Enyo et al.
halogen, the observed relative absorbances of the nitrile
versus CdN stretch can be used as a (qualitative)
measure for the relative ratios of 3-XN versus 4-XN.
Thus, the fact that the CN triple bond stretch of 4-FN
was not detected provides the second piece of evidence
that, in the case of F, the ratio [3-XN]/[4-XN] is higher
than that in the cases of Cl and Br.
The Z-6-XN, X ) Cl, Br, isomers are intriguing since
they are formally derived from their
1A′-2-XN isomers
by a geometric distortion of the CCX bond angle. On the
other hand, their charge distributions are quite different,
and they should be described by different wave functions.
This isomerization can be visualized as a transfer of a
Br atom from the carbene center to the nitrene. This
bond-breaking bond-forming process would leave behind
an excited vinylidene (with the two unpaired electrons
occupying different orbitals), which after electronic re-
organization would have the electronic distribution of
Z-6-BrN. We speculate that this transformation involves
a conical intersection, but due to the size of the active
space involved we have not been able to locate it. Thus,
it is not surprising that our DFT efforts to find the
Although the computational results agree with the
experimental results that a mixture of 3-XN and 4-XN
should be obtained, the above analysis suggests that the
actual photochemistry may be more complicated than
what Figure 3 implies.26 According to Figure 3, if there
is no equilibration between E- and Z-isomers, then the
former should give exclusively 3-XN and the latter 4-XN,
making the [3-XN]/[4-XN] ratio equal to the ratio [E-2-
XN]/[Z-2-XN]. One possible explanation for the observed
results is that, in the case of X ) F, formation of the
E-isomer is for some reason relatively preferred, even
though it is energetically less stable than its Z counter-
part. While this is attractive, it shifts the problem to
explaining why the nature of the halogen affects the E/Z
ratio of 2-XN. Another possibility is that E- and Z-
isomers are in a photoequilibrium and that in the case
of X ) F the Z to E isomerization is relatively more
efficient than the E to Z one, compared to the cases of X
) Cl and Br. Finally, it is of interest to note that 3-FN
is 28 kcal/mol more stable than 4-FN, while in the cases
of X ) Cl and Br 3-XN is only 15-16 kcal/mol more stable
than 4-XN. Thus, phenomenologically the higher [3-XN]/
[4-XN] ratio in the case of X ) F correlates with a
relatively higher stability of 3-FN compared to 4-FN.
Whatever the case may be, the reason for the dependence
of the [3-XN]/[4-XN] ratio on the nature of the halogen
remains currently unresolved.
(d) Other Intermediates. As mentioned above, the
formation of Z-6-BrN during the photolysis of 1-BrN,
although not definitive, seems possible. Careful analysis
of the IR spectrum of the photoproduct from 1-ClN
(Figure 3a) failed to reveal any detectable amounts of Z-6-
ClN. This is compatible with the finding that Z-6-BrN
lies 2.7 kcal/mol lower in energy than the Z-isomer of
1A′-2-BrN, but Z-6-ClN lies 4.7 kcal/mol higher in energy
than its isomer Z-1A′-2-ClN.
Species Z-6-XN, X ) Cl, Br, are closed shell, unlike
2-XN, X ) Cl, Br. Their geometric isomers, E-6-XN, may
be thought of as quinoidal imine vinylidene compounds.
However, these are significantly less stable than their Z
counterparts by 19.7 and 23.0 kcal/mol for X ) Cl, Br,
respectively. Comparison of the geometrical parameters
between E- and Z-isomers of 6-XN, X ) Cl, Br (Figure
4)- and of the computed charges of their halogens27
suggests that the Z-6-XN, X ) Cl, Br, may be thought of
as cyclic internal ylides29 that result from the internal
trapping of the nitrenocarbene diradical. In contrast, Z-6-
ClC (vide infra) is better viewed as a vinylidene.
