Resonance structures carbenes

Two stannenes have been synthesized by the reaction of a stannylene with a boranediylborirane (Eq. 34).85 The boranediylborirane has been shown to react toward suitable reagents as though it were the carbene,101 which is only slightly higher in energy than the boranediylborirane.102 The reaction occurs at room temperature in pentane solution. The resulting stannene has a considerable contribution from the ylide resonance structures. The carbene arising from the boranediylborirane is extremely electrophilic, and therefore the stannenes can be considered formally to be adducts of the stannylene as Lewis base and the carbene. 

The matrix IR spectra of la and several isotopomers (cU-la, l80-la) reveal details of the electronic structure of the carbene.23 In particular the red-shift of the C=0 stretching vibration (compared to p-benzoquinone) below 1500 cm-1 indicates a substantial contribution of the phenoxyl/phenyl resonance structure to the wave function of la. The C2V symmetry of the carbene was experimentally revealed by measuring the IR dichroism of partially oriented samples of matrix-isolated la. The orientation of la in an argon matrix was achieved by irradiation with linearly polarized light. 

The BE-matrices of hydrogen cyanide and hydrogen isocyanide (notice that since the neutral "carbenic" resonance structure has not the octets fully completed, the dipolar resonance structure is taken instead) are ... 

Fig. 2 Electronic configuration and resonance structures of heterocyclic carbenes containing an X7C carbene center...

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. 

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. 

The formation of the observed products can be explained by the following catalytic cycle (Scheme 8). Addition of the nucleophilic carbene leads to adduct I, followed by proton transfer to give conjugate enamine Ha. Homoenolate equivalent Ha (see also resonance structure lib) can add to the aldehyde reaction partner providing zwitterion III and after... 

The delocalized dipolar resonance formulae postulated on p. 104 for 1 and 4 are justified on the basis of the spHtting of the carbene levels and the mild nucleophihcity of 4. For the 4 +2 carbene 2, the carbene energy-level splitting is very small and no clear electrophilic character has been detected for 2. Since 3 clearly behaves as an electrophil, a reasonable contribution from a delocalized resonance structure seems to be involved for 3. Although no clear indication of electrophilic character has been observed for 2, we believe that 2 is also best described by a delocalized resonance structure. 

You might have interpreted the molecule s skeleton to be that shown in resonance structures 1-61 through 1-63. However, none of these structures is expected to be stable. Structure 1-61 has a large number of charged atoms, 1-62 has an uncharged carbon with only six valence electrons (a carbene, see Chapter 4), and 1-63 has a carbene carbon and a nitrogen... 

Several approaches exist to understanding the structure of metal carbenes. Perhaps the simplest and most familiar is resonance theory. Structures 10-13 represent several possible contributing resonance structures of metal carbenes. Structures 10-12 seem to be important contributors for Fischer carbene complexes, as indicated by experiment and calculations. 

There exists a relevant analogy between Schrock carbene complexes and ylides. The term ylide typically refers to a dipolar compound where carbon is attached to a nonmetal such as N, P, or S. An ylide shows nucleophilic character at the carbon, with two resonance structures providing significant contribution to the overall structure of the molecule, (i.e., R Y=CR2 R Y+- CR2). For an introduction to the chemistry of ylides, see M. B. Smith,... 

The enr/o-adduct, endo- 6, formed from cyclopentadiene, spontaneously rearranged to a benzene derivative 7. Of the two carbene resonance structures, the cyclopropanes derive from alkynyl-bromocarbene only. 

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]. 

This increase in acidity is mainly the result of the weaker 77-donor effect of the MeS (c7r = - 0.15) " compared to that of the MeO group (ctr = - 0.43) " which leads to a weaker stabilization of the neutral thia compared to the oxa carbene complexes. The higher pK values of entries 1-3 compared to that of 5 can be attributed to 77-donation by the respective oxygens in entries 1-3 that is stronger than in 5. In the case of entry 3, the ethyl group is a better stabilizer of the oxyanion resonance structure of the carbene complex (140). For the cyclic carbene complexes (entries 1 and 2), the 77-donor effect is enhanced further because, by virtue of the cyclic structures of entries 1 and 2, the oxygen is locked into a position for better 77-overlap with carbene carbon (142). Cr NMR data are in agreement with this assessment. ... 

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