Photochemistry of crystalline chlorodiazirines: The influence of conformational disorder and intermolecular Cl...N=N interactions on the solid-state reactivity of singlet chlorocarbenes

Article abstract of DOI:10.1021/jp026641t

A photochemical study was carried out with 3-substituted 3-chlorodiazirines with 4-biphenyl- (4a), (4-biphenyl)methyl- (4b), 2-(4-biphenyl)ethyl- (4c), and 1,1-dimethyl-1-(4-biphenyl)methyl (4d) substitueras. The chlorodiazirines were prepared from the co

Full text of DOI:10.1021/jp026641t

J. Phys. Chem. A 2003, 107, 3287-3294
3287
Photochemistry of Crystalline Chlorodiazirines: The Influence of Conformational Disorder
and Intermolecular Cl‚‚‚NdN Interactions on the Solid-State Reactivity of Singlet
Chlorocarbenes†
Carlos N. Sanrame, Christopher P. Suhrada, Hung Dang, and Miguel A. Garcia-Garibay*
Department of Chemistry and Biochemistry, UniVersity of California, Los Angeles, California 90095-1569
ReceiVed: July 30, 2002; In Final Form: January 17, 2003
A photochemical study was carried out with 3-substituted 3-chlorodiazirines with 4-biphenyl- (4a), (4-biphenyl)-
methyl- (4b), 2-(4-biphenyl)ethyl- (4c), and 1,1-dimethyl-1-(4-biphenyl)methyl (4d) substituents. The
chlorodiazirines were prepared from the corresponding amidines (7) by a procedure involving oxidation with
tert-butyl hypochlorite under phase-transfer catalysis. The crystalline nature of 4a-d was established by
differential scanning calorimetric analysis, which revealed melting endotherms prior to thermal decomposition.
Photochemical results in crystalline solids were analogous to those observed in solution, and the products
were analyzed in terms of the corresponding singlet-state chlorocarbene intermediates (5a-d). Irradiation of
4a in solution and in crystals resulted in formation of azine 9a by reaction of carbene 5a with its precursor.
Equally selective, diazirine 4d gave alkene 6d as the only product by a 1,2-Ph migration from carbene 5d.
In contrast, irradiation of compounds 4b and 4c resulted in formation of two alkenes by 1,2-H shifts and
formation of azines by reactions of the carbenes with their precursors. The low selectivity of 4b was rationalized
in terms of structural data from single-crystal X-ray diffraction analysis, which revealed two disordered diazirine
conformers and close Cl‚‚‚N contacts between adjacent molecules. Rapid conformational equilibration in the
solid state was also suggested by solid-state 13C CPMAS NMR. Similar structural effects are also postulated
to account for the solid-state reactivity of 4c.
1. Introduction
SCHEME 1
We have shown that triplet arylalkyl carbenes 2T generated
in crystals of their arylalkyldiazo precursors 1 undergo 1,2-H
shifts from their thermally populated singlet states 2S with
remarkably high yields and stereoselectivities (Scheme 1a).1
Although nanosecond lifetimes have been measured in solution
for several 1,2-diphenylethylidenes (2),1d kinetic estimates in
crystals reveal time constants that range from hours at 77 K to
several seconds at 273 K. Since the photochemical formation
of the carbene 2 in the solid state should be very fast, slow
reaction rates may be due to crystal effects on the rates of
intersystem crossing or on the rate of the 1,2-H shift. To discern
which of these alternatives, if any, may be correct, we decided
to explore the solid-state reactivity of similar systems with
carbenes that have a singlet ground state (Scheme 1b).
Given the large amount of mechanistic information available
from solution and matrix isolation studies on singlet-state
halocarbenes,2,3 we chose to study the reactivity of chlorocar-
benes 5S in crystals of 3-chlorodiazirines 4. On the basis of
our previous experience with crystalline derivatives of related
compounds,1 we set out to prepare a series of chlorodiazirines
with a 4-biphenyl moiety (4a-d, Chart 1)4 and to investigate
their photochemistry in solution and in the solid state. When
very low yields of 4a-d were obtained using the standard
Graham oxidation procedure,5 we developed a simple modifica-
tion based on the use of a phase-transfer catalyst (PTC) and
tert-butyl hypochlorite. Pure solid samples were characterized
by differential scanning calorimetry and, in the case of 4b, by
X-ray difraction and solid-state NMR. Reactions in the solid
state are consistent with the formation of singlet carbenes that
react under the constraints imposed by the crystal lattice of the
chlorodiazirine precursor. However, partial product formation
from the excited-state chlorodiazirines2f,34 cannot be ruled out
from the results of this study.
2. Results
2.1. Synthesis of Chlorodiazirines. Our initial attempts
toward the synthesis of 4b were based on the hypochlorite
oxidation of amidines reported by Graham,5 as shown by the
sequence of reactions in Scheme 2. However, while amidine
hydrochloride 7b (R ) PhPhCH2) was obtained in good yields
* To whom correspondence should be addressed. E-mail: mgg@
chem.ucla.edu.
