Control of carbene reactivity by crystals. A highly selective 1,2-H shift in the solid-to-solid reaction of 1-(4'-biphenylyl)-2-phenyldiazopropane to (Z)-1-(4'-biphenylyl)-2-phenylpropene

Article abstract of DOI:10.1021/ja9633225

Ruby red polycrystalline samples of 1-(4'-biphenylyl)-2-phenyldiazopropane (1a) efficiently transform into colorless (Z)-1-(4'-biphenylyl)-2-phenylpropene [(Z)-3a] in up to 96% yield in a highly selective photochemical solid-to-solid reaction. The solid state reaction was shown to proceed via a carbene intermediate, and it was formulated as a 1,2-H shift with unprecedented stereoselectivity. It was shown that irradiation of la in inert solvents gives products via 1,2-H shifts [(Z)-3a and (E)-3a] and 1,2-Ph migrations [(Z)-4a and (E)-4a] in yields that vary strongly with the polarity of solvent. Irradiation of 1a in ethanol gave similar products along with ethers 5a from insertion of the carbene into the EtO-H bond.

Full text of DOI:10.1021/ja9633225

J. Am. Chem. Soc. 1997, 119, 1859-1868
1859
Control of Carbene Reactivity by Crystals. A Highly Selective
1,2-H Shift in the Solid-to-Solid Reaction of
1-(4′-Biphenylyl)-2-phenyldiazopropane to
(Z)-1-(4′-Biphenylyl)-2-phenylpropene
Steve H. Shin, Deniz Cizmeciyan, Amy E. Keating, Saeed I. Khan, and
Miguel A. Garcia-Garibay*
Contribution from the Department of Chemistry and Biochemistry, UniVersity of California,
Los Angeles, California 90095
ReceiVed September 23, 1996X
Abstract: Ruby red polycrystalline samples of 1-(4′-biphenylyl)-2-phenyldiazopropane (1a) efficiently transform
into colorless (Z)-1-(4′-biphenylyl)-2-phenylpropene [(Z)-3a] in up to 96% yield in a highly selective photochemical
solid-to-solid reaction. The solid state reaction was shown to proceed via a carbene intermediate, and it was formulated
as a 1,2-H shift with unprecedented stereoselectivity. It was shown that irradiation of 1a in inert solvents gives
products via 1,2-H shifts [(Z)-3a and (E)-3a] and 1,2-Ph migrations [(Z)-4a and (E)-4a] in yields that vary strongly
with the polarity of solvent. Irradiation of 1a in ethanol gave similar products along with ethers 5a from insertion
of the carbene into the EtO-H bond.
I. Introduction
Scheme 1
In their pioneering work, Doetschman and Hutchison showed
that the crystalline solid state may exert a very high degree of
control over the chemical fate of diphenylcarbene.1 Using
crystalline 1,1-diphenylethylene as a host and a combination
of X-ray crystallographic analysis and EPR and ENDOR
spectroscopies, they were able to describe two competing
bimolecular reactions with a high degree of structural and kinetic
accuracy. Given the potential demonstrated by their early work,
it is surprising that no other reports have followed in what seems
to be an area of great potential for organic chemistry. A possible
deterrent for such studies comes from the uncertainty of an a
priori knowledge of suitable packing arrangements required for
bimolecular, carbene-carbene, carbene-olefin, or carbene-
precursor reactions that may or may not give rise to alkenes,
cyclopropanes and azines.2 Thus, in order to carry out a
systematic study of carbene reactivity in the crystalline solid
state with no need for specific packing arrangements, we have
begun an investigation of unimolecular carbene reactions. On
the basis of expectations of crystallinity, structural variability,
and experimental convenience, we chose as a starting point a
reaction model involving the intermediacy of 1,2-diarylalky-
lidenes (2). As other arylalkylcarbenes, these highly reactive
intermediates are capable of undergoing 1,2-H and 1,2-alkyl
shifts as well as 1,2-Ph migrations (Scheme 1). We envisioned
the formation of carbenes 2 in crystals of their readily available
1,2-diaryldiazoalkane precursors 1. With several examples, we
have found that the solid-state reactivity of 1,2-diaryldiazoal-
kanes is quite general. However, it is the purpose of this paper
to present a very detailed analysis of the solid-state reactivity
of 1-(4′-biphenylyl)-2-phenyl-diazopropane, 1a (Ar ) 4′-
biphenylyl, R ) Me, and Ar′ ) Ph), which represents in many
ways an ideal model system. Compound 1a gives high-melting
crystals with excellent diffraction properties, and a detailed
comparison of results obtained in solution and in the solid state
demonstrates an unprecedented degree of control by the
crystalline medium.6 We have found that the reaction may
proceed at ambient temperatures entirely in the solid phase with
a very high selectivity and up to 95% conversion.
II. Results and Discussion
The photochemistry of 1,2-diaryldiazoalkanes has been
studied in detail in fluid solutions, frozen solvents, and
amorphous glassy matrices by the groups of Pomerantz and
Whitherup3 and Platz and Tomioka4 and by ourselves.5 We
will describe here the first detailed analysis carried out in the
crystalline solid state.6 It has been shown that 1,2-diaryldiaz-
(3) Pomerantz, M.; Witherup, T. H. J. Am. Chem. Soc. 1973, 95, 5977-
5988.
(4) (a) Platz, M. S. In Kinetics and Spectroscopy of Carbenes and
Biradicals; Platz, M. S., Ed.; Plenum Press: New York, 1990; pp 143-
212. (b) Tomioka, H.; Hayashi, N.; Sugiura, T.; Izawa, Y. J. Chem. Soc.,
Chem. Commun. 1986, 1364-1366. (c) Tomioka, H.; Hayashi, N.; Izawa,
Y.; Senthilnathan, V. P.; Platz, M. S. J. Am. Chem. Soc. 1983, 105, 5053-
5057. (d) Tomioka, H.; Ueda, H.; Kondo, S.; Izawa, Y. J. Am. Chem. Soc.
1980, 102, 7817-7818.
(5) (a) Garcia-Garibay, M. A. J. Am. Chem. Soc. 1993, 115, 7011-7012.
(b) Garcia-Garibay, M. A.; Theroff, C.; Shin, S. H.; Jernelius, J. Tetrahedron
Lett. 1993, 52, 8415-8418.
(6) For a communication of this work see: Shin, H. S.; Keating, A. E.;
Garcia-Garibay, M. A. J. Am. Chem. Soc. 1996, 118, 7626-7627.
X Abstract published in AdVance ACS Abstracts, February 1, 1997.
(1) Doetschman, D. C.; Hutchison, C. A. J. Chem. Phys. 1972, 56, 3964-
3982.
(2) For some leading reviews on carbene chemistry see: (a) Brinker, U.
H. AdVances in Carbene Chemistry; JAI Press, Inc.: New York, 1994. (b)
Nickon, A. Acc. Chem. Res. 1993, 26, 84-89. (c) Platz, M. S. In Kinetics
and Spectroscopy of Carbenes and Biradicals; Platz, M. S., Ed.; Plenum
Press: New York, 1990; pp 143-212. (d) Schuster, G. B. AdV. Phys. Org.
