Excited Precursor Reactivity, Fast 1,2-H Shifts, and Diffusion-Controlled Methanol Insertion in 1,2-Diphenylalkylidenes

Article abstract of DOI:10.1021/jo990172i

The photochemistry of several arylalkyldiazo compounds has been investigated by steady-state direct and triplet sensitized irradiations and by nanosecond laser flash photolysis. Steady-state photolyses in methanol yielded methyl ethers and alkenes as the only significant products. Ethers were formed by denitrogenation of the diazo compound followed by carbene insertion into the MeO-H bond, and alkenes were obtained by 1,2-R (R = H,Ph) migrations both in the free carbene and in the singlet excited state of the diazo precursor. Evidence for the exited-state precursor reaction came from differences in product yields as a function of excitation wavelength and excited-state multiplicity. The yields of methyl ethers increased with increasing excitation wavelengths and were greatest upon triplet-state sensitization but were never the sole products formed despite expected diffusion-controlled insertion rates. Accordingly, the formation of pyridine ylides analyzed by laser flash photolysis in the presence of high concentrations of the base yielded relatively modest transient absorptions. The kinetics of the 1,2-H reaction, analyzed by time-resolved detection of stilbene in the case of 1,2-diphenylethylidenes, displayed fast and slow components. The fast component was attributed to stilbene that formed from the excited-state precursor and from the originally formed singlet carbene within the 20 ns pulse. The long-lived component was attributed to a spin-state equilibrated carbene with a lifetime of 70-80 ns in fluorocarbon solvents. Analysis of the long-lived component by a Stern-Volmer treatment upon addition of MeOH gave a lower limit for the reaction rate constant with a value of k_1,2-H > 5 × 10⁹ s^-1 in a mixed Freon-methanol solvent. It is concluded that the combined effect of phenyl substituents at the carbon bearing the migrating group and high solvent polarity can lead to intramolecular 1,2-H shifts and 1,2-Ph migrations that are fast enough to compete with diffusion-controlled intermolecular MeO-H insertion in concentrated MeOH solutions.

Full text of DOI:10.1021/jo990172i

J . Org. Chem. 1999, 64, 5139-5147
5139
Excited P r ecu r sor Rea ctivity, F a st 1,2-H Sh ifts, a n d
Diffu sion -Con tr olled Meth a n ol In ser tion in
1,2-Dip h en yla lk ylid en es
Krista Motschiedler,† Anna Gudmundsdottir,‡,§ J ohn P. Toscano,‡,| Matthew Platz,*,‡ and
Miguel A. Garcia-Garibay*,†
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095,
Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, Department of Chemistry,
University of Cincinnati, Cincinnati, Ohio 45221, and Department of Chemistry, J ohns Hopkins
University, Baltimore, Maryland 21218
Received J anuary 29, 1999
The photochemistry of several arylalkyldiazo compounds has been investigated by steady-state
direct and triplet sensitized irradiations and by nanosecond laser flash photolysis. Steady-state
photolyses in methanol yielded methyl ethers and alkenes as the only significant products. Ethers
were formed by denitrogenation of the diazo compound followed by carbene insertion into the
MeO-H bond, and alkenes were obtained by 1,2-R (R ) H,Ph) migrations both in the free carbene
and in the singlet excited state of the diazo precursor. Evidence for the exited-state precursor
reaction came from differences in product yields as a function of excitation wavelength and excited-
state multiplicity. The yields of methyl ethers increased with increasing excitation wavelengths
and were greatest upon triplet-state sensitization but were never the sole products formed despite
expected diffusion-controlled insertion rates. Accordingly, the formation of pyridine ylides analyzed
by laser flash photolysis in the presence of high concentrations of the base yielded relatively modest
transient absorptions. The kinetics of the 1,2-H reaction, analyzed by time-resolved detection of
stilbene in the case of 1,2-diphenylethylidenes, displayed fast and slow components. The fast
component was attributed to stilbene that formed from the excited-state precursor and from the
originally formed singlet carbene within the 20 ns pulse. The long-lived component was attributed
to a spin-state equilibrated carbene with a lifetime of 70-80 ns in fluorocarbon solvents. Analysis
of the long-lived component by a Stern-Volmer treatment upon addition of MeOH gave a lower
limit for the reaction rate constant with a value of k1,2-H > 5 × 109 s-1 in a mixed Freon-methanol
solvent. It is concluded that the combined effect of phenyl substituents at the carbon bearing the
migrating group and high solvent polarity can lead to intramolecular 1,2-H shifts and 1,2-Ph
migrations that are fast enough to compete with diffusion-controlled intermolecular MeO-H
insertion in concentrated MeOH solutions.
I. In tr od u ction
stituent effects and environmental factors.15,16 At the
same time, laser flash photolysis experiments have
confirmed unusual dynamics and kinetic complexities17-19
suspected from classical product analysis studies.20,21
Among these complexities, the formation of carbene-like
products from nitrogenated precursors (Scheme 1, path
Several interesting aspects of arylalkylcarbene reactiv-
ity have been recently unveiled with the application of
fast kinetic methods and increasingly powerful compu-
tational resources. Improved calculations of carbene
structures,1,2 singlet-triplet gaps,3-9 and transition
states10-14 have led to a better understanding of sub-
(10) Sulzbach, H. M.; Platz, M. S.; Schaefer, H. F.; Hadad, C. M. J .
Am. Chem. Soc. 1997, 119, 5682-5689.
† University of California.
(11) Keating, A. E.; Garcia-Garibay, M. A.; Houk, K. N. J . Phys.
