KETENE-FORMING ELIMINATION REACTIONS
689
When the b-R group was changed from H to Ph
Product studies
for R2NH-promoted eliminations from 1 and 2 in MeCN,
b and jblgj values remained nearly the same, although the
rate was retarded by the Ph-group probably because of
the steric effect. Here again, the relative insensitivity of
the ketene-forming transition state to the b-R group
variation is noted. On the other hand, when the same
reactions were carried out in R2NH/R2NHþ2 –70 mol%
MeCN(aq), the extent of proton transfer decreased and
the degree of Ca—OAr bond cleavage increased by the
same variation of the b-R group. The difference between
the b-R group effects in the two base-solvent systems can
be attributed to the anion-solvating ability of the solvent.
In 70 mol% MeCN(aq), where the developing negative
charge on the leaving group oxygen atom can be
stabilized by forming a stronger hydrogen bonding with
water and R2NHþ2 , a significant amount of charge density
could be transferred toward the Cb —Ca bond to enhance
the double bond character and increase the charge density
on the leaving group oxygen atom. The transition state
would then be more symmetrical with similar extents
of Cb —H and Ca—OAr bond cleavage and more double
bond character. In MeCN, however, the charge density at
the b-carbon may be stabilized by the b-Ph group rather
than to transfer toward the Cb —Ca bond to form the
partial double bond because the leaving group oxygen
atom cannot be stabilized by hydrogen bonding. This
would predict that the transition state structure would not
be significantly altered and the extents of Cb —H
and Ca—OAr bond cleavage would remain nearly the
same.
The product of the reaction between 2a and piperidine in
MeCN was identified as described.14 From this reaction,
3,3-diphenylpropionic acid piperidylamide (3) was
obtained in 89% yield. For all reactions, the yields of
aryloxides as determined by comparing the absorbance of
the infinity absorbance of the samples from the kinetic
studies with those of the authentic aryloxides were in the
range of 96–99%.
Control experiments. The stabilities of 1 and 2 were
determined as reported.14,16,19 Solutions of aryl pheny-
lacetates 1a, 1b, 2a, and 2b were stable for at least 2
weeks in MeCN solution at room temperature. However,
the solutions of 1c and 2c were stable for only 6 days.
Acknowledgement
This work was supported by KOSEF 2006–03792.
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In conclusion, we have studied the ketene-forming
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EXPERIMENTAL
Materials
Aryl phenylacetates 1 and 2 were available from previous
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dissolving R2NH in MeCN.
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Kinetic studies
Reactions of 1 and 2 with R2NH in MeCN were followed
by monitoring the increase in the absorbance of the
aryloxides at 428–430 nm with a UV–Vis spectropho-
tometer as described.14,16,19
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Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 685–689
DOI: 10.1002/poc