Chem. 1986, 22, 311-362.
S0002-7863(96)03322-7 CCC: $14.00 © 1997 American Chemical Society

1860 J. Am. Chem. Soc., Vol. 119, No. 8, 1997
Shin et al.
Table 1. Literature Data for Some 1,2-Diaryldiazoalkanes and
Scheme 2
1,2-Diarylalkylidenes
compd
R1
R2
mp (°C)
Da (h/c)
Ea (h/c)
ref
1a
1b
1c
1d
Ph
H
H
Me
H
Me
Et
93
0.470
0.497
0.498
na
0.029
0.029
0.027
na
this work
oil
oilb
oil
17
4c
5
H
a Zero field splitting parameters for the corresponding carbene. b We
could obtain poor quality crystals that melt at 19 °C.
Scheme 3
oalkanes display a very rich solution photochemistry where
product ratios depend strongly on temperature, solvent polarity,
and physical state. Most experimental observations reported
in the literature regarding the chemistry of 1,2-diaryldiazoal-
kanes can be explained in terms of a reaction scheme originally
proposed by Tomioka (Scheme 2). The proposed mechanism
been proposed that 12 can undergo fast 1,2-H shifts (and slower
1,2-Ph migrations), while 32 can only undergo 1,2-Ph migrations
by way of a bridged spirocyclooctadienyl biradical (Scheme
2).4a,b Pronounced changes in the 3:4 ratio as a function of
temperature have been interpreted in terms of changes in the
equilibrium populations of the two spin states.4c Similarly,
changes in the 3:4 ratio as a function of solvent polarity have
been interpreted in terms of a solvent-induced stabilization of
the polar singlet carbene that affects the singlet-triplet energy
gap, ∆GST, as well as the spin-state equilibrium constant.5
This mechanistic interpretation requires that temperature and
solvent effects on the intrinsic reaction rates k1,2-H and k1,2-Ph
are smaller than on the equilibrium dynamics of the spin states
of the carbene, kST and kTS. While limited experimental
information suggests that these conditions may be possible, the
mechanism in Scheme 2 is only tentative and alternative
interpretations of experimental effects cannot be ruled out. This
brief analysis stresses the complexity of a system where at least
three precursors (11*, 2S, and 2T), two of which may equilibrate
(2S and 2T), may be involved in the formation of each of the
observed products.
(1) Synthesis and Characterization of 1-(4′-Biphenylyl)-
2-phenyldiazopropane, 1a. While studies of several 1,2-
diaryldiazoalkanes (e.g., 1b-1d) have been reported in the
literature (Table 1), these compounds have been described as
thermally unstable viscous oils.4a,c We found that a 4-phenyl
substituent on the 1,2-diaryldiazopropane chromophore results
in a high-melting and highly crystalline solid that can be stored
indefinitely in the dark at -20 °C without traces of chemical
decomposition.
1
assumes quantitative denitrogenation of 1* (Scheme 2, path
1) to produce a 1,2-diphenylalkylidene 2.4 Recent reports with
other arylalkyldiazoalkanes and diazirines suggest that formation
of products may also occur from the excited state of the
nitrogenated compound (path 2).7 Work in progress in our group
with similar compounds suggests that path 2 in 1,2-diarylalky-
lidenes contributes with values that range between 0 and 30%.8
In analogy with several thoroughly studied diarylcarbenes
(e.g., diphenylcarbene),9,10 it has been proposed that the singlet
excited state of 1 forms the singlet carbene 12, which can rapidly
3
equilibrate with the lower energy triplet 2 (Scheme 2). The
main products in inert solvents include stilbenes 3 and 1,1-
diarylethenes 4, the formation of which can be formulated in
terms of well-known carbenic 1,2-H shifts2b,10 and 1,2-Ph
migrations, respectively.11 Changes in the relative yields of 3
and 4 as a function of temperature and solvent polarity can be
consistently explained in terms of environmental perturbation
of a sensitive spin-state equilibrium.5 This model makes the
reasonable assumption that 1,2-diphenylalkylidenes have prop-
erties similar to those of well-known diaryl carbenes.9 Reactions
are assumed to occur from a spin state pre-equilibrated carbene
that displays spin-state specific reactivity as suggested by the
Skell-Woodworth12 rules and the Bethell13 mechanism. It has
(7) (a) Platz, M. S.; White, W. R.; Modarelli, D. A.; Celebi, S. Res.
Chem. Intermed. 1994, 20, 175-193. (b) Celebi, S.; Leyva, S.; Modarelli,
D. A.; Platz, M. S. J. Am. Chem. Soc. 1993, 115, 8613-8620. (c) Modarelli,
D. A.; Morgan, S.; Platz, M. S. J. Am. Chem. Soc. 1992, 114, 7034-7041.
(d) White, W. R., III; Platz, M. S. J. Org. Chem. 1992, 57, 2841-2846.
(8) Motschiedler, K. R.: Toscano, J. P.; Garcia-Garibay, M. A. Tetra-
hedron Lett., in press.
(9) (a) Closs, G. L.; Rabinow, B. E. J. Am. Chem. Soc. 1976, 98, 8190-
8198. (b) Eisenthal, K. B.; Turro, N. J.; Sitzmann, E. V.; Gould, I. R.;
Hefferon, G.; J., L.; Cha, Y. Tetrahedron 1985, 41, 1543-1554. (c) Griller,
D.; Nazran, A. S.; Scaiano, J. C. Tetrahedron 1985, 41, 1525-1530. (d)
Schuster, G. B. AdV. Phys. Org. Chem. 1986, 22, 311-362.
The diazo compound 1a [1-(4′-biphenylyl)-2-phenyldiazo-
propane] was prepared from biphenyl and phenylacetyl chloride
by the sequence of reactions shown in Scheme 3. Compounds
6 and 7 were isolated in 71% and 93% yield, respectively, after
purification by column chromatography. The hydrazone 8 was
obtained in 67% yield by crystallization from ethanol, and
(12) Woodworth, R. C.; Skell, P. S. J. Am. Chem. Soc. 1959, 81, 3383-
3386.
(13) (a) Bethell, D.; Hayes, J.; Newall, A. R. J. Chem. Soc., Perkin Trans.
2 1974, 1307-1312. (b) Bethell, D.; Stevens, G.; Tickle, P. J. Chem. Soc.,
Chem. Commun. 1970, 792-794.
(10) (a) Schaefer, H. F., III. Acc. Chem. Res. 1979, 12, 288-296. (b)
Liu, M. T. H. Acc. Chem. Res. 1994, 27, 287-294.
(11) (a) Liu, M. T. H. J. Phys. Org. Chem. 1993, 6, 696-698. (b) Nickon,
A.; Bronfenbrenner, J. K. J. Am. Chem. Soc. 1982, 104, 2022-2023.