Chem. 1998, 102, 8467-8476.
‡ The Ohio State University.
§ University of Cincinnati.
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Am. Chem. Soc. 1996, 118, 1535-1542.
| J ohns Hopkins University.
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10.1021/jo990172i CCC: $18.00 © 1999 American Chemical Society
Published on Web 06/19/1999

5140 J . Org. Chem., Vol. 64, No. 14, 1999
Motschiedler et al.
Sch em e 1
fast enough to compete with the bimolecular process.
However, results from the Ohio State University with
1-phenyldiazoethane and other arylalkyl diazoalkanes24,36
question the validity of those assumptions. In particular,
it was shown that 1-phenyldiazoethane undergoes an
excited-state 1,2-H shift that bypasses the carbene (path
2) and that the carbene itself undergoes 1,2-H shift
reactions that are much slower than previously assumed
(i.e., k1,2-H e 6 × 106 s-1).36 If this were also the case for
carbenes such as 2, the conversion of 2S to 3 could not
compete with the reaction from 2S to 5 in neat alcohols.
Carbene-like products 3 (and 4) might then be formed
only from an excited diazo reaction as indicated by Path
2.
In this paper, we report triplet sensitized steady-state
and laser flash photolysis experiments with the five
arylalkyl compounds below, addressing the feasibility of
concurrent 1,2-R shifts and diffusion-controlled bimo-
lecular reactions. Because methyl group migration is
comparatively slow, compound 8 only undergoes 1,2-H
shifts, as does compound 7. Compound 11 can only
undergo 1,2-Ph migrations for the same reason. Com-
pounds 9a , 9b, and 10 can, in principle, undergo both
reactions. It is suggested that 1,2-H(Ph) shifts in excited-
state 1,2-diphenyldiazoalkanes account for approximately
0-30% of the products observed and that 1,2-H(Ph)
migrations from the carbenes compete with diffusion-
controlled trapping when the rate of the 1,2-rearrange-
ment is increased by polar solvents24 and “bystander”
substituents at the carbon bearing the migrating group.11,37
2), which was originally suspected in gas-phase studies
over 30 years ago,22 has been established in several
cases.17,23-25 This information suggests the revision and
refinement of some kinetic schemes involving reactions
that have previously been exclusively assigned to the
carbene (Scheme 1, path 1).
In particular, work at UCLA has recently addressed
the photochemistry of 1,2-diphenyldiazoalkanes such as
1 (Scheme 2) in several solvents,26,27 and in the crystalline
state.28 Arylalkylcarbenes 2 produced from 1 in glassy
matrices and in crystals give EPR signals characteristic
of carbenes with a triplet ground state.29-31 It is known
that product selectivities from diazo compounds such as
1 are highly susceptible to subtle changes in experimental
conditions, which has been interpreted in terms of
carbene processes, i.e., following path 1 in Scheme 2.
Whereas product selectivities in the liquid phase depend
strongly on solvent and temperature, reactions in the
highly rigid and organized environment of a crystalline
diazo precursor can be highly selective.15,28,31 Using
Scheme 2 as analyzed in detail below, we drew some
interesting conclusions regarding the effects of solvent
polarity on the singlet-triplet energy gap of arylalkyl-
carbenes such as 2.26,27,32 Our studies spanned solvents
from pentane to methanol and assumed that a free spin-
equilibrated carbene 2 may undergo intramolecular 1,2-H
shifts and 1,2-Ph migrations to give products 3 and 4 in
competition with bimolecular reactions with alcohols to
give ethers such as 5. Although it had been known for
some time that singlet halocarbenes33 and thermally
populated singlet diarylcarbenes34,35 react with MeOH at
diffusion-controlled rates (i.e., kMeOH ≈ 0.5 × 1010 M-1 s-1),
it was assumed that 1,2-H shifts giving rise to 3 were
Resu lts
(22) Frey, H. M.; Stevens, I. D. R. J . Chem. Soc. 1965, 1701-1705.
(23) Fox, J . M.; Guillen Scacheri, J . E.; J ones, K. G. L.; J ones, M.,
J r.; Shelvin, P. B.; Armstrong, B.; Sztyrbicka, R. Tetrahedron Lett.
1992, 33, 5021-5024.
P r ep a r a tion of Dia zo Com p ou n d s a n d P r od u ct
Ch a r a cter iza tion . All the diazo compounds used in this
study were obtained in high purity from the analogous
aryl ketone hydrazones by oxidation with yellow HgO and
base.38 Compounds 7, 8, and 9a were prepared from
commercially available acetophenone, isobutyrophenone,
and deoxybenzoin, respectively. Aryl ketones used to
prepare compounds 10 and 11 were obtained by mono-
and dialkylation of deoxybenzoin with MeI by conven-
tional methods.39 The R,R-dicyclohexylacetophenone de-
rivative 9b was synthesized as shown in Scheme 3,
starting from R,R-dicyclohexylacetic acid (12). The aceto-
phenone chromophore was selected as an intramolecular
sensitizer for triplet excitation experiments, and the two
(24) Celebi, S.; Leyva, S.; Modarelli, D. A.; Platz, M. S. J . Am. Chem.
Soc. 1993, 115, 8613-8620.
(25) Liu, M. T. H. Acc. Chem. Res. 1994, 27, 287-294.
(26) Garcia-Garibay, M. A. J . Am. Chem. Soc. 1993, 115, 7011-
7012.
(27) Garcia-Garibay, M. A.; Theroff, C.; Shin, S. H.; J ernelius, J .
Tetrahedron Lett. 1993, 52, 8415-8418.
