Nonclassical carbenes

In the light of experimental difficulties associated with the identification of intermediates, a MINDO/3 quantum-mechanical study of the singlet state ( a2) of the cyclobutylidene to methylenecyclopropane rearrangement has been carried out. It has been proposed that the whole process is initiated by electrophilic attack from the C3 methylene group of cyclobutylidene at the empty p atomic orbital on the Cl carbene site, so that a shift of electron density towards Cl can take place to give the bicyclobutane-like nonclassical carbene intermediate 4. Finally, the bicyclobutane intermediate 4 undergoes a symmetry-allowed conrotatory bond-fission process to generate methylenecyclopropane. The activation enthalpy calculated for the two steps is 8 kcal mol-1.2... 

The complete retention of the original T position in the product from the liquid-phase reaction indicates that automerization is slow in comparison writh the collision frequency in the condensed phase, in agreement wdth theoretical estimates (18) of the activation energy for Equation 7, ranging from 44 to 77 kcal mol While no direct information is provided by the decay experiments as to the specific mechanism of automerization (H vs. CH-group migration), the results nevertheless exclude any significant intervention of the nonclassical carbene structure... 

These trials showed a reluctance of carbene 46 to undergo intramolecular fragmentation to 47 also in the condensed phase. The hydrolysis reaction of 46 with trace amounts of H20, giving 55f, was evident in each medium. Yet, the formation of 5-norbornen-2-yl-substituted products iso-55 in protic media is noteworthy. Unsaturated iso-55 cannot directly stem from carbene 46 (i.e., no 46-> iso-55) it derives from carbocation 58. So, carbene protonation is indicated (46 —> 58) 69-120 135 However, the typical product ratio that is characteristic of nonclassical carbocation 58 (i.e., 55 iso-55 — ca. 3.5) was not observed, even in the presence of diethyl fumarate (DEF).136,137 Thus, the paucity of iso-55 implies a deviation due to ion pairing of 58 with its counterion. 

Olah emphasizes that the division of cations into classical and nonclassical is frequently arbitrary, since in many cations there is an intenn liate range of delocalization ( partial carbonium-ion character ) as in the 2-methylnorbomyl ion. The author does not want to name classical ions carbonium because it is restricted to highest valeiK state carbocations this requirement is nwt by penta-and tetracoordinate carbocations but not trivalent ones. On the other hand, while in the formation of other onium ions the atom of the donor (nitrogen, oxygen etc.) increases its covalence by one unit upon addition of the acceptor (electrophile), in the formation of a classical ion the covalence of the carbon atom decreases from 4 to 3. As for the name carbenium ion, in the author s opinion it reflects the logical relationships between the carbene and the carbeiunm ion, between the alkene and the carbenium ion ... 

Singlet cyclobutylidene (7) is found to have a C, symmetric nonclassical structure at the DFT, MP2, and CCSD(T) levels (Figure 9). The distances to pentacoordinated C4 are between 1.61 and 1.84 A. The structure of 7 is very similar to that of the well studied isoelectronic cyclobutyl cation. Both the carbene and the carbocation profit from hyperconjugative interaction of the Cl -C4 and C3-C4 bonds with the formally empty p orbital on C2 in spite of the associated distortion of the molecular framework. 

The use of high speed computers has allowed one to carry out sophisticated and reliable ab initio calculations even on medium size molecules. Such calculations are not only reproducing physical properties of chemical systems, but are also able to predict the stability of nonclassical bonding and structures. One of the interesting examples of this type is silylenes, compounds containing divalent silicon, which were considered unstable species even a few years ago. However, in 1994 the first stable member 1 of this class of compounds was synthesized, following the synthesis of the analogous stable carbene. ... 

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