Chemistry Reference
In-Depth Information
scopy and detailed quantummechanical calculations indicated that the Cl-N bond is a
localized three-electron coordination, strong enough to hold the chlorine in place but
weak enough to permit it to attack tertiary C-H bonds [169]. Quinoline and acridine
templates in
96
and
97
, respectively, could also use this nitrogen coordination of a
chlorine atom to direct steroid chlorination [170]. In a particularly interesting example,
both nitrogen positions in the template in
98
were used for chlorine coordination,
producing the 9,17-dichloroproduct
99
in quantitative yield [171]. We have reviewed
the use of heterocycles as radical relay templates [172].
The templates can be simply coordinated rather than attached. For example, com-
plex
100
directed the radical relay chlorination to C-9, although the process was not as
clean as with the attached templates [173]. We also used template-directed chlorina-
tions to determine the conformations of flexible chains, just as we had previously with
the benzophenone probes [174]. Also, by use of a set of tandem free radical chain
reactions we could direct the formation of carbon-bromine and carbon-sulfur bonds,
again with geometric control by the attached template [175].