(28) Shin, H. S.; Keating, A. E.; Garcia-Garibay, M. A. J . Am. Chem.
Soc. 1996, 118, 7626-7627.
(29) Trozzolo, A. M.; Wasserman, E. In Structure of Arylcarbenes;
Moss, R. A., J ones, M. J ., Eds.; J ohn Wiley & Sons: New York, 1975;
Vol. 2, pp 185-206.
(30) Tomioka, H.; Hayashi, N.; Izawa, Y.; Senthilnathan, V. P.; Platz,
M. S. J . Am. Chem. Soc. 1983, 105, 5053-5057.
(31) Shin, S. H.; Cizmeciyan, D.; Keating, A. E.; Khan, S. I.; Garcia-
Garibay, M. A. J . Am. Chem. Soc. 1997, 119, 1859-1868.
(32) Motschiedler, K. R.; Toscano, J . P.; Garcia-Garibay, M. A.
Tetrahedron Lett. 1997, 38, 949-952.
(33) Kirmse, W. In Advances in Carbene Chemistry; Brinker, U. H.,
Ed.; J AI Press Inc.: New York, 1994; Vol. 1, pp 1-57.
(34) Griller, D.; Nazran, A. S.; Scaiano, J . C. J . Am. Chem. Soc. 1984,
106, 198-202.
(35) Eisenthal, K. B.; Turro, N. J .; Sitzmann, E. V.; Gould, I. R.;
Hefferon, G.; Langan, J .; Cha, Y. Tetrahedron 1985, 41, 1543-1554.
(36) Sugiyama, M. H.; Celebi, S.; Platz, M. S. J . Am. Chem. Soc.
1992, 114, 966-973.
(37) Nickon, A. Acc. Chem. Res. 1993, 26, 84-89.
(38) Smith, L. I.; Howard, K. L. Org. Synth. 1955, 3, 351-352.
(39) Pomerantz, M.; Witherup, T. H. J . Am. Chem. Soc. 1973, 95,
5977-5988.

Photochemistry of Arylalkyldiazo Compounds
J . Org. Chem., Vol. 64, No. 14, 1999 5141
Sch em e 2
Sch em e 3
attempts at analyzing the selectivity of the cis- and trans-
stilbene products because photoisomerization occurs
under the reactions conditions. Compound 11 has no
R-hydrogen and, as expected, gives 1,1,-diphenyl-2-meth-
ylpropene from a 1,2-Ph migration as the main observ-
able product (>98%).
Dir ect a n d Sen sitized Ir r a d ia tion s in Meth a n ol.
Although little is known about diazo compound photo-
physics, it is thought that the triplet excited diazo
compound will form the triplet carbene without concerted
1,2-R reactions such as those observed in the singlet
excited-state reactions. It is also expected that neither
the triplet excited-state diazo nor the triplet carbene will
react with singlet carbene traps such as methanol or
pyridine.26,40 As indicated on the right side of Scheme 2,
the triplet carbene may be reached by excitation and
intersystem crossing of a suitable sensitizer such as
benzophenone, which then transfers triplet energy to the
diazo compound.20,41-43 Most compounds studied were
sensitized with benzophenone in the spectral window
between 360 and 400 nm where there is little absorption
by the aryldiazo chromophore.32 To investigate the pos-
sibility of interference by the intermolecular sensitizer,
we analyzed compound 9b with an acetophenone unit
built into the molecule (Scheme 3). Direct irradiation of
all compounds was carried out in the UV region at λ >
290 and at the weak diazo n,π* transition at λ > 450 nm
where the aryl ketone chromophore in 9b cannot absorb.
The results in Table 1 give the yields of products from
1,2-H shifts, 1,2-Ph migrations, and methyl ether forma-
tion under the three different excitation conditions.
bulky cyclohexyl groups were added to protect it from
the reaction conditions used to prepare the aryldiazo
chromophore.
Denitrogenation products were obtained by steady-
state irradiation of compounds 7-11 in dilute (2-5 mM)
deoxygenated benzene solutions with λ > 290 nm. Ir-
radiation under these conditions results in almost ex-
clusive excitation of the very intense π,π* transition of
the diazo chromophore. Products obtained in benzene are
consistent with those reported in the literature, which
may be construed in terms of carbene 1,2-H shifts and
1,2-Ph migrations. Azines formed by reaction of the
carbene and diazo compound were not detected in sig-
nificant amounts. Product yields were quantified by gas
chromatography, and their retention times were estab-
lished by co-injection with authentic samples. As indi-
cated in Table 1, compounds 7 and 8 lead to styrene and
2,2-dimethylstyrene, respectively, from formal 1,2-H
shifts.24 Irradiation of compounds 9a and 9b led to the
corresponding stilbenes from formal 1,2-H shifts. Despite
having one R-aryl group, only traces (<1-2%) of 1,1-
diarylethanes from 1,2-Ar migrations were observed. As
reported by Tomioka,30 compound 10 undergoes concur-
rent 1,2-H shifts and 1,2-Ph migrations in 74% and 26%
yields, respectively. In the case of 9 and 10, we made no
As shown in Table 1, direct irradiation of compounds
7-11 in MeOH yields the methyl ethers from RO-H
insertion reactions as the main products in competition
with the alkene products observed in benzene. Alkenes
were observed from diazo compounds possessing either
or both 1,2-H and 1,2-Ar migration pathways. That
triplet sensitization occurs cleanly under the conditions
(40) Griller, D.; Nazran, A. S.; Scaiano, J . C. Tetrahedron 1985, 41,
1525-1530.
