Copper‐Catalyzed Addition of Nucleophilic Silicon to Aldehydes

Abstract

The transition metal-catalyzed transfer of silicon nucleophiles [1] onto various electrophiles, has recently gained considerable attention, [2] owing to the development of readily available silicon pro-nucleophiles. In particular, the reagent developed by the Suginome group, which possesses a SiÀB linkage has become one of the major sources of nucleophilic silicon. [1a] This new method is particularly attractive, as it provides a facile access to metal-silicon reagents that render the use of any activating agents unnecessary.These metal-silicon intermediates allow the catalytic transfer of a silicon nucleophile onto various electrophiles; our interest lies in the addition of such species to aldehydes, thus generating a-hydroxysilanes. This type of C À Si bond formation as well as the hydroxy group is of crucial importance in organic synthesis. Moreover, a stereogenic center is formed in this process, thus generating possible opportunities for asymmetric catalysis.Optically active a-hydroxysilanes are a class of chiral organometallic compounds that contain a functional group. These molecules and their derivatives have been used for stereocontrolled C À C bond formation and rearrangements, which resulted in a wide variety of chiral organic compounds. [3][4][5][6][7] The majority of literature-known methods for the preparation of a-hydroxysilanes are based on the asymmetric reduction of acylsilanes [8] or the hydrogenation of enolsilanes. [9] However, the synthesis of acylsilanes usually requires several steps. [10] An alternative, but less used, approach is the one previously described, which includes the addition of a silicon nucleophile to a carbonyl compound. [11][12][13][14] Hiyama et al. introduced a fluoride-catalyzed SiÀ Si bond cleavage followed by the addition of released silicon nucleophiles to aldehydes. [11] However, yields were partially diminished by [1,2]-Brook rearrangement. Barrett and Hill [13] elaborated a practical procedure based on the addition of easy-to-form Me 2 PhSiLi to aliphatic and aromatic aldehydes, but the use of such strongly basic nucleophiles is not without problems for functionalized aldehydes.Recently, Oestreich described the racemic 1,2-addition of a silicon nucleophile to imines and aldehydes, and proposed a mechanism involving a CuÀSi intermediate as the silyl transfer species. [2d, 15] The mild copper-mediated generation of nucleophilic silicon from Si À B compounds might therefore be useful for catalytic one-step access to enantiomerically pure a-hydroxysilanes from readily available aldehydes.Herein, we report the first enantiomeric version of the 1,2addition of a silicon nucleophile to aromatic and aliphatic aldehydes catalyzed by copper(I) complexes.Our investigation started with benzaldehyde, which was selected as a model substrate for initial screening. The reaction of 1 a with Me 2 PhSiBpin (2; = Si À B, pin = pinacolato) [16] in the presence of CuF(PPh 3 ) 3 ·2 MeOH (3) [17] as the copper catalyst was completed in 2 h at room temperature, and resulted in the isola...

Decarbonylative Silylation of Esters by Combined Nickel and Copper Catalysis for the Synthesis of Arylsilanes and Heteroarylsilanes

“…As the nature of the base plays ac ritical role for the success of the decarbonylative cross-coupling,various bases were evaluated. Although different co-catalysts could be utilized (entries [10][11][12], the best result was obtained with copper(II) fluoride,w hich confirmed that the copper-based co-catalyst plays acrucial role in activating the SiÀBbond. Although different co-catalysts could be utilized (entries [10][11][12], the best result was obtained with copper(II) fluoride,w hich confirmed that the copper-based co-catalyst plays acrucial role in activating the SiÀBbond.…”

“…With these considerations in mind, we began to search for potential catalysts and nucleophiles for the decarbonylative silylation of esters.Owing to their air stability,facile synthesis, and environmentally benign nature,s ilylborane compounds, which serve as efficient silicon nucleophiles in coppercatalyzed transmetalation, [11] have shown high efficiencya nd remarkable potential in various organic transformations. [12] Herein, we present the discovery of an ew method for the nickel- [13] and copper-catalyzed decarbonylative silylation of esters by using silylborane compounds as coupling partners, which also features aw ide substrate scope (Scheme 1).…”

Enantio‐, Regio‐ and Chemoselective Copper‐Catalyzed 1,2‐Hydroborylation of Acylsilanes

“…However most reports of such chiral reductionr eactions dependo nt he use of stoichiometric amounts of chiral reducing agents [3m, p, s, 5] or transition metals [6] and thus suffer from poor atom economya nd importantc osts. [7d] Besides asymmetricr eduction reactions, a-hydroxysilanes are accessible with high enantiomeric excess through copper-catalysis by addition of Suginome's reagent [8] to aldehydes [9] (Scheme1a) and addition of Grignardr eagents to acylsilanes [10] (Scheme 1b)a ffording secondary and tertiary a-hydroxysilanes,r espectively. [4b, c, 7] For instance Arai et al reported an extremelye fficient ruthenium-catalyzed hydrogenation reaction of acylsilanes under high hydrogen pressure.…”

“…[15] Among the BINAP family, L1 gave (a-hydroxyallyl)silane 2a with 46 %e nantiomeric excess (Table 1, entry 1). DTBM-Segphos L3,w hich gave the best results in ap revious studyo na-hydroxysilanes [9] disappointingly yielded 2a with a3 9% ee only (entry 3). DTBM-Segphos L3,w hich gave the best results in ap revious studyo na-hydroxysilanes [9] disappointingly yielded 2a with a3 9% ee only (entry 3).…”

Copper(I)‐Catalyzed Highly Regio‐ and Stereoselective Hydrosilylation of Terminal Alkynes with Boryldisiloxane

“…Even the sterically crowded tert-butylethyne showed (Table 2, In the absence of MeOH, the hydrosilylation of propargyl alcohol 2 m with boryldisiloxane 1 also proceeded smoothly under the optimized conditions. A proton was effectively provided by 2 m itself, and after hydrolysis [21] desired product 3 m was conveniently isolated by SiO 2 flash chromatography in 90 % yield.…”