in the photodenitrogenation of azoalkanes, should on sym-
metry grounds (Salem-Dauben-Turro theory)2 generate the
1DZ diradical, in support of the stepwise extrusion of
molecular nitrogen.3 Recent viscosity studies have confirmed
experimentally the intervention of the 1DZ diradical as a bona
fide intermediate.4
Extensive computational work on the parent DBH (1)
provided a detailed mechanistic scenario for the photochemi-
cal evolution of the 1DZ diradical and its subsequent
chemical transformation to the inverted housane 2; the salient
features are displayed in Scheme 2.5
is enhanced along pathway B versus pathway A to generate
the C2-symmetric 1DR, diminution of the diastereoselectivity
is expected, as observed experimentally for DBH-d2 (1) in
more viscous solvents. Should pathway C be more enhanced,
the consequences would be reclosure of the endo-axial 1DZ
to the azoalkane along pathway D, which should manifest
itself in a lower quantum yield for the photodenitrogenation
in more viscous solvents. Such a study of the dependence
of the quantum yield of azoalkane disappearance on solvent
viscosity has not yet been conducted but should provide
valuable insight into the photodenitrogenation mechanism;
in particular, it would allow assessment of the feasibility of
reversible ring closure of the 1DZ diradical along the
pathways C and D. Reversible formation of a diazenyl-radical
species has been documented for both the photolysis7 and
thermolysis8 of acyclic azoalkanes through traditional cage-
effect studies for the intermolecular process but has yet to
be demonstrated for the intramolecular case of bicyclic
azoalkanes such as the parent DBH and derivatives.
Scheme 2
In this work, we report the photodenitrogenation efficiency
as a function of viscosity (η) in nujol-isooctane mixtures
for the parent DBH (1) and the structurally more elaborate
azoalkane 3. The photolysis of the latter DBH derivative
leads to the anti-4 (retention) and syn-4 (inversion) housanes
(Scheme 3). Our results imply that the lifetimes of the
On n,π* excitation, the DBH (1) generates initially the
1
exo-axial DZ intermediate by passage through a conical
Scheme 3
intersection. Thereafter, along pathway A (major route), the
1DZ species carries sufficient momentum to convert the exo-
axial to the exo-equatorial conformer; the latter is predestined
for N2 loss to afford the inverted housane inV-2 along the
SH2 trajectory. Alternatively, pathway B competes to generate
1
the C2-symmetric DR diradical on denitrogenation, which
cyclizes in equal amounts to the inV-2 and ret-2 housanes.
The third pathway C entails the rotational change of exo-
axial to the endo-axial 1DZ diradical, with subsequent
reclosure to the azoalkane 1. For the parent DBH (1),
pathway C may be neglected, since the quantum yield of
DBH consumption is unity.6
1
intermediary DZ diradicals determine whether a viscosity
effect is experimentally observable.
The above reaction scheme is consistent with viscosity
studies,4 in that frictional forces oppose the conformational
motion necessary for pathway A and diminish the population
of the exo-equatorial DZ species so that other pathways
may compete. Thus, if denitrogenation of the exo-axial 1DZ
In preliminary experiments, we examined the photodeni-
trogenation quantum yields of azoalkane 3 in solvents of
similar low viscosity, i.e., isooctane (Φ ) 0.61 ( 0.03),
acetonitrile (Φ ) 0.68 ( 0.03), and methanol (Φ ) 0.12 (
0.01). These data reveal that whereas solvent polarity
(isooctane versus acetonitrile) has only a nominal effect on
the photodenitrogenation efficiency of azoalkane 3, hydrogen
bonding (methanol) dramatically diminishes the Φ value.
Such hydrogen-bonding effects are well-documented in
azoalkane photolysis;9 consequently, protic polar solvents
such as alcohols had to be avoided for the viscosity variation
in our quantum-yield studies. For this reason, we employed
nujol-isooctane mixtures, an aprotic solvent system of low
1
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