† Part of the special issue “George S. Hammond & Michael Kasha
Festschrift”.
10.1021/jp026641t CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/27/2003

3288 J. Phys. Chem. A, Vol. 107, No. 18, 2003
Sanrame et al.
TABLE 1: Yields of 3-Substituted 3-Chlorodiazirine in
CHART 1
Side-by-Side Reactions by Use of a Phase-Transfer Catalyst
(PTC) and by Standard Graham Oxidationa,b
3-substituent
PTC (%)
Graham (%)
1-biphenylmethyl (4b)
1-naphthylmethyl (4e)
benzyl (4f)
35
14
47
∼1
∼1c
11
a Reactions were carried out to completion. b Yields determined by
1H NMR using an internal standard. c We were unable to reproduce a
yield of 15% reported in ref 15.
when most of the starting material had reacted. Analysis by 1H
NMR showed that diazirine 4b was a minor constituent of the
reaction mixture. Hoping to increase the amount of oxidant
present in the organic phase, we replaced NaOCl with tert-butyl
hypochlorite (t-BuOCl). Under this variation, 8b was dissolved
in a biphasic system consisting of CH2Cl2 and 1.0 M NaOH in
brine containing 20 mol % TBAHS. To this mixture was slowly
added a solution of t-BuOCl in CH2Cl2. After only 1 h, 8b
disappeared, and 1H NMR analysis of the crude product mixture
showed a significant proportion of diazirine 4b. Control
experiments demonstrated that the presence of TBAHS, base,
and chloride ion in the aqueous phase are all necessary for
optimum yields.
SCHEME 2
Optimal reaction conditions determined with 7b11 using the
PTC were subsequently applied to the synthesis of chlorodiaz-
irines 4a-d (Scheme 2). Although product yields improved
substantially, those of compounds 4b and 4d remained relatively
modest (25 and 13%, respectively). Knowing that low yields
(5-22%) have been reported for less hydrophobic benzylic
diazirines using the Graham procedure,12-14 it was of interest
to carry out a side-by-side comparison of the two methods.
Along with the synthesis of 4b, we decided to test the formation
of 3-(1-naphthylmethyl)-3-chlorodiazirine (4e) and 3-benzyl-
3-chlorodiazirine (4f), both of which have been previously
reported (Table 1). Since there are slight differences in the
Graham oxidation reported for those compounds, we followed
precisely the procedure reported for the synthesis of 3-(1-
naphthylmethyl)-3-chlorodiazirine.15 Reaction yields in Table
1, measured by 1H NMR using an internal standard, show that
experimental conditions using the PTC method result in
significantly higher yields than those with the traditional method.
Yield improvement from ca. 1% using the Graham method to
as much as 35% by the PTC method in the case of 4b suggest
that the modified procedure should be broadly applicable to
compounds that are highly hydrophobic and with large molec-
ular weights such as those required for our studies in nonpolar
molecular crystals.
2.2. Photochemistry in Solution and in the Solid State.
Compounds 4a-d were purified by column chromatography
and isolated as white solids with microcrystalline appearance.
Despite the explosive character associated with chlorodiazirines,
small quantities (20-100 mg) were easy to handle and stable
for months when protected from light at -10 °C.16 Dilute
benzene solutions and polycrystalline samples of 4a-d were
irradiated with a 400 W medium pressure with a Hanovia lamp
using a λ > 360 nm glass filter. The extent of conversion and
the identity of the photoproducts were determined by 1H NMR
(Table 2). Solution photolyses carried out in argon-purged
benzene at ambient temperature for 2 h gave high conversion
values (88-99%). Control experiments in solution showed that
photoisomerization of the isomeric alkenes does not occur under
the reaction conditions. Photolyses in the solid state for 30 min
to 6 h gave conversions between 26 and 99%, depending on
the sample. Finely powdered samples were placed between
from the corresponding nitrile,6 chlorodiazirine 4b was obtained
in 0-5% yields from 7b upon oxidation with sodium hypochlo-
rite in dimethyl sulfoxide (DMSO) and pentane. The major
product of this reaction was chloroamidine 8b, which precipi-
tated out of the reaction mixture in 60% yield along with several
other unidentified components. Given that chloroamidines are
intermediates in the oxidation mechanism (Scheme 2),5,7 we
reasoned that low yields in the case of 4b may reflect the low
solubility of 8b. At the same time, the oxidation efficiency may
be diminished by rapid reaction of hypochlorite with DMSO,8
which was easily established by iodometric titration.
