Markovnikov Hydrosilylation of Alkenes: How an Oddity Becomes the

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Perspective

Markovnikov Hydrosilylation of Alkenes: How an Oddity Becomes the Goal Maciej Zaranek, and Piotr Pawluc ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03104 • Publication Date (Web): 13 Sep 2018 Downloaded from http://pubs.acs.org on September 13, 2018

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ACS Catalysis

Markovnikov Hydrosilylation of Alkenes: How an Oddity Becomes the Goal Maciej Zaranek, Piotr Pawluc* Faculty of Chemistry and the Center for Advanced Technology, Adam Mickiewicz University in Poznań, Umultowska 89 B/C, 61-614 Poznań, Poland

ABSTRACT Over the years, hydrosilylation of terminal alkenes has emerged as one of the most prominent applications of homogeneous catalysis. While most of the relevant reports concern βselective hydrosilylation yielding linear products which are of industrial importance, the opposite selectivity is also gaining increasing interest and sets a scene for next challenges. Markovnikov hydrosilylation of alkenes, especially in its asymmetric variant, has become the aim of development of new catalytic systems successfully implementing base metal complexes – one of the most prominent trends in contemporary catalysis. In this Perspective, we present the current state of this topic and the way it has been achieved, with special emphasis put on the issues still unresolved and prospective directions of development based on the trends present in literature, but without unnecessary attention to some details of only historical significance.

KEYWORDS Hydrosilylation, Markovnikov selectivity, asymmetric catalysis, base-metal complexes, transition-metal catalysis, catalyst design

ACS Paragon Plus Environment

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1. Background Hydrosilylation ranks among the most important industrial applications of homogeneous catalysis, providing access to organosilicon compounds, commonly used for production of silane coupling agents, cross-linkers and polymers. Transition metal-catalyzed hydrosilylation of terminal alkenes has been extensively studied in the last decades with strong emphasis on development of classical anti-Markovnikov-selective catalytic systems leading to industrially viable products.1,2 In this context, selective Markovnikov hydrosilylation of terminal alkenes is still a challenge and has been rarely reported. Over the past decades, however, a series of transition-metal and main group-element catalysts have been successfully developed to address the selectivity issues, rendering Markovnikov hydrosilylation even more interesting. . Recent comprehensive review on the synthetic and mechanistic aspects of hydrosilylation reaction has been published almost ten years ago by Marciniec and co-workers.3 A number of excellent specialized reviews have appeared periodically during the last decade, however, all of them have been written in a similar way.1,2,4–7 While being great at describing the field from catalytic point of view, none of them has been focused on selectivity as the main criterion of content selection. Only very recently, a combined review on asymmetric hydrofunctionalization of alkenes over base metal catalysts has been published by Chen and Lu, who discussed also the relevant reports on hydrosilylation.8 The lack of more overall reviews on Markovnikov-selective hydrosilylation of terminal alkenes has prompted us to present the recent advances in this field. Our Perspective is focused on the most attractive results of the regioselective hydrosilylation of terminal alkenes leading to Markovnikov addition products, published mostly in the last decade with particular emphasis on the application of this process in enantioselective synthesis. Alkene (olefin) hydrosilylation can lead to two typical products, as depicted in the Scheme 1.


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ACS Catalysis

Scheme 1. Possible products in a model hydrosilylation reaction system. The first product, known as β, linear, or anti-Markovnikov one, is formed most often in majority when transition metal catalysis is applied, and has been a subject of most reports.1–3,9 It is the product of particular importance to industry.2,10 On the other hand, this article is focused on the second product, referred to as α, branched or Markovnikov one, whose efficient synthesis poses more problems. The selectivity of transition-metal catalyzed hydrosilylation can be traced back to the mechanism proposed by Chalk and Harrod (Scheme 2).11 In this unmodified variant of the original mechanism, the alkene undergoes insertion to the metal-hydrogen bond of the intermediate hydrido complex. The mode of insertion determines the regioisomer formed. Since the central atom of any active species is sterically hindered to a certain degree by ancillary ligands, 1,2-insertion is strongly favored kinetically, and thus, the anti-Markovnikov product is obtained (hydrosilylation seldom requires thermal activation and therefore is governed purely by kinetics). In the modified version of this mechanism, in which the alkene inserts into M-Si bond,12 the relation between insertion modes and resulting isomers is reversed. Notably, as there is no intrinsic property completely disabling formation of any of the two isomers, it is always possible that the other one is formed as a by-product. Most recently, Zhu et al. have described in more detail the role of specific π-π stacking interactions between the ligand and styrene reagent that can be of key importance in establishing Markovnikov selectivity of base-metal catalyzed hydrosilylations.13


