ISSN:
0009-2940
Keywords:
Carbonyl ylides, generation from syn1-bishomofuran
;
Thiocarbonyl ylides, generation from syn1-bishomothiophene
;
Ylides, carbonyl and thiocarbonyl, 1,3-dipolar cycloaddition, electrocyclization, sigmatropic 1,4-hydrogen shift of
;
Chemistry
;
Inorganic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Reactivity of s̰1-Bishomoaromatic Heterocyles: Electrocyclic Reactions and Cycloadditions of syn- and anti1-Bishomofuran and -BishomothiopheneProfessor William von E. Doering zum 75. Geburtstag gewidmet.The title compounds syn-4a, b and anti-4a, b, synthesized by the CuCl-catalyzed reaction of homofuran 1a or homothiophene 1b with diazomethane, are very different in their thermal stability. syn-4a isomerizes to 2,3-dihydrooxepine (7a) readily at 130°C whereas the rearrangement anti-4a → 7a requires a temperature of about 370°C. A similar difference in thermal reactivity is observed between syn-4b and anti-4b. But syn-4b undergoes in addition to the rearrangement to 2,3-dihydrothiepine (7b) a dimerization (to 12?) on thermolysis in [D6]benzene solution at 140°C, whereas mainly isomerization to 9b, cis-10b, and 11 (product of the elimination of sulfur from cis-10b) occurs on thermolysis in the gas phase. The formation of 7a and 7b, respectively, is explained by the electrocyclic ring opening of syn-4a or syn-4b leading to the carbonyl ylide 5a or the thiocarbonyl ylide 5b followed by a sigmatropic 1,4 hydrogen shift. To rationalize the additional products in the thermolysis of syn-4b an electrocyclic conrotatory ring closure in 5b leading to the highly strained episulfide trans-10b is assumed to complete with the hydrogen shift. Under the reaction conditions trans-10b is unstable and either dimerizes in solution or isomerizes in the gas phase. The formation of 9b in the thermolysis of syn-4b indicates that a two-step pathway may compete with the concerted process. syn-4a, b react with various π acceptor-substituted olefins to form cycloadducts of the general type 6a, b. The cycloaddition occurs stereospecifically cis with respect to the configuration of the starting olefin and non-stereoselectively with respect to the endo/exo ratio. The kinetic analysis of the reaction of syn-4a with fumaronitrile (FN), N1-phenylmaleinimide (NPMI) or dimethyl dicyanofumarate (DCFM) shows unambiguously, that the adducts are formed by a 1,3-dipolar cycloaddition of the intermediate carbonyl ylide 5a rather than a homo-Diels-Alder reaction of syn-4a (in contrast to the cycloaddition of homofuran 1a). A comparison between the enthalpies of reactions syn-4a → 5a (ΔHR ≤ 22.4 kcal/mol) and syn-4d → 5d (ΔHR ≥ 47.5 kcal/mol) shows the carbonyl ylide to be a 1,3-diradical, highly resonance-stabilized by the heteroatom (resonance energy R.E. ≍ -25 kcal/mol).
Additional Material:
4 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/cber.19931260228
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