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Access to Enantioenriched Benzylic 1,1-Silylboronate Esters by Palladium-Catalyzed Enantiotopic-Group Selective Suzuki– Miyaura Coupling of (Diborylmethyl)silanes with Aryl Iodides Junghoon Kim, and Seung Hwan Cho ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03979 • Publication Date (Web): 06 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018
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Access to Enantioenriched Benzylic 1,1-Silylboronate Esters by Palladium-Catalyzed Enantiotopic-Group Selective Suzuki–Miyaura Coupling of (Diborylmethyl)silanes with Aryl Iodides Junghoon Kim, and Seung Hwan Cho* Department of Chemistry, Pohang University of Science and Technology (POSTECH), 37673, Pohang, Rep. of Korea ABSTRACT: This work describes the palladium-catalyzed enantiotopic-group selective Suzuki–Miyaura cross-coupling of (diborylmethyl)silanes with aryl iodides. The combination of a Pd(TFA)2 and rev-Josiphos-type ligand bearing a 3,5bis(trifluoromethyl)phenyl as benzylic phosphine substituent in the presence of NaI as an additive and NaOMe as a base promote the reaction to high efficiency and enantioselectivity. This method provides a convenient approach for synthesizing chiral benzylic 1,1silylboronate esters from readily accessible reagents. Synthetic applications including stereospecific C–O, C–N, and C–C bond forming reactions of boron group are also demonstrated. KEYWORDS: palladium catalysis, cross-coupling, enantioselectivity, rev-Josiphos, silicon, boron Scheme 1. Approaches for Synthesizing Chiral 1,1Silylboronate Esters. *
(a) Stoichiometric approaches for the preparation of enantioenriched 1,1-silylboronates alkyl
X
R
R2 Bpin
+
(R2 = SiR3 or alkyl) (Blakemore) Et Et O O
R
Li
N
=
N
N
Li
LG alkyl
*
1
X
OCb
PhMe2Si
with R3Si Bpin
OMe N
N Li
iPr
(Aggarwal)
iPr
Li
Cl Si Me Me *
LG
(Aggarwal, X u) X
Si * Bpin Me Me
*
R1
Li
*
X
*
Chiral molecules that contain two different main group elements at the same sp3 carbon center are highly versatile intermediates in organic synthesis.1 Such reagents provide significant advantages for increasing molecular diversity and complexity through stereospecific and/or iterative carbon– carbon and carbon–heteroatom bond forming reactions.2 Among them, chiral 1,1-silylboronate esters are particularly appealing because of their stability, low toxicity, and ease of handling. Nevertheless, despite their potential usefulness, only a limited success for the preparation of chiral 1,1-silylboronate esters have been reported.3-5 For example, Aggarwal3a,b and Xu3c demonstrated the stereocontrolled homologation of chiral lithiated carbamates with silylborane (Scheme 1a). Blakemore et al. reported that chiral -silylmethyllithium carbamate, prepared in the presence of a chiral bis(oxazoline)ligand, reacted with alkylboronates to afford 1,1-silylboronate esters with low to moderate enantioselectivity.3d Aggarwal et al. elegantly improved the stereoselectivity by reacting a lithiated -chloromethylsilane, tethered with a chiral (methoxymethyl)pyrrolidinomethyl group, with alkylboronates.3e However, these methods required the use of stoichiometric enantioenriched reagents.4 In this context, Hoveyda and co-workers disclosed a copper-catalyzed enantioselective proto-boryl addition to vinylsilanes in the presence of B2pin2 and MeOH (Scheme 1c).5a Recently, Morken et al. reported the Pt-catalyzed enantioselective hydrosilylation of alkenylboronates with good to moderate enantioselectivity (Scheme 1d).5b Despite these notable advances, the development of complementary method for synthesizing 1,1silylboronate esters is still desirable. Herein, we describe a highly enantiotopic-group selective Pd-catalyzed Suzuki– Miyaura cross-coupling of (diborylmethyl)silanes with aryl iodides. This process represents the attractive features of efficiently utilizing (diborylmethyl)silane as a new type of 1,1,1-triorganometalics in enantiotopic-group selective reaction.6 1,1-Diborylalkanes have been used in several enantiotopic-
*
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
OTIB
with alkyl Bpin
with alkyl Bpin
(b) Catalytic approaches for the preparation of enantioenriched 1,1-silylboronates (Hoveyda)5a R
PhMe2Si
cat. [Cu]/L* MeOH B2pin2
(Morken)5b Bpin PhMe2Si *
(c) This work I R1
Bpin
+ pinB
broad substrate scope
SiR3 high yields
R
cat. [Pt]/L* HSiMe2Ph
R
pinB
Bpin
cat. Pd/L* NaOMe, NaI R1
SiR3
excellent enantioselectivity
group selective C–C bond forming reactions.7-10 Within this field, Morken and Hall have independently demonstrated that a palladium catalyst ligated to a chiral phosphoramidite ligand could promote the enantiotopic-group selective cross-coupling of 1,1-diborylalkanes with aryl10a,10b or vinyl10c halides in the presence of excess aqueous NaOH. Based on these seminal precedents, we envisioned palladium-catalyzed enantiotopicgroup selective cross-coupling of (diborylmethyl)silanes with aryl halides might afford chiral benzylic 1,1-silyl boronate esters. In fact, the group of Endo reported the racemic version of cross-coupling of (diborylmethyl)silanes with aryl bromides
ACS Paragon Plus Environment
ACS Catalysis 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
Scheme 2. Optimization Study for Pd-catalyzed Enantiotopic-Group Selective Cross-Coupling of 2a with Aryl Electrophiles
X
Bpin +
MeO
pinB
SiMe2Ph
Pd(TFA)2 (5.0 mol %) ligand (5.0-10 mol %) NaOMe (3.0 equiv) with or w/o NaI (1.0 equiv)
Bpin SiMe2Ph
1,4-dioxane, RT, 3 h MeO
1
2a
3a
a) Effects of chiral phosphine ligands with 4-iodoanisole (1a)a O
Ar
Ar
O P NMe2 O
O O
O
Me P N Me
O
Ar Ar
O
PAr2 PAr2
O
O Ar = 3,5-tBu-4-MeOC6H2(L3) 91%, 6% ee (w/o NaI)
Ph
Ar = 4-Me-C6H4(L1)b 99%, 20% ee (w/o NaI)
b
(L2) 31%, 0% ee (w/o NaI) Me
Me
Fe
Ph
PCy2 PPh2
Fe
Me
PPh2 P(tBu)2
Fe
(L5) 28%, 0% ee (w/o NaI)
(L4) 32%, 7% ee (w/o NaI)
PPh2 PCy2
(L6) 74%, 84% ee (w/o NaI) 99%, 84% ee (with NaI)
b) Substituent (R1) effects of rev-Josiphos type ligands with 4-iodoanisole (1a)a Me P(R1)2
Fe
PCy2
1
R =
R
R = OMe (L7) 54%, 42% ee (with NaI)
R R = CF3 (L9) 99%, 96% ee (with NaI)
R = CF3 (L8) 92%, 88% ee (with NaI)
R = Me (L10) R 43%, 63% ee (with NaI)
c) Reactions with other aryl (pseudo)halidesa Br MeO
MeO
MeO 1a-Br
OTf
Cl
1a-Cl
with L6, 99%, 40% ee (with NaI) with L6,