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Scheme 2. Regioselectivity of hydrosilylation as a consequence of steric hindrance of a metal center in the Chalk-Harrod mechanism. 2. General approaches 2.1 The genesis Markovnikov hydrosilylation has been a subject of research for more than four decades. Most of the reaction systems developed are worth mentioning purely because of their significance as pioneering ones. The first reports on α-selective hydrosilylation are limited to trichlorosilane. Bennett and Orenski did in fact obtain branched product of hydrosilylation of styrene in the presence of nickel(II) complexes, but high selectivity was accompanied by good yields only in the presence of catalytic systems containing copper(I) chloride as co-catalyst (Table 1).14 Such a hydrosilylation product, although can be Si-alkylated by a standard nucleophilic substitution, finds little use. Parallelly, Kumada was working with the same reagents and reported that using [NiCl2(dmpf)] (dmpf being a rigid 1,1’-bis(dimethylphosphino)ferrocene ligand), the only product of hydrosilylation is formed in the anti-Markovnikov (β) manner.15 Nickel complexes bearing simple phosphines were also used by Hetflejš but gave α products only as minor ones.16


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ACS Catalysis

Table 1. Selected results of hydrosilylation of styrene with trichlorosilane, by Bennett and Orenski.a

#

Catalytic system

Overall yield, %

Selectivity, % α

β

1

[NiCl2(PPh3)2]

10

80

20

2

[NiCl2(PPh3)2] + CuCl

75

95

5

3

[NiCl2(PBu3)2]

20

95

5

4

NiSO4 + 2PPh3 + CuCl

65

90

10

5

NiCl2 + 2PBu3 + CuCl

60

90

10

6

NiBr2 + 2PPh3 + CuCl

70

95

5

7

[NiCl2(dppe)] + CuCl b

70

85

15

a

Conditions: 0.2 mmol of styrene, 0.2 mmol of HSiCl3, 21 wt% of [Ni], 100 mg CuCl; neat, 140-175 °C; b dppe = 1,2-bis(diphenylphosphino)ethane; Kumada has also approached the enantioselective hydrosilylation in HSiCl3–based systems using palladium(II) complexes containing chiral menthyl- and neomenthylphosphine.17,18 His assumption was that incorporation of chiral substituent into the ligand would induce desired chirality in the product via diastereoisomeric intermediates. The experiments involving styrene, cyclopentadiene, 1,3-, and 1,4-cyclohexadiene resulted in formation of exclusively branched adducts in moderate to high yields (41-87%), however, with poor or no enantiomeric excess (up to 22%). 2.2. Lanthanide and early transition metal complexes The complexes of early transition metals, especially lanthanides, constitute an important and coherent group of good α hydrosilylation catalysts that represent the first of the trends in this field. In 1991, Tanaka observed formation of up to 72% of Markovnikov products of


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hydrosilylation of styrene with phenylsilane over neodymium pentamethylcyclopentadienyl complexes.19 Other silanes or olefins did not repeat this selectivity and this research direction was abandoned. The observation that lanthanide metal center could provide more space to facilitate 2,1-insertion was exploited by Marks et al. in the first report on deliberately α-selective hydrosilylation over lanthanide complexes with extended range of olefinic substrates.20 The intention of the authors was, again, to produce enantiopure silanes by introducing chiral menthyl side group to one of cyclopentadiene rings (Scheme 3). This goal was moderately achieved as the highest ee in hydrosilylation of 2-phenyl-1-butene with PhSiH3 (65% R) was observed for 1(R).

Scheme 3. Example of Sm(III) complexes used by Marks. The selectivity towards α hydrosilylation and turnover frequency were observed to increase with increasing cationic radii of used metals and with opening of the coordination sphere by bridging cyclopentadiene rings (Chart 1) The dependence of the rate of hydrosilylation on cationic radius of a lanthanide has been very well documented by Roesky and co-workers,21 although aliphatic olefins they examined gave exclusively linear products and the selectivity towards α adduct in hydrosilylation of styrene was 65%.

Chart 1. Dependencies of selectivity and catalytic efficiency on spatial parameters of lanthanide complexes, obtained by Marks.


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Table 2 summarizes the results of Marks’ research. Substituted styrenes maintained the selectivity but aliphatic alkenes tended to give a mixture of isomers or pure anti-Markovnikov adducts, if bulky enough (entries 15 and 16). Using chiral catalysts 1(R)/1(S), it was possible to perform hydrosilylation of 2-phenyl-1-butene with moderate enantioselectivity (entries 9, 10). Table 2. Hydrosilylation of different alkenes with phenylsilane over samarium precatalysts, [Sm], selected results reported by Marks et al.a

#

Alkene

[Sm]

Yield, α, % %

ee, %

1

styrene

3

98

>99

-

2

4-methoxystyrene

3

98

>99

-

3

4-fluorostyrene

3

80

>99

-

4

2-methoxystyrene

3

98

>99

-

5

α-methylstyrene

2-Sm

0

-

-

3

85

>99

-

2-Sm

0

-

-

8

3

98

>99

9

1(R)

98

>99

68 R

10

1(S)

