C O M M U N I C A T I O N S
3
for ground-state reactions. Our total SKIE is large, kH/kD ) 2.17
( 0.03, and the proton inventory plot is clearly curved, indicating
that the transition state involves at least two weakly bonded protons
“in flight”. The relationship between the kn-n data pairs is
commonly analyzed by the Kresge-Gross-Butler (KGB) relation,
eq 1,21,22 where the parameters æiq ) (D/H)i/(D/H)L2O are the
isotopic fractionation factors for the exchangeable positions i ) 1
‚‚ ν in the transition complex.
T1(1). The formation of 3 from T1(1) amounts to release of the
substrates HX; the lifetimes of T1(1) thus define the release rate.
Formation of p-hydroxybenzyl alcohol (6) has not been previously
reported. It is a signature of the elusive spirodione 4, the lifetime
of which must be shorter than its rate of formation under the reaction
conditions. Nevertheless, the spirodione 4 must be a real intermedi-
ate, because it is trapped at high water concentrations, yielding
p-hydroxyphenylacetic acid (2).
ν
Acknowledgment. This work was supported by the Swiss
National Science Foundation (J.W.) and by NIH grants P50
GM069663 and R01 GM72910 (R.S.G.).
k /k )
(1 - n + næ q)
(1)
∏
n
H
i
i
Supporting Information Available: Step-scan FTIR spectra, NMR
of 5, and details of the calculations. This material is available free of
It is here assumed that isotopic fractionation in the reactant state
is negligible. Then the fractionation factor for the ith transition-
state site, æiq, becomes equal to the reciprocal isotope effect (kH/
kD)-1 generated at that site. It is significantly less than 1 only for
protons being transferred in the transition state or bonded to a
positively charged oxygen as in H3O+. Equation 2 then reduces to
some simplified limiting cases: (a) a single site generates the entire
isotope effect of 2.17, as in a single proton-transfer reaction (eq 2,
ν ) 1 and kH/kD ) 2.17); (b) two sites generate roughly equal effects
as in a double proton-transfer reaction (eq 2, ν ) 2; kH/kD ) 2.171/2
) 1.47); (c) three sites generate roughly equal effects as in general-
base catalysis by water19c (eq 2, ν ) 3; kH/kD ) 2.171/3 ) 1.30);
(d) a large number ν of sites generate the effect as in a large change
in solvation, so that each effect is small and thus the dependence
of kn on n becomes exponential, eq 3.
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In conclusion, the sequence of intermediates occurring in the
photo-Favorskii rearrangement of p-hydroxyphenacyl derivatives
1 in aqueous solutions is now established (Scheme 1). The proton
inventory data indicate that diethyl phosphate release from T1(1a)
proceeds in concert with deprotonation and requires the assistance
3
of at least two water molecules.7c The primary biradical 3 has
hitherto escaped detection by optical spectroscopy, because its
absorption is so weak and its lifetime is similar to or, at low water
concentrations, shorter than that of its strongly absorbing precursor,
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