Chemistry Reference
In-Depth Information
SCHEME 11.3.
toluene at ambient temperature gives a singlet nitrene which forms adducts in low
yields (Scheme 11.3). At cryogenic temperatures, singlet nitrene undergoes ISC to
triplet nitrene. This latter nitrene, generated in the solid state experiences photo-
chemical hydrogen abstraction to give a radical pair (linked by a line in Scheme 11.3)
that collapses to form a C
H insertion product. A particularly high yield of insertion
adduct was obtained for the 2,6-difluorophenyl azide and PFPhN
3
. The intermediates
proposed in the mechanism, radical pair and triplet nitrene, were detected and
monitored as a function of time by EPR spectroscopy.
Platz et al.
21
investigated the photochemistry of PFPhN
3
in alkane, alkene,
aromatic, and amine solvents at room temperature (Scheme 11.4), and the results
are summarized in Table 11.1. The nitrene generated gave adducts with all the
solvents in moderate to good yields indicating that this intermediate was highly
reactive. The yields of pentafluoroaniline (PFA) and decafluoroazobenzene (DFAB)
were quite low for most of the reactions. Triplet-sensitized photolysis gave quite
different product distribution, larger amounts of PFA and DFAB were observed and
smaller amounts of insertion adducts.
An interesting ylide-type product (Scheme 11.4) was obtained on photolysis of
PFPhN
3
in pyridine.
21
This ylide was a yellow compound with
394 nm in
CH
2
Cl
2
. Its absorption maximum varied only slightly with solvent polarity. The
structure of this colored ylide was solved by X-ray crystallography. Due to its intense
absorptivity, this ylide was an ideal probe to study the dynamics of
1
PFPhN. In
subsequent investigations, Platz et al.
22
studied a series of FPhN
3
by laser flash
photolysis (LFP) using the pyridine molecular probe.
l
max
ΒΌ
