Copper‐Catalyzed 1,2‐Addition of Nucleophilic Silicon to Aldehydes: Mechanistic Insight and Catalytic Systems

Abstract

Activation of the Si-B inter-element bond with copper(I) alkoxides produces copper-based silicon nucleophiles that react readily with aldehydes to yield α-silyl alcohols (that is, α-hydroxysilanes) after hydrolysis. Two independent protocols were developed, one employing a well-defined NHC-CuOtBu complex and one using the simple CuCN-NaOMe combination without added ligand. The mechanism of the aldehyde addition was investigated in detail by stoichiometric and catalytic experiments as well as NMR spectroscopic measurements. The primary reaction product of the addition of the Si-B reagent and the aldehyde (a boric acid ester of the α-silyl alcohol) and also the "dead-end" intermediate, formed in the competing [1,2]-Brook rearrangement, were characterized crystallographically. Based on these data, a reasonable catalytic cycle is proposed. The NHC-CuOtBu catalytic setup performs nicely at elevated temperature. A more reactive catalytic system is generated from CuCN-NaOMe, showing fast turnover at a significantly lower temperature. Both aromatic and aliphatic aldehydes are transformed into the corresponding α-silyl alcohols in good to very good yields under these mild reaction conditions.

Copper(I)‐Catalyzed Regioselective Addition of Nucleophilic Silicon Across Terminal and Internal Carbon–Carbon Triple Bonds

“…We had used the copper(I)‐catalyzed transmetalation approach in the racemic variants of 1,2‐additions11a, 12a and allylic substitution13a without any additional ligands. Reaction times increased significantly in the presence of both phosphines13a and N‐heterocyclic carbenes11a, 12b, 13b as supporting ligands, particularly documented by the enantioselective procedures recently developed by us 12b. 13b For that reason, and in contrast to the work of Hoveyda and co‐workers15 (cf.…”

Recent advances in Cu-catalyzed C(sp3)–Si and C(sp3)–B bond formation

“…The Oestreich group [42] Driven by the success of earlier results, the authors utilized 78 for reductive couplings between ketones 93 and imines 97 as electrophiles to form unsymmetrical 1,2-diols 94-96 and 1,2amino alcohols 98-100, respectively. Palladium was no longer needed for these transformations.…”

Synthesis, Structure, and Reactivity of an NHC Silyl Gold(I) Complex

“…Most noteworthy are the gold(I) fluoride and hydride complexes reported by Nolan, Sadighi and Toste. [19][20][21][22][23] Unlike silyl copper(I) complexes which are well known and commonly involved in catalytic transformations, [24][25][26] analogous silyl gold(I) species remain scarce. NHC boryl gold(I) complexes have been reported 27,28 but to the best of our knowledge, NHC silyl gold(I) complexes are hitherto unknown.…”