98

>99

65 S -

6 7

2-phenyl-1-butene

11

1,1-diphenylethene

3

92

>99

12

2-vinylnaphthalene

3

96

>99

13

1-hexene

2-Sm

5

5.2

-

3

74

76

-

3

96

0b

-

14 15

vinylcyclohexane


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16

2-ethyl-1-butene

98

3

0b

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-

a

Conditions: [alkene]:[PhSiH3]:[Sm] = 1:1.2:0.005, benzene-d6; RT; reaction initiated with H2 infusion; b linear product was formed exclusively Observations by Marks were further examined and confirmed by Molander et al., however, without any improvement in the catalytic performance of lanthanide(III) cyclopentadiene complexes in catalyzing addition of PhSiH3 to aliphatic alkenes.22 Tilley and Gountchev were the first to use different kind of ligands in rare-earth-metal catalyzed hydrosilylation.23 Their bis(silylamido) yttrium complex 4 underwent transformation into hydride-bridged dimer 5 upon reaction with phenylsilane (Scheme 4), which was a good indication of its potential in the field here discussed.

Scheme 4. Transformation of methyl yttrium(III) complex upon reaction with phenylsilane, by Tilley and Gountchev. Catalyst 4 led to results very similar to those reported by previous researchers, giving more linear products in the presence of 1-hexene than styrene. It was, however, able to convert norbornene into chiral norbornylphenylsilane with 90% enantiomeric excess (Table 3). Table 3. Summary of yttrium(III) complex 4 capabilities, by Tilley.a

Alkene

Silane

TOF, h-1

α sel., %

ee,

# 1

1-hexene

PhSiH3

100

8

-

Ph2SiH2

4.3

99%) under mild conditions.24 Under the same conditions, 1-hexene gave exclusively a product of anti-Markovnikow hydrosilylation. Zirconocene was used by Takahashi et al. in an interesting reaction system exhibiting selectivity depending on the order of addition of the reagents (Scheme 5).25

Scheme 5 Protocol-controlled regiodivergent hydrosilylation, by Takahasi et al. Although initially not catalytic, this system was optimized and showed another unexpected behavior, being controllable by the ratio of 6 to the organolithium reagent. In general, the greater the RLi content, the higher the selectivity towards the branched products (Chart 2).


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ACS Catalysis

98

100

98 92

81

α sel. (%)

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

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50

27

0

0

0 2

2,6 n-BuLi

3

2

0 3,2 s-BuLi

4

2

3,2 PhLi

Chart 2. Summary of hydrosilylation over ZrCl2Cp2/RLi catalyst, by Takahashi et al. A) Influence of [RLi]:[6] ratio on selectivity; B) products obtained using [RLi]:[6] = 3. Although apparently not mainstream any more, lanthanide-catalyzed hydrosilylation is still being developed. Most recently, Cui et al. have reported an ene-diamido complex 7 of samarium able to promote regiodivergent hydrosilylation of alkenes.26 Substituted styrenes were transformed exclusively into Markovnikov products, whereas unconjugated alkenes formed only the respective products of β-addition (Chart 3).

Chart 3. Sm(III) ene-diamido complex used by Cui et al. and representative results of hydrosilylation obtained with its aid. This behavior is another illustration of the phenomenon often called “aryl-directed effect”, which determines the selectivity of many lanthanide-catalyzed reactions.27 In the mechanism of alkene hydrosilylation over lanthanide (and other d0 metal) catalysts, a rapid and irreversible


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ACS Catalysis

alkene insertion into a metal-hydride bond is proposed to be followed by a turnover limiting σbond metathesis between the lanthanide alkyl intermediate and hydrosilanes (Scheme 6).

Scheme 6. Mechanism of hydrosilylation catalyzed by d0 metals and the structure proposed for the transition state key to “aryl-direction”. The unique feature of lanthanide-catalyzed hydrosilylation of conjugated aryl alkenes is its potentially excellent regioselectivity towards Markovnikov products. As the olefin insertion reaction defines the regiochemistry of the final product, the styrene insertion takes place to orient the sterically encumbered lanthanide center towards a tertiary carbon center. It is believed that acting as a Lewis base, the arene π system interacts with the electrophilic lanthanide atom of the catalyst in the Lewis acid-base manner, leading to a benzylic organolanthanide intermediate, which then undergoes σ-bond metathesis with a hydrosilane to yield the Markovnikov hydrosilylation product. It was perhaps best visible in the report of Okuda et al. who showed that even π-donating properties of 1,5-hexadiene could be enough to shift the selectivity.28 What is important for chiral lanthanide catalytic systems, the σ-bond metathesis presumably occurs with retention of configuration, providing the observed products with remarkably high ee’s considering the overall nature of the transformation. 2.3.Catalysts based on metal hydrides


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The supremacy of rare earth metals as catalysts of Markovnikov-selective hydrosilylation was broken by the work of Buch, Brettar, and Harder.29 They pointed to a new research direction, presenting a group of s-block metal complexes (Scheme 7). This work is to the day the reference point for benchmarking new catalysts.