Control of Carbene ReactiVity by Crystals
J. Am. Chem. Soc., Vol. 119, No. 8, 1997 1861
Scheme 4
Table 2. Product Distribution from Photolysis of 1a in Several
Solventsa
solvent
hexane
acetonitrile
ethanol
(Z)-3a
(E)-3a
4a (Z+E)
5a
16
39
22
10
18
10
53 + 21
32 + 11
4 + 1
0
0
64
a Irradiation at 0 °C with λ > 380 nm.
conformational preferences of π-systems with R-alkyl groups
(ketones, alkenes, arenes).15
The packing diagram of 1a viewed along the short b axis
(Figure 1) depicts columns of molecules running along the
crystallographic c axis (vertical axis) with pairwise centrosym-
metric interactions that position the diazo group of one molecule
directly above the biphenylyl group of another. Face to face
interactions between phenyl groups of neighboring columns and
edge-to-face interactions between biphenylyl and phenyl sub-
stituents may also be discerned in the packing structure.
(2) Solution Photolysis and Product Characterization.
Photochemical irradiation of diazo 1a in deoxygenated hexanes
and acetonitrile led to formation of both E and Z isomers of
1-(4′-biphenylyl)-2-phenylpropene (3a) and 1-(4′-biphenylyl)-
1-phenylpropene (4a) (Scheme 4). No products from the much
slower 1,2-Me shift reaction or intramolecular C-H insertion
were observed. The relative product yields established by GC
analysis were highly solvent dependent (Table 2). It was also
shown in a different experiment that irradiation of 1a in pure
ethanol leads to compounds 3a and 4a along with 64% yield of
ethers 5a. The identification of olefins 3a and 4a and ethers
5a was carried out by coinjection of authentic samples inde-
pendently synthesized from 4-phenylbenzophenone and from
1-(4′-biphenylyl)-2-phenylpropanone (7) using conventional
functional group transformations as indicated in Scheme 4.
Compounds (E)-3a and (Z)-3a were separated by fractional
recrystallization, but numerous attempts to separate (E)-4a and
(Z)-4a by fractional crystallization and by preparative TLC and
HPLC on various solid supports were unsuccessful. The
stereochemical identity of (E)-3a and (Z)-3a was suggested by
1H NMR NOE measurements, and it was unambiguously
established by X-ray diffraction analysis of the (Z)-isomer,
which was also the predominant photoproduct in the solid state.
The X-ray structure of (Z)-3a was solved in the space group
P21/c with two independent molecules per asymmetric unit
Figure 1. X-ray structure and packing diagram of compound 1a.
oxidation of 8 with HgO in pentane as a solvent in the presence
of MgSO4 and EtOK yielded pure diazo 1a in 81% yield. Ruby
red crystals of 1a were obtained by slow evaporation of pentane
at -20 °C as long needles. While differential scanning
calorimetry (DSC) traces with slow heating rates showed slow
thermal decomposition (Vide infra), a sharp melting point of
93-94 °C was obtained on a hot stage apparatus when melting
was accomplished with rapid heating rates. Compound 1a was
fully characterized by spectroscopic methods both in solution
and in the solid state. The spectroscopic signatures of the diazo
group14 were observed in its characteristic red color with a λmax
at 515 nm (Figure 3), a sharp stretching band at 2033 cm-1 in
the IR spectrum, and a very shielded signal at 79.4 ppm in the
13C NMR.
Single-crystal X-ray diffraction revealed the space group
P21/c with one molecule per asymmetric unit. The details of
data collection and refinement are included in the Supporting
Information. The molecular structure of 1a is characterized by
having the 4′-biphenyl, diazo, and methyl substituents in a nearly
coplanar arrangement, while the R-hydrogen and phenyl groups
are located above and below this plane with torsion angles
N2)CsCsH and N2)CsCsPh of 127° and -115.5°, respec-
tively (Figure 1). The planarity between diazo and the biphe-
nylyl system probably comes from maximized conjugation
between the two π-systems while the coplanarity of methyl and
diazo groups may be understood in terms of dipole-induced
dipole interactions such as those proposed to account for the
(14) Patai, S. The Chemistry of the Diazonium and Diazo Groups; John
Wiley & Sons: New York, 1978.
(15) (a) Wiberg, K., B. J. Am. Chem. Soc. 1986, 108, 5817-5822. (b)
Wiberg, K. B.; Murcko, M. A. J. Comput. Chem. 1988, 9, 488-494.

1862 J. Am. Chem. Soc., Vol. 119, No. 8, 1997
Shin et al.
Figure 3. UV-vis spectra of compounds 1a (dotted line), (Z)-3a
(dashed line), and (E)-3a (full line). The cutoff wavelengths of glass
filters used in various experiments are indicated by vertical lines.
Figure 4. Fluorescence spectra in methylcyclohexane at 77 K of (a)
purified (Z)-3a and (b) samples of 1a irradiated at 77 K to give primarily
(Z)-3a (not corrected for detector response).
cm-1) and kinetic behavior nearly identical to those reported
by Trozzolo for 1,2-diphenylethylidene (1b)17 and by Platz and
Tomioka for 1,2-diphenylpropylidene4c (1c, Table 1). The
lifetime of this triplet in MCH glasses was relatively short (few
seconds), and its detection required continuous illumination at
77 K. This signal has been assigned to carbene 32a, which we
suggest is the ground state of 2a.
Figure 2. X-ray molecular structures of compound (Z)-3a showing
the two conformers present in the asymmetric unit and the packing
diagram along the b axis.
Following the detection of carbene 2a by EPR, we attempted
the detection of its T-T fluorescence at 77 K. While the T-T
fluorescence of numerous diarylcarbenes and naphthylcarbenes
has been observed over the last 30 years,18 the emission of
arylalkylcarbenes remains to be documented.19 While we could
not observe a transient signal that we could assign to carbene
2a, we noticed the growth of a strong emission signal at λ )
440 nm. We found that the spectrum obtained by irradiation
of diazo 1a at 77 K was nearly identical to that obtained from
purified samples of stilbene (Z)-3a (Figure 4).
It was shown that irradiation of 1a at 77 K in methylcyclo-
hexane gives rise to alkenes (Z)-3a and (E)-3a in ca. 45 and
15% yields, respectively, along with a mixture of products from
reaction with the matrix (ca. 40%).4 In agreement with previous
observations by Tomioka et al., no products from the sterically
demanding 1,2-Ph migration were observed.4d It was also
shown that the fluorescence of the product mixture corresponded
to that of the cis- and trans-stilbenes (Z)-3a and (E)-3a.
Growth of the fluorescence signal of the photoproducts as a
function of time offers an opportunity to analyze the kinetics
(Figure 2). The details of data collection and refinement are
included in the Supporting Information.
As observed with other 1,2-diaryldiazoalkanes, product ratios
in solution are highly solvent dependent. However, in order to
obtain reliable product ratios care must be exercised in the
selection of excitation wavelengths. In order to prevent cis-
trans isomerization16 by secondary photochemical reactions of
the stilbene (3) and diarylethene chromophores (4), all irradia-
tions were carried out with λ > 380 nm. It can be seen in the
UV spectrum in Figure 3 that irradiation through a 380 nm cutoff
filter results in exclusively excitation of the weak (ꢀmax ) 57 L
M-1 cm-1) and long-wavelength n,π* absorption (λmax ) 515
nm) of the diazo chromophore. It was shown that product ratios
under these conditions are constant as a function of irradiation
time and reaction progress. It was also shown that stereochemi-
cally pure (or enriched) samples of these compounds remained
photostable upon irradiation with λ > 380 nm.
