Carbene like resonance structures

The cation, formed by heterolysis of the C—Cl bond in 2-chlorobiphenyl, has been represented as a carbene-like resonance structure with the unshared electron pair at position 2 and the positive charge in the other ring358. 

Inspired by the contribution of the carbene-like resonance structure, the homopolymerization of isocyanide giving rise to the formation of poly(iso-cyanide) has attracted much attention [3, 4]. On storage, or distillation, isocyanides that lack bulky AT-substituents tend to form solid materials, which had been supposed to be poly(isocyanide)s. However, this polymerization , (or resinification), largely depended upon the nature of the glass surface of the apparatus used for storage or distillation and, therefore, was poorly reproducible. Moreover, no structural information was provided for these materials, making the evaluation of the polymerization systems difficult. The historical background has already been overviewed by Millich in two reviews published in 1972 and 1980 [3, 4]. 

Bifunctional activation of CO leads to carbene-like resonance structures of the type shown in Equation 2-81. 

Although not the central subject of this review, several thorium dihaptocarbamoyl complexes ( 4) Th(n2-CONR2)/ were also examined with respect to thorium hydride-catalyzed reduction. Under 0.75 atm H2 and over the course of several days at temperatures as high as 100°C, no hydrogenation was observed. These results are in accord with other spectral, structural, and chemical data (14) indicating the importance of carbamoyl resonance hybrids O and P, and that the carbene-like reactivity is significantly reduced in comparison to the acyls (14) ... 

The first nitrile ylide stable enough to be isolated (i.e., 1) has been prepared by the carbene/nitrile method (1). For this dipole, the anionic component is stabilized by electron delocalization and the nitrilium component by the steric bulk of the adamantyl group to such an effect that it has a melting point of 230 °C. The X-ray structure showed that the nitrile ylide moiety is close to linear and much like the resonance structure shown below. 

Thermolysis of diazoalkynes yields alkynyl carbenes like vinyl carbenes, these undergo carbene-to-carbene rearrangements very readily. Propargylene (241) was prepared in 1971, and it was found that the singlet form reacted with alkenes only at Ci, the position vacated by nitrogen, but the triplet form reacted at either Ci or C3. Propargylene and its methyl and phenyl derivatives showed electron paramagnetic resonance, and the predicted structure is of C2 symmetry, best described in terms of a diradical valence structure. 

Chugaev obtained carbene complexes like these as early as 1915, but the right structure was only assigned much later. Acetylides L M-C=CR are unexpectedly good bases via their resonance form L M+=C=C -R. They can react with acid in alcohol solution to give the carbenes shown in equations (7) and (11). An intermediate vinylidene cation probably undergoes nucleophilic attack by the alcohol. In this case, the usual order of attack of equations (1) and (2) - nucleophile, then electrophile - is inverted. 

A more reasonable hypothesis is that the transition state is imbalanced, as shown in equation (102), but that there is a structural feature characteristic of carbene complexes, absent from other carbon acids, that masks the imbalance by reducing a. The most likely candidate is the 7r-donor effect of the methoxy group. Inasmuch as the contribution of 151 leads to resonance stabilization of the carbene complexes, this resonance is expected to add to the intrinsic barrier of proton transfer. This is because, as is true for resonance effects in general, its loss at the transition state should be ahead of the proton transfer. As Z becomes more electron... 

The electron-deficient Fischer carbene carbon receives donation from the lone pair(s) of the ir-donor substituents, denoted OR(lp). Structure 11.1 shows how the M(d,) and OR(lp) orbitals compete for IT donation to the carbene carbon. This can be described in valence bond (VB) language by resonance between 11.2 and 11.3. The real structure often resembles 11.3 rather than 11.2, as shown by the long M-C and short C-O bonds found by X-ray studies. For electron counting purposes, we regard the Fischer carbene as an L-type hgand like CO. The true M=C bond order is much less than 2, thanks to the contribution of 11.3. 

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