Ylide, Halonium

The reaction of crotyl bromide with ethyl diazoacetate once again reveals distinct differences between rhodium and copper catalysis. Whereas with copper catalysts, the products 125 and 126, expected from a [2,3] and a [1,2] rearrangement of an intermediary halonium ylide, are obtained by analogy with the crotyl chloride reaction 152a), the latter product is absent in the rhodium-catalyzed reaction at or below room temperature. Only when the temperature is raised to ca. 40 °C, 126 is found as well, together with a substantial amount of bromoacetate 128. It was assured that only a minor part of 126 arose from [2,3] rearrangement of an ylide derived from 3-bromo-l-butene which is in equilibrium with the isomeric crotyl bromide at 40 °C. 

Electrophilic carbene complexes can also react with organic halides to yield halonium ylides. Reaction of acceptor-substituted carbene complexes with allyl... 

On the basis of these results, a mechanism (Scheme 8.10) involving the intermediacy of a silver-carbene 54 was proposed in which the insertion product arises from the formation of the halonium ylide 55, followed by a 1,2 shift (55 —> 26, or 51 or 52). Alternatively, if the substrate and thus the halonium ylide 56 contain a (3-hydrogen, this could be removed by an intramolecular deprotonation with concomitant loss of halide resulting in formation of the olefin 57 and the a-haloacetate 53. At this stage, no independent evidence has been obtained to support this pathway thus this mechanism is purely speculative (see text below). Indeed, although the pathway has been depicted as involving metal-free intermediates, it is quite likely that this is not the case, but this awaits independent experimental verification. 

Ochiai and coworkers developed a new synthetic approach to various iodonium ylides 429 by the inter-molecular transylidation reactions between halonium ylides under thermal or catalytic conditions (Scheme 2.125) [566,567], The transylidations of bromonium 428 to iodonium 429 ylides proceed under thermal... 

Halonium ylides afforded cyclopropanes. Ochiai et al. synthesized halonium ylides and examined cyclopropanation with cyclooctatetraene [60]. Chloronium ylides 61c smoothly underwent progress of cyclopropanation and bicyclic cyclopropane 63 was obtained in 72% yield (Scheme 1.36). 

The first isolated adduct between an aminocarbene and a Lewis acid was obtained by reacting an imidazol-2-ylidene (IV) with iodopentafluorobenzene (Scheme 8.22). In contrast to classical halonium methylides (obtained from transient electrophilic carbenes and halogen centers), which feature a characteristically small C—X—C angle, this adduct has an almost linear C—I—C framework (178.9°). This hnding is consistent with a 10e-I-2c (hypervalent) bonding at the iodine center, and a reverse ylide nature. 

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