(3) Measurements in Low-Temperature Glasses. Product
selectivity in solution is rather low, and variations in product
yields in Table 2 are complex. In order to explain these
observations, the identity of the precursor(s) of the observed
products was investigated. Evidence in support of pathway 1
was obtained by irradiation of 1a in rigid glasses at 77 K in the
cavity of the ESR spectrometer. We observed a triplet signal
with zero-field splitting parameters (D ) 0.470 and E ) 0.029
(17) Trozzolo, A. M.; Wasserman, E. In Structure of Arylcarbenes; Moss,
R. A., Jones, M., Jr., Eds.; John Wiley & Sons: New York, 1975; pp 185-
206.
(18) (a) Trozzolo, A. M.; Gibbons, W. A. J. Am. Chem. Soc. 1967, 89,
239-243. (b) Scaiano, J. C.; Weir, D. Can. J. Chem. 1988, 66, 491-494.
(c) Johnston, L. J.; Scaiano, J. C. Chem. Phys. Lett. 1985, 116, 109-113.
(19) From several arylalkylcarbenes recently analyzed, we have only been
able to assign emission from phenyl-tert-butylcarbene: Shin, S. H.; Garcia-
Garibay, M. A. Unpublished results.
(16) Turro, N. J. Modern Molecular Photochemistry; Benjamin/Cum-
mings Publishing Co.: Menlo Park, 1978; Chapter 12.

Control of Carbene ReactiVity by Crystals
J. Am. Chem. Soc., Vol. 119, No. 8, 1997 1863
step when the R-deuterio compound was analyzed. While this
value is smaller than expected for a primary isotope effect at
this temperature, it is consistent with those reported for other
carbene 1,2-H shifts,22-24 for which there is growing evidence
suggesting the involvement of quantum mechanical tunneling.
While the above results provide evidence for product forma-
tion via a carbene intermediate in rigid glasses at low temper-
1
atures, the involvement of an excited-state reaction of 1a* in
fluid media at higher temperatures cannot be discounted. Triplet
sensitization studies in progress in our group with compounds
1
that bypass 1a* suggest that products from the diazo singlet
excited state in solution (pathway 2) may have a contribution
of ca. 30% yield.8 Nonetheless, with strong evidence for 2a as
an intermediate, the formation of 3a and 4a can be formulated
in terms of carbenic 1,2-H shifts and 1,2-Ph migrations,
respectively, while formation of 5a (Scheme 4) may be
formulated in terms of insertion of 2a into the polar O-H bond
when ethanol is present.25 If we assume that the excited state
of the diazo does not react with the solvent, results in ethanol
indicate that at least 64% of the reaction goes through a trappable
carbene intermediate.7 Trends in product yields as a function
of solvent polarity are in agreement with the mechanism in
Scheme 2.4,5 We propose that the more polar solvent stabilizes
the singlet state and increases its equilibrium population, thus
favoring the formation of products 3a via a singlet 1,2-H shift
at the expense of 4a, which would form primarily via a triplet
state 1,2-Ph migration. Polar solvents are also known to
stabilize the transition state for 1,2-H reactions,26 which may
also contribute to the changes in product yield observed.
(4) Photolysis in Crystalline Samples of 1a.27 A large
number of solid-state photochemical reports have described
studies where the extent of reaction is systematically limited to
low conversion values. It is assumed that reactive crystals will
degrade at high conversion and the reaction selectivity will be
adversely affected. This may be true in many cases. However,
studies by Hasegawa et al.,28 Novak et al.,29 and Choi et al.30
have demonstrated that a careful selection of experimental
conditions may have important consequences on the progress
of the reaction. In particular, excitation at the longest possible
wavelengths helps avoid absorption by the photoproduct and
ensures the longest possible penetration depth. Photochemical
reactions under these conditions probably dissipate less heat,
occur in a more homogeneous manner, and have a better
opportunity to maintain their crystal control.
Figure 5. Kinetics of 1a by fluorescence of the stilbene products as
a function of time at two light intensities in ethanol at 77 K (5% of the
total data points shown). The line indicates the fit to the kinetic scheme
shown on top.
of the reaction (Figure 5). A primary photochemical process
is expected to give linear intensity vs. time plots with a reaction
velocity proportional to the number of quanta absorbed and to
the quantum yield of reaction.20 In contrast, products from a
long-lived intermediate (i.e., several seconds) should be detect-
able after an induction period, and their formation should
continue after illumination is stopped. Thus, the signals of the
stilbene products were monitored as a function of time using
the spectrofluorimeter to excite both the reactant and the product
at λ ) 320 ( 10 nm. While a strict kinetic analysis was not
attempted, signals assigned to the photoproducts (largely stilbene
3a) grow with the behavior characteristic of a two-step process
involving a dark intermediate (Figure 5).21 The growth of the
product fluorescence was fit to the kinetic model shown in
Figure 5. The first step [i.e., k1(I)] represents the generation of
the carbene and is light-intensity dependent, while the second
step (k2) represents the overall (thermal) reaction of the carbene.
While we assume that the 1,2-H shift occurs from the singlet
state, we do not know whether intersystem crossing or reaction
from the singlet carbene is rate determining. Variations in light
intensity over a factor of 90 accomplished with different slit
settings qualitatively reveal the expected dependence of k1(I).
The values of k2 displayed a smaller intensity dependence, which
we postulate may be due to photochemistry of the carbene, as
suggested by reports from McMahon and Chapman in their
studies of phenylmethylcarbene.22 The values for k2 obtained
in methylcyclohexane glasses at 77 K in low-intensity experi-
ments are estimated to be ca. 0.002-0.005 s-1. These are
qualitatively consistent with the EPR measurements where a
short carbene lifetime is suggested by the requirement of
continuous illumination to maintain an observable spin con-
centration. To support our assignments, we carried out experi-
ments with deuterated samples of 1a. Although isotope effects
in carbene 1,2-H shift reactions are known to be anomalous,22,23
we expected that the proposed kinetic scheme would display a
primary isotope effect in the second step of the reaction. In
fact, a value of τD/τH ) 1.5 ((0.2) was calculated for the second
(24) (a) McMahon, R. J.; Chapman, O. L. J. Am. Chem. Soc. 1987, 109,
683-692. (b) Moss, R. A.; Ho, G.-J.; Liu, W.; Sierakowski, C. Tetrahedron
Lett. 1992, 33, 4287-4290. (c) Moss, R. A.; Xue, S.; Ma, Y. Tetrahedron
Lett. 1996, 30, 4287-4290. (d) Moss, R. A.; Xue, S.; Ma, Y. Tetrahedron
Lett. 1996, 37, 1929-1932.
(25) (a) Tomioka, H.; Hayashi, N.; Sugiura, T.; Izawa, Y. J. Chem. Soc.,
Chem. Commun. 1986, 1364-1366. (b) Griller, D.; Liu, M. T. H.; Scaiano,
J. C. J. Am. Chem. Soc. 1982, 104, 5549-5551. (c) Warner, P. M.; Chu, I.
S. J. Am. Chem. Soc. 1984, 106, 5366-5367. (d) Moss, R. A.; Shen, S.;
Wlostowski, M. Tetrahedron Lett. 1988, 29, 6417-6420.
