Hydrocyanation. Cyanosilylation. and Related Additions

Zinc-catalysed asymmetric hydrosilylation of ketones and imines has been reviewed.  

A bidentate cyclopentadienyl-functionalized NHC complex of nickel(II) catalyses hydrosilylation of aldehydes, allowing quantitative reduction in 5 min at 25 A transient nickel hydride complex, ( Cp-NHC)NiH, is implicated as the active species. 

Cyclic a-fluorinated ketones undergo solvent-assisted addition of TMSCN in DMF, giving TMS-promoted cyanohydrins in good yields and up to 91% de, with the c/i -product predominating. Interaction of the carbonyl of DMF with TMSCN is proposed to activate the reagent, by weakening the Si-CN bond. Simple reduction of the product with lithium aluminium hydride affords the corresponding fluorinated 1,2-amino alcohols. 

Hydrosilylation of ketones in high ee has been achieved under ambient conditions in toluene using a silane and a zinc catalyst in which the metal ion is chelated simultaneously by a chiral 1,2-diamine and a chiral 1,2-diol. Although the catalyst could not be isolated in crystalline form, a combination of diethylzinc, diamine and diol shows evidence for it by H-NMR, and addition of the silane results in a new peak at 5 = 4.50 ppm, consistent with Zn-H this peak decreases on addition of ketone. CD spectra are also reported, and extensive DFT calculations support the hydride formation, as well as preorganization of substrate and catalyst via an N-H---0=C hydrogen bond. 

DFT has been used to model the hydrosilylation of ketones catalysed by NHC-Cu(I) hydrides. Using CuF as pre-catalyst, a four-centre metathesis TS is identified. 

A review of asymmetric hydrocyanation, cyanosilylation, and hydrophosphonylation of carbonyl and imino groups covers catalyses by metals species, organics, and enzymes.  

A chiral Li-Ru complex, characterized by X-ray crystallography, is a highly efficient catalyst for hydrocyanation of aldehydes by HCN at -78 °C up to 99% ee is achieved with catalyst loading as low as 0.05%.  

Aldehydes are enantioselectively cyanobenzoylated by 1 1 mixtures of BINOLAM (a diamino-BINOL) and Ti(OPr-04. A mechanistic study has identified HCN and isopropyl benzoate in the reaction. Evidence for an indirect mechanism, involving enantioselective /tydrocyanation followed by 0-benzoylation, is presented. Changing to higher ratios of metal - that is, BINOLAM/Ti(OPr-04 of 1 2 up to 1 5 - gives a reversal of enantioselection. The relative roles of monomeric and dimeric complexes have been examined to understand this switch.  

An unsymmetrical salen ligand bearing a Lewis base catalyses Ti(OPr-i)4-promoted addition of TMSCN to benzaldehyde with as little as 0.05 mol% loading, quantitative conversion is achieved in 10 min at ambient temperature. Another salen catalyst - a bifunctional salen-phosphine oxide-Ti(IV) combination - promotes enantioselective cyanosilylation of aldehydes. Fine tweaking of the structure of another series of bifunctional chiral salen-Ti(IV) complexes allows the enantioselectivity to be reversed. Biaryl-bridged salen-titanium complexes are also highly efficient catalysts, one example giving 87% ee at room temperature.  

A C2-symmetric copper-bound NHC gives excellent ees in fast room-temperature hydrosilylation of dialkyl and aryl alkyl ketones.  

Diastereoselectivities in the tetrabutylammonium cyanide-catalysed cyanosilylation of cyclic a -epoxyketones are dependent on ring size, with a switchover in selectivity between flve-membered and larger rings being explained through computation of 

A range of Lewis bases catalyse the addition of TMSCN to aldehydes, with phosphines and amines the most efficient.247 Kinetic studies indicate that the orders of aldehyde, Lewis base (LB), and TMSCN are 1, 1, and 0, suggesting an Me3Si-LB+ CN ion pair as an intermediate. However, chiral phosphines and amines gave very 

A chiral alumi n i u m - s a I e n - PI13 PO combination catalyses addition to ketones in up to 92% ee the catalyst system essentially acts as a Lewis acid-Lewis base bifunctional system.248 A similar chiral manganese(III)-salen-Ph3PO method is comparable.249 

Another effective catalyst is 1,1,3,3-tetramethylguanidine, which works well at 0.1 mol% loading in solvent-free conditions at ambient temperature.250 

Aliphatic aldehydes have been converted to their (R)-cyanohydrins using a bipha-sic system to accommodate hydroxynitrile lyase enzyme (from the Japanese apricot, Prunus mume) as the enantioselective catalyst.251 

---