Photoisomerizations of protonated 5-methylhex-3-en-2-one and 4-methylpent-2-enoic acid. Evidence for charge localization during photoisomerization

Abstract

.J. Chcm. 61, 856 (1983).The photoisomcrizations of protonated 5-mcthylhcx-3-en-2-one and 4-methylpent-2-cnoic acid have been exalnined. In each case a relatively rapid c~i,s/trrrtl.s ison~crization about the carbon-carbon partial doublc bond and a slower conversion to protonated dihydrofuran 2H or prvtonatcd lactonc 5H, respcctivcly, were observed. The transformation of the acyclic cations to the five-membered ring cations was shown to occur by a photo-initiated. intramolecular hydride shift from the y to P carbon atom. This shift is fully in accord with an excited state twisting/chasge localization model for the excited states of these systems. Control experiments and the thermal chemistry of these ions is described.

The importance of ground state conformation in the photoisomerizations of protonated enones

“…The low temperature 'H nmr spectra of these cations were consistent with the indicated structures, Table 1 (8). In the case of 1 , 2 , and 6 the spectra obtained were identical to those previously reported (3,14).…”

“…'H nmr spectra with those of other protonated dihydrofurans (3,9). These isomerizations proceeded in high yield with no other products being detected.…”

Divergent Reaction Pathways of a Cationic Intermediate: Rearrangement and Cyclization of 2-Substituted Furyl and Benzofuryl Enones Catalyzed by Iridium(III)

“…A 1,2-deuteride shift from the isopropyl chain of 2- d 1 to form 3- d 1 was confirmed using 2 H NMR spectroscopy. Simpler enones with branched alkyl chains are known to experience a 1,2-hydride shift upon treatment with protic acid and generate carbocation-trapped products …”