Radical ions in photochemistry. 18. The photosensitized (electron transfer) tautomerization of alkenes; the 1,1-diphenyl alkene system

Arnold, Donald R.; Mines, Shelley A.

doi:10.1139/v87-384

Cited by 12 publications

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“…In fact, the strength of the base is important. This effect was observed previously in the case of the 1,l-diphenyl alkene tautomerization (2). While tautomerization of the diphenyl alkenes was catalyzed by pyridine (pKa = 12.3 (21)), the reaction was significantly more efficient when the stronger base lutidine (pKa = 15.4) was used.…”

Section: Discussionsupporting

confidence: 73%

“…Alternatively, an allylic carbon-hydrogen bond of the freely rotating methyl group can overlap with the SOMO and deprotonation is uninhibited. This type of stereoelectronic effect, controlling the reactivity of radical cations, has been observed before (2,24).…”

Section: Discussionsupporting

confidence: 68%

“…The energy (eq. [2]) stored within the radical ion pair in step 3a (75 kcal mol-') is greater than the triplet energy of both the electron acceptor (1,4-dicyanobenzene, 70.1 kcal mol-' (17)) and of the donor (phenyl conjugated alkenes, ca. 50 kcal mol-I (7a)).…”

Section: Discussionmentioning

confidence: 93%

“…In the previous study involving tautornerization of 1,ldiphenyl alkenes, lutidine (2,6-dimethylpyridine) was used as the base (2). Collidine was found to be more effective than lutidine for these reactions involving monophenyl alkenes.…”

Section: Resultsmentioning

confidence: 99%

“…We have reported a general photochemical method for the tautomerization of 1,l-diphenyl substituted alkenes to nonconjugated isomers (2). The mechanism proposed for this process involves the initial formation of the alkene radical cation and the sensitizer radical anion, induced by irradiation of the sensitizer and mediated by the cosensitizer.…”

Section: Introductionmentioning

confidence: 99%

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Radical ions in photochemistry. 21. The photosensitized (electron transfer) tautomerization of alkenes; the phenyl alkene system

Arnold¹,

Mines²

1989

Can. J. Chem.

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. MINES. Can. J. Chem. 67, 689 (1989). Alkenes, conjugated with a phenyl group, can be converted to nonconjugated tautomers by sensitized (electron transfer) irradiation. For example, irradiation of an acetonitrile solution of the conjugated alkene 1-phenylpropene, the electron accepting photosensitizer 1,4-dicyanobenzene, the cosensitizer biphenyl, and the base 2,4,6-trimethylpyridine gave the nonconjugated tautomer 3-phenylpropene in good yield. Similarly, 2-methyl-1-phenylpropene gave 2-methyl-3-phenylpropene, and l-phenyl-1-butene gave E-andZ-1-phenyl-2-butene. The reaction also works well withcyclic alkenes. For example, 1-phenylcyclohexene gave 3-phenylcyclohexene, and 1-(phenylmethylene)cyclohexane gave 1-(phenylmethyl)cyclohexene. The proposed mechanism involves the initial formation of the alkene radical cation and the sensitizer radical anion, induced by irradiation of the sensitizer and mediated by the cosensitizer. Deprotonation of the radical cation assisted by the base gives the ambident radical, which is then reduced to the anion by the sensitizer radical anion. Protonation of the ambident anion at the benzylic position completes the sequence. Reprotonation at the original position is an energy wasting step. Tautomerization is driven toward the isomer with the higher oxidation potential, which is, in the cases studied, the less thermodynamically stable isomer. The regioselectivity of the deprotonation step is dependent upon the conformation of the allylic carbon-hydrogen bond. The tautomerization of 2-methyl-1-phenylbutene gave both 2-phenylmethyl-1-butene and 2-methyl-1-phenyl-2-butene (E and Z isomers), while 2,3-dimethyl-1-phenylbutene gave only 3-methyl-2-phenylmethyl-1-butene. In the latter case, steric interaction of the methyls on the isopropyl group prevents effective overlap of the tertiary carbon-hydrogen bond with the singly occupied molecular orbital, thus inhibiting deprotonation from this site.Key words: photosensitized, electron transfer, alkene, tautomerization, radical cation. , on peut transformer des alcknes conjuguCs avec un groupement phCnyle en tautomkres qui ne le sont pas. Par exemple, l'irradiation du phCnyl-1 propkne, un alckne conjuguC, en solution dans I'acCtonitrile, en prCsence de dicyano-1,4 benzkne agissant comme photo-sensibilisateur acceptant des electrons, de biphCnyle agissant comme cosensibilisateur et de trimCthy1-2,4,6 pyridine agissant comme base, fournit de phCnyl-3 propkne, le tautomkre qui n'est pas conjuguCe, avec un bon rendement. De la m&me manikre, le methyl-2 phCnyl-1 propkne fournit du mCthyl-2 phCnyl-3 propkne alors que le phCnyl-1 butkne-1 conduit i un melange des phCnyl-1 butknes-2-(E) et -(a. La rCaction donne aussi de bons rtsultats avec les alcknes cycliques. Par exemple, le phinyl-1 cyclohexkne fournit du phCnyl-3 cyclohexkne alors que le (phCny1mCthylkne)-1 cyclohexane conduit au (phCnylmCthy1)-1 cyclohexene. Le micanisme proposC implique la formation initiale du cation radical de l'alckne et l'anion radical du sensibilisateur qui est induit...

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Section: Discussionsupporting

confidence: 73%

Section: Discussionsupporting

confidence: 68%

Section: Discussionmentioning

confidence: 93%

Section: Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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