Vinyl halides failure

Rhodium(II) pivalate has also been recommended for the cyclopropanation of vinyl halides with ethyl diazoacetate 78). As Table 8 shows, yields with this catalyst are far higher and reaction conditions milder than with copper. Failures are noted,... 

Halide anions have also been employed to facilitate the cyclization of weakly nucleophilic terminal vinyl ir-nucleophiles. For example, the butenylamine (85) undergoes Mannich cyclization in the presence of excess Nal to provide the 4-iodopiperidine (87) in excellent yield. The success of this cyclization should be contrasted with the failure of related amines to cyclize in formic acid with formaldehyde (Scheme 25). A detailed study " of the effect that nucleophile concentration has on the outcome of Mannich cyclizations provides deilnitive evidence that the cyclization of iminium ions with alkenes is not a concerted process, but rather proceeds via a cationic intermediate capable of partitioning between product formation and reversal to the starting iminium ion. A bridged cation or ir-complex, e.g. (86) in equation (8), is a reasonable description of this intermediate. 

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