To increase the yield of the desired chlorodiazirine, we
explored experimental factors expected to increase the solution
concentrations of 8b, the oxidant, and chloride ion. We showed
that intermediate 8b is very soluble in pyridine, dimethylfor-
mamide (DMF), and DMSO and somewhat less soluble in
dioxane, MeCN, and MeOH. In all cases, the solubility
diminished when the aqueous oxidant and chloride ion were
added. We also established that MeOH, DMF, MeCN, and
pyridine react with hypochlorite more slowly than DMSO. With
these facts in mind, we tested the reaction using each of those
solvents with the ratio of solvent to water adjusted to obtain
the highest concentration of 8b. Additionally, due to their greater
solubility, LiOCl and LiCl were used in place of the corre-
sponding sodium salts. Unfortunately, these changes did not
increase the yield of 4b.9
To facilitate the co-dissolution of the hydrophobic substrate
and the ionic reagents, we decided to explore a microhetero-
geneous system consisting of an aqueous solution, a water-
immiscible organic solvent, and a PTC. In our first trial, we
used conditions similar to those employed previously for the
oxidation of alcohols and amines by NaOCl in a biphasic system
using tetrabutylammonium hydrogen sulfate (TBAHS) as a
PTC.10 Initially, 8b was placed in a mixture of CH2Cl2 and
aqueous NaOCl (1.6 M) saturated with NaCl and containing
20 mol % TBAHS. The reaction was worked up after 12 h,

Photochemistry of Crystalline Chlorodiazirines
J. Phys. Chem. A, Vol. 107, No. 18, 2003 3289
TABLE 2: Product Distribution Obtained by Photolysis of
TABLE 3: Product Distribution from Photolysis of
Chlorodiazirine 4b in the Solid State between 298 and 66 K
Chlorodiazirines 4a-d at 298 Ka
normalized product distribution (%)
product distributiona (%)
conditions conv (%)
(Z)-6
(E)-6
9
temp (K)
conv (%)
Z-alkene
E-alkene
azine
4a
4b
C6D6
solid
C6D6
solid
solid
C6D6
solid
solid
C6D6
solid
>95
>95
88
26
65
97
32
90
96
100
100
0
14
22
0
298
298
160
135
115
100
66
95
77
63
72
70
73
44
38
35
40
43
51
55
59
44
46
39
47
44
41
39
18
19
21
10
6
4
2
21 (15)b
48
79 (85)b
38
41
37
4c
74 (68)b
78
26 (32)b
12
10
37
52
100
100
11
a Product distribution was determined by H NMR. The estimated
integration error is (5%.
1
4d
99
a Normalized product yields determined by H NMR. b Results in
cyclohexane as reported in ref 19a.
1
complex than in solution and changed slightly as a function of
reaction progress. In the case of 4b, the ratio of (Z)-6b to (E)-
6b reversed from 21:79 in solution to 48:38 in the solid state.
The ratio of (Z)-6c to (E)-6c increased slightly from 74:26 in
solution to 78:12 in the solid state. Reactions carried out to
higher conversion values in crystals showed small changes in
the (Z):(E) ratios with an increase in the yield of azine.
SCHEME 3
2.3. Variable Temperature Photolysis in Crystals of 4b.
It is possible that formation of azines and two 4-phenyl-â-
chlorostyrene isomers will depress the melting points of 4b,c
so that reaction at ambient temperature may occur in liquid
phases. To document this as the possible origin of the low
selectivity observed, we investigated crystals of chlorodiazirine
4b in a series of photochemical experiments carried out between
298 and 66 K. The reactions reported in Table 3 were carried
out with films prepared with 3 mg of microcrystals spread
between two quartz plates, placed in a cryostat cooled with N2
gas, and irradiated with the filtered output (λ > 360 nm) of a
1000 W Hg-Xe lamp. Photolyses carried out with this setup
for 60 and 30 min at 298 K gave 95 and 77% conversion,
respectively. Reactions carried out for 1 h at 160 K and below
gave only small variations on the extent conversion, which was
close to 70% in most cases. The relatively small changes in
reaction selectivity as a function of temperature between 66 and
130 K refute the possibility of melting as an explanation for
the low selectivity in crystals of 4b. The most significant change
as a function of temperature involves the relative yields of the
cis-1,2-H product (Z)-6b and azine 9b. It may be noted that
small changes in reaction selectivity and product yields over a
very large temperature range are a clear indication of processes
that occur with very small differences in activation energies
and low absolute activation energy values.
2.4. X-ray Analysis of Diazirine 4b. Single crystals of
3-chlorodiazirines 4a,c,d have been difficult to obtain.15,21
However, thin plates of 4b with reasonable diffraction quality
were obtained by slow solvent evaporation from pentane
solutions kept at -10 °C. Diffraction data were collected at
100 K, and the structure of 4b was solved in the monoclinic
space group P21/c with only one independent molecule per
asymmetric unit.22 The molecular structure of 4b is characterized
by a planar biphenyl group with the chlorodiazirine being
orthogonal to the aromatic plane, as described by a dihedral
angle C(1)-C(2)-C(3)-C(4) of 93° (Figure 2a,b). The packing
structure of 4b has biphenyl and diazirine groups segregated in
layers that take advantage of aromatic face-to-edge interactions
and Cl‚‚‚N interactions23 (Figure 2, bottom). However, as
illustrated in Figure 2, the three-membered ring of chlorodiaz-
irine 4b crystallizes conformationally disordered with two
rotamers having 75:25 occupancies. The best refinement model
gave R1 ) 0.054 for reflections with I > 2σ(I) and wR2 )
0.1588 for all reflection data. The molecular structures were
microscope slides, exposed to filtered UV light, and dissolved
in CDCl3 for analysis. Signals corresponding to the biphenyl
alkenes (Z)-6b and (E)-6b were very similar to those previously
reported in the literature for the phenyl analogues and were
confirmed by comparison to those of authentic samples prepared
independently. Signals corresponding to (Z)-6c and (E)-6c were
assigned by analogy to those of (Z)-6b and (E)-6b. Small
amounts of azines 9a-c and alkene 6d were isolated from small
scale photolysis of the corresponding diazirines and were
characterized by conventional methods.
