Benzyltrimethylsilane and Trimethylsilyldiphenylmethane
J. Phys. Chem. A, Vol. 103, No. 46, 1999 9183
TABLE 3: Decay Rate Constants of Transient Species
Produced by the 266-nm Light Photolysis of
partially be ascribed to the difference in stability of the 1,3-
silyl-shifted intermediates at these temperatures. Activation
energy from the deformed excited state to the ortho-rearranged
isomer of 2 may be also responsible for this difference. The
activation energy is considered to be small because the latter
was produced even in the photolysis at 130 K. The reaction
mechanism of 2 at room temperature is summarized in Scheme
2.
Trimethylsilyldiphenylmethane at Room Temperaturea
o-rearranged
photo-Fries rearranged
intermediates [s-1
]
intermediates [s-1
]
b
solvent
Ar bubbling
O2 bubbling
1,3-shift
1,5-shift
CH
MeOH
29
25
28
26
3.6
6.5 × 10-2
3.0 × 105
2.5 × 105
a Error, (10%. b Reference 24.
Acknowledgment. The authors thank Profs. H. Matsumoto,
T. Kudoh, and H. Shizuka of Gunma University for their
valuable suggestions and discussion. This work was partly
supported by a Grant-in-Aid for Scientific Research on Priority-
Areas, “The Chemistry of Inter-element Linkage” (No.09239104)
and “Photoreaction Dynamics”, from the Ministry of Education,
Science, Sports and Culture of Japan.
by oxygen and decay unimolecularly to give ortho-hydroxyac-
etophenone.24 According to the resemblance in the lifetime and
spectral shape of absorption band the broad transient absorption
band is attributable to the 1,3-silyl-shifted isomer of 2.
To examine the formation of the rearranged isomer, product
analysis was carried out by measuring the GC-MS of the
photoproducts obtained by irradiation with 266-nm light in
MeOH at room temperature. Three peaks denoted as A, B, and
C were observed in the GC chart as shown in Figure 9a with
relative intensities of 6.5%, 91%, and 2.5%, respectively. The
MS spectrum of the most intense peak, B, shown in Figure 9c
is essentially the same as that of parent molecule 2. The MS
spectrum of peak A (Figure 9b) gave typical fragmentation peaks
at 168 m/z (molecular ion), 167 (molecular ion - 1), and 91
(benzyl ion) and is essentially the same as the MS spectrum of
diphenylmethane. Therefore, peak A is ascribable to diphenyl-
methane produced from 2 by losing a trimethylsilyl group. In
the MS spectrum of peak C shown in Figure 9d, the most
prominent peak is observed at 239 m/z which is ascribable to
the molecular ion - 1. Also, a trimethylsilyl ion is observed at
73 m/z and, in addition, moderately intense signals at 165 and
152 m/z commonly observed for A and B are observed for C.
These results indicate that the peak C is ascribable to the species
having a diphenylmethyl skeleton and a trimethylsilyl group.
Thus, peak C is ascribable to the rearranged isomer of
trimethylsilyldiphenylmethane. These results are consistent with
our assignment of the broad absorption band to ortho-rearranged
intermediates. The ortho-trimethylsilyldiphenylmethane which
is considered to be responsible for the peak C can be formed
when a 1,3-hydrogen shift takes place in ortho-rearranged
intermediates. Ortho-rearranged intermediates, however, may
preferentially undergo a reverse 1,3-silyl group shift to form
parent molecule, 2, as suggested by a relatively weak peak of
C.
References and Notes
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It is interesting that the photochemical reaction of 2 at room
temperature is different from that at 77 K. This difference may