Free-rotor effect

Upon low conversion direct photolysis the cis isomer (10) gave only the cis isomer (12) and the trans isomer (11) gave only the trans isomer (13). The triplet sensitized reaction of (10) and (11) gave rise only to cis-trans isomerization. Thus the di-ir-methane photorearrangement from the triplet state cannot compete with triplet state deactivation via cis-trans isomerization (Zimmerman has termed this the free rotor effect). Several other examples of regio-specilicity and stereospecificity in di-w-methane photoreactions are as followsa8 a3) ... 

In order to determine whether the partial suppression of the free-rotor effect was required for the success of the ODPM rearrangement process, the study was extended to the aldehyde 33 [51], When 33 is irradiated (15 min), under similar conditions to those used for 29, the cyclopropyl aldehyde 34, resulting from an ODPM rearrangement, was obtained, as the tran -diastereoisomer, in 90% isolated yield. This result demonstrated clearly that the ODPM reactivity of (3,y-unsaturated aldehydes is not restricted to cyclic compounds, such as 29, but can also be extended to acyclic derivatives. Therefore, the suppression of the free-rotor effect is not essential for the success of the rearrangement and the reaction is probably controlled by both the excitation of the molecule to the TiCtt, -it )... 

A plausbile explanation reason is that these ketones have relatively less strained double bonds and undergo and efficient E—>Z isomerization, thereby dissipating the excited state energy, termed as the free rotor effect. In another report [27] it was indicated that the 1,2-acyl shift can... 

The photoinertness of 2,4-diisopropylidenecyclo-butanone is explained as a "free rotor" effect. In other words, in the S state neither a-bond can be cleaved very facilely, so intersystem crossing occurs. In T, excitation is localized on the C=C bond and is deactivated... 

The problem of the free-rotor effect and the efficient dissipation of triplet energy is still of interest. Hixson and Tausta42 have found that the excited singlet state of the naphthyl olefin (53a), a typical substrate for a di-7r-methane reaction, undergoes cw-Zra/w-isomerization to (53b) and conversion into the cyclopropane (54). Continued irradiation leads to the formation of cyclopropane at the expense of as-olefin (53b). Sensitized irradiation gave very efficient... 

Unlike the situation in the singlet excited state, cts-fraws-isomeriza-tion is very effective from the triplet state. 9,18,20,48) When the carbon-carbon double bond is not geometrically constrained (f. e. the bond is part of an acyclic system or not in a small ring) m-fra s-isomerization can become so efficient that it is (almost) the only triplet process observed and the other triplet processes do not compete. This effective process has been called the /rcc rotor effect for. y-unsaturated ketones in view of the similarity to the concept proposed earlier for flexible di-it-methane systems. 89) Flexible double bonds thus provide an efficient pathway for dissipation of the energy of the triplet state. The free rotor effect is a chemical pathway of intersystem crossing. The effect is nicely borne out... 

The free rotor effect apparently has not much influence on the reactivity of the triplet of PP.80) Although no oxa-di-w-methane product is found, efficient =0.19) [2 -f-2]cydoaddition to 158 and 159 takes place. 

Hancock, K.G. and Grider, R.O., A free rotor effect in P,y-unsaturated ketone photochemistry. The diverging singlet and triplet reactions of 3-ethylidene-2,2,5,5-tetramethylcyclohexanone Tetrahedron Lett., 1367,1972. 

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