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Abstract
Rates of acid-catalyzed hydrolysis of the vinyl ether group of 4-methoxy-1,2-dihydronaphthalene were measured in dilute aqueous perchloric acid solutions and also in a series of carboxylic acid buffers. These data give the hydronium-ion isotope effect kH/kD = 3.39 and provide a Bronsted relation with the exponent α = 0.70. The hydronium ion catalytic coefficient for this reaction is less, by a factor of 2.4, than that for the corresponding reaction of 3-methoxyindene. This rate difference is consistent with the coplanar arrangement of the vinyl and benzene groups in indene but non-coplanar disposition of these groups in dihydronaphthalene, and the consequent reduced ability of the benzene ring of dihydronaphthalene to stabilize the adjacent positive charge being put there in the rate-determining step of the vinyl ether hydrolysis rreaction.
Highlights
Indene, 1, is believed to be a planar molecule,[4] in contrast to its higher homolog, 1,2dihydronaphthalene, 2, whose nonaromatic ring is puckered and whose vinyl group is out of coplanarity with its benzene ring by some 15.5 OMeARKIVOC 2002 (iv) 12-17 OMeRates of addition of electrophiles to the vinyl group of 1,2-dihydronaphthalene should be lower than those of electrophilic addition to the vinyl group of indene
This follows from the fact that such addition generates positive charge on the vinyl carbon atom adjacent to the benzene rings of these molecules, where it can be stabilized by delocalization into these rings, and such stabilization is at its maximum when the vacant p-orbital of the carbon atom taking on this positive charge is parallel to the p-orbitals making up the benzene ring πsystems
We have carried out additional measurements of the hydrolysis of 4-methoxy-1,2dihydronaphthalene, 4, and have found that the rate difference predicted by the conformational argument made above is realized
Summary
1, is believed to be a planar molecule,[4] in contrast to its higher homolog, 1,2dihydronaphthalene, 2, whose nonaromatic ring is puckered and whose vinyl group is out of coplanarity with its benzene ring by some 15.5 OMeARKIVOC 2002 (iv) 12-17 OMeRates of addition of electrophiles to the vinyl group of 1,2-dihydronaphthalene should be lower than those of electrophilic addition to the vinyl group of indene. Acid-catalyzed vinyl ether hydrolysis is a reaction that occurs by rate-determining electrophilic addition of a proton to the β-carbon atom of the vinyl ether group,[6] eq 1, CH2=CHOR + H+ → CH3CHOR+
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