Versatile Reactions Undergone by Carbenes and Nitrenes in Noble Gas Matrices at Cryogenic Temperatures

Abstract

Abstract A series of phenyldiazomethanes and phenyl azides bearing proximate substituents matrix-isolated in Ar at cryogenic temperatures have been photolyzed. Products are characterized mainly by IR spectroscopy in combination with ab initio theoretical calculations. In most cases, several intermediates are produced as a result of interaction between the divalent centers and the proximate functional group; they are photo-interconvertible under these conditions. Phenylcarbenes and phenylnitrenes having hydrogen donors such as CH3, OH, and NH2 groups at the ortho position have been shown to abstract hydrogen to form o-quinonoid compounds. Both the carbene and nitrene undergo oxygen transfer from the nitro group at the ortho position to form the corresponding nitroso compounds. While phenylcarbene interacts with o-CO2Me group to generate the corresponding carbonyl ylide, phenylnitrene undergoes 1,4-MeO shift in its excited states to produce 5-carbonyl-6-methoxyimino-1,3-cyclohexadiene. Interesting differences in the reactivity between carbenes and nitrenes are noted; these are discussed in terms of the electronic configuration, singlet-triplet energy gap and tunneling. 1,3-Bis(diazo) ketones incorporated in five- and seven-membered rings are prepared and photolyzed under matrix conditions. While the seven-membered ring bis(diazo)ketone affords phenanthryne by way of 9,10-cyclopropa-phenanthren-3′-one, the five-membered analogue generates 1,2-cyclopropabenzocyclobutadien-3′-one, which does not undergo photodecarbonylation to form benzocyclobutenyne. Finally, a series of bis(divalent) species conjugatively connected through aromatic rings are generated and characterized. In both p- and o-phenylene systems, there is a significant interaction between the benzene nucleus and the divalent substituents. The species are best regarded as p- and o-quinonoidal biradical structures, respectively, having a singlet ground state with a low-lying excited triplet state.