Di-n-methane photorearrangement

Abstract After a brief introduction and summary of various methods of asymmetric induction in organic photochemistry, the main part of the review covers the solid-state ionic chiral auxiliary approach to asymmetric photochemical synthesis. Application of this technique to the Norrish type II reaction, as well as to the di-n-methane and oxa-di-n-methane photorearrangements, and the cis,trans-photoisomerization of diarylcyclopropane derivatives is presented and discussed. 

Demuth, M., Lemmer, D., Schaffner, K., Electron Spin resonance and IR Evidence for Intermediates in the Di n methane Photorearrangement of a Naphthobarrelene like Compound after Low temperature UV Irradiation, 7. Am. Chem. Soc. 1980, 102, 5407 5409. 

The di-n-methane photorearrangement of barrelene-type molecules such as 253 has been studied extensively in solution and in the solid state (Scheme 41). Although, the asymmetric induction was very low in solution with salts... 

This regioselective example of a di-n--methane photorearrangement is of particular importance because in this case both it moieties have very similar singlet energy levels. This suggests that excitation localization does not control the reaction. 

Diarylcyclohex-2-enones undergo a different photorearrangement to bicydol3.1.0]hexan-2-ones, in which an aryl substituent migrates from C-4 to C-3 (4.641. This reaction finds a parallel in the di-n-methane rearrangement of 3-phenylalkenes (see p. 54). It is usually efficient (<> — 0.1-0.2), it occurs by way of the (n.Jt ) triplet... 

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

The rearrangement is analogous to the di-n-methane rearrangement and is considered as 1,2-shift of imino group from C(3) to C(4), followed by a o bond formation between C(3) and C(5). For example, the photorearrangement of P,y-unsaturated imine 55 gives cyclopropyl imine 56, which on hydrolysis gives cyclopropane aldehyde 57 [62]. 

Pt A), 63-84 (c) Evans, S.V., Garcia-Garibay, M., Omkaram, N., Scheffer, J.R., Trotter, J., and Wireko, F. (1986) Use of chiral single crystals to convert achiral reactants to chiral products in high optical yield application to the di-Jt-methane and Norrish type II photorearrangements. 

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