Scheme 7. S-block metal complexes used by Harder et al. It was shown that early main-group metals coordinated with suitable ligands could

be

effective in hydrosilylation of conjugated double bonds (Chart 4). Interestingly, when the reaction took place in a THF solution or with potassium catalyst 11, other mechanisms took over and only the products of anti-Markovnikov hydrosilylation were detected. Products

Reagents

SiH2Ph

+

SiH3 SiH2Ph

10% / 7 (5%); 16h, 50oC >98% / 8(10%); 2h, 50oC >98% / 9 (2.5%); 2h, 50oC >98% / 10 (5%); 2h, 50oC >98% / 8 (2.5%); 3h, 50oC, THF >98% / KH (25%); 4h, 50oC

SiH3

SiH2Ph

>98% / 8 (2.5%); 98% / 9 (2.5%); 98% / 8 (0.5%); 1.5h, 50oC

SiH3

SiH2Ph

20% / 8 (2.5%); 24h, 50oC >98% / 9 (2.5%); 2.5h, 20oC

SiH2

SiHPh

>98% / 9 (2.5%); 24h, 20oC

+

+

+

Conversion / Conditions

Chart 4. Selected representative results of hydrosilylation using s-block metal catalysts, by Harder. Reaction in neat, if solvent is not given. In general, these authors proposed a metal hydride to be formed during initiation and to act as the active species in the desired catalytic cycle (Scheme 8, upper part). For 11, a mechanism was


12

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proposed in which a pentacoordinated silylate is formed from the metal hydride and turns into metal silylide upon elimination of dihydrogen. According to Harder et al., it is unlikely for the silylate itself to actively participate in catalyzing this transformation. Ph

SiMe3

Ph

SiH2Ph NMe2

PhSiH3

Si

[M]

[M]

H

N

Ph Main cycle Ph

[M]

Si H

3

Ph

Ph

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

ACS Catalysis

Ph

PhSiH3

SiH2Ph

observed product H2 [M]

Ph

H Ph

Ph Si H H H

H

Ph H Si

H H

SiH2Ph

Ph Ph

[M]

Ph

[M]

Ph

SiH2Ph [M]

Ph

Ph

Ph PhSiH3

Ph H H

Ph H Si

PhH2Si

Ph Ph

H [M]

Scheme 8. Possible mechanisms of hydrosilylation catalyzed by s-block metal complexes, by Harder. Another hydride source, NaHBEt3 used extensively in many hydrosilylation protocols to generate active complexes in situ of the reaction systems, was itself proven to be a potent catalyst of Markovnikov-selective hydrosilylation of conjugated alkenes and allyl glycidyl ether.30 It worked well with phenyl- and diphenylsilane, worse with triphenyl- and dimethylphenylsilane, and did not work with purely aliphatic silanes (Chart 5).


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Chart 5. Selected representative results of hydrosilylation using NaHBEt3 as catalyst, after Zaranek et al. A mechanism based on formation of a resonance-stabilized carbanion was proposed (Scheme 9).

Scheme 9. A mechanism of NaHBEt3-catalysed hydrosilylation of styrene, proposed by Zaranek et al.

2.4 Contemporary approaches The contemporary approaches to Markovnikov hydrosilylation concentrate mostly around base-metal catalysis. It would be, however, unfair to name this paragraph after those catalysts


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ACS Catalysis

since the ones based on PGMs are still being successfully improved. This section does not cover reports on enantioselective hydrosilylation to which the next paragraph is devoted. Zargarian et al. have observed formation of hydrosilylation products when using nickel(II) indenyl complexes,31,32 however, complete Markovnikov-selectivity was observed only in the reaction of styrene and phenylsilane, whereas hydrosilylation of 1-hexene ended up exclusively with the linear product. This catalytic system was not examined further for its potential in hydrosilylation, as it was not the main scope of the research. The same complex was a potent catalyst of styrene polymerization which was inhibited in the presence of hydrosilanes. Valerga et al. have proposed a set of nickel(II) methylallyl NHC complexes (Scheme 10) all of which were active catalysts of hydrosilylation of styrene and 4-methylstyrene with phenylsilane, with 12-14 being the ones most selective towards Markovnikov addition (Table 4).33

Scheme 10. Nickel(II) catalysts exhibiting the best selectivity of the set, after Valerga et al. Table 4. Selected best results of hydrosilylation with PhSiH3 using nickel(II) complexes, after Valerga et al.a #

Olefin

Cat.

Yield, %

α, %

1

styrene

12

40

92.5

2

13

65

96

3

14

60

96.7

12

60

96.7

4

4-methylstyrene


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5

13

53

60.8

6

14

60

96.7

a

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Conditions: [alkene]:[PhSiH3]:[cat] = 1:2:0.01; 1,2-dichloroethane, 60 °C, 4 h

Methylallyl analogues of these complexes were generally less selective towards α addition and attempts to hydrosilylate α-methylstyrene ended up with obtaining only the linear product in poor yield. Excellent yields and selectivity were obtained by Komine et al. using structurally simple diallyl ether monophosphine palladium complex 15.34 It was established that such a system operated according to the Chalk-Harrod mechanism at room temperature and was efficient for Markovnikov hydrosilylation of electron-deficient alkenes with mostly aromatic tertiary silanes, i.e., Ph3SiH, MePh2SiH, Me2PhSiH, and Me2HSiCl (Chart 6).