(41) Kopecky, K. R.; Hammond, G. S.; Leermakers, P. A. J . Am.
Chem. Soc. 1961, 83, 1397-1398.
(42) Kopecky, K. R.; Hammond, G. S.; Leermakers, P. A. J . Am.
Chem. Soc. 1962, 84, 1015-1019.
(43) Moritani, I.; Yamamoto, Y.; Murahashi, S.-I. Tetrahedron Lett.
1968, 5697-5701.

5142 J . Org. Chem., Vol. 64, No. 14, 1999
Motschiedler et al.
Ta ble 1. P er cen t Yield s fr om Stea d y-Sta te P h otolysis of Ar yld ia zo Com p ou n d s 7-11 in Ben zen e a n d u p on Dir ect a n d
Sen sitized Ir r a d ia tion s in Meth a n ola ,b
used is suggested by the very similar (within experimen-
Sch em e 4
tal error) results with compounds 9a and 9b, and we
conclude that there are no complications when the
reaction is sensitized in the presence of 50 mM benzo-
phenone. Quantitative phosphorescence quenching of the
triplet aryl ketone in 9b by the diazo chromophore in
rigid glasses at 77 K indicates that intramolecular energy
transfer occurs efficiently. Significant differences in
product yields for a given diazo precursor were observed
not only as a function of the multiplicity of the excited
state of the diazo compound, determined by direct and
sensitized irradiations, but also as a function of irradia-
tion wavelength. An increase of about 10% in the yield
of ethers is observed for compounds 7-9a when the
excitation wavelength changes from that of the π,π*
transition in the UV (λ > 290 nm) to that of the n,π*
transition in the visible (λ > 460 nm). A further increase
in the yield of ethers when the reaction is carried out
with triplet sensitization is strong evidence for 1,2-H
reaction from the singlet state of the diazo compound.
In the case of 1-phenylethylidene, for example, the yield
of 1-phenyl-1-methoxyethane increases from 77% at λ >
reactions as shown in Scheme 4. The fraction of 1,2-H
shift products via Path 2, x, can be calculated relative to
the fraction of products via Path 1, or 1 - x, by assuming
that Paths 1 and 3 give identical yields from a spin-state
equilibrated carbene. The yields of 1,2-H shifts and
MeO-H insertion from the “true carbene” in neat metha-
nol are known from the sensitized experiments and may
290 to 87% at λ > 450 to as much as 94% when the
experiment is conducted with benzophenone sensitiza-
tion. Changes in product ratios as a function of wave-
length are smaller for compounds 10 and 11. In the case
of compound 10, the ratio of products from 1,2-H and 1,2-
Ph increases from 2.84 in benzene to 4.67 in MeOH at λ
> 290 to 6.0 in MeOH upon triplet sensitization. Com-
be labeled a and b, respectively. These can be used to
pound 11 undergoes a 1,2-Ph migration in competition
describe the ratio y of 1,2-H and insertion products
with the MeO-H insertion. The yield of ether increases
obtained upon direct irradiation.
from ∼45% upon direct photolysis to 68% when the
With y, a, and b known from direct and sensitized
experiments, respectively, solution for x gives the fraction
of product from the excited diazo. As indicated in Table
1 in the column labeled Path 1, the percentage of carbene
calculated in this manner for compounds 7-11 ranges
between 100% (x ) 1.0) for 9b to 60-70% (x ) 0.6-0.7)
for compound 10. Although the extent of the excited-state
reaction is greater at the shorter wavelengths for com-
pounds 7-9, the wavelength dependence of compounds
10 and 11 is comparatively smaller.
reaction is triplet sensitized.
The relative contributions to product formation from
excited-state and carbene reactions may be estimated
with the assumption that triplet sensitization experi-
ments represent the true reactivity of the free carbene
intermediate. For this analysis, it is assumed that the
singlet carbene can achieve a steady-state concentration,
and the total yield is set equal to the sum of all the
products, which are expressed as fractions. The simplest
case is that of compounds possessing only singlet-state

Photochemistry of Arylalkyldiazo Compounds
J . Org. Chem., Vol. 64, No. 14, 1999 5143
Ta ble 2. Rela tive In ten sities a n d λm a x of P yr id in e Ylid es
Obser ved by LF P a
F igu r e 1. Transient absorption spectra of compounds 7, 9a ,
and 9b by LFP in Freon-113 in the presence of pyridine.
from experiments carried out with 2-methyl-1-phenyl-
diazopropane 8, even after addition of 2-5 M pyridine.
Measurements with compounds 9a , 9b, 10, and 11 under
identical conditions also gave weak ylide signals with
their λmax displaced to shorter wavelengths (Table 2).
Whereas ylides derived from compounds 9a and 10 have
a unique λmax at 430 nm, the transient from compound
9b has a second maximum at 350 nm, which may be
attributed to triplet-triplet transitions involving the aryl
ketone chromophore (see Figure 1). Compound 11 showed
no absorption in the 450 nm range upon laser flash
photolysis and has a single band with a λmax of 355 nm
that is very similar to that previously observed in the
ylide formed by trapping phenyl-tert-butyl carbene.24 It
has been pointed out that this unusually short wave-
length of absorption may be the result of a strong
perturbation of the ylide chromophore by the severe steric
hindrance of the neighboring tetrasubstituted carbon.
