl I

Si Rh

II

Si

Me

III Me Si

5 mol% [RhCl(cod)]2 10 mol% TMS-segphos

(106) toluene, 80 °C, 48 h 91% SiMe3 O O O

P P

SiMe3 SiMe3

2

O SiMe3

2

TMS-segphos

Cramer observed that tert-cyclobutanols 21 equipped with a 2-silylphenyl group at C3 position rearranged by a Rh/chiral bidentate ligand L5 complex through β-C elimination and 1,4-silicon shift from phenyl to alkyl to give indanol derivatives 22 with high diastereo- and enantioselectivity (Eq 107).145 The complex rearrangement is understood by β-carbon elimination from alkoxyrhodium complex IV to generate alkylrhodium V, subsequent oxidative addition of the Ar–Si bond to the rhodium center to give trivalent rhodacycle VI in preference to an activation of the opposite Ar–H bond, and reductive elimination to form arylrhodium VII accompanied by the carbon–silicon bond formation. Finally, an intramolecular 1,2-addition of the phenylrhodium to ketone carbonyl gives alkoxyrhodium trans-VIII, which is finally protonated to give rise to products 22. This transformation is applied to synthesis of tricycles 23 using the Rh/L5 catalyst (Eq 108). Starting compound cis-


44

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ACS Catalysis

21 is considered to be first converted to 22 upon treatment with the same Rh catalyst and Cs2CO3 through a Si–Me bond cleavage. HO R

2.5 mol% [RhOH(cod)]2 6.0 mol% L5

Si R

(107)

mesitylene, 100 °C Me

Si Me trans-22

OH trans-21 F

O

F

O

F

O

F

O

R = Et, Si = SiMe3 82%, d.r. = 6:1, 97% ee R = t-Bu, Si = SiMe3 82%, d.r. = 20:1, 96% ee R = CH=CH2, Si = SiMe2(CH=CH2) 75%, d.r. = 19:1, 99% ee

PPh2 PPh2

L5 Si

Si

Rh

R Me

O Rh

O Me

IV

R

V

P Si P Rh

Rh

Rh O R

Me O R

O Me Si

VI

1) 2.5 mol% [RhOH(cod)]2 6.0 mol% L5 mesitylene SiMe3 100 °C, 5 h OH 2) 5 mol% Me Et [RhOH(cod)]2 Cs2CO3 (3.0 eq) cis-21 mesitylene 130 °C, 8 h

R Me

VII

Si

trans-VIII

Et OX

Et O SiMe2 (108)

Me SiMe3

Me 23, 57%, 96% ee

X = H, cis-22 X = Rh, cis-VIII


45

ACS Catalysis

8. MISCELLANEOUS 8.1. Synthesis of 1,1-Difluoroethenes The combined system of the Ruppert-Prakash reagent and a copper catalyst was successfully

applied

to

synthesis

of

1,1-difluoroethenes.

Diaryl-

and

alkyl(aryl)diazomethanes react with CF3–SiMe3 in the presence of catalytic amounts of CuI and CsF to give 2,2-disubstituted 1,1-difluoroethenes (Eq 109).146 When CF3CF2–SiMe3 is used, 1-fluoro-1-trifluoromethylethenes are produced. Similar transformation is the reaction of diazo compounds with CF2Br–SiMe3 catalyzed by tetrabutylammonium bromide (TBAB) to give 1,1-difluoroethenes as reported in 2016.147 This reaction is applied electron-deficient aldehydes as well as ketones. Br

CF 3 SiMe3

+

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 46 of 69

Br 5 mol% CuI 5 mol% CsF

N2

1,4-dioxane/NMP (10:1) rt, 10 h

(109)

F 2C

(2 eq) Br

75%

Br

8.2. Si–C Bond-cleaving Annulation Tobisu and Chatani observed in 2009 that a Si–Me bond was cleaved by an organorhodium complex.148 They demonstrated that intramolecular annulation of 2-boryl-2’trimethylsilylbiphenyl underwent through Si–Me bond cleavage by the rhodium catalyst to give dibenzosiloles (Eq 110). This system was applied to the intermolecular annulation of ortho-borylated arylsilanes with alkynes via Si–Et bond cleavage to form benzosiloles (Eq 111). The mechanism for the intermolecular reaction (Eq 111) is understood in terms of transmetalation of arylboronic acids with a catalytically active hydroxyrhodium generated from rhodium(I) chloride with H2O and a base to generate an arylrhodium complex. Subsequent insertion of an alkyne into the aryl–Rh bond gives vinylrhodium complex IX,


