The reactivity of chloro- and fluoro(N-methyl-3-pyridinium)carbenes was examined by laser flash photolysis, where the halo(pyridinium)carbenes formed ylides with pyridine, acetonitrile, and acetone. Although the halo(pyridinium)carbenes reacted within the time of the laser pulse, their relative reactivities with a series of alkenes could be obtained from quenching experiments by using carbene-pyridine ylides. Their relative order of reactivity with the alkenes and their poor overall selectivity showed that the halo(pyridinium)carbenes are strongly reactive electrophilic species. Computational studies demonstrated that the alkene (HOMO)-carbene (LUMO) interaction is predominant in halo(pyridinium)carbene-alkene reactions, supporting the electrophilic nature of these species.

[reaction: see text] Herein we report the first direct observation of reactive carbenes within the cavities of cation-exchanged Y zeolites. Chloro(phenyl)- and bromo(phenyl)carbenes were generated upon laser photolysis of 3-halo-3-phenyldiazirines incorporated within dry zeolites and the absolute reactivity of the carbenes was investigated as a function of counterbalancing cation and coincorporated quenchers in order to elucidate the behavior of these intermediates within zeolites. Product analysis performed upon thermolysis of the diazirine in Y zeolites yielded products that were identified as those derived from the carbene.

A comprehensive series of halo(pyridyl)carbenes was generated by laser flash photolysis of the appropriate diazirines. Only the chloro- and bromo(2-pyridyl)carbenes and the chloro- and bromo(3-pyridyl)carbenes could be directly observed, but the reactivity of all nine halo(pyridyl)carbenes could be directly studied using the standard and a modified pyridine-ylide approach. The carbenes were all ambiphilic, being highly reactive toward both electron-rich and election-deficient alkenes. Second-order rate constants for these reactions ranged from 2.9 x 10(6) to 3.5 x 10(9) M(-1) s(-1) and depended on both the position of the nitrogen atom within the pyridine ring and the nature of the halogen group, as well as the electrophilicity or nucleophilicity of the alkene. A reactivity trend with respect to the location of the nitrogen within the pyridine ring was observed, with the 4-pyridyl carbenes being the most reactive followed by the 2-pyridylcarbenes and then the 3-pyridylcarbenes being the least reactive. This observed reactivity trend is consistent with the pyridyl ring acting as an overall electron-withdrawing group. The results also show that resonance delocalization of electron density into the nitrogen atom of the 4-pyridyl- and 2-pyridylcarbenes in the transition state significantly reduces the effect of the adjacent halogen (F, Cl, or Br) on the reactivity of the pyridyl carbenes with a series of alkenes.

A number of para-substituted chloro(aryl)carbenes are generated within the cavities of a series of dry alkali metal cation-exchanged zeolites (LiY, NaY, KY, RbY, and CsY) upon laser flash photolysis of the corresponding diazirine precursor. The absolute reactivity of the chloro(aryl)carbene is found to be strongly dependent on both the nature of the electron-donating and -withdrawing properties of the aryl substituent and the nature of the zeolite charge-balancing cations. The results strongly suggest that two opposing mechanisms for capture of the carbene can occur depending on whether the zeolite framework behaves as a nucleophilic reagent or an electrophilic reagent in its reaction with the carbene center. Hammett relationships for the decay of the carbene as a function of aryl substituent and zeolite counterion versus the sigma+ substituent parameter support a change in mechanism as the carbene center toggles between being electron poor and electron rich. For the electron-poor chloro(4-nitrophenyl)carbene, a framework adduct is proposed upon reaction of the nucleophilic [Si-O-Al]- bridge with the carbene center, and for the electron-rich chloro(4-methoxyphenyl)carbene, an adduct with the tight Li+ cation is proposed.

The synthesis of naphthylarylethanes via the photodecarboxylation of naphthylmethyl arylacetate esters is reported where the aryl group is able to stabilize a charge transfer reaction. The reaction proceeds via intramolecular charge transfer from the donor to acceptor, thereby enhancing a pathway to produce, within the solvent cage, the desired diarylethane products. These in-cage naphthylarylethanes are produced in good yields, in a single photochemical step, with the use of cyclohexane as a solvent providing optimal yields.