La ser F la sh P h otolysis in th e P r esen ce of P yr i-
d in e. Although arylalkylcarbenes have no useful spec-
troscopic transitions for direct detection by laser flash
photolysis, they form strongly absorbing ylides (6, Scheme
2) upon reaction with pyridine.19,44 Ylides formed from
sterically unencumbered arylalkyl carbenes such as
1-phenylethylidene have a broad absorption band with
a λmax at 450 nm (Figure 1).17,36 It is known that pyridine
does not react with the excited state of the diazo precur-
sor or with the triplet carbene, but it reacts with many
singlet carbenes at near diffusion controlled rates. One
of the most useful experimental parameters in the
analysis of carbenes by the pyridine ylide method is the
maximum yield of ylide obtained at high concentrations
of pyridine. The yields of ylide determined for a series of
analogous compounds is proportional to the efficiency of
carbene formation, provided that the extinction coefficient
for ylide absorption does not vary within the series. Ylide
yields may be used to determine the extent of excited-
state reactions competing with carbene formation when
the corresponding carbenes are sufficiently long-lived and
the extinction coefficients of their ylides are very similar.
This strategy was attempted with compounds 9-11 in
parallel experiments with compounds 7 and 8, which had
been measured previously.24
The maximum intensities of the transient absorption
at high concentration of pyridine, AY∞, obtained from
compounds 8-11 in the 430-450 nm range were used
to estimate the yields of ylide at high concentrations of
pyridine. The yields of ylide measured for compounds 7
and 8 were consistent with those reported in the litera-
ture.24 By comparing the AY values from compounds
∞
9-11 to those of 7 and 8 obtained under identical
conditions, we were able to compute their corresponding
trappable carbene yields (Table 2). If these values were
taken as the absolute yields of carbene, one would
conclude that 1,2-diphenyldiazoalkanes give yields of 1,2-
diphenylethylidenes that are systematically lower than
the yields of carbenes obtained with phenyldiazoalkanes
by about 30%. For this conclusion to be correct, it is
necessary that 1,2-H shifts in 9-11 are slow compared
with bimolecular trapping by pyridine. However, it is
shown below that 1,2-H shifts do seem to compete with
bimolecular trapping in the presence of MeOH, a polar
solvent which is expected to accelerate 1,2-H shifts.
Nanosecond laser flash photolysis experiments were
carried out with samples having an OD between 0.5 and
0.9 at an excitation wavelength of 308 nm from a XeCl
excimer laser (150 mJ , 18 ns) in Freon-113 solvent. As
reported in ref 31, addition of pyridine to 1-phenyldiazo-
ethane 7 gave a strong ylide signal with λmax at 450 nm
that reached saturation at about 2 M pyridine in Freon-
113. Also as reported, a much weaker signal was detected
(44) J ackson, J . E.; Soundararajan, N.; Platz, M. S.; Liu, M. T. H.
J . Am. Chem. Soc. 1988, 110, 5595-5596.

5144 J . Org. Chem., Vol. 64, No. 14, 1999
Motschiedler et al.
F igu r e 2. LFP of 9b detected at 300 nm upon 266 nm
excitation in pure Freon-113 (top trace) and with 6 M MeOH
(bottom trace).
F igu r e 3. Stern-Volmer plot of the maximum absorbance of
stilbene from compound 9b with MeOH as a quencher.
Sch em e 5
in the absence of MeOH suggests that the elementary
rate constant for the 1,2-H shift may be large enough to
contribute to the fast component.
The slow component (τ ) 1/kslow ) 70-80 ns) may be
attributed to stilbene formation from a spin-state equili-
brated free carbene which, in the presence of MeOH, will
have a rate constant given by
kslow ) KST k1,2-H + KST kMeOH[MeOH]
(1)
As indicated in Scheme 5, KST is the singlet-triplet
equilibrium constant defined as KST ) [S]/[T], k1,2-H is
the elementary rate of 1,2-H shifts from the singlet
carbene, and kMeOH is the bimolecular rate constant for
MeOH trapping. Both KST and k1,2-H will likely vary with
methanol concentration.26,27,36 The first term in eq 1 is
responsible for the 70-80 ns lifetime (1/kslow) of the long-
lived component, and the second term is responsible for
its quenching by added MeOH. Although precise rate
constants upon addition of MeOH were difficult to fit, a
Stern-Volmer analysis with the absorbance values of the
slow component from 9b gave linear plots with kqτ values
of 0.21 M-1 s-1 (Figure 3). If one assumes diffusion-
controlled values for singlet carbene MeOH insertion (i.e.,
kq ) kMeOH ≈ 109 M-1 s-1), one may calculate the lifetime
of the singlet carbene to be τS ≈ 0.21 ns in Freon-113 at
ambient temperature, assuming that KST and k1,2-H do
not vary with methanol concentration. This lifetime
reflects the time “spent” periodically by the spin-
equilibrated carbene in the singlet state, which is the
time during which it is available for reaction with MeOH
in Freon as a solvent. This lifetime is determined by the
rates of intramolecular reaction and singlet-to-triplet
intersystem crossing as given by eq 2:
Therefore, we were prompted to question the validity of
the assumption that the 1,2-H shifts did not compete with
pyridine trapping in Freon-113 as well.
F la sh P h ot olysis Mea su r em en t s of St ilb en e
Gr ow th in Com p ou n d s 9a a n d 9b. Using a XeCl
excimer laser as the excitation source,45 we were able to
detect the real-time growth of stilbene from compounds
9a and 9b in the 290-300 nm range (Figure 2). With
absorbances as high as 0.25 OD units, time constants for
stilbene growth in Freon-113 were calculated to be ca.
