Cyclopropyl carbenoid intermediate

Although Echavarren et al. reported similar conversions by platinum catalysis, lower selectivity was observed [165-167]. Computational studies then suggested the initial formation of a cyclopropyl carbenoid intermediate 213, as shown in Scheme 8.28. 

A rationale for the heteroatom effect has recently been provided for the reaction of 1,6-enynes based on detailed mechanistic studies,318 314 especially DFT calculations performed by several groups (Scheme 90) 183>319321 From what has been disclosed so far, an interplay of cationic and carbenoid species is obvious in these processes. Nowadays, there is a good consensus to adopt the involvement of cyclopropyl metallacarbene intermediates. 

Analysis of triene structures in leukotrienes or the dienes in the HETEs reveals in all cases that a cis,trans dienes (partial structure) is commonly shared. The Merck Frosst group has recently described a methodology for the delivery of cis,trans dienes in the synthesis of various HETEs. Scheme 2.9 depicts the basic route whereby a-diazoketones 25 react, via a carbenoid species, with furan to produce an unstable cyclopropyl furan intermediate 26. This reaction is efficiently... 

Because of these and other useful molecules containing three-membered rings, methods to make them are important as well as interesting. Most chemical syntheses of compounds containing cyclopropyl groups make use of the addition of a carbene, or carbene equivalent, to an aikene. What do we mean by carbene equivalent Usually, this is a molecule that has the potential to form a carbene, though it may not actually react via a carbene intermediate. One such example is a zinc carbenoid formed when diiodomethane is reacted with zinc metal it reacts with alkenes just as a carbene would—it undergoes addition to the 7t bond and produces a cyclopropane. 

The inherent energy content of the cyclopropane ring demands that the method of introduction of a cyclopropyl subunit itself relies either on highly reactive intermediates or on irreversible or energetically, if not entropically, favored processes. Thus the synthesis of cyclopropane derivatives can be classified into three major categories 1,3 bond forming sequences (equation 4) carbene or carbenoid routes (equation 5) and rearrangement pathways (equations 6 and... 

Beside carbenes and carbenoids, metal-catalyzed cyclopropanation using other types of intermediate is also feasible. Treatment of 5-methyl-5-phenyl-4-methylene-l,3-dioxolan-2-one with palladium(O) catalyst in the presence of norbomene produced the cyclopropyl ketone in excellent yield (Scheme 8) (60). The active intermediate was proposed to be a Zwitterion... 

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