Intramolecular electrophilic metal-carbene intermediate

In the following examples, formation of 11,333 13,334 15,335-336 and 17,335,337 intramolecular carbenoid interaction with a C-C double bond predominates over the respective alternative reaction pathway (carbonyl ylide formation, interaction with a phenyl ring, insertion into an aliphatic C-H bond), when the less electrophilic metal-carbene intermediate is involved. 

The dominant role of the traditional copper catalysts, generally used under heterogeneous conditions, has not been challenged as yet. Only a few reports shed light on the efficiency of alternative catalysts. Copper(II) triflate allows high-yield intramolecular cyclopropanation of y,8-unsaturated diazoketone 182160) it is superior to CuS04 (53 % yield 192 ) or Rh2(OAc)4160). The solvent is crucial for an efficient conversion If the reaction is carried out in ether, the solvent competes with the double bond for the electrophilic metal carbene to give 184, presumably via an oxonium ylide intermediate. 

Terminal alkynes readily react with coordinatively unsaturated transition metal complexes to yield vinylidene complexes. If the vinylidene complex is sufficiently electrophilic, nucleophiles such as amides, alcohols or water can add to the a-carbon atom to yield heteroatom-substituted carbene complexes (Figure 2.10) [129 -135]. If the nucleophile is bound to the alkyne, intramolecular addition to the intermediate vinylidene will lead to the formation of heterocyclic carbene complexes [136-141]. Vinylidene complexes can further undergo [2 -i- 2] cycloadditions with imines, forming azetidin-2-ylidene complexes [142,143]. Cycloaddition to azines leads to the formation of pyrazolidin-3-ylidene complexes [143] (Table 2.7). 

A variety of transition metal-carbene complexes have been prepared and characterized. None of these are known to efficiently effect intermolecular C-H insertion. An electrophilic iron carbcne complex can, however, participate in intramolecular C-H insertions (Section I.2.2.3.2.I.). More commonly, transition metal complexes are used to catalyze intramolecular C-H insertion starting with a diazo precursor. In these cases, the intermediate metal carbene complexes are not isolated. 

Cyclopropanation of alkenes can also be accompUshed via other transient electrophilic intermediates that are possibly metal complexes of carbenes. Copper, rhodium, ruthenium, cobalt, or palladium catalyze the decomposition of diazocarbonyl compounds [62] which, in the presence of alkenes, gives cyclopropyl derivatives. The intramolecular example shown in Equation 7.37 uses rhodium bis(l-adamantate) dimer as the catalyst [63]. 

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