On the Constitutional Stability ofη1-Allylmetal Compounds
摘要:
Many eta(1)-allymetal species undergo a rapid haptotropic rearrangement. The stereochemical outcome of reactions of these allylmetal species with aldehydes depends on whether the haptotropic rearrangement is faster or slower than the reaction with the aldehyde. We present here a test system based on kinetic resolution, by which just this information becomes available. Thus, haptotropic rearrangement of cyclohexenyllithium, -magnesium chloride, -titanium triisopropoxide, and -titanium tetraisopropoxide, was found to be faster than addition to the aldehyde 9. Borotropic rearrangement in cyclohexenyldiethylborane was found to be slower than the addition to the aldehyde 9.
The perfluoroalkylation of lithium enolates of chiral N-acyloxazolidinones with perfluoroalkyl iodides mediated by triethylborane proceeds with good diastereomeric excess (55%–93% de).
Triethylborane-mediated reactions of lithium enolates derived from chiral N-acyloxazolidinones with ethyl difluoroiodoacetate allows easy access to α-(ethoxycarbonyl)difluoromethylated carboximides with good diastereomeric excess (86->98%de). The stereochemistry of the (ethyoxycarbonyl)difluoromethylated carboximides indicates that the (ethoxycarbonyl)difluoromethyl radical generated from ethyl difluoroiodoacetate and triethylborane reacts preferentially on the si face of the lithium enolates.
The introduction of an EtO2CCF2- group into lithium enolates of chiral N-acyloxazolidinones with ethyl difluoroiodoacetate mediated by triethylborane proceeds with good diastereomeric excess (86->98% de).
Lithium enolates of chiral N-acyloxazolidinones reacted with dibromodifluoromethane and bromodifluoromethane to give α-bromodifluoromethyl and α-difluoromethyl carboximides, respectively, with good diastereomeric excess. These reactions proceed not via a radical mechanism but via an ionic chain mechanism involving insertion of difluorocarbene.
The trifluoromethylation of lithium enolates of chiral N-acyloxazlidinones with iodotrifluoromethane mediated by triethylborane proceeds with good diastereomeric excess (62–86% de).