(26) Sugiyama, M. H.; Celebi, S.; Platz, M. S. J. Am. Chem. Soc. 1992,
114, 966-973.
(27) For some leading reviews in solid-state organic chemistry see: (a)
Ramamurthy, V.; Venkatesan, K. Chem. ReV. 1987, 87, 433-481. (b)
Desiraju, G. R. Organic Solid State Chemistry; Elsevier: Amsterdam, 1987;
p 550. (c) Scheffer, J. R.; Garcia-Garibay, M.; Nalamasu, O. Org.
Photochem. 1987, 8, 249-347. (d) Toda, F. Acc. Chem. Res. 1995, 28,
480-486. (e) Paul, I. C.; Curtin, D. Y. Science 1975, 187, 19-26. (f)
Schmidt, G. M. J. Solid State Photochemistry; Ginsburg, D., Ed.; Verlag
Chemie: New York, 1976. (g) Hasegawa, M. AdV. Phys. Org. Chem. 1995,
30, 117-171.
(20) Wagner, P. J. In CRC Handbook of Organic Photochemistry;
Scaiano, J. C., Ed.; CRC Press: Boca Raton, FL, 1989; pp 251-269.
(21) Steinfeld, J. I.; Fransisco, J. S.; Hase, W. L. Chemical Kinetics and
Dynamics; Prentice Hall: Englewood Cliffs, 1989; Chapter 2.
(22) McMahon, R. J.; Chapman, O. L. J. Am. Chem. Soc. 1987, 109,
683-692.
(23) (a) Storer, J. W.; Houk, K. N. J. Am. Chem. Soc. 1993, 115, 10426-
10427. (b) Dix, E. J.; Herman, M. S.; Goodman, J. L. J. Am. Chem. Soc.
1993, 115, 10424-10425.
(28) Chung, C.-M.; Hasegawa, M. J. Am. Chem. Soc. 1991, 113, 7311-
7316.
(29) Enkelmann, V.; Wegner, G.; Novak, K.; Wagener, K. B. J. Am.
Chem. Soc. 1994, 115, 10390-10391.
(30) Choi, T.; Cizmeciyan, D.; Khan, S.; Garcia-Garibay, M. A. J. Am.
Chem. Soc. 1995, 118, 12893-12894.

1864 J. Am. Chem. Soc., Vol. 119, No. 8, 1997
Shin et al.
Scheme 5
Scheme 6
Figure 6. Changes in the normalized yields of (Z)-3a (squares), (E)-
3a (circles), and 4a (triangles), as a function of reaction progress upon
irradiation of crystalline 1a at 0 °C with λ > 350 nm (top frame) and
with λ > 380 nm (bottom frame).
Photolysis of polycrystalline powdered samples and single
crystals of 1a were carried out at 0 °C and at ambient
temperatures with a 450 W water-cooled medium-pressure
mercury arc Hanovia lamp and with a 1000 W Hg-Xe Oriel
lamp. Although consistent results independent of crystal size
were obtained at these temperatures, we found that excitation
wavelengths have a strong influence on the reaction. Experi-
ments were carried out with cutoff filters with λ > 350 or λ >
380 nm to determine the influence of absorption by the product
(if any) at the shorter wavelength. Product analyses were carried
out by GC as a function of UV exposure. Since the unreacted
diazo decomposes thermally in the GC column into the same
products obtained by photolysis, all partially reacted samples
were treated with excess dimethyl acetylenedicarboxylate
(DMAD), which was added to the solution containing the
product mixture and the internal standard. It was shown that
DMAD removes the diazo compound quantitatively in less than
5 min at ambient temperatures to form (presumably) the
corresponding pyrazolines (Scheme 5). Although the latter
decompose thermally in the GC column they present no
interference because of their different GC retention times.
Results obtained by irradiation at λ > 350 nm are plotted in
Figure 6 (top frame) as reaction progress versus normalized
yield, or selectivity, for each of the three products detected [(Z)-
3a, (E)-3a, and traces of one of the isomers of 4a]. Irradiation
to very low conversion values (ca. 0.5-5%) resulted in the
nearly exclusive formation of (Z)-3a with only traces of (E)-3a
and 4a. Continued irradiation and product accumulation from
ca. 5.0% up to 50-60% conversion resulted in decreased
reaction selectivity with (Z)-3a accounting for ca. 75-80% of
the product mixture. Microscopic analysis of samples as a
function of reaction progress showed no liquid phases, although
changes in color from red to pink to white were evident as the
product accumulated. As shown in the final portion of the graph
in Figure 3, a very noticeable change in the selectivity of the
reaction occurred between 60 and 100% conversion. Although
compound 4a remained at the trace level, by the time the
reaction was nearly complete, compound (E)-3a had become
the major product. Mass balance shows that the rise in the yield
of (E)-3a and the fall in the yield of (Z)-3a are due to
photoisomerization of (Z)-3a. In fact, this was demonstrated
with several control experiments summarized in Scheme 6.
Compound (Z)-3a was photolyzed in three different media:
benzene solution, pure recrystallized samples, and as it is formed
in photolyzed crystals of 1a. In solution, with a Pyrex filter (λ
> 290 nm), the photostationary state is 45% (Z)-3a and 55%
(E)-3a. With a 350 nm filter, we measured 38% (Z)-3a and
62% (E)-3a. With a 380 nm filter we observed no photoi-
somerization. In recrystallized samples of (Z)-3a there was no
photoisomerization observed with λ > 290, λ > 350, or λ >
380 nm. Finally, a similar set of experiments was carried out
with samples of 1a that had been irradiated with λ > 380 nm
and that contained ca. 95% of (Z)-3a (Vide infra). In these
samples, isomerization was observed when the solid was
exposed through cutoff filters that selected for λ > 290 and λ
> 350 nm, but not when exposed to λ > 380 nm.
The above results show that the relative yields of compounds
(Z)-3a and (E)-3a at high conversion reflect their photostationary
state in the solid state.31 The UV absorption spectra of 1a, (Z)-
3a, and (E)-3a in the solid parallels their absorption in
cyclohexane. Selective excitation of (Z)-3a occurs with λ >
350 nm but not with λ > 380 nm (Figure 3). While cis-trans
photoisomerization of alkenes in the solid state is rare, it is not
unprecedented.31 However, differences in reactivity between
recrystallized samples of (Z)-3a and samples obtained by
irradiation of crystals of 1a suggest two entirely different solid
phases. As discussed below, samples of “as formed” (Z)-3a
probably exist in a metastable phase that is quite different from
the one it crystallizes in under equilibrium conditions.
As expected from the above discussion, irradiation of 1a with
λ > 380 nm gave extremely high selectivity with only small
amounts of byproducts (Figure 6, bottom frame). As in the
previous experiment, red solids slowly turned white while
microscopic analysis revealed no apparent intervention of fluid
phases. The importance of the crystalline environment on the
selectivity of the reaction was also shown by comparison of
the results obtained in crystals with λ > 380 nm with those
from experiments carried out with amorphous solid samples.