70 and 80 ns from 9a and 9b, respectively. These values
reflect the lifetimes of the diazo precursor, the free
carbene, and/or the excited diazo, as limited by their
intramolecular 1,2-H reactions. Experiments run with 9b
upon addition of up to 6 M MeOH and in the pure alcohol
solvent quenched only 60% of the total signal, uncovering
a nontrappable reaction path that forms stilbene within
the time resolution of the instrument, as limited by the
20 ns laser pulse (Figure 2).
Laser flash photolysis experiments at 308 nm with 9b
promote transitions of the aryl ketone and aryldiazo
groups so that concurrent singlet and triplet excited-state
chemistry may be expected. The fast component may be
attributed a priori to the singlet excited-state reaction,
labeled kH* in Scheme 5, and the slow component may
be attributed to the reaction from the spin-equilibrated
carbene. However, an observed time constant for stilbene
growth of 80-90 ns in Freon-113 for the slow component
τS ) 1/(k1,2-H + kST) ) 0.21 ns
(2)
The value of 0.21 ns for the lifetime of the singlet carbene
constitutes an upper limit for the time constant of the
singlet-state 1,2-H shift, which gives a lower limit of
k1,2-H > 5 × 109 s-1 in Freon-113 at ambient temperature.
However, it is important to note that methanol, being
polar, may reduce the singlet-triplet separation26,27 and
increase the absolute rate constant k1,2-H relative to
Freon-113.36 Thus, the magnitude of k1,2-H > 5 × 109 s-1
for carbene 15 deduced in Freon-113 containing methanol
may be substantially larger than the value of this rate
constant in a nonpolar solvent such as pure Freon-113.
Previously,36 we reported a k1,2-H < 6 × 106 s-1 for
carbene 16 derived from 7 in heptane. We attribute the
large difference in k1,2-H values for carbenes 15 and 16
to the polar solvent and to the presence of the second
(45) Soundararajan, N.; J ackson, J . E.; Platz, M. S.; Liu, M. T. H.
Tetrahedron Lett. 1988, 29, 3419-3422.

Photochemistry of Arylalkyldiazo Compounds
J . Org. Chem., Vol. 64, No. 14, 1999 5145
phenyl group which weakens the migrating C-H bond
and stabilizes a developing positive charge at the alpha
carbon.
excited states of the precursor is possible and cannot be
strictly eliminated. However, the most likely candidate
for the noncarbene precursor is the excited state of the
precursor.
With respect to the generality of the wavelength and
multiplicity dependence of the product yields, although
excited-state 1,2-H and 1,2-alkyl migrations have been
postulated before, our results with compounds 10 and 11
are the first experimental evidence suggesting the oc-
currence of excited-state 1,2-Ph migrations. Compound
9b was the only example for which no differences in
product yields were observed upon direct and sensitized
irradiations. By accessing the carbene surface from the
singlet and the triplet state, this compound strongly
suggests that a 1,2-H shift and a MeOH insertion in pure
methanol may occur concurrently from a spin-state equili-
brated (or nearly spin-equilibrated) carbene. The lack of
multiplicity dependence for 9b probably comes from
perturbations brought about by the ketone chromophore.
Although the UV spectrum of 9b is almost a perfect
match with the summed spectra of R,R-dicyclohexyl
acetophenone (12) and 1,2-diphenyldiazoethane (9a ),
there may be enough mixing to modify the excited-state
reactivity of the diazo chromophore. Interestingly, the
similarity of the chemical results from sensitized irradia-
tions in compounds 9a and 9b indicates that perturba-
tions to the singlet carbene by the aryl substituent are
not very important.
Unimolecular reaction rates of 5 × 109 s-1 in methanol
can compete efficiently with diffusion-controlled and
pseudo-unimolecular reactions. Thus, we conclude that
the fast component of stilbene formation in methanol, and
perhaps in Freon-113, will contain contributions not only
from an excited-state reaction but also from the singlet
carbene before it undergoes intersystem crossing. This
result indicates that kinetic resolution of excited diazo
and carbene dynamics will require analysis in the
picosecond time scales.
Discu ssion
Differences in direct and sensitized photochemical
experiments are not consistent with a common precursor
for product formation under all experimental conditions.
Because we expect that the triplet-state surface of the
diazo compound should be dissociative and have little
access to stable closed-shell molecules, we postulate that
triplet sensitization is devoid of excited-state reactions
other than denitrogenation to give a free carbene inter-
mediate. We propose that results from sensitization
experiments in Table 1 represent the true reactivity from
a free carbene intermediate.
Turning our attention to direct irradiations, it is
interesting to note that differences in product ratios as
a function of irradiation wavelength are rare for poly-
atomic molecules in condensed media, as they constitute
a violation to the Kasha-Vavilov rule.46 The rule states
that all photochemical processes are independent of
excitation wavelength, as they should all take place from
the lowest excited state. Exceptions to the rule occur
when the rates of internal conversion are slow by virtue
of a large energy gap between the electronic-states
involved. Accordingly, in the case of arylalkyldiazo
compounds, there is an energy gap of about 30 kcal/mol
between the n,π* transition in the visible and the π,π*
transition in the UV.
As shown in Table 1, product yields at λ > 450 nm are
closer to those obtained upon sensitization than product
yields obtained with λ > 290 nm. The differences in
product yields as a function of irradiation wavelength
may occur from differences in the efficiency of the excited-
state reaction from S1 and S2, from differences in
intersystem crossing yields and differences in carbene
yields from singlet and triplet excited states, or from
generation of carbenes in different electronic states.
Because it is unlikely that singlet diazo compounds will
undergo intersystem crossing to the triplet state, differ-
ences in product ratios probably do not reflect differences
in carbene yields from singlet and triplet excited states.
