Dichlorocarbene amine catalysis

The reaction of dihalocarbenes with isoprene yields exclusively the 1,2- (or 3,4-) addition product, eg, dichlorocarbene CI2C and isoprene react to give l,l-dichloro-2-methyl-2-vinylcyclopropane (63). The evidence for the presence of any 1,4 or much 3,4 addition is inconclusive (64). The cycloaddition reaction of l,l-dichloro-2,2-difluoroethylene to isoprene yields 1,2- and 3,4-cycloaddition products in a ratio of 5.4 1 (65). The main product is l,l-dichloro-2,2-difluoro-3-isopropenylcyclobutane, and the side product is l,l-dichloro-2,2-difluoro-3-methyl-3-vinylcyclobutane. When the dichlorocarbene is generated from CHCl plus aqueous base with a tertiary amine as a phase-transfer catalyst, the addition has a high selectivity that increases (for a series of diolefins) with a decrease in activity (66) (see Catalysis, phase-TRANSFEr). For isoprene, both mono-(l,2-) and diadducts (1,2- and 3,4-) could be obtained in various ratios depending on which amine is used. 

In addition, there are a few examples of heterogeneous nonaqueous sonochemistry, in both liquid-liquid and liquid-solid systems. Two recent reports have utilized ultrasonic agitation in place of or along with phase transfer catalysis for the preparation of dichlorocarbene from aqueous NaOH/CHCl3 (166), and for N-alkylation of amines with alkyl halides (167). Along the same lines, several papers have appeared in which... 

Phase transfer catalysis has been applied to the Hofmann isocyanide synthesis, where dichlorocarbene is generated in the presence of a primary amine yields of 40—60% were reported. The synthesis of toIuene-/ -sul-phonylmethyl isocyanide and some alkylated analogues has been described. Isocyanides react with alcohols in thepresence of thalIium(iii)or mercury (u) nitrate to give carbamates the latter salt, while giving lower yields, is more flexible than the former, which is restricted to the use of methanol. ... 

Reactions of carbenes other than cyclopropanation can also be performed, and recent examples include the ring expansion of five-ring heterocycles, such as the indoles (18), to their six-membered counterparts (19), and the production of formamides from secondary amines (Scheme 7), both with dichlorocarbene. The latter method is of interest because of its relation to the catalysis of dichlorocarbene generation by tertiary amines in two-phase systems. Recent work indicates that such catalysis is possible because the carbene, after generation at the phase boundary, is transferred into the organic phase (to undergo reaction) in the form of the N-ylid adduct (20). 

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