Chart 6. Pd(0) complex and representative α hydrosilylation products, by Komine et al. The effect of para-substitution of styrene indicates strong dependence of this reaction on electron-withdrawing properties of the alkene. It is also worth noting that the best silane turned out to be the bulkiest one – Ph3SiH - which often is far less reactive than the other members of its family. Huang et al. have devised a new cobalt(II) phosphinoiminopyridine complex 16 able to catalyze Markovnikov hydrosilylation of a variety of aliphatic alkenes bearing range of functional groups (halogen, amide, amine, ester, etc) mostly with phenylsilane (Chart 7).35 Successful use of


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ACS Catalysis

diphenylsilane and triethoxysilane required reduction of the complex by addition of 2 mol% of NaHBEt3.

Chart 7. Co(II) complex and representative products of hydrosilylation with its aid, after Huang et al. A reaction of styrene gave α and β addition products in nearly equal amounts (51:49). On the other hand, analogous complex of Fe(II) upon reduction with NaHBEt3 led exclusively to antiMarkovnikov hydrosilylation. The potential of Co(II) catalysis has been extended by Ge et al. who have devised twocomponent catalytic systems for regiodivergent hydrosilylation of both aromatic and aliphatic alkenes (Chart 8).36 While Co(acac)2 / xantphos 17 system has been shown to promote Markovnikov addition of PhSiH3 to vinylarenes, in the reaction of non-conjugated alkenes, it gave exclusively linear products. This, in turn, prompted these authors to try a different type of ligand – MesPDI 18, which led to the desired α selectivity for wide range of functionalized olefins.


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SiH2Ph

F3C

O F3 C

96%

O PPh2

SiH2Ph

SiH2Ph

1 mol% Co(acac)2

SiH2Ph

17 F C 3

1 mol% CF3 SiH2Ph

O

91%

92%

SiH2Ph

SiH2Ph

PPh2

THF, RT, 3h

Page 18 of 41

91%

94%

96%

NC

SiH2Ph

SiH2Ph

SiH2Ph S

O

O

87%

82% H2N

72% (2% cat)

SiH2Ph

3 mol% Co(acac)2

SiH2Ph

88%

N Mes

N

N 3 mol%

18 Mes

Toluene, 60 oC, 24h

O

SiH2Ph

AcO

92%

SiH2Ph Me3Si

95%

SiH2Ph S

65%

SiH2Ph

O

SiH2Ph

O

59%

91%

PhH2Si EtOOC

O

98%

O

SiH2Ph

N 61%

91%

87% ( / = 68:32)

COOEt

Chart 8. Co(II) catalytic systems and representative products of Markovnikov hydrosilylation of functionalized vinylarenes (top) and aliphatic alkenes (bottom), after Ge and co-workers. Recently, the group led by Ge have reported a highly regioselective 1,2-hydrosilylation of conjugated dienes under previously established conditions using Co(II) / xantphos 17 catalytic system.37 The yields obtained were generally very good and selectivity was usually higher than 95% in favor of 1,2-addition (Chart 9).

Chart 9. Co(II) catalytic system used by Ge et al. and representative products of Markovnikov hydrosilylation of 1,3-dienes.


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On the basis of their experiments, the same authors proposed also a mechanism explaining observed unusual isomerization of (Z)-dienes to products of the opposite (E) geometry. No experiments have been made to explore possible enantioselectivity of such catalytic systems. Year 2018 brought two important reports on successful Markovnikov hydrosilylation over iron catalysts. First, complex 19 synthesized by Zhu et al., upon alkylation with ethylmagnesium bromide, turned out to be efficient catalyst for α-selective hydrosilylation of aromatic alkenes under mild conditions (Chart 10).13 Second, Lu et al. reported on an iron(II) catalyst for highly enantioselective hydrosilylation,38 which is discussed in section 3.

Chart 10. Fe(Darphen) complex 19 devised by Zhu et al. and representative results of Markovnikov hydrosilylation of styrene derivatives obtained with its aid. In the course of the research, Zhu et al. performed also DFT calculations to clarify the origin of selectivity in their catalytic system. It was concluded that π-π interaction between phenanthroline aromatic system and the phenyl ring of styrene is a key factor lowering the energy of the transition state leading to more stable intermediate product of 2,1-insertion (Scheme 11).


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Scheme 11. Regioselectivity-determining step of the mechanism calculated by Zhu et al. Most recently, manganese joined the group of base metals capable of catalyzing Markovnikov hydrosilylation. Trovitch and co-workers have shown that β-diketiminate dinuclear complex of Mn, upon reaction with sodium triethylborohydride, turned into respective hydrido form 20 which catalyzed hydrosilylation of alkenes in a manner similar to that of lanthanide-based and Zhu’s iron catalysts (Chart 11).39

Chart 11. Manganese(II) dinuclear complex synthesized by Trovitch et al. and representative results obtained with its aid. For their reaction, Trovitch et al. proposed an interesting mechanism in which the H-bridged dimer 20 is cleaved on the way of alkene insertion and subsequently re-formed after σ-bond metathesis with phenylsilane (Scheme 12).