The generation of different electronic states of the car-
bene from different electronic configurations in the
The extent of excited-state reaction estimated with the
pyridine ylide method (28%) and by methanol trapping
and triplet sensitization (20%) for compound 7 are in
excellent agreement with each other. However, the extent
of excited-state reaction estimated by MeOH trapping in
the case of compound 8 is much smaller (44%) than that
suggested by the maximum yield of pyridine ylide in
Freon-113 (∼95%). Disagreement may occur if sensitiza-
tion is incomplete, if there are reaction pathways from
the triplet excited state that bypass the carbene (reducing
the carbene yield), or if the carbene reaction with MeOH
were much slower than the reaction with pyridine. We
believe that these factors are unlikely. It is more likely
that the extinction coefficients of the pyridine ylides vary
over the series of compounds studied, as do their absorp-
tion maxima, and overestimate the extent of pathway 2.
It is expected that methyl and phenyl groups at the
migrating origin and polar solvents will accelerate the
carbene 1,2-H shifts37 to the extent that they may become
competitive with intermolecular trapping. In the case of
compounds 9-11, there is no correlation between the
extent of the excited-state reaction estimated by triplet
sensitization in methanol and by the pyridine ylide
method in Freon-113 solvent. While carbene yields
estimated in methanol are high and range between 60%
for 11 to 100% for 9b (Table 1), the corresponding values
with pyridine in Freon-113 are as low as 4% for 11 and
14% for 9b (Table 2). Differences observed with neat
MeOH and pyridine-containing Freon may arise from
differences in solvent polarity, which are known to affect
both the singlet-triplet interconversion rates and equi-
libria, as well as the rate of the singlet state 1,2-R
reactions. It is also possible that ylides from 9-11 may
have somewhat different spectral properties as compared
to those of 7, so that a calculation of their ylide yields
may require a correction by differences in their extinction
coefficients at the wavelength of measurement.
(46) Turro, N. J . Modern Molecular Photochemistry; Benjamin/
Cummings Publishing Co.: Menlo Park, 1978.

5146 J . Org. Chem., Vol. 64, No. 14, 1999
Motschiedler et al.
pump-thaw cycles or by bubbling Ar through the sample for
at least 20 min prior to irradiation with a 400 W medium-
pressure Ace-Hanovia Hg arc lamp. A 150 W PTI medium-
pressure Hg arc lamp with a monochromator was also used
for some experiments. Samples were immersed in a controlled
temperature bath. Direct irradiations at λ > 290 nm were
carried out with Pyrex filters, and for λ > 470 nm a cutoff filter
was used. A combination of a 360 nm cutoff and a 300-400
nm band-pass from Melles Griot was used for sensitized
irradiation when not using the PTI lamp. Samples were
irradiated until the characteristic pink color of the diazo
compound had completely disappeared, or when necessary,
they were quenched with a small amount of dimethylacetylene
dicarboxylate (DMAD) to remove unreacted diazo. The samples
were analyzed in duplicate by GLC, and the average values
are reported.
Gen er a l P r oced u r e for Syn th esis of Hyd r a zon es. All
hydrazones were prepared from the corresponding ketone,
which was either purchased or synthesized by methods
described below. In a typical reaction, the ketone is heated to
reflux in EtOH. A slight excess of hydrazine is added slowly,
and the reaction is allowed to reflux for 1-12 h. Reaction
progress is monitored by IR, TLC, and/or GC, and more
hydrazine may be added as needed until the reaction is
complete. The solution volume is reduced and then cooled to
induce crystallization of the hydrazone. The white crystals are
filtered and washed several times with cold EtOH, followed
by recrystallization from CHCl3/EtOH if needed.
Gen er a l P r oced u r e for Syn t h esis of Dia zo Com -
p ou n d s. Diazo compounds were prepared from the appropri-
ate hydrazone by HgO oxidation. In a typical reaction, 10-25
mg of the hydrazone is suspended in 25 mL of pentane, along
with 5 equiv of MgSO4 and a large excess of HgO. Several
drops of anhydrous concentrated KOH/EtOH are added, and
the reaction is allowed to stir in the dark at room temperature
for several hours. Reaction progress is monitored by IR and/
or GC. When the reaction is complete, the pale pink pentane
solution is filtered several times to remove all traces of
mercury, and solvent is removed to give dark pink fluid or
crystals. A quantitative yield is assumed. Samples are stored
in the dark at 0 °C.
Con clu sion s
Although some of the assumptions involved in the
triplet sensitization experiments and the methanol trap-
ping rates will have to be confirmed, interpretation of
the data in terms of an excited-state reaction (∼30%) and
competition between carbene 1,2-H(Ph) shifts and diffu-
sion-controlled trapping reactions seems to be very
robust. This interpretation is supported by the well
documented role of substituents at the migrating origin,
which have a very strong accelerating effect on the rates
of 1,2-H shifts and 1,2-Ph migrations. For instance,
whereas methylchlorocarbene has a 1,2-H-limited life-
time of 330 ns47 (or ∼700 ns48), the lifetimes of propyl-
chlorocarbene,48 benzylchlorocarbene49 and R-methylben-
zylchlorocarbene47 are reduced to 17, 18, and 2 ns,
respectively. The combined effect of a phenyl and a
methyl group at the migrating origin accelerate the 1,2-H
shift by a factor of ca. 150. Transition-state energies with
zero point energy corrections calculated at the B3LYP/
6-311**G//B3LYP/631G* level recently published11 ac-
count well for these rate differences and confirm the
importance of stabilizing a developing positive charge at
the migrating origin. The same conclusion was reached
experimentally from the effect of solvents on the rate of
the reaction.36 The rate of 1,2-H shift in phenylethylidene
is increased by more than a factor of 30, from k1,2-H < 6
× 106 s-1 in hydrocarbons to 2 × 108 s-1 in acetonitrile.