The results of solution photolyses in Table 1 were consistent
with literature reports for analogous compounds with a benzyl
rather than 4-phenylbenzyl substituents and can be understood
in terms of the expected carbene intermediates (Scheme
3).2,12-14,17-19 In inert solvents and in the absence of trapping
reagents, biphenylchlorocarbene 5a reacts with its own precursor
to form azine 9a as the only observable product (Scheme 3, eq
1).17,20 In contrast, halocarbenes 5b-d have migrating R-groups
that undergo fast intramolecular rearrangements. Chlorocarbenes
5b,c give rise to alkenes (Z)-6 and (E)-6 by facile 1,2-H shifts
(Scheme 3, eqs 2 and 3),12,14,18-20 and carbene 5d gives rise to
6d by an efficient 1,2-Ph migration (eq 4).13
Reactions in the solid state at 298 K differed from those in
solution only in the cases of 4b,c. The reactions of 4a,d were
as clean in crystals as in solution, yielding azine 9a and alkene
6d, respectively. Product formation in crystals of 4b,c was more

3290 J. Phys. Chem. A, Vol. 107, No. 18, 2003
Sanrame et al.
C(1)-N(2′) ) 1.556 for the minor one. While normal NdN
bonds range from 1.228 to 1.293 Å, those of the major and
minor occupancies of 4b are NdN ) 1.404 and N′dN′ ) 1.237
Å, respectively.
Trends in bond distances suggest a dispersion of electronic
density between sites corresponding to the long-bonded chlorine
atom of one conformer and the two short-bonded nitrogen atoms
from the other. However, additional sources of disorder,
including dynamic rotation or libration of the diazirine group,
cannot be ruled out. The general features of the molecular
structure of 4b, including a flat conformational surface that may
facilitate rotational disorder, are qualitatively corroborated by
semiempirical quantum mechanical calculations with the AM1
method (C-Cl ) 1.745 Å, NdN ) 1.228 Å, C-N ) 1.473
Å).26 The structures of the two rotamers in the crystal are
described by dihedral angles Cl-C(1)-C(2)-C(3) of +82.9
and -68.4°, respectively, which correspond to local shallow
minima located between at ca. +75 and -75° in the calculated
structure. Rotation about the C(1)-C(2) bond passes through
the global minimum with Cl-C(1)-C(2)-C(3) of 180° and
over a barrier of <1.0 kcal/mol, suggesting libration as a
possible dynamic mechanism. Rotational disorder in the crystal
should be facilitated also by the small size and nearly globular
shape of the 3-chlorodiazirine group, which makes it insensitive
to steric interactions along different orientations within the
lattice,27 and by a network of nonbonded Cl‚‚‚N interactions23
that can be satisfied with close neighbors on either direction of
the rotational disorder (Figure 3).
Figure 1. Changes in selectivity as a function of temperature upon
irradiation of chlorodiazirine 4b with λ > 350 nm.
The presence of two reactant conformers and close diazirine-
to-diazirine contacts help account for the results of the solid-
state reaction. The two conformers orient the C-Cl bonds from
one molecule toward the diazirine nitrogens of their near
neighbors so that they can experience close Cl‚‚‚N interactions
(Figure 3). The halogensnitrogen supramolecular bonds are
thought to reflect weak charge-transfer interactions and have
been well-documented in various crystal structures23 and in
solution.28 Although there is significant scatter in the X-ray
literature data, the Cl‚‚‚N bond is characterized by distances of
3.1-3.5 Å and C-Cl‚‚‚N angles near 180°. Given that both
atoms are present in the chlorodiazirine group, the possibility
of self-complementing interactions is conceivable. However, an
ab initio HF 3-21G* geometry optimization of the parent
3-chlorodiazirine dimer reveals a structure with a single interac-
tion consisting of a linear C-Cl‚‚‚N angle of 178.5° and a
Cl‚‚‚N distance of 3.15 Å (Figure 3, left) which is qualitatively
similar to the C-Cl‚‚‚N contacts that are possible in the crystal
structure of 4b (Figure 3, right).