The latter samples were prepared by melting crystals of 1a and
rapidly quenching the melt in a liquid N2 bath. The vitreous
samples were treated in an identical manner as polycrystalline
(31) For examples of cis-trans isomerization in solids see: (a) Bregman,
J.; Osaki, K.; Schmidt, G. M. J.; Sonntag, F. I. J. Chem. Soc. 1964, 2021-
2030. (b) Griffin, G. W.; O’Connel, E. J.; Kelliher, J. M. Proc. Chem. Soc.
1964, 337. (c) Saigo, K. Bull. Chem. Soc. Jpn. 1996, 69, 779-784. (d)
Kinbara, K.; Kai, A.; Maekawa, Y.; Hashimoto, Y.; Naruse, S.; Hasegawa,
M.; Saigo, K. Mol. Cryst. Liq. Cryst. 1996, 276, 141-151.

Control of Carbene ReactiVity by Crystals
J. Am. Chem. Soc., Vol. 119, No. 8, 1997 1865
Scheme 7
Figure 7. X-ray structures of compound 1a, its major solid-state
photoproduct (Z)-3a, and their overlap.
samples irradiated with the 380 nm cutoff filter. After 3 h of
photolysis, 72% of the diazo was converted, and the samples
were analyzed. The relative yields of (E)-3a, (Z)-3a, and (E +
Z)-4a were 29%, 51%, and 20% (3% + 16%), respectively.
Irradiation and microscopic examination of single crystal
specimens was carried out with λ > 380 nm. Single crystalline
plates of ca. 0.5 × 2 × 3 mm were fixed onto a thin film of
apiezon grease on a glass surface with care that no other crystal
faces were contaminated. After only 15 min of irradiation, the
crystals developed superficial fractures along the long crystal
axis. After 0.5 h of irradiation, the ruby-red plates started to
display a white coating inhomogeneously distributed over the
crystal surface. Continued irradiation led to cloudy white
crystals with some longitudinal fractures. While a small amount
of a fine white powder observed along the periphery of irradiated
crystals suggests the possibility of micro explosions to release
trapped nitrogen, dissolution of crystals at the end of the
experiments displayed a vigorous evolution of nitrogen gas.
(5) Solid-State Reaction Mechanism. EPR detection of
carbene 2a was accomplished with several single-crystal
specimens that had their long axis aligned perpendicular to the
magnetic field. Very long-lived signals (hours) covering a field
width of ca. 4000 G were observed after the sample was exposed
for 30 s to the output of a filtered 1 kW Oriel lamp at 77 K.
Signals disappeared upon warming to ambient temperatures but
could be obtained upon further exposure, and the cycle could
be repeated. Analysis of the solid-state reaction by fluorescence
detection showed that the carbene is stable for hours at 77 K
but the fluorescence of stilbenes 3a grows in rapidly at ambient
temperatures. A detailed kinetic analysis as a function of
temperature is currently in progress.
These results confirm that formation of (Z)-3a in the solid
state occurs via the intermediacy of a carbene, 2a, which
undergoes a stereoselective 1,2-H shift. It is consistently agreed
that carbene 1,2-H shifts have highly demanding transition states
where the migrating hydrogen must overlap with the empty
p-orbital of the sp2-hybridized carbene carbon (Scheme 7).32
We propose that reaction control originates from the confor-
mation of the reactant as determined by crystallization and from
the structural barriers imposed by the crystal lattice that prevent
conformational equilibrium and ensure a very high similarity
between the diazo, the carbene, the transition state, and the final
product. A Newman representation of several conformers of
1a and 2a along with a projection of their corresponding
products in Scheme 7 makes a good graphical representation
of this proposal. While all conformers may be available in
solution media, only one is relevant in the solid state. In
addition to individual activation energies, which are expected
to be low,33 product ratios in solution are determined by a subtle
balance of conformational populations and isomerization rates,
as well as singlet-triplet populations and intersystem crossing
rates. Further complications occur in solution due to contribu-
tions from excited-state reactivity.7 In contrast, in the solid-
state reaction, the structure of the reactant and the rigidity of
the crystal lattice make many of these arguments less important
by restricting the structural possibilities of the reaction.
In spite of a high concentration of diazo compound, crystals
of 1a prevent intermolecular processes that are common in
concentrated solutions. The packing diagram of 1a (Figure 1)
explains the absence of intermolecular reactions that would give
rise to azines or alkenes by reaction of carbenes with diazo
compounds34 or with themselves,35 respectively. The distance
between prospective carbene carbons and the terminal diazo
nitrogen of the closest neighboring molecules (required for azine
formation) is 6.35 Å. The closest distance between prospective
carbene carbons (required for alkene formation) is 7.38 Å. Not
only are these distances too long but the diazo groups of adjacent
molecules do not have the required alignment for bimolecular
reactions to occur. A close packing structure prevents molecular
diffusion and rotation, which could lead to these products.
After obtaining the X-ray structure of photoproduct (Z)-3a
and comparing it with that of diazo 1a, we confirmed that the
pictorial representation of the solid state reaction in Scheme 7
is supported beyond our most optimistic expectations. For our
analysis we have assumed that carbene 2a can only experience
relatively minor relaxation so that in the crystal it retains the
overall shape of the precursor. Thus, the X-ray molecular
structure of diazo compound 1a corresponds closely to that of
conformer 1a Pre-(Z)-3a in Scheme 7. This structure possesses
the biphenyl and phenyl groups gauche to each other with the
R-hydrogen deviating from the expected orientation by ca. 37°.
Side-by-side comparison and overlap of the X-ray structures
of the reactant and the product also shows outstanding similari-
ties (Figure 7). With the exception of the migrating hydrogen,
in going from the reactant to the product, there are only small
atomic displacements and the solid state reaction effectively
constitutes a least motion process. A recent computational
(33) Sugiyama, M. H.; Celebi, S.; Platz, M. S. J. Am. Chem. Soc. 1992,
114, 966-973.
(34) Example of carbenes reacting with diazo compounds: Griller, D.;
Majewski, M.; McGimpsey, W. G.; Nazran, A. S.; Scaiano, J. C. J. Org.
Chem. 1988, 53, 1550-1553.
(35) Example of carbenes reacting with themselves: (A) Zimmerman,
H. E.; Paskovich, D. H. J. Am. Chem. Soc. 1964, 86, 2149-2160. (b)-
Nazran, A. S.; Lee, F. L.; Gabe, E. J.; Lepage, Y.; Northcote, D., J.; Park,
J. M.; Griler, D. J. Phys. Chem. 1984, 88, 5251-5254.
(32) (a) Evanseck, J. D.; Houk, K. N. J. Phys. Chem. 1990, 94, 5518-
5523. (b) Schaefer, H. F., III. Acc. Chem. Res. 1979, 12, 288-296. (c)
Kyba, E. P.; John, A. M. J. Am. Chem. Soc. 1977, 99, 8330-8332.

1866 J. Am. Chem. Soc., Vol. 119, No. 8, 1997
Shin et al.
