Stereochemical Analysis of an Aromatic Triplet Di-?-methane Rearrangement

Article abstract of DOI:10.1002/hlca.19860690821

It is shown that (-)-(S)-N,N-dimethyl-2-(1'-methylallyl)aniline((-)-(S)-4), on direct irradiation in MeCN at 20 deg C, undergoes in its lowest-lying triplet state an aromatic di-?-methane (ADPM) rearrangement to yield (-)-(1'R,2'R)- and (+)-(1'R,2'S)-N,N-dimethyl-2-(2'-methylcyclopropyl)aniline((-)trans- and (+)-cis-7) in an initial trans/cis ratio of 4.71 +/- 0.14 and in optical yields of 28.8 +/- 5.2percent and 15 +/- 5percent, respectively.The ADPM rearrangement of (-)-(S)-4 to the trans- and cis-configurated products occurs with preponderance of the path leading to retention of configuration at the pivot atom (C(1') in the reactant and C(2') in the products) for (-)-trans-7 and to inversion of configuration for (+)-cis-7, respectively.The results can be rationalised by assuming reaction paths which involve the occurence of discrete 1,4- and 1,3-diradicals (cf.Schemes 10, 12, and 13).A general analysis of such ADPM rearrangements which allows the classification of these photochemical reaction in terms of borderline cases is presented (Scheme 14).It is found that the optical yields in these 'step-by-step' rearrangements are determined by the first step, i.e. by the disrotatory bond formation between C(2) of the aromatic moiety and C(2') of the allylic side chain leading to the generation of the 1,4-diradicals.Moderation of the optical yields can occur in the ring closure of the 1,3-diradicals to the final products, which may take place with different trans/cis-ratios for the individual 1,3-diradicals.Compounds (-)-trans-7 as well as (+)-cis-7 easily undergo the well-known photochemical trans/cis-isomerization.It mainly leads to racemization.However, a small part of the molecules shows trans/cis-isomerization with inversion of configuration at C(1'), which is best explained by a photochemical cleavage of the C(1')-C(3') bond.