the selectivities are lower than for the corresponding benzylic
arenesulfonates (7.3 for 3-chlorobenzyl arenesulfonates and 4.9
for 4-methylbenzyl toluene-p-sulfonates). However, we know
that about half of the initially formed benzyl carbenium
ion from the parent benzyl azoxytoluene-p-sulfonate (1a) is
trapped by the adjacent nitrous oxide molecule to give the
benzyloxydiazonium ion, 2 in Scheme 1.8 This then leads on to
give principally a further solvent-derived substitution product
and, in the absence of a base, only a very low yield of benzalde-
hyde. Thus, we are unable to ascribe the present overall ratio
of alcohol to ether wholly to the selectivity of the benzylic
carbenium ion, and we have insufficient data from the
reactions of 1f, 1c, and 1b to calculate the extents of involve-
ment of the corresponding substituted benzyloxydiazonium
ions (2).
Spherisorb) HPLC was used with manual injections (Rheodyne
valve with 20 µl loop), aqueous methanol flow rate = 1.5 cm3
minϪ1; the UV detector was set at 257 nm.
Acknowledgements
We thank EPSRC for an equipment grant (H. M.) and a
studentship (I. M. G.), and Professor M. Mishima (Kyushu
University) for helpful discussions.
References
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Experimental
TFE (Fluorochem) was refluxed over polyphosphoric acid for
1.5 h then fractionally distilled from molecular sieves (type 3A).
Preparative details and structural assignments are available as
electronic supplementary information.‡
Kinetics
Rates of solvolysis in 1 : 1 (v/v) TFE–H2O were measured by
monitoring the decrease in UV absorbance at a suitable wave-
length in the range 230–260 nm in the thermostatted cell com-
partment of a Pye-Unicam SP-800 spectrophotometer fitted
with a platinum resistance thermometer and interfaced to an
Apple microcomputer.5,6 Initial substrate concentrations were
usually ca. 10Ϫ5 mol dmϪ3. Each reaction was monitored for 4–5
half-lives with the collection of 80–100 absorbance readings.
First-order rate constants were calculated by a non-linear least
squares programme; standard deviations on individual rate
constants were generally <1% and reproducibility was normally
3%. Activation parameters and rate constants at a common
temperature of 25.0 ЊC were calculated by a computer version
of the Eyring equation using average rate constants from tripli-
cate determinations at each of four temperatures covering a
30 degree range.
9 I. M. Gordon and H. Maskill, J. Chem. Soc., Chem. Commun., 1989,
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10 R. F. Borch, M. D. Bernstein and H. D. Durst, J. Am. Chem. Soc.,
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11 R. S. Tipson, J. Org. Chem., 1944, 9, 235.
12 T. E. Stevens, J. Org. Chem., 1964, 29, 311.
13 H. Maskill, The Physical Basis of Organic Chemistry, Oxford
University Press, 1985.
14 4-Cyanobenzyl azoxytoluene-p-sulfonate was also prepared and
fully characterised, but did not yield a solvolytic rate constant;
presumably, its very slow solvolytic reaction was compromised by
competing non-solvolytic modes of decomposition.
Product analysis5,8
HPLC was carried out using Gilson instrumentation and a UV
detector connected to a Pye-Unicam PU 4810 computing inte-
grator. Glass distilled water and HPLC grade methanol were
filtered before use. The three substituted benzyl azoxytoluene-p-
sulfonates were each solvolysed at least twice for at least 10 half-
lives, and each reaction mixture was analysed directly at least
5 times to give averaged product analyses. Reverse phase (C-18
15 I. Lee, Adv. Phys. Org. Chem., 1992, 27, 57.
16 Y. Tsuno and M. Fujio, Adv. Phys. Org. Chem., 1999, 32, 267.
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J. Chem. Educ., 1975, 52, 76; H. Maskill, Structure and Reactivity in
Organic Chemistry, Oxford University Press, 1999.
2062
J. Chem. Soc., Perkin Trans. 2, 2001, 2059–2062