2.5. Solid-State 13C CPMAS NMR.29,30 Additional evidence
for the two diazirine conformers and their possible equilibration
in the solid state may be available from NMR chemical shifts,
signal coalescence as a function of temperature, or from
relaxation properties. The solid-state 13C CPMAS NMR spec-
trum of 4b obtained at ambient temperature is consistent with
the high-resolution spectrum obtained in solution (Figure 4, top).
To help confirm assignments in the solid state, we used an
interrupted decoupling sequence to remove the signals of the
protonated carbons.31 Signals at 50, 132, and 137 ppm were
not affected and could be assigned to the nonprotonated diazirine
and the three aromatic ipso carbons. The signal at 43 ppm could
be assigned unambiguously to the benzylic carbon.
Figure 2. (a) ORTEP diagram of a side view of chlorodiazirine 4b
with thermal ellipsoids drawn at 50% probability suggesting libration
of the phenyl group about the C(9)-C(12) axis. (b) ORTEP diagram
of a top view of 4b showing a highly disordered chlorodiazirine group.
(c) Packing diagram (bottom) showing close contact between neighbor-
ing diazirine groups, which explains the formation of the azine in the
crystalline solid state.
reasonable in every respect, except for the chlorodiazirine group
which showed systematic deviations from the expected geom-
etries.15,21,24,25 Strong overlap between the chlorine atom of one
conformer and the diazirine nitrogens of the other near the same
crystallographic site severely diffuses the electron density and
causes systematic deviations in the C-Cl, C-N, and NdN
distances. While the C-Cl bond in the conformer with the
largest occupancy was close to the normal values of 1.716-
1.778 Å15,21,24,25 [C(1)-Cl(1) ) 1.686 Å], that of the minor
occupancy was shorter by 0.16 Å [C(1)-Cl(1′) ) 1.556 Å].
The C-N and NdN bonds were longer than normal for both
occupancies. Literature C-N distances fall within 1.462-1.490
Å, and those obtained for 4b were C(1)-N(1) ) 1.503, C(1)-
N(2) ) 1.502 for the major conformer and C(1)-N(1′) ) 1.758,
Given the X-ray evidence for the two conformers in the
crystal, unique signals for benzylic and diazirine carbons in the
13C NMR spectrum at 298 K imply that there is not enough
resolution between the two conformers or that isomerization

Photochemistry of Crystalline Chlorodiazirines
J. Phys. Chem. A, Vol. 107, No. 18, 2003 3291
Figure 3. (Left) Structure of the HF 3-21G* optimized C-Cl‚‚‚N interaction in 3-chlorodiazirine. (Right) Schematic representation from the
crystal structure of chlorodiazirine 4b, illustrating close C-Cl‚‚‚N interactions with dClsN < 3.5 Å for a reference molecule (boldface) shown in
two possible orientations (I and II). In arrangement I, all the C-Cl bonds point down and only two short C-Cl‚‚‚N contacts are possible. In
arrangement II, the C-Cl bonds point up and four C-Cl‚‚‚N close contacts are possible.
Figure 5. DSC curves for 4a-d illustrating melting followed by
Figure 4. Solid-state CPMAS 13C NMR spectra of 4b obtained at 298
exothermic denitrogenation (scanning rate 5 °C/min).
(top) and at 133 K (middle). The spectrum at the bottom was obtained
at 133 K by direct polarization (Bloch decay).
a mechanism based on relaxation processes within the spin-
locking field of the cross-polarization experiment. It is known
occurs in the fast exchange limit to give a single average peak.
that molecular motions in the 103-104 s-1 dynamic range can
To explore the latter, we measured a series of 13C NMR spectra
interfere with cross-polarization by diminishing the magnitude
as a function of temperature between 298 and 133 K. Illustrated
in Figure 4 are the spectra measured at 298 (top) and at 133 K
(middle) using the CPMAS sequence. The bottom spectrum was
of the 1H-13C dipolar coupling and by facilitating the decay of
the spin-locked magnetization via a spin-lattice relaxation (T1F)
in the rotating frame.30b This interpretation is consistent with
measured at 133 K without cross-polarization but with high-
the dynamic behavior of symmetric groups such as methyl-,
power decoupling and magic angle spinning.28 Spectra collected
tert-butyl-, adamantyl-, etc., which can undergo rapid molecular
down to 173 K (not shown) resemble very closely the spectrum
at 298 K, showing no changes over a 125 K range. However,
as illustrated by the middle spectrum in Figure 4, there was a
clear change in the CPMAS spectrum at 133 K, which is near
the experimental limit of our temperature control. The signal
assigned to the chlorodiazirine carbon at 50 ppm almost
disappears at the baseline. One possible explanation would be
that 133 K may be near the coalescence temperature so that
reorientations in the solid state.27
2.6. Differential Scanning Calorimetric Analysis. Although
we were unable to grow X-ray quality crystals of compounds
4a,c,d, we were able to establish their crystallinity by differential
scanning calorimetry (DSC).33 As shown by the thermograms
of 4a-d in Figure 5, an endothermic peak was assigned to
melting (m) and a broad exothermic transition to thermal
denitrogenation and reaction (d). In the case of 4a, a broad
decomposition peak (d) centered at ca. 90 °C overlaps with a
conformational equilibration has slowed toward the slow-
exchange regime of conformers with distinguishable chemical
weak melting peak at 77 °C (m). The thermograms of
shifts.32 Although this can only be confirmed with measurements
compounds 4b-d revealed melting endotherms between 58 and
at lower temperatures, alternative interpretations based on the
effects of conformational motions on the cross-polarization
75 °C and the onset of decomposition starting at 90-100 °C.
