Geometrically strained carbenes: Interdependence among geometry, spin multiplicity and reactivity

Abstract

The singlet and triplet states of cyclobutenylidene 1 and benzocyclobutenylidene 2 have been studied computationally (using ab initio and DFT methods) in order to assess the effect of angle strain on the S-T gap of vinyl- and phenylcarbenes. It is found that both carbenes have a singlet ground state. Cyclobutenylidene 1 has a more significant singlet-triplet gap (-25 kcal mol-1) than benzocyclobutenylidene 2 (-14.5 kcal mol-1). The strong preference of 1 for a singlet ground state may be understood if it is viewed as bicyclobut-1-ene with a considerably puckered ring. Singlet benzocyclobutenylidene (12) is computed to have also a puckered cyclobutene ring albeit less pronounced. The lowest isomerization path available for singlet cyclobutenylidene (11) is the formation of vinylacetylene, which is predicted to have a barrier of around 9 kcal mol-1. Our calculations suggest that singlet benzocyclobutenylidene (12) lies in a rather deep potential well and should be observable under suitable experimental conditions. However, under low-temperature Ar matrix conditions no evidence for the formation of carbene 2 was obtained from the photolysis of precursor 15. On the other hand, laser flash photolysis (LFP) of precursor 16 enables us to observe benzocyclobutenylidene at room temperature. The lifetime of carbene 12 is determined to be 0.6-0.7 μs in cyclohexane. The mCXY value for carbene 12 is determined to be 0.43, revealing a strong electrophilic nature and ranking 12 second among the electrophilic carbenes listed in the carbene philicity spectrum.