1
transition structure connecting A′-2-XN with 6-XN, X
) Cl, Br, have also not been successful.30
Biscarbene 2-ClC. (a) Computational Consider-
ations. The presence of two carbene subunits gives rise
to four different isomers: EE, EZ, ZE, and ZZ. Their
relative energies and computed geometries appear in
Tables S3 and S4. Much of the geometrical characteristics
and relative energetics discussed above for the carbeno-
nitrenes 2-XN are applicable in this case as well (Scheme
2, Z ) CH). The four different isomers of 2-ClC exhibit
quite similar geometries. Thus for a given state the bond
lengths differ in general by 0.01 Å or less among the four
isomers. An exception is the C5-C6 bond length of the
S1 state, which varies by 0.04 Å, with EE-isomer having
the shortest and ZZ-isomer the longest. B3LYP and
MCSCF predict similar geometries with the exception of
the S1 state of EE-2-ClC, which does not exist at the
MCSCF level, because it ring-closes “spontaneously” to
benzochlorocyclobutadiene (3-ClC).
The ground state (S1) is a singlet biradical, with the
Z,E conformer preferred by the three levels of theory. The
Q-S1 splitting is about 22-23 kcal/mol (i.e., around 5
kcal/mol lower than that of carbenonitrenes 2-XN). The
energy difference between singlet and triplet A′′ states
is ∼18 kcal/mol, similar to that between open-shell
singlet and triplet phenylcarbene (22 kcal/mol).31,32 The
halogen substituent stabilizes both A′′ states, resulting
in the T2 state being about 18 kcal/mol above the ground
(27) The computed charges for the halogens are relatively more
positive for the Z isomer than that for the E. The computed charges
for the Br in Z-1A′-2-BrN, E-6-BrN, Z-6-BrN based on the atomic polar
tensor [28] (Mulliken population analysis) are: -0.19 (0.04), -0.25
(0.01), and -0.04 (0.30), respectively. For the Cl in Z-1A′-2-ClN, E-6-
ClN, Z-6-ClN these are: -0.26 (0.14), -0.32 (0.06), and 0.00 (0.43),
respectively.
(28) Cioslowski, J. J. Am. Chem. Soc. 1989, 111, 8336.
(29) The ylide of a very electrophilic carbene (tetrakis(trifluorom-
ethyl)-cyclopentadienylidene) with p-bromotoluene has been synthe-
sized and characterized by 19F NMR. Janulis, E. P., Jr.; Arduengo, A.
J., III. J. Am. Chem. Soc. 1983, 105, 3563.
(30) The biradical is described by a spin-broken symmetry wave
function while Z-6-XN is a closed-shell singlet. Attempts to locate the
transition state were unsuccessful, as the calculation seemed to wander
from the open-shell surface to the closed one. Apparently an MCSCF
treatment is required, and it is quite possible that a conical intersection
connects the two singlet surfaces. However, this also may not be
enough since the vibrational mode connecting the two isomers may
need to be taken into account at the same time. Whatever the case,
this is beyond the scope of the current work.
(26) The reported calculations refer to thermal reactions, which are
unlikely to be taking place under our experimental conditions. It is
more likely that the diradicals 2-XN are extremely photochemically
reactive and once formed are rapidly converted to the observed
products. Alternatively, excited states of the precursors 1-XN or
intermediates resulting from the loss of one nitrogen molecule (e.g.,
nitrenediazirines) could give rise to the observed products by simul-
taneous loss of nitrogen. While these more likely processes may give
product ratios that reflect the ground-state proclivities of the diradicals,
this can neither be proved nor disproved by the current data.
(31) Nicolaides, A. Mol. Phys. 2004, 103, 1047.
(32) The lowest singlet of phenylcarbene is of the σ2 (closed-shell)
type in contrast to phenylnitrene, which is of the σ1π1 type. Platz, M.
S. Acc. Chem. Res. 1995, 28, 487.
7750 J. Org. Chem., Vol. 70, No. 19, 2005