A Stereospecific Access to Allylic Systems Using Rhodium(II)−Vinyl Carbenoid Insertion into Si−H, O−H, and N−H Bonds
摘要:
Rhodium-catalyzed decomposition of alpha-vinyldiazoesters in the presence of silanes, alcohols, ethers, amines, and thiols have been shown to produce the corresponding alpha-silyl, alpha-hydroxy, alpha-alkoxy, alpha-amino, and alpha-thioalkoxy esters in generally good yield with a complete retention of the stereochemistry of the double bond of the diazo precursor. An extension of the process in homochiral series has also been devised using either a chiral auxiliary attached to the ester function or achiral alpha-vinyldiazoesters and Doyle's chiral catalyst Rh-2(MEPY)(4). In the former approach, pantolactone as chiral auxiliary gave diastereoselectivities of up to 70%, while the second approach produced the desired allylsilane with ee as high as 72%. On the other hand, Rh-2(MEPY)(4)-catalyzed insertion into the O-H bond of water led to poor or no enantioselectivity in good agreement with recent literature reports.
“<i>Syn-Effect</i>” in the Conversion of (<i>E</i>)-α,β-Unsaturated Esters into the Corresponding β,γ-Unsaturated Esters and Aldehydes into Silyl Enol Ethers
The stereochemistry in the conversion of (E)-α,β-unsaturated esters into the corresponding β,γ-unsaturated esters, and that in the conversion of aldehydes into the silyl enol ethers, were investigated. The Z/E ratios of the resulting β,γ-unsaturated esters and the silyl enol ethers varied according to the γ-substituents of the (E)-α,β-unsaturated esters and the α-substituents of the aldehydes, respectively