A sharper transition for the melting of 4b is consistent with its
sequence can be easily tested without changing the temperature.
higher crystallinity.
As shown on the bottom spectrum in Figure 4, a 13C NMR
spectrum acquired by direct excitation and high-power decoup-
3. Discussion
ling (Bloch decay) restores the intensity of the diazirine carbon
relative to that of the benzylic carbon. This result discounts an
explanation based on signal coalescence (splitting) and suggests
The photochemical results with biphenyl-substituted chlo-
rodiazirines 4a-d in dilute benzene solutions are very similar

3292 J. Phys. Chem. A, Vol. 107, No. 18, 2003
Sanrame et al.
to those reported in the literature for the phenyl analogues, where
singlet chlorocarbenes have been detected by laser flash
photolysis with the pyridine ylide method.2e,34 The analogous
results observed in crystals suggest the formation of the same
reactive intermediates.
Photoreactions in diazirine crystals proceed smoothly and are
relatively clean. Aryldiazirine 4a gives only azine 9a by reaction
of carbene 5a with a neighboring diazirine molecule4,17 and
(dimethylbenzyl)chlorodiazirine 4d gives the R-chlorostyrene
6d by a selective 1,2-Ph migration in the singlet state of 5d.13
The lack of azine formation in crystals of 4d implies a very
efficient solid-state 1,2-Ph shift, a disfavored intermolecular
reaction, or a combination of the two. Unfortunately, we were
not able to grow single crystals for X-ray analysis to determine
whether the formation of azines is structurally feasible.
Figure 6. (Top) Neighboring molecules of diazirine 4b shown in one
of the two disordered conformers present in the crystal lattice. (Bottom)
Hypothetical structure of azine 9b derived from the structure of the
dimer shown on top by assuming a least motion process. The top
structures were taken from the X-ray coordinates, and the bottom one
was derived directly from the one above followed by minimization
with the AM1 method (constraining only the planarity of the biphenyl
group).
In contrast to 4d, reaction selectivities in crystals of benzylic
diazirines 4b,c were significantly lower with concurrent 1,2-H
shifts and bimolecular azine formation. While reactions in
crystals tend to be significantly more selective than analogous
reactions in solution, exceptions occur for reactions that occur
at defect sites, when product accumulation causes a deterioration
of the crystal lattice, when microscopic liquid phases occur by
depression of the melting point or in crystals that are disor-
dered.35 Low selectivities at low conversion values and at very
low temperatures in the case of 4b are in agreement with the
disordered and packing arrangement shown by X-ray analysis,
and a similar explanation is likely in the case of diazirine 4c.
to be at a distance of 1.3 Å in the methylene-diazirine ylide.20b
Although this is much shorter than the 3.20 Å between the
corresponding atoms in the diazirine crystal, small molecular
displacements are known to occur in many reactions, making
the future detection and analysis of an ylide plausible.
3.2. Singlet-State 1,2-H Shift. In addition to 1,2-H shifts
from singlet carbenes, it has been suggested that formation of
chloroalkenes may also occur by a reaction involving concerted
N2 elimination and 1,2-H migration from the excited-state
chlorodiazirine precursors.2f,34 Although this pathway was
proposed to account for ca. 26% of the chlorostyrene formed
in the case of benzylchlorodiazirine in solution,18d product
analysis offers no information which may reveal whether a
similar pathway may contribute in the solid-state reactions of
compounds 3b,c. With regard to the carbene rearrangement, it
is known that 1,2-H shifts occur from conformations where the
σ bond of the migrating hydrogen is parallel to the empty
p-orbital in sp2-hybridized singlet carbene (Scheme 4). The
transition state is characterized by displacement of the hydrogen
across the forming double bond and by planarization of the
R-carbon.19b,40,41 As shown in Scheme 4, two alkene geometric
isomers arise from syn- and anti-conformations of the carbene
intermediate.18,39,40
The stereoselectivity of the rearrangement is influenced by
the population of the carbene conformers, by whether they can
equilibrate and by their individual rate constants.20,36,40,42 A flat
conformational profile calculated for rotation about the benzylic-
diazirine carbon-carbon bond in 4b suggests that there should
be very small, if any, conformational bias in the precursor (Eanti
≈ Esyn). Similarly, no conformational preference is expected in
the carbene.26 Recent computational studies with several chloro-
carbenes by Keating et al. using B3LYP/6-31G* structural
optimizations with zero point energies revealed very close syn-
and anti-carbene conformers.40,43 Differences in conformational
and transition-state energies for benzyl chlorocarbene, an
excellent model for the biphenyl analogue 5b, are illustrated in
Figure 7. The difference calculated between the lowest energy
anti-conformer and a near-lying syn-structure in BnCCl was only
0.7 kcal/mol. Cis- and trans-transition states (cis- and trans-
TS) for 1,2-H shifts in benzyl chlorocarbene obtained by the
same method were 8.8 and 6.8 kcal/mol above the syn- and
anti-conformers, respectively, in agreement with reaction ster-
eoselectivities in solution where a small preference for the (E)-
alkene (trans-TS) in BnCCl is observed.18
3.1. Azine Formation. Azines form by reaction of singlet
carbenes with their nitrogenated precursors in a near-diffusion-
controlled process.20,36 Azine formation in solution becomes
important in the absence of alternative reactions, when very high
precursor concentrations are used or when thermally activated
rearrangements are slowed at low temperatures.17,18a,20 The
formation of azines 9b,c in crystals of 4b,c was not anticipated.