Scheme 8
analysis based on the calculation of steric energies along the
ab initio gas-phase reaction coordinate suggests that planariza-
tion during rehybridization of the R-carbon and a small displace-
ment of the phenyl group do not cause strongly adverse
nonbonding interactions between the reactant and the crystal
lattice.36
(6) Study of Crystal Changes as a Function of Reaction
Progress. Organic reactions in crystals may be extremely
complex non-linear processes.37 While crystal packing and
molecular structure control the reactivity and the selectivity of
reaction, changes in composition due to reaction may affect the
structure of the crystal. It is not uncommon that high selectivi-
ties and specificities may only be observed at low conversion
values because of destruction of the crystal lattice. In this
context, the solid-state transformation of 1a into (Z)-3a is
remarkable because the selectivity of the reaction is maintained
to nearly quantitative conversion values (see Figure 6). We
have carried out a thorough spectroscopic and thermal analysis
to elucidate the nature of the phase changes occurring in this
reaction.
The foundation for understanding the consequences of product
accumulation in solid-state reactions was briefly laid down by
Schmidt in reference to numerous examples of cinnamic acid
derivatives.38 Schmidt proposed that product molecules go into
solid solution in the lattice of the reactant until they reach their
solubility limit, when they separate into their own phase
(Scheme 8). It should be pointed out that separation of a liquid
phase (melting) is expected to occur if the temperature of the
reaction is above the eutectic temperature of the reactants and
the products. By careful spectroscopic and thermal analysis
we have recently observed that efficient solid-state photochemi-
cal reactions may involve a solid solution present only in a
steady state.30,38 An alternative, but infrequent, situation
involves the so-called single crystal-to-single crystal, or topo-
tactic transformations.39 These are ideal situations where
crystals of the reactant smoothly transform into crystals of the
product by means of a solid solution that covers their entire
composition diagram. The crystal structure of the reactant and
the crystal structure of the product in this case are isomorphic,
and they constitute a single phase with the greatest potential
for reaction control.
Figure 8. 13C CPMAS spectra of crystalline samples of 1a (bottom)
and (Z)-3a (top) along with partially reacted samples to 25, 45, and
60% conversion (as labeled).
1a crystallizes with only one molecule per asymmetric unit while
(Z)-3a crystallizes in two different conformers that differ in the
torsion angle of their biphenyl substituents, which are 1.6° and
-33.6°, respectively. The high molecular similarity between
1a and one conformer of (Z)-3a is shown in Figure 7. A similar
structural match may be obtained in all respects, except for the
relatively soft biphenyl torsion, between 1a and the other
conformer of (Z)-3a.41 We propose that the excellent structural
overlap of molecules of 1a and (Z)-3a allows for a high solid-
state solubility of the product in crystals of the reactant, but
the poor structural overlap of their packing arrangements
prevents them from being part of a continuous solid phase. It
was our goal to determine whether the solid phase of “as
formed” (Z)-3a is the same as or different from that obtained
after recrystallization.
Spectral analysis of the solid-state reaction was carried out
as a function of conversion by 13C CPMAS NMR.42 Spectra
were collected with the TOSS sequence for spinning sideband
suppression. A stack plot with spectra of the starting material
(bottom), the recrystallized product (top), and three intermediate
values (25, 45, and 60% conversion) is shown in Figure 8. The
solid-state spectrum of 1a qualitatively matches the spectrum
obtained in solution. Seven out of 18 nonequivalent aromatic
signals are resolved in the range between 120 and 145 ppm,
while the diazo, methine, and methyl carbons resonate at ca.
60.1, 33.6, and 21.6 ppm, respectively. The spectrum of the
product [(Z)-3a] on the top of Figure 8 displays eight aromatic
signals corresponding to 28 aromatic and vinylic carbons of
The X-ray structures of 1a and (Z)-3a cannot be the end points
of a continuous phase involving a solid solution,40 and the solid
state reaction of 1a cannot be topotactic. Although both
compounds crystallize in space group P21/c their packing
arrangements are different (see Figures 1 and 2). Compound
(36) Keating, A. E.; Shin, S. H.; Houk, K. N.; Garcia-Garibay, M. A. J.
Am. Chem. Soc., in press.
(37) Garcia-Garibay, M. A.; Constable, A. E.; Jernelius, J.; Choi, T.;
Cizmeciyan, D.; Shin, S. H. In Physical Supramolecular Chemistry;
Echegoyen, L., Kaifer, A., Eds.; Kluwer Academic Publishers: Dordrecht,
1996; pp 289-312.
(38) Schmidt, G. M. J. J. Chem. Soc. 1964, 2014-2021.
(39) (a) Wegner, G. Pure Appl. Chem. 1977, 49, 443-454. (b) Thomas,
J. M. Nature 1981, 289, 633. (c) Theocharis, C. R.; Jones, W. In Organic
Solid State Chemistry; Desiraju, G., Ed.; Elsevier: Amsterdam, 1987; pp
47-67.
(41) It has been proposed that planar biphenyl is a time-average
structure: Baudour, J. L. Acta Crystallogr. B 1991, 47, 935-949.
(42) (a) Fyfe, C. A. Solid State NMR for Chemists; CFC Press: Guelph,
Ontario, 1983. (b) Yannoni, C. S. Acc. Chem. Res. 1982, 15, 201-208. (c)
Schaefer, J.; Stejskal, E. O. Top. Carbon-13 NMR Spectrosc. 1979, 3, 283-
324.
(40) Kitaigorodski, A. I. Mixed Crystals; Springer Verlag: Berlin, 1984.

Control of Carbene ReactiVity by Crystals
J. Am. Chem. Soc., Vol. 119, No. 8, 1997 1867
Figure 10. First-order plot for irradiation of microcrystalline samples
of 1a as a function of time at 25 °C with λ > 380 nm. A0 and At refer
to the IR absorbance values at the diazo stretch at time ) 0 and time
) t.
precede the melting endotherm. The latter observation suggests
that small quantities of product disturb the crystal lattice of 1a
to the extent that it no longer stabilizes the diazo compound. It
also seems from these results that a thermal reaction in the solid
state may be possible if carried out with care (to prevent
melting), and work is currently in progress to determine the
selectivity of the thermal reaction.
From the above results one may conclude that the solid-state
reaction of 1a proceeds in a single phase and that the solid phase
of “as formed” (Z)-3a is not related to that obtained after
recrystallization. Since the selectivity of the solid state reaction
for (Z)-3a remains unaltered, regardless of the composition of
the solid, we propose that the “as formed” product phase retains
most of the structural properties of the initial phase of the
reactant. Although some order may be lost as the reaction
proceeds, we conclude that the conformation of the diazo
compound and its environment retain the influences that dictate
the course of the solid-state reaction.
Experiments with microcrystalline samples in optically clear
KBr matrices with constant illumination at λ > 380 nm also
give evidence of a relatively homogeneous process with
excellent apparent first-order plots. Figure 10 was obtained with
conversion values calculated from FT-IR absorbance of the diazo
signal. It is expected that the slope of the plot should depend
on the number of quanta absorbed, given by the intensity of
the irradiating beam, and on the quantum yield of the reaction.
The data show that the apparent rate of disappearance of 1a is
the same at the early stages of the reaction and at the end. This
is not always the case with solid-state reactions. Some become
less efficient as the product accumulates, and similar plots bend
upward.43 Other reactions appear to become more efficient, as
their first-order plots bend downward, suggesting that melting
may occur as the reaction goes on.