46

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ACS Catalysis

cyclization of which proceeds with Si–Me bond cleavage possibly via σ-bond metathesis to produce the benzosilole and methylrhodium complex X. The complex releases methane upon hydrolysis. When a boryl-dimethylphenylsilylethene is applied to the reaction with alkynes, tri-substituted silole is formed via selective cleavage of Si–Ph bond (Eq 112).94b A suitable chiral phosphine ligand (S,S)-QuinoxP* successfully achieves asymmetric annulation of (2borylphenyl)dimethylisopropylsilane to give an Si-chiral benzosilole of 98% ee (Eq 113).148b 5 mol% [RhCl(cod)]2 Et3N (2 eq) Me3Si

(110)

1,4-dioxane/H2O (100:1) 100 °C, 15 h

B(OH)2

Si Me2 97%

Ph

B(OH)2 + SiMe3

Ph

Ph

5 mol% [RhCl(cod)]2 DABCO (2 eq) 1,4-dioxane/H2O (100:1) 80 °C, 15 h

Si Me2

(2 eq) Ph

Ph

O B Ph

Rh

Si Me2

Me

Ph

+ Rh Me

X

Ph O

+ Ph

SiMe2 Ph

Ph

B(OH)2 +

Ph (2 eq)

Ph

5 mol% [RhCl(cod)]2 DABCO (2 eq) 1,4-dioxane/H2O (15:1) 80 °C, 15 h

(2 eq)

SiMe2i-Pr

(111)

95%

Ph

Me Si Me IX

Ph

Ph

(112)

Ph

5 mol% [RhCl(C2H4)2]2 11 mol% (S,S)-QuinoxP* Na2CO3 (2 eq) 1,4-dioxane/H2O (5:1) 80 °C, 15 h

Ph

Si Me Ph 69%

Ph

* Si Me

(113)

i-Pr

46% (98% ee)

Me N

P

t-Bu

P t-Bu Me (S,S)-QuinoxP* N

Cyclization of 2-bromo-2’-trimethylsilylbiaryl also occurs via Si–Me bond cleavage under palladium(0) catalysis using an aldehyde and LiOt-Bu. This method makes various benzosilolo[2,3-b]indoles readily accessible (Eq 114).149 ortho-Silylated arylboronic acids react with alkynes via C–Si bond cleavage to give benzosiloles (Eq 115).150 A similar transformation is effected with a rhodium catalyst (vide infra). 2-(Trimethylsilyl)aryl triflates were applied to this sort of C–Si bond-cleaving annulation with alkynes (Eq 116).151 In this


47

ACS Catalysis

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Page 48 of 69

particular case, KBr instead of aldehydes is effective for this reaction due possibly to involvement of a Ar–Pd–Br intermediate to cleave Si–Me bond.

5 mol% [PdCl(allyl)]2 10 mol% Pt-Bu3 Ph

(114)

4-NO2-C6H4CHO (1 eq) LiOt-Bu (3 eq) toluene, 120 °C, 12 h

N Me3Si

Br

N Ph

Si Me2

95%

Br

Ph

Ph

2.5 mol% [PdCl(allyl)]2 10 mol% Pt-Bu3

SiMe3 (1.2 eq) +

4-NO2-C6H4CHO (1 eq) LiOt-Bu (3 eq) toluene, 120 °C, 24 h

Ph

Si Me2

(115)

Ph

90%

OTf

LiOt-Bu (3.0 eq) KBr (2.0 eq) toluene, 120 °C, 24 h

OMe + Ph

Ph

2.5 mol% [PdCl(allyl)]2 10 mol% Pt-Bu3

SiMe3

Ph

MeO

Si Me2

(116)

Ph

86%

(1.2 eq)

8.3. Si–C Bond Cleavage Followed by Addition to C–C Unsaturated Bond An N-bis(trimethylsilyl)methyl amide behaves as a methylation reagent in the presence of a palladium catalyst by amide oxygen-assisted desilylation, as disclosed by Brown in 2008.152 For example, alkenylarenes are methylated smoothly by disilylmethylurea 24 using Pd(OAc)2 and benzoquinone in acetic acid, and propenylbenzenes are produced (Eq 117). Methyl groups are transferred cleanly to alkenes.

