Chemistry Reference
In-Depth Information
SCHEME 11.6.
Several factors that favor ISC, from singlet nitrene to lower-energy triplet
nitrene, were explained by Platz et al.
21
In the photolysis of PFPhN
3
, the addition
of small amounts of ethyl iodide reduced the yield of toluene adducts and
increased the yield of PFA and DFAB, even in the presence of isoprene, a typical
triplet excited state quencher. Therefore, ethyl iodide accelerates singlet to triplet
ISC by external heavy atom effect.
12
The addition of methanol also had a dramatic
effect on the products obtained. The yields of toluene adducts were reduced while
the yield of PFA increased. The fact that methanol is promoting singlet to triplet
ISC was supported by time-resolved spectroscopy. LFP of PFPhN
3
in methanol
gives a transient absorption spectrum that greatly resembles the spectrum
obtained by photolysis at low temperature (77 K). This catalytic effect was
explained in terms of hydrogen bonding (Scheme 11.7).
21
Low-frequency bend-
ing modes are provided by hydrogen bonding between singlet nitrene and
methanol molecules. In methanol, ISC is accelerated since the singlet-triplet
energy gap is reduced.
SCHEME 11.7.