Figure 9. DSC traces of crystalline samples of 1a (upper right), (Z)-
3a (lower right), and partially reacted samples to 25, 45, and 60%
conversion (left).
the two distinct molecules in the crystal of (Z)-3a. Signals
corresponding to the methyl groups (one per conformer) appear
resolved at 29.0 and 27.8 ppm, respectively.
The progress of the reaction is clearly visible in spectra
obtained at intermediate stages. As the reaction proceeds,
signals of the reactant show increasing broadening while signals
corresponding to the product start to grow in. Interestingly,
product signals are also quite broad and not at all recognizable
as those of recrystallized (Z)-3a at the top of the figure. The
spectra of partially converted 1a cannot be reproduced by linear
additions of spectra corresponding to the reactant and the
products. These results contrast very sharply with previous
observations in our group where a solid-state reaction forms a
separate solid phase of the product with clear 13C NMR signals
that grow sharp from the beginning.30,38 We also noticed that
the spectra continue evolving as a function of conversion,
suggesting that solid-state reaction of 1a proceeds by a
continuous-phase mechanism rather than by a phase-separation
mechanism. While similar qualitative observations were made
by solid-state FT-IR (not shown), confirmation of this may be
obtained by differential scanning calorimetry (DSC).
DSC measurements that paralleled the 13C CPMAS NMR
spectra were carried out with pure and partially reacted samples
(Figure 9). The thermogram of the reactant is characterized by
a sharp melting endotherm at 94 °C followed by a strong
exotherm assigned to the thermal denitrogenation of the diazo
compound. It is interesting that no thermal decomposition
occurs below the melting point with heating rates between 2
and 10°/min. Constant temperature runs with samples main-
tained at 90 °C (which is only 4 °C below their melting point)
decomposed after 12 min, suggesting a high thermal stability
for compound 1a when in the crystalline state. Crystals of
product (Z)-3a display a sharp melting endotherm at 103 °C
with no other transitions that may be assigned to liquid
crystalline phases, which are relatively common for biphenylyl-
substituted compounds.
The thermal behavior of partially reacted samples of 1a is
extremely complex but quite illustrative and interesting. First
of all, the lack of a eutectic transition clearly implies that
reaction occurs through a continuous solid phase. Secondly,
the absence of melting transitions below ca. 60 °C indicates
that the reaction proceeds entirely in the solid state. Thirdly,
even at a point where only small quantities of (Z)-3a may be
present, it is noticed that the exothermic transition comes to
III. Conclusions
We have shown that the crystalline solid state may control
the reactivity of aryldiazoalkanes to levels that have not been
accomplished with any other media. Although we have also
shown strong evidence for a carbene intermediate, we may not
discount the partial contribution to product formation of an
excited-state reaction of the diazo compound. We have sug-
gested that reaction control in the crystal derives from confor-
mational factors that determine orbital alignment and from
environmental factors that maintain a strong structural similarity
between the reactant, the intermediate, the transition state and
the final product. Spectroscopic and thermal analysis of changes
(43) Garcia-Garibay, M. A. PhD Thesis, University of British Columbia,
1988.

1868 J. Am. Chem. Soc., Vol. 119, No. 8, 1997
Shin et al.
slides. The slides were clamped together and irradiated for about 3 h
in an ice bath (0 °C) with the 380 nm filter. The sample was removed,
dissolved in ether, and analyzed by GC. The yields of (Z)-3a, (E)-3a,
and 4a (E and Z isomers) were 51%, 29%, and 20% (3% + 16%),
respectively.
in the crystalline solid as a function of reaction progress suggest
a continuous phase. With spectroscopic, thermal, and chemical
(cis-trans isomerization) measurements it was shown that the
solid accumulated at the end of the reaction is different from
that obtained after recrystallization. Finally, it was clearly
demonstrated that the importance of experimental conditions
(temperature, wavelength) on the selectivity of solid-state
reactions should not be underestimated.
Medium-Scale Solid-State Photolysis. A sample of 400 mg of 1a
was ground into a fine powder. This powder was placed on a watch
glass, covered with a 380 nm filter, and photolyzed. At varying time
intervals (15-60 min) irradiation was stopped and the sample removed
for analysis. A 13C CPMAS solid state NMR was obtained after each
photolysis period. The sample was returned to the photolysis setup
for further irradiation after 7 mg of the sample was removed. About
5 mg of those were used for DSC (differential scanning calorimetry)
and 1 mg for solid-state FT-IR in a KBr matrix. The final 1 mg of
sample was dissolved in diethyl ether and treated with DMAD to react
with any remaining 1a. The resulting solution was analyzed by GC to
determine the products of photolysis. At the end of the experiment,
the remaining sample was recrystallized from hexanes to give crystals
of pure (Z)-3a. The results form this experiments are shown in Figures
8 (CPMAS 13C NMR) and 9 (DSC). Yields determined by FT-IR were
consistent with yields obtained by GC.
IV. Experimental Section
We describe here the procedures for solution- and solid-state
experiments. Instrumentation details, the preparation and characteriza-
tion of compounds shown in Schemes 3 and 4, and photoisomerization
experiments are described in detail in the Supporting Information.
Solution Photolysis. Approximately 1 mg of 1a was dissolved in
1 mL of solvent. The solution was placed in an NMR tube, and N2
gas was bubbled through the solution for 20 min to deoxygenate the
sample. Photolysis was carried out in an ice bath (0 °C) with a 380
nm cutoff filter. Samples were irradiated for 30 min after which time
the characteristic red color of 1a had disappeared. Upon completion
of photolysis, dimethyl acetylenedicarboxylate (DMAD) was added to
react with any remaining 1a. GC analysis revealed the composition
of the product mixture, and the results are shown in Table 2.
Microscale Solid-State Photolysis. A sample of 20 mg of 1a was
ground into a fine powder and mixed with 5 mg of ground crystals of
triphenylmethanol, which served as a standard to determine extent of
conversion of 1a. Powdered samples were placed between two
microscope slides in a polyethylene bag immersed in an ice bath (0
°C). Photolyses were performed with 350 and 380 nm cutoff filters.
Approximately 1 mg of the irradiated sample was removed for analysis
every 15-60 min. The removed portion was dissolved in diethyl ether,
and DMAD was added to react with unreacted 1a. The resulting
solution was analyzed by GC. The results of these experiments are
shown in Figure 6. Microscale photolysis was also carried out with
an amorphous solid sample of 1a. The sample was obtained by
grinding, fast melting, and liquid N2 quenching of 5 mg of 1a. The
glassy material thus obtained was sandwiched between two microscope
Acknowledgment. Support by the National Science Founda-
tion and the Petroleum Research Fund, administered by the
American Chemical Society, is gratefully acknowledged. The
National Science Foundation is also acknowledged for a
graduate research fellowship to A.E.K.
Supporting Information Available: Experimental descrip-
tion of the preparation and characterization of compounds in
Schemes 3 and 4. Description of photoisomerization experi-
ments in solution and in the solid state. Details of X-ray data
collection and structure refinements of compounds 1a and 3a,
solution, and CPMAS solid-state 13C NMR data of compounds
1a and 3a (11 pages). See any current masthead page for
ordering and Internet access instructions.
JA9633225