20

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Scheme 12. Mechanism proposed by Trovitch et al. for manganese-catalyzed Markovnikov hydrosilylation of aromatic alkenes.

3. Enantioselective hydrosilylation All monosubstituted (terminal) alkenes show an intrinsic property of being prochiral, and thus, hydrosilylation at the α carbon very early prompted attempts of stereocontrol. The first reports on moderate success dating back to the 1970s and other ones have been already mentioned, however, it was not until the 1990s and the break of the millennium for a complete success to be announced. Profound studies on axially chiral monophosphine ligands with a binaphthyl backbone (MOP) have been conducted by Hayashi and co-workers and published in a set of papers from 1991 to 2001.40–44 It is worth mentioning that this group, led formerly by Kumada, have also succeeded in asymmetric hydrosilylation of α-methylstyrene, yet in the anti-Markovnikov manner.45 A mixture of [{Pd(allyl)Cl}2] and MOP ligand 21 allowed initially enantioselective hydrosilylation of aliphatic terminal alkenes (Chart 12, upper part),40,41 but later the catalytic system was improved to work with various substituted styrenes,42 and eventually became fully functional when MOP 22 was used (Chart 12, lower part).43


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Chart 12. Selected examples of asymmetric hydrosilylation over Pd(II) MOP complexes, after Hayashi and co-workers. Numbers in parentheses indicate selectivity towards α-addition, if given by authors. Since then, the concept of axially chiral monophosphines have evolved and soon a class of phosphoramidite ligands has been introduced to hydrosilylation by Johannsen et al.46 They have enriched already chiral binaphthol (BINOL) backbone with a chiral substituent at the nitrogen atom resulting in ligand 23, however, it was further determined that only the absolute stereochemistry of the former determines the stereochemistry of products. Chart 13 shows that very good enantioselectivity in this catalytic system was retained even in prolonged reaction of electron-deficient 3-nitrostyrene at slightly elevated temperature. SiCl3

SiCl3 O2N

Pd

O

Cl +

Cl

O

Pd

P N

144h, 40 oC, 94%, 95% ee

16h, 87%, 99% ee

Cl

SiCl3

SiCl3 Cl

0.5 mol% 0.125 mol% [olefin]:[silane] = 1:1.2, neat, RT

23 40h, 89%, 96% ee

CF3 SiCl3

40h, 74%, 95% ee

SiCl3

60h, 91%, 98% ee

SiCl3

SiCl3

F3C 60h, 40oC, 88%, 98% ee

40h, 75%, 97% ee

40h, 95%, 86% ee


22

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Chart 13. Phosphoramidite-based catalytic system used by Johannson et al. along with the products obtained with its aid. Significantly better reaction rates were achieved by Zhou et al. using ligand 24 in which the binaphthol backbone was replaced by a 1,1’-spirobiindane one (Chart 14).47

Chart 14. Phosphoramidite-based catalytic system used by Zhou et al. along with the products obtained with its aid. Zhang and Fan enriched the classical phosphoramidites with bulky substituents at 3 and 3’ positions of the BINOL backbone.48 It resulted in further increase in activity with a slight drop in enantioselectivity of such catalytic systems (Chart 15). The authors postulated that bulkiness of ligand 25 prevented coordination of the second molecule to the Pd center, which left a vacant coordination site for the substrate. SiCl3

SiCl3

O

16h, 80% ee

8h, 96% ee

Pd Cl

O Cl +

Pd

SiCl3

SiCl3

P N

O 16h, 93% ee

O 0.125 mol% 0.25 mol% [olefin]:[silane] = 1:2, neat, -20 oC

MeO

SiCl3

25

16h, 89% ee

SiCl3 Br

16h, 89% ee

SiCl3

SiCl3 Br

16h, 91% ee

24h, RT, 87% ee

SiCl3

16h, 90% ee

F3C Cl

F3 C

16h, 87% ee

Chart 15. Phosphoramidite-based catalytic system used by Zhang and Fan along with the products obtained with its aid.


