Q2(c). Singlet and triplet carbenes with cis-pent-2-ene

Introduction. A carbene R₂C: has a divalent carbon with two non-bonding electrons. In the singlet state they are paired in one sp² orbital (an empty p orbital remains); in the triplet state they are unpaired in two orbitals, so the carbene is a diradical. Addition to an alkene gives a cyclopropane, and the spin state decides the stereochemistry (Skell's rule).

Scheme: Singlet: stereospecific. Triplet: non-stereospecific (both 1-ethyl-2-methylcyclopropanes).

1. Singlet carbene (e.g.:CH₂ from photolysis of diazomethane in the liquid phase, or:CCl₂) adds in one concerted step: both new C–C bonds form together, so the alkene has no time to rotate. cis-Pent-2-ene gives only cis-1-ethyl-2-methylcyclopropane: stereospecific syn addition.
2. Triplet carbene (e.g.:CH₂ made by benzophenone-sensitised photolysis of diazomethane, or in the gas phase diluted with an inert gas) adds stepwise. The first C–C bond gives a 1,3-diradical whose two electrons have parallel spins.
3. The ring cannot close until one spin inverts (spin inversion, intersystem crossing). This is slower than rotation about the former C=C bond, so the diradical loses its geometry.
4. Ring closure then gives a mixture of cis- and trans-1-ethyl-2-methylcyclopropane: non-stereospecific.
5. Each cyclopropane is formed as a racemate (attack from either face of the alkene is equally likely).

Conclusion. Singlet carbenes add to cis-pent-2-ene stereospecifically to give only cis-1-ethyl-2-methylcyclopropane; triplet carbenes add through a rotating 1,3-diradical and give both cis and trans isomers. The stereochemical result is used as a test of carbene multiplicity.

Application. Stereospecific singlet cyclopropanation (dihalocarbenes, Simmons–Smith carbenoids) is used to make cyclopropane insecticides such as the pyrethroids.