Isobenzofuran, 1,3-diphenyl-, trapping

The interesting tricyclic compound 55 49), which has ( )-cycloheptene moiety, was found to be very labile and could only be trapped by condensing with diphenyl-isobenzofuran. 

Benzyne trapping agents 2,5-Di-p-anisyl-3,4-diphenylcyclopentadienone. 1,3-Diphenyl-isobenzofurane. Tetraphylcyclopentadienone. 

Bicyclo[3.2.0]hept-l-ene (151) can be prepared from the tosylhydrazone of norbornan-7-one via norbornan-7-ylidene 150 in 74% as a stable olefin (144). Similarly, 152 has been obtained from norborn-2-en-7-one in a yield of 67% (144a). Introduction of a further double bond leads to the strained triene 153, which has been prepared from 105b (Scheme 3). Compound 153 dimerizes readily at room temperature to a 1 1 mixture of hydrocarbons 154 (145). The most likely structures of these dimers arise from a formal [2 + 2]cycloaddition of the bridgehead double bonds of 153 with each other. Support for the intermediacy of 153 comes from trapping experiments with 1,3-diphenyl-isobenzofuran, which yield a mixture of stereoisomers via reaction with the bridgehead double bond of 153. 

Numerous examples of unstable bridgehead olefins trapped by dienes such as 2,5-diphenyl-3,4-isobenzofuran, furan, and similar compounds have been reported (96). However, systematic studies for probing the reactive double bond and strain effects are rare. 

When an attempt was made to produce rrcm.v-cycloheptene from trans-1,2-cycio-heptenethionocarbonate (8) by the same procedure, only cfy-cycloheptene was obtained. However, the reaction of trans-1,2-cycloheptenethionocarbonate (8) with trimethyl phosphite in the presence of 2,5-diphenyl-3,4-isobenzofurane (9) as trapping agent gave an adduct (10), m.p. 150°, isomeric with the known adduct of the benzofurane with cii-cycloheptene, m.p. 185°. It is apparent, therefore, that trans-cycloheptene is the primary product from trans- 1,2-cyclohcptenethionocarbonate. 

With only five methylene units available to bridge the tmns double bond, the molecule is highly strained and very reactive. Isolation of. E-cycloheptene has not been possible, but evidence for its formation has been obtained by trapping experiments. The alkene is generated in the presence of a reagent expected to react rapidly with it— in this case, the very reactive Diels-Alder diene 2,5-diphenyl-3,4-isobenzofuran. The adduct that is isolated has the structure and stereochemistry anticipated for that derived from E-cycloheptene. The lifetime of -cycloheptene has been measured after generation by photoisomerization of the Z-isomer. The activation energy for isomerization to Z-cycloheptene is about 17 kcal/mol. The lifetime in pentane is on the order of minutes at... 

Enantiomeric (E)-cyclooctene (20E) was first resolved in 1963 through its diastereomeric platinum(II) complex. Synthesis of optically active 20E has been the subject of intensive study since 1968. The first preparation involves the treatment of enantiopure (E)-cyclooctane-l,2-thiocarbonate with triisooctyl phosphate or of (E)-cyclooatane-l,2-trithiocarbonate with l,3-dibenzyl-2-methyl-l,3,2-diazaphospholi-dine. Following analogous synthetic routes, enantiomeric (E)-cycloheptene (18E) can be produced and trapped by 2,5-diphenyl-3,4-isobenzofuran as an optically active adduct. In 1973, the circular dichroism spectrum of enantiopure 20E vapor was recorded in the vacuum UV region down to 150 nm. The first enantiodifferentiating Z-E photoisomerization of cyclooctene sensitized by chiral benzenecarboxylates appeared in 1978. Transfer of chiral information from sensitizer to substrate occurs within the exciplex intermediate. ... 

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