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The report by Beller et al. using H8-binaphthol-based phosphoramidites as ligands for palladium(II) active complexes (Chart 16) is the most recent example of enantioselective hydrosilylation of alkenes inspired by earlier achievements in this field.49 This system was, however, applied only to trichlorosilane, similarly to those reported earlier.17,18

Chart 16. Phosphoramidite-based catalytic system used by Beller et al. along with chiral products obtained with its aid. Optimization

of

reaction

conditions

made

it

viable

to

use

0.25 mol %

of

[{Pd(methylallyl)Cl}2] precatalyst along with 0.5 mol % of phosphoramidite ligand 26 to hydrosilylate various styrene derivatives without solvent with usually good enantiomeric excess. Noteworthy is that unconjugated allylbenzene was transformed nearly exclusively into the linear anti-Markovnikov addition product and bulkiness in general hinders enantioselectivity. The use of phosphonites structurally similar to both Beller’s phosphoramidites and Hayashi’s MOPs led to very inconsistent results of ee varying from 1 to 94%.50


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Good selectivity in the reaction of styrene with HSiCl3 was exhibited by analogous system with triptycenyl monophosphine, devised by Fukushima et al.51 It was not, however, examined in hydrosilylation besides this only example. Another approach to phosphine ligands was taken by Weber and Jones, who synthesized bidentate chiral phosphines with a pendant heterocycle based on η6-benzenetricarbonylchromium(0) (Scheme 13).52 The results obtained with the use of catalytic systems comprising Pd(II) source and the ligand 27 in hydrosilylation of substituted styrenes with trichlorosilane varied significantly and seemed to be highly temperaturedependent. X

P Cr OC

CO

CO

X = O (27), S, NH, NCH3

Scheme 13. General structure of phosphine ligands used by Weber and Jones. Nishiyama

and

co-workers

attempted

enantioselective hydrosilylation

using chiral

bis(oxazolinyl)phenyl rhodium acetate complexes.53 The results obtained using complex 28 (Chart 17) revealed how challenging it was to use silanes other than trichlorosilane. Excellent enantioselectivity was usually accompanied by moderate selectivity towards branched products. Electron withdrawing effect seemed to have a positive impact on selectivity, contrary to the results of hydrosilylation over Pd/phosphine systems.


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Chart 17. Rh(III) catalyst used by Nishiyama et al. along with selected results obtained with its aid. The year 2017 brought a report of Buchwald et al. on a copper catalyst for enantioselective hydrosilylation of vinylarenes and vinyl heterocycles (Chart 18).54 This relatively simple catalytic system comprising copper(II) acetate and diphosphine ligand (S,S)-Ph-BPE 29 was able to convert a group of substituted styrenes into the corresponding α-adducts of diphenylsilane with very high ee. Although the enantioselectivity was lower with vinyl heterocycles and in the reaction on larger scale, switching silane to phenylsilane recovered the original effectiveness in the former case. It was the first report on successful highly enantioselective hydrosilylation with the use of silanes without electronegative substituents.


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Chart 18. Cu catalytic system for enantioselective hydrosilylation and selected representative products obtained with its aid, after Buchwald et al.: (A) styrene derivatives with Ph2SiH2, (B) vinyl heterocycles with Ph2SiH2, (C) products of oxidation of PhSiH3 adducts to vinyl heterocycles. Some extremely high enantioselectivities have been simultaneously reported by Lu and coworkers using cobalt(II) complex 30 as a catalyst.55 It contained the N3-donor ligand motif often found in the first-row-metal hydrosilylation catalysts, whose fragment was a chiral 4benzyloxazoline (Chart 19).

Chart 19. Co(II) catalyst 30 and selected representative products of enantioselective hydrosilylation, after Lu et al. This cobalt(II) precatalyst needed activation by threefold excess of sodium tert-butanolate and exhibited very good tolerance towards wide spectrum of functional groups, especially sulfurcontaining vinyl heterocycles and unactivated terminal alkenes with functional groups such as alcohol, ketone, and halide, without apparent drop in activity. The authors suggested a mechanism in which an active silyl complex is formed via in situ reduction by the silane reagent


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assisted by the alkoxide. Such an activation pathway was highlighted and explored further by Thomas et al.,56 and once more applied by Lu et al. in enantioselective hydrosilylation over iron(II) catalyst 31.38 Remarkably, it has been the first reported example of asymmetric hydrosilylation of extended range of functionalized mostly aliphatic substrates aided by an iron complex (Chart 20).

Chart 20. Iron(II) catalyst 31 and selected examples of products of enantioselective hydrosilylation, after Lu. 4. Summary On the basis of discussed experimental reports, there are few important factors influencing Markovnikov addition selectivity to hydrosilylation reactions that we would like to envisage. The unique role of lanthanide-based catalysts in Markovnikow hydrosilylation is often related to the stability of Ln3+ oxidation state and ease of generation of Ln-H bond, which makes the classical oxidative addition/reductive elimination pathway inaccessible and forces a different reaction mechanism through olefin insertion/σ-bond metathesis. In this case, the enhanced influence of steric factors is also observed; the selectivity increases with increasing cationic radii of used metals and with opening of the coordination sphere, also when bridging ligands are used.