Since changes in size and volume upon 1,2-H shifts are
relatively small, we expected solid-state rearrangements at
ambient temperature to be nearly as fast as in solution. The
rate constants for 1,2-H shifts of benzylic37 and secondary
hydrogens38 are (6.7-5.9) × 107 s-1 for analogous carbenes,
suggesting that molecular displacement required for bimolecular
azine formation should be unable to compete with the rigid
environment of crystalline solids. Thus, barring unexpectedly
slow 1,2-H shifts in crystals, the concurrent formation of azines
and alkenes in crystals of 4b,c implies a very favorable
predisposition for the bimolecular reaction.39
The formation of azine 9b in crystals of diazirine 4b can be
easily rationalized upon inspection of the crystal structure.
Although the crystallographic disorder in 4b precludes a rigorous
geometric analysis, the crystal structure reveals diazirine nearest
neighbors within distances and orientations that favor the
reaction (Figures 3 and 6). It is expected that formation of azine
9b will proceed by denitrogenation of a diazirine molecule
followed by reaction between the resulting singlet carbene and
a close neighbor in the crystal. The distance between the
diazirine carbon of one molecule and a diazirine nitrogen of its
closest neighbor is only 3.20 Å (Figure 6). A computational
transformation of nearby diazirine neighbors into the minimized
structure of the azine product can be carried out with no
significant displacement of either biphenyl group (Figure 6,
bottom). The distance between the diazirine carbons changes
from 3.88 Å in the putative reactants to 3.56 Å between the
corresponding carbon atoms in the product. It is interesting to
note that formation of an intermediate ylide20 may be possible.
Calculations suggest the carbene-carbon and diazirine-nitrogen
It seems clear that the lack of solid-state selectivity observed
in the 1,2-H shift of 4b, and very likely 4c, is controlled by the

Photochemistry of Crystalline Chlorodiazirines
J. Phys. Chem. A, Vol. 107, No. 18, 2003 3293
SCHEME 4
4. Conclusions
This work aims at understanding the factors that affect the
rate of the 1,2-H shift of singlet carbenes in crystalline solids.
As a first step, we have investigated the suitability of several
biphenyl-substituted chlorodiazirines as crystalline precursors,
including their preparation, crystallization, and reaction control.
Having first identified serious synthetic problems associated with
the low solubility of nonpolar reactants and intermediates under
the conditions of the standard Graham procedure for the
synthesis of 3-chlorodiazirines, we implemented a simple
modification that proceeds in reasonable yields. We showed that
the photochemical reactivity of biphenyl chlorocarbenes 5a-d
in solution is similar to that of their phenyl analogues. Reactions
of biphenyl chlorocarbene 5a and R,R-dimethyl-(4-phenyl)-
benzyl chlorocarbene 5d in crystals of their diazirine precursors
proceeded in high yields to give azine 9a and alkene 6d,
respectively, as the only products. In contrast, low selectivities
were observed in reaction of carbenes 5b,c, which give rise to
azines 9b,c along with (Z)- and (E)-alkenes 6b,c from 1,2-H
shifts. Product formation in the case of 5b could be rationalized
from the single-crystal X-ray structure of diazirine 4b. Two
chemically relevant features of the crystal structure are the
rotational disorder of the chlorodiazirine group and the presence
of an extended 2D network of nonbonded ClsN interactions.
Preliminary evidence of the dynamic nature of the rotational
disorder was obtained by solid-state 13C NMR experiments,
suggesting the need of a dynamic mechanistic model, rather
than one derived from the static equilibrium structures available
from X-ray diffraction. It appears that facile conformational
equilibration of the chlorodiazirine, and perhaps also the carbene,
results in formation of both (Z)-6b and (E)-6b from competing
1,2-H shifts. Similarly, the apparent mobility of adjacent
diazirine groups leads to the facile formation of azine 9b at
ambient temperature and to its almost complete suppression
below ca. 100 K. Work in progress suggests that conformational
disorder and a network of nonbonded Cl-N interactions are
frequent motifs in crystalline 3-chlorodiazines.
