Carbenes, coupling

Fundamentally, the high nucleophilicity of stable carbenes, coupled with the tendency to increase the coordination number at the carbenic carbon, renders their chemical behavior very similar to that of electron-rich phosphines. As such, they have aroused considerable interest as ligands in p-, cl-, and / -block chemistry. Earlier reviews (7a,b) have covered some aspects of the main group chemistry of stable nucleophilic carbenes however, there has been considerable recent activity in this field, and thus a comprehensive, up-to-date review was considered desirable. 

It has been reported that thermolysis of the bis(diazomethyl)phosphines lo leads to the formation of the diazaphospholes 64. This can be explained by either an intramolecular [2 + 3] cycloaddition process involving the diazo group, or a diazo-carbene coupling reaction. 

Not observed were chloroethane (CH3-CH2C1) and vinyl chloride (CH2=CHC1). An interesting finding was that 1,1-DCA reacted much too slowly to represent an intermediate in the formation of ethane. The authors postulated a scheme involving successive one- or two-electron reduction steps to form radicals and carbenes to explain the absence of other observable intermediates, as well as the formation of products originating from radical or possibly from carbene coupling. Try to constmct such a hypothesized reaction scheme yourself. 

Figure 5.28 Carbene coupling with 2VE ligands examples...

The mechanism of the reaction is not known photoinduced CO loss and photoinduced CO-carbene coupling to yield a ketene intermediate were considered. 

The complex [Y(L) N(SiMe3)2 2] (L = l-f t7-butyl-3-(2-fi t7-butylaminoethyl)imidazol-2-ylidene) showed a Cc(carbene) resonance at 186 ppm with a V -C(carbene) coupling of 54.7 Hz, larger than any Vyc value teported for an s-bound yttrium alkyl or an yttrium-NHC adduct <2003AGE5981>. In comparison, the J c value fot Y[N(SiHMe2)2]3(N,N -dimethyl-NHC) was 49.6 Hz <19970M682>. 

Hofmann, P., Hammerle, M. The Mechanism of the Dotz Reaction - Chromacyclobutenes by Alkyne-Carbene Coupling. Angew. Chem., Int. Ed. Engl. 1989, 28, 908-910. 

Clement ND, CaveU KJ (2004) Transition-metal-catalyzed reactions involving imidazoUum sal t//V4 Ictcrocycl ic carbene couples as substrates. Angew Chem Int Ed 43 3845-3847... 

Carbene coupling involving such obtained iridium-carbene species thus explains the observed formation of ethene (see Rg. 37), in analogy with ethene formation upon oxidation of [Re (Cp)Me(NO)(PPh3)] (117). 

The magnitudes of Jpt-c (carbene) were not found to follow the exact trans-influence order previously established (37, 39) for other organoplatinum (2+) complexes. However, it was agreed that changes in hybridization of the Pt-C (carbene) bond are monitored by Jpt-c- For phosphorus-carbene couplings it was noted that Jp-pt-c (trans) > Jp-pt-c (cis). Data for these 1,3-di-organoimidazolidin-2-ylidene complexes are given in Table XXXVIII. 

The direct conversion of carbyne complexes TpW( = CR)(CO)2 (R = Ph, C6H4Me-4) to cationic c/ -dicarbonyl -phosphinocarbene (metallaphosphacyclo-propene) complexes can be achieved by the addition of chlorodiorganophosphines in the presence of sodium tetraphenylborate or thallium hexafluorophosphate (Scheme 51)/ In the absence of these sodium salts, nucleophile (Cl )-induced carbonyl-carbene coupling affords neutral -phosphinoketene complexes. 

Scheme 77 1,2-Acyl shift followed by diazoalkane carbene coupling...

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