Solution photochemistry of 4-alkylidene-3,3,5,5-tetramethyl-Δ1-pyrazolines

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Abstract
Direct photolysis of 4-((2H6)isopropylidene)-3,3,5,5-tetramethyl-1-pyrazoline (11) is believed to proceed via a singlet trimethylene-methane (TMM). It gives mainly the ‘least motions’ product ([2H6]isopropylidene)-2,2,3,3-tetramethylcyclopropane (12). This implies that the singlet TMM adopts a bis-orthogonal geometry (21) rather than the mono-orthogonal geometry (20) expected. Direct photolysis of 13 different α-methylated 4-alkylidenepyrazolines (5) shows that, in this series, formation of the ‘least motions’ product (22) is normally preferred. In the initially formed bis-orthogonal biradicals, the ability of C–ĊMe2 rotation to compete with ring closure seems to be determined by steric factors. Generally, the tendency to form the ‘least motions’ product (22) is greatest in those cases where the alkylidene group bears two substituents (X, Y ≠ H), less where there is one (X = H, Y ≠ H), and least of all where there is none at all (X, Y = H). The two main exceptions to this least motions rule are for 4-(2′,2′,3′,3′-tetramethylcyclopropylidene)-3,3,5,5-tetramethyl-1-pyrazoline (27) and for 3,3,5,5-tetramethyl-1pyrazolin-4; ylidenemalononitrile (32). In both of these cases there may be special factors which destabilise the ‘least-motions’ product. By way of contrast to the direct photolysis, triplet sensitised photolysis of 4-([2H6]isopropylidone)-3,3,5,5-tetramethyl-1-pyrazoline (11) gives a statistical mixture of the deuteriated methylenecyclopropanes (12) and (13). This is consistent with the expected planar geometry (19 or the kinetic equivalent thereof) of the triplet TMM biradical. Some examples of the photochemical interconversion of methylenecyclopropanes and one example of a photochemical reduction of a pyrazoline are also reported.

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