Photochemistry in a Crystalline Cage. Control of the Type-B Bicyclic Reaction Course:  Mechanistic and Exploratory Organic Photochemistry^1,2

Abstract

Our current theories on crystal lattice control of organic photochemistry were subjected to studies of type-B rearrangement of bicyclo[3.1.0]hex-3-en-2-ones. A first finding was that the solid state photochemistry differed dramatically from that in solution. One of our past observations in this bicyclic photochemistry in solution was that the six-membered ring, type B, zwitterion was a ubiquitous intermediate. This intermediate invariably underwent a preferential migration of an aryl group to carbon-2 relative to carbon-4 with formation of a 2,3-disubstituted phenol, a result deriving from electronic effects. In contrast, the crystal lattice photochemistry revealed a regioselectivity depending on the surrounding lattice rather than electronics. Perhaps an even more dramatic difference was an observation of the dependence of reactant stereochemistry. Thus, in solution, for 6,6-disubstituted bicyclics with two different groups at C-6, a common zwitterion is formed and the same photoproduct is formed independent of reactant stereochemistry. In crystal lattices the endo and exo stereoisomers of the 6,6-disubstituted bicyclics react differently and the group originally endo tends to migrate after three-ring opening to the zwitterion. The experimental results were paralleled with a theoretical analysis. This consisted of generation of a “mini crystal lattice” with sufficient lattice molecules to completely surround a central, reacting electronically excited state molecule. Then computational extraction of the central molecule and replacement by a transition structure afford a model of the reacting excited state inside the crystal lattice. Overlap of this species with the lattice neighbors and energy computations are then possible. These permit prediction and understanding of excited state crystal lattice reactivity on a quantitative basis.

Substituent and solvent influence on the nature of the oxyallyl intermediate in the rearrangement of bicyclo[3.1.0]hex-3-en-2-ones

“…Beginning from the intermediate of the parent system, compound 2a, the simplest derivatives add a methyl, a methoxy or a vinyl substituent in C 2 or C 6 (compounds 2b, 2d and 2f ). The idea was to stabilise the possible zwitterionic intermediate allowing a delocalization or a compensation of the positive charge that some authors proposed localized at the carbon atoms adjacent to the carbonyl group [25,29,[50][51][52][53]. The derivative with a methoxy group in C 3 was considered (compound 2c) because this group seems to favour the cleavage of the cyclopropane internal bond [53][54][55].…”

“…The idea was to stabilise the possible zwitterionic intermediate allowing a delocalization or a compensation of the positive charge that some authors proposed localized at the carbon atoms adjacent to the carbonyl group [25,29,[50][51][52][53]. The derivative with a methoxy group in C 3 was considered (compound 2c) because this group seems to favour the cleavage of the cyclopropane internal bond [53][54][55]. On the other hand, vinyl substituents in C 5 appears to protect this bond from photocleavage [54] so derivative 2g was also included in our series for comparison purposes.…”

Meta−Ortho Effect in Organic Photochemistry: Mechanistic and Exploratory Organic Photochemistry

“…In a variety of our publications we have noted that there are two different factors that control photochemical reactivity. (A) One is the nature of the excited-state hypersurface with molecules preferring low-energy pathways that avoid large barriers. ,3a, (B) The second is the probability of radiationless decay to a given photoproduct via an efficient conical intersection (or avoided crossing) in the case of a singlet, or via efficient intersystem crossing in the case of a triplet. 2b,− As has been made clear in these discussions, in complex rearrangements one most often encounters control by low-energy pathways, and such pathways do find a late conical intersection to use . It is for this reason that “organic mechanistic intuition” has been so successful in dealing with photochemical reactions, since the low-energy pathways are those predicted by the organic chemist.…”

Direct observation of deuterium migration in crystalline-state reaction by single-crystal neutron diffraction. II. 3–1 Photoisomerization of a cobaloxime complex

“…Since the 2-cyanopropyl group is produced in the original crystalline lattice by photoirradiation, the 2-cyanopropyl group with a more stable con®guration would be produced. In order to estimate the energy of the 2cyanopropyl group produced in the original lattice, a mini crystal lattice model was applied (Zimmerman & Zhu, 1995;Zimmerman & Sebek, 1997). A mini crystal lattice composed of the 36 original 3-cyanopropyl complexes was computationally generated by referring to the known crystal structure of the 3-cyanopropyl complex.…”