Chemistry Reference
In-Depth Information
The template-directed steroid functionalizations are remarkably selective. After all,
the C-9 and C-14 hydrogens are very close (1,3 diaxial on ring C). In a theoretical paper
we discussed this selectivity, and concluded that an extra factor in the selectivity may be
more than the ability of a chlorine atom to simply collide with one or the other of these
hydrogens [176]. Instead, the trajectory of the collision may be important, requiring a
straight C-H-Cl alignment. This would help select among such closely spaced hydro-
gens.
1.4.4
Nitrene Insertions
We briefly explored the use of nitrene insertions directed by geometric placement of an
appropriate nitrene precursor, which was generated by photolysis of an azide.
Although some nitrenes readily rearrange, we saw a class of phosphoryl nitrenes
that performed intermolecular insertions rather than rearrangements [177]. However,
when such a nitrene
102
was generated in cyclohexane solution by photolysis of
101
it
preferentially attacked the solvent, forming
103
, rather than performing an intramo-
lecular insertion reaction into the neighboring benzylic C-H bond [178]. Intramole-
cular product
105
was indeed seen when azide
101
was decomposed thermally in cy-
clohexane in the presence of a rhodium salt. Thus a free nitrene is so reactive that the
normal preference for intramolecular reactions is not seen - it reacts on essentially
every collision and is surrounded by solvent. Rhodium complexed nitrene
104
, a “ni-
trenoid“, is less reactive and shows the entropy advantage of an intramolecular reac-
tion with geometric control.
Such metal-complexed nitrenes were also generated by the reaction of (tosyliminoio-
do)benzene (
106
) with Mn(
III
)- or Fe(
II
)-tetraphenylporphyrin,
107
, in a mimic of cy-
tochrome P-450 but with a tosylimino group instead of an oxygen atom on the metals
(
108
) [179]. It was able to functionalize cyclohexane solvent, by nitrogen insertion into a
C-H bond to form
109
. Furthermore, the metalloporphyrins also catalyzed an intra-
molecular nitrogen insertion converting
110
into
111
[180].
