Relative rates of insertion reactions

7arbene Carbene precursor Alkane Secondary C-H Primary C-H Tertiary C-H Primary C-H Ref. 

The intramolecular insertion of alkylcarbenes appears to involve an eclipsed conformation the energy of which depends on the number of alkyl-alkyl interactions. As a consequence of steric effects, highly-branched alkylcarbenes display an apparent selectivity in favor of primary C-H bonds whereas less-branched structures insert preferentially into seconday C-H bonds . For the example given in scheme (42), 

Carbenoids, i.e. species intermediate in a-elimination reactions and catalytic decomposition of diazoalkanes, have so far failed to give products of inter-molecular insertion. In contrast, alkylcarbenoids do insert into /3 and y C-H bonds. They show a greater selectivity in favor of secondary and tertiary C-H bonds than to free alkylcarbenes (Table (c/. p. 381 for isotope 

The data collected in Table 19 have been arranged according to the type 

2-Diphenylcyclopropylidene, generated thermally (spontaneously) from the diazo compound, shows a similar reversal in reactivity with highly substituted olefins In this case the authors have attributed the reversal to steric hindrance during addition of the free carbene. In contrast, steric assistance may operate in the addition of dihalocarbenes to highly substituted olefins. The preferred yw-addition of several halocarbenes and halocarbenoids (Table 19) indicates favorable non-bonded interactions between halogen and alkyl groups. 

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IV. Reactivity of Cycloalkenecarbenes A. Relative Rate of Insertion Reactions... 

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