Ph

H N

Me N

SiMe3

5 mol% Pd(OAc)2 BQ (1 eq)

+

AcOH, 70 °C, 1 h

SiMe3 O 24 (0.5 eq) Me

Me +

85% (Isolated) 95% (NMR)

92

Me :

4

:

(117)

+ Me 4

Cationic rhodium-catalyzed intramolecular alkynylsilylation of alkynes in 1alkynylsilyl-8-alkynyl-naphthalene gives silatricyclic products via alkynyl–Si bond activation (Eq 118).153 Oxidative addition of an alkynyl–Si bond to an alkyne-coordinated cationic rhodium is the initial step for the alkynylsilylation. Use of a substrate containing two alkynylsilyl groups in the presence of an additional chiral ligand, (R,S,S)-phosphoramidite


48

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ACS Catalysis

and in the absence of MeCN leads to a high degree of asymmetric annulation to give an Sichiral silacycle (Eq 119).

Me2 Si Ph

4 mol% [RhCl(C2H4)2]2 16 mol% NaB(3,5-(CF3)2C6H3)4 8 mol% PPh3

SiMe2

Ph

(118)

MeCN (1 eq) CH2Cl2, 40 °C, 16 h 82%

Cy Si

Ph Ph

5 mol% [RhCl(C2H4)2]2 20 mol% NaB(3,5-(CF3)2C6H3)4 10 mol% (R,S,S)-phosphoramidite

Cy * Si

Ph Ph (119)

CH2Cl2, 40 °C Ph

59% (94% ee (+))

O P N O Ph (R,S,S)-phosphoramidite

8.4. Silylmethyl C–H Activation Recently, C–H bond in a silylmethyl group is shown to be selectively cleaved by a palladium and iridium to open an entry to novel synthetic transformations. For example, 2trimethylsilyl-substituted biaryls containing bromine at 2- and 2’-positions apparently underwent the SiCH2–H bond cleavage (Eq 120).154 Similar substrates with a triethylsilyl group undergo SiCH(CH3)–H bond cleavage in preference to SiCH2CH2–H bond cleavage. It is considered that oxidative addition of C–Br bond to a palladium catalyst gives a Pd(II) complex then selective sp3C–H activation takes place to form a palladacycle instead of C–Si bond cleavage as is the case of Eq 114.149 Finally, reductive elimination provides the annulation products. The major differences between C–Si bond cleavage (Eq 114, 115, and 116) and sp3C–H bond cleavage (Eq 120) at the aryltrimethylsilyl moiety are attributed to the difference of palladium catalyst {Pd(II)Cp(allyl) or Pd(0)(PPh3)4} and base (NaOt-Bu or LiOt-Bu). However, details remain to be clearified.


49

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Page 50 of 69

5 mol% Pd(PPh3)4 10 mol% Pt-Bu3 20 mol% TBAB Ph

N Br

Me3Si

NaOt-Bu (3 eq) toluene, 120 °C, 12 h

(120) N Ph

Si Me2

91%

Ph

PdLn

N N Ph

PdLn

Me3Si Br

Si Me2

Ohmura and Suginome demonstrated sp3C–H borylation of methylsilanes took place by means of bis(pinacolato)diboron and an iridium/Me4phen catalyst. For example, methylchlorosilanes were borylated under the Ir-catalyzed conditions. After treatment with isopropyl

alcohol,

they

isolated

borylmethyl(isopropoxy)silanes

(Eq

121).155a

Alkyldichloromethylsilanes also are applicable to the borylation. The computational study on this C–H borylation suggests that the chlorosilyl group acts apparently stabilizing the Ir(V) intermediate by a combination of the silicon α-effect and the high electronegativity of the chlorine substituent to assist C–H activation.156 Substrates without chlorine, namely, methyltrialkylsilanes such as tetramethylsilane, obviously need higher catalyst loading and higher reaction temperature to give the corresponding borylmethylsilanes (Eq 122).155b When tetraethylsilane is used in the borylation, CH3 in ethyl reacts selectively to give 2borylethyl(triethyl)silane (Eq 123) albeit in a lower yield. This indicates that the silylmethyl C–H bond preferentially participates in the borylation due possibly to smaller steric hindrance and the α-silyl effect. The reactivity order of the C–H borylation is Ar−H > Ar−CH3

>

Si−CH3

>

alkyl−CH3.

In

contrast

to

chloro(trimethyl)silane

and

dichloro(dimethyl)silanes, methyltrichlorosilane fails to effect the borylation. In contrast, methytrialkoxylsilanes are effectively borylated: for example, methyltris(neopentyloxy)silane is converted to the corresponding borylmethylsilane in high yield (Eq 124).155c A catalytic amount of KOt-Bu accelerates the borylarion. The product is readily converted to borylmethyltris(trimethylsiloxy)silane by treatment with trimethylsilanol/TBAF.


