Carbenes alkene complexes

III. Rearrangements in the Excited State vs. Carbene-Alkene Complexes. .. 57... 

III. REARRANGEMENTS IN THE EXCITED STATE VS. CARBENE-ALKENE COMPLEXES... 

Of all the carbenes considered in this chapter, benzylchlorocarbene (10a) has produced the most debate, from carbene-alkene complexes vs. excited state... 

Photolysis ofbenzylchlorodiazirine (3) in the presence of tetramethylethylene (TME) is known to produce ( )- and (Z)-/l-chlorostyrene (4) and the cyclopropane (5). Plots of [5]/[4] vs [TME] are curved, consistent with the existence of two pathways for the formation of the alkenes (4). Benzylchlorocarbene (BnClC ) was generated by laser flash photolysis of the phenanthrene (6) in the presence of TME. In this case, plots of [5]/[4] vs [TME] are linear, mling out the possibility that the second pathway to the alkenes (4) involves reaction of a carbene-alkene complex. Time-resolved IR spectroscopy revealed that diazirine (3) rearranges to the corresponding diazo compound, but this process is too inefficient to account for the curvatures. It is proposed that the second pathway to alkene formation involves the excited state of the diazirine. 

The 1,2-H shift in 53 is of interest in connection with the possible intervention of kinetically significant carbene-alkene complexes and 1,2-H shifts that may occur in excited states of the nitrogenous carbene precursor as well as in the car-benes. ° " These questions will be further discussed in Section 2.4. 

Complexes were also considered for other carbenes and even suggested for reactions in alkane solution( ) A counterproposal, backed by the inability of theory to find support for such stable complexes, held that the second source was not the carbene-alkene complex but instead was the diazo compound, formed from isomerization of excited diazirine. " Other LFP studies reinforced the need for two intermediates, but could not finally resolve the question of carbene complexes ver-sus diazo compound. However, the question is now settled in this case,... 

Note that despite the death of the carbene-alkene complex in the study of benzylchlorocarbene (53) (see above), benzene is able to modulate the intramolecular reactivity of ferf-butylcarbene. ° Some sort of complex must be involved here. Benzene complexes with carbenes have been proposed before. Kahn and Goodman found a transient species on photolysis of diazomethane in benzene, and attributed it to a complex. Moss et al. found that benzene modulated the ratio of intramolecular rearrangement to intermolecular addition for three different carbenes (53), chloropropylcarbene, and chlorocyclopropylcarbene, and proposed that a carbene-benzene complex 70 favored the intramolecular rearrangement (Scheme 7.31). Their proposal was bolstered by ab initio calculations that found such stable complexes for CCI2 and CH3CCI. 

Generation of Alkyl and Cycloalkyl Carbenes - Photolysis or thermolysis of a series of alkylchlorodiazirines (16) (Scheme 7) in the presence of alkenes, such as tetramethylethene, results in 1,2-H shifts, giving the corresponding vinyl chorides (18), in competition with additions of the carbenes (17) to the alkenes, yielding cyclopropanes (19). The mechanism of these reactions is discussed in the light of results obtained from photoacoustic calorimetry, and the ratio of vinyl chloride to cyclopropane seems to depend on the excited states of the carbene precursors and also on carbene-alkene complexes. Similar reactions of related diazirines have been investigated by flash photolysis. 

Our initial explanation of the negative activation energies postulated that the carbene-alkene additions involved the intermediate and reversible formation of loose carbene/alkene change-transfer complexes. The partitioning of these intermediates between cyclopropane formation and reversion to PhCCl and alkene would determine the observed rate constants and their temperature dependence. [95] However, although carbene-arene Jt complexes do appear to modulate the chemistry of some carbenes in solution, carbene-alkene complexes have not been supported by theoretical studies. [96]... 

Metallocycloalkanes.—Recently accumulated evidence requires the alkene metathesis reaction to involve the interconversion of mixed carbene-alkene complexes with metallocyclobutanes. With Pt, a highly stereospecific rearrangement of a four-membered ring is observed and work on the mechanism is reported. In the reaction in Scheme 8 and with diphenyl-substituted rings the inter-... 

Warner, P.M., On Carbene Alkene Complexes, Tetrahedron Lett., 25,4211,1984. 

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