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Additionally, extremely high regioselectivity observed for aromatic alkenes in the presence of lanthanide-based catalysts is connected with the so-called aryl-directed effect or specific π-π stacking interactions between the ligand and aromatic reagent, which is of key importance in olefin insertion regioselectivity leading to formation of Markovnikov hydrosilylation products. In the case of s-block metal catalysis, it appears that selectivity is driven mostly by formation of more stable intermediates. Location of the charge at benzylic carbon atom is strongly favored and leads to formation of Markovnikov products. Simple palladium(0) precursors, in the presence of bulky axially chiral monophosphines, promote Markovnikov addition of trichlorosilane to aromatic olefins with high enantiomeric excess as the bulkiness of such ligands prevents premature formation of coordinatively saturated Pd complexes, leaving a vacant site for the substrate. Base metal complexes (Co, Fe) containing bulky N3-donor ligands seem to be valuable alternative to previously devised Markovnikov hydrosilylation catalysts, especially as they exhibit unrivalled potential of asymmetric catalysis when equipped with suitable chiral motifs. The unique regioselectivity can be associated also with ability of metal alkyl complexes to isomerize to more thermodynamically stable branched isomer.57 The asymmetric iron-catalyzed alkene hydrosilylation is a breakthrough in transformation of unactivated terminal alkenes. It is an important leap forward in asymmetric iron catalysis, as this field is believed to be particularly challenging due to many unpredictable aspects of iron chemistry.58 5. A Perspective Although the beginnings of Markovnikov-selective hydrosilylation catalysis are associated with catalytic systems based on organolanthanide and palladium complexes, currently it is the


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area in which base-metal catalysts have found their place, as in the last years we witnessed an outbreak of such reports. Markovnikov hydrosilylation of terminal alkenes has opened a new opportunity of enantioselective catalysis. Classical asymmetric alkene hydrosilylation catalytic systems based on expensive precious metals such as palladium or rhodium have been successfully replaced by non-precious metals. The new generation catalysts tolerate a wide range of silanes, and are not limited to trichlorosilane as the former catalysts. By developing new earth-abundant transition metal catalysts (Fe, Co, Cu), with suitable chiral ligands, further applications of the Markovnikov hydrosilylations would include the synthesis of enantiopure secondary alkylsilanes, which can be further converted to synthetically versatile secondary alcohols, silanols and other valuable products. In the recent years, earth-abundant transition metal catalysts have played an important role in the field of Markovnikov hydrosilylation. However, despite a rapid development in this field, there are still a number of challenges to be addressed. • Although many of the base-metal catalysts exhibit high Markovnikov regioselectivities, their activities vary, depending on the nature of the silane and alkene. Most of the research on catalysis has been unfortunately limited to hydrosilylation of styrene derivatives by primary or secondary silanes (RSiH3 or R2SiH2), so development of new efficient catalytic systems for functionalized alkene hydrosilylation with commercially relevant tertiary silanes (used for the synthesis of silane coupling agents and polymers) would be beneficial.1,10 • Several early main group metal catalysts (based on calcium, strontium and potassium) have emerged recently, offering activity and regioselectivity similar to those of previously reported organolanthanide catalysts. Nevertheless, asymmetric hydrosilylation of olefins over early main


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group metal catalysts has not been reported as they do not possess bulky chiral ligands. Moreover, the nature of reactions involving these complexes seems not to allow for such modification to be successful. • Reported catalytic systems often suffer from catalyst sensitivity towards moisture and oxygen, complexity and high cost of the ligand used. The synthesis of highly active, commercially viable non-precious metal catalysts would significantly contribute to the development of Markovnikov hydrosilylation protocols. • Development of hydrosilylation catalysts other than early- or late- transition metals would be a welcome addition to this field. One of the most promising recent developments in catalysis of hydrosilylation has been the discovery of catalytic activity of trialkylhydroboranes in Markovnikov hydrosilylation of aromatic alkenes, vinylsilanes and selected allyl derivatives.

At a certain point, the question might appear what is the reason why to ever bother about enantioselectivity of hydrosilylation. The possible use in pharmaceutic science appears a good answer.59,60 While silanes by themselves do not imply any bioactivity, they are considered potentially useful in drug development for several reasons. Silicon is the element most similar to carbon, yet having slightly different properties and size. Therefore, substitution of a carbon atom in a known molecule with a silicon one can lead to a modest alteration of spatial properties of the subject molecule and potentially great difference in its metabolic pathway.61 Examples of molecules both analogous to natural ones and designed from scratch are known for their bioactivity (Chart 21).


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Chart 21. Examples of silicon-containing bioactive compounds having international nonproprietary names. There is no doubt that the possibility of control over formation of a next stereogenic center will be a great chance in creating more complex, possibly better targeted molecules. In view of the above, all the endeavor of scientific community is far more than art for art's sake, leading the research into an uncharted land of new challenges and promising perspectives. Author Information Corresponding Author *E-mail: [email protected] Notes Authors declare no competing financial interests. Acknowledgements National Science Centre (Poland) grant No. UMO-2016/23/B/ST5/00177 is gratefully acknowledged. References (1)

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Johansson, T.; Weidolf, L.; Popp, F.; Tacke, R.; Jurva, U. In Vitro Metabolism of Haloperidol

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Markovnikov Hydrosilylation of Alkenes: How an Oddity Becomes the

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