Figure 7. Relative B3LYP/6-31G* energies (kcal/mol; zpe corrected)
for the syn- and anti-conformers of benzyl chlorocarbene, which can
be used as a model for compound 5b. Data taken from ref 40a.
structural disorder and conformational dynamics present in their
crystals. Photolysis of crystalline 4b at 298 K over a wide range
of conversion values and reactions carried out at very low
temperatures, where melting is unlikely, are consistent with
results determined by the structure of the crystal. While each
of the two diazirine conformers observed by X-ray diffraction
could lead stereospecifically to each alkene isomer, the equili-
bration of carbene conformers in the solid state cannot be ruled
out.
The effect of temperature on the solid-state reaction of 4b is
remarkably small. Matrix isolation studies by Wierlacher et al.
suggest that 1,2-H shifts proceed rapidly by a tunneling
mechanism even at 10-20 K.14 Assuming that formation of the
carbene determines the formation of products in the solid state,
the relatively small changes in chemical yields from 298 to 66
K suggest a nearly barrierless photodissociation of N2 from the
diazirine excited singlet state. A relatively small change in
reaction selectivity from 298 to 66 K is characterized by a small
increase in the yield of (Z)-6b and a small decrease in the yield
of azine 9b. These observations are consistent with modest
changes in the relative populations and rates of conformational
equilibration of diazirine and carbene conformers. Since azine
formation in solution actually increases at lower temperatures18a
(indicating a smaller barrier than those of the 1,2-H shifts), a
relatively slow decrease in the yield of azine in crystals is
consistent with a small structural barrier in the product-
determining step. A small increase in the yield of (Z)-6b may
also reflect a structural bias. We propose that a rigid, or slowly
equilibrating, conformation of the diazirine and carbene in the
crystal should favor the formation of the (Z)-product due to Cl-
C(1)-C(2)-C(3) dihedral angles that are less than 90°, when
values of 180° are required to favor the (E)-isomer.
Acknowledgment. This work was supported by NSF Grants
CHE-0073431, CHE9871332 (X-ray), and DMR9975975 (Solid-
State NMR). C.N.S. is a grateful recipient of a fellowship from
CONICET (Argentina). We thank Dr. Robert Taylor (UCLA)
for support with the solid-state NMR experiments.
Supporting Information Available: General experimental
procedures and spectral data of amidines 7b and 8b, diazirines

3294 J. Phys. Chem. A, Vol. 107, No. 18, 2003
Sanrame et al.
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T.; Izawa, Y. J. Chem. Soc., Chem. Commun. 1986, 1364. (b) Tomioka,
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(21) 3-[(p-Nitrophenoxy)methyl]-3-chlorodiazirine is reported to be a
very stable and highly crystalline solid. Its X-ray structure analysis, including
electron density deformations, have been analyzed by: (a) Cameron, T. S.;
Bakshi, P. K.; Borecka, B.; Liu, M. T. H. J. Am. Chem. Soc. 1992, 114,
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(22) Selected crystal data. For 4b: C14H11ClN2, MW ) 242.70,
monoclinic, space group P21/c, a ) 8.4274(12) Å, b ) 5.6755(8) Å, c )
25.158(4) Å, â ) 96.963(3)°, V ) 1194.4(3) Å3, Z ) 4, Fcalcd ) 1.350
Mg/m3, F(000) ) 504, λ ) 0.71073 Å, µ(Mo KR) ) 0.296 mm-1, T )
100(2) K, crystal size ) 0.4 × 0.15 × 0.1 mm3. Of the 6572 reflections
collected (1.63 e θ e 28.25°), 2588 [R(int) ) 0.0275] were independent
reflections; max/min residual electron density +397 and -383 e nm-3, R1
) 0.0543 (I > 2σ(I)) and wR2 ) 0.1588 (all data).
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Reddy, D. S.; Ovchinnikov, Y. E.; Shishkin, O. V.; Struchkov, Y. T.;
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(26) A portion of the energy surface of diazirine 4b consisting of rotation
about the Cl-C(1)-C(2)-C(3) and C(1)-C(2)-C(3)-C(4) dihedrals was
investigated with the AM1 method using the Hyperchem package (Hyper-
chem Inc. Gainsville, Fl). These calculations suggest a flat potential with
a lowest energy anti-conformer having Cl-C(1)-C(2)-C(3) ) 180° and
close-lying syn-conformers with Cl-C(1)-C(2)-C(3) ) (75° that are only
1 kcal/mol above. These results are consistent with calculations reported
in ref 14. Please see Supporting Information.
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4a-d, alkenes 6b-d, and azines 9a-c, X-ray acquisition and
refinement data for diazirine 4b, and contour diagram of the
AM1 conformational surface of 4b. This material is available
free of charge via the Internet at http://pubs.acs.org.
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