50

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ACS Catalysis

Me n-Bu

2.5 mol% [Ir(OMe)(cod)]2 5 mol% Me4phen

O

Cl

H +

Si

B O

cyclohexane 80 °C, 12 h

2

(4 eq) Me n-Bu

i-PrOH Et3N

Cl Si

B(pin)

Me

cyclohexane

n-Bu

Si

Oi-Pr B(pin) (121)

82% 5 mol% [Ir(OMe)(cod)]2 10 mol% Me4phen

O Me3 Si

H +

B O

2


Me3Si

B(pin) (122) 82%

(10 eq)

Et3Si

5 mol% [Ir(OMe)(cod)]2 10 mol% Me4phen

O +

B O

H

2

Et3Si (123)


B(pin)

(4 eq)

52% 5 mol% [Ir(OMe)(cod)]2 10 mol% Me4phen

O (RO)3Si

H+

B O

2

R = CH2t-Bu (1 eq)

(Me3SiO)3Si

2.5 mol% KOt-Bu cyclohexane 110 °C, 18 h

(RO)3Si

B(pin) (124)

70%

Me3SiOH (15 eq) 30 mol% TBAF

B(pin)

THF, 50 °C, 15 h

71%

A novel 1,4-silicon/halogen exchange reaction is observed by lithiation of 4-(orthobromophenyl)-5-trimetylsilylpyrazole 25 followed by iodination to give 26 (Eq 125).157 The net transformation is understood by bromine/lithium exchange, an intramolecular attack of phenyllithium XI to the silicon center to generate a bridged silicate XII and then pyrazolyllihtium XIII which is quenched by I2 to give 26.158 This then undergoes palladiumcatalyzed cyclization via C–H bond activation on the silylmethyl moiety in a manner similar to the case in Eq 120 to finally give a tricyclic silacycle through another C–H activation. In this annulation reaction, no C–Si bond activation is observed even in the absence of NaOt-Bu, indicating that a combination of pivalic acid and Cs2CO3 promote a concerted-metalationdeprotonation pathway to cleave the C–H bond in SiMe3 selectively. N N Br SiMe3 25

1) t-BuLi (2.5 eq) THF, -78 °C THP

N N Li

2) I2

THP 30 mol% PivOH Me3Si I Cs2CO3 (1.4 eq) xylene, 135 °C 26, 73%

THP

SiMe3 XI

5 mol% Pd(OAc)2 10 mol% PCy3

N N

Li

Si Me3 XII

N N THP

N N Me3Si

N N

(125) THP

Me2Si 98%

THP

Li XIII


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9. CONCLUSION AND OUTLOOK This review article has discussed briefly on synthetic reactions of organosilicon reagents focusing on a series of reactions according to type of reaction and kind of metal catalyst, as the same reaction is often effected by different metal catalysts. Preparative methods discussed herein assist to construct a wide variety of organosilicon chemicals, especially siliconcontaining pharmaceuticals. Initial studies stimulated research on transition metal-catalyzed reactions of tetragonal organosilicon reagents such as cross-coupling, C–H arylation and alkenylation, photo-mediated radical reactions, and C–Si bond-cleaving transformations. Many synthetic reactions are known nowadays which are catalyzed by palladium, nickel, copper, rhodium, silver, and gold, though each metal catalyst exhibits characteristic reactivity, scope, and limitations. Among all, palladium catalysts show the most versatile catalytic activities and are employed for various types of cross-coupling reaction including the polymerization reaction for synthesis of π-conjugated framework like polyarylenes. The transition metal-catalyzed cross-coupling polymerization with organosilicon reagents compares well to widely-used boron- or stannane-based polyarylene synthesis and in some cases overrides them. Transition metal-catalyzed reactions with perfluoroalkylsilanes are becoming a fundamental tool for the construction of various fluorinated functional molecules for pharmaceuticals, agrochemicals, and electronic materials. Continuing progress in the silicon-based organic transformations will make it possible that stable tetraorganosilanes will find more applications through a well-devised activation. In addition, replacement of noble metal catalysts to common metals, and facile sp3C–sp3C bond forming reactions will be the future research topics. As noted at the beginning, in addition to rich abundance of silicon on the surface of the earth, organosilicon reagents have great advantages over other metal reagents with respect to stability, solubility, non-toxicity, easy-handling, accessibility, and selectivity. Thus, it is always expected that silicon-based


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catalytic reactions will contribute to basic research and manufacturing of biologically active agents and materials, essential for the sustainable society in the future.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT TH appreciates financial support of grant-in-aids for Scientific Research (S) (No. 21225005) from JSPS and the ACT-C project from JST. YM thanks grant-in-aids from JSPS for young scientists (B) (No. 25870747 to YM) and the Asahi Glass Foundation.

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