To conclude, although 1,2-H shifts in 1-phenylethylidene
are fairly slow, it is reasonable that rates of 1,2-H shifts
in 1,2-diphenylethylidenes in methanol should have rate
constants in the subnanosecond regime when the effect
of the phenyl group at position 2 (∼150-fold) and the
effect of solvent polarity (∼30-fold) are taken into ac-
count.
Exp er im en ta l Section
Syn th esis of Com p ou n d s 7, 8, 9a , a n d 10. The prepara-
tion of these compounds has been previously reported.25,32
Syn th esis of Com p ou n d 9b. Compound 9b was prepared
as shown in Scheme 3. The preparation and characterization
of all compounds is described below.
Gen er a l. UV spectra were obtained on a Varian Cary 2300
spectrophotometer and a Beckman DU 650 spectrophotometer.
GC data was obtained on a Hewlett-Packard 5890 Series II
gas chromatograph, with an HP 3396 Series II integrator. An
HP-1 cross-linked methyl silicone gum column and an HP-
20M Carbowax 20M column were used. Both are 25 m × 0.20
mm, with a 0.20 µm film thickness. A Spex Fluorolog spec-
trofluorimeter with double grating monochromator was used
to obtain fluorescence and phosphorescence spectra. Front
surface illumination and detection was used. Phosphorescence
measurements used a pulsed lamp with a 10 µs pulse width.
A custom sample holder held a Dewar with a Pyrex cold finger
to keep the samples at 77 K during the analysis. The
nanosecond laser flash photolysis apparatus at The Ohio State
University has been described in detail in ref 18.
Ma ter ia ls. Dicyclohexylacetic acid (99%), deoxybenzoin
(97%), and isobutyrophenone (97%) were purchased from
Aldrich. Acetophenone, benzaldehyde, and toluene (Certified
A.C.S.) were purchased from Fisher. All commercial reagents
were of the highest grade available. Solvents were dried
according to accepted literature methods and were kept over
4 Å molecular sieves.
2,2-Dicycloh exyl-1-(4′-m eth yl)-p h en yleth a n on e. A mix-
ture of dicyclohexylacetic acid (2.5 g, 11.1 mmol), and 3.3 g
(2.0 mL, 27.8 mmol) of thionyl chloride was refluxed in 25 mL
methylene chloride for 1.5 h. The solvent and unreacted SOCl2
were removed by rotary evaporation. The resulting acid halide
was dissolved in 20 mL of toluene. A 2.5 g (18.7 mmol) portion
of AlCl3 was added slowly, and the reaction was stirred for 1
h. The reaction was quenched by pouring into several volumes
of water, and the aqueous layer was extracted twice with ether.
The combined organic layers were washed twice with water
and dried over MgSO4. Removal of the solvent gave a yellow
oil. Crystallization from pentane gave 2.4 g (72%) in two crops
of white crystals, mp 55-60 °C. 1H NMR (CDCl3, 200 MHz) δ:
0.76-1.32 (m, 12H), 1.62-1.94 (m, 10H), 2.40 (s, 3H), 3.28 (t,
J ) 7.3 Hz, 1H), 7.24 (d, J ) 8.2 Hz, 2H), 7.87 (d, J ) 8.2 Hz,
2H). 13C NMR (CDCl3, 90 MHz) δ: 21.53, 26.44, 26.51, 26.76,
29.81, 31.85, 37.83, 55.73, 128.26, 129.16, 137.73, 143.31,
205.08. FTIR (KBr pellet) cm-1: 1663. EI HRMS: calculated
for C21H30O 298.2297, found 298.2288.
2,2-D ic y c lo h e x y l-1-(4′-b r o m o m e t h y l)-p h e n y le t h a -
n on e (13).33 The Friedel-Crafts adduct (1.50 g, 5.03 mmol),
1.34 g (7.55 mmol) of N-bromosuccinimide (NBS), and 0.12 g
(0.50 mmol) of benzoyl peroxide were dissolved in 15-25 mL
of CCl4. The solution was heated to vigorous reflux for up to
24 h, until the reaction showed no further progress by TLC
and GC. The reaction mixture was cooled and filtered to
remove solids, and then solvent was removed to give an oil
which was 80% pure by GC analysis. Flash chromatography
was used to purify the mixture, but some ketone starting
Gen er a l P r oced u r e for An a lytica l P h otolysis. Solutions
for photolysis were prepared by dissolving the diazo compound
in MeOH to make a 5 mM solution. For sensitized experiments,
benzophenone was added to make the solutions 25 mM in
sensitizer. Samples were deoxygenated by 5-10 freeze-
(47) Liu, M. T. H.; Bonneau, R. J . Am. Chem. Soc. 1996, 118, 8098-
8101.
(48) LaVilla, J . A.; Goodman, J . L. J . Am. Chem. Soc. 1989, 111,
6877-6878.
(49) Liu, M. T. H.; Bonneau, R. J . Am. Chem. Soc. 1990, 112, 3915-
3919.

Photochemistry of Arylalkyldiazo Compounds
J . Org. Chem., Vol. 64, No. 14, 1999 5147
1
material remained. H NMR (CDCl3, 200 MHz) δ: 0.80-1.23
MHz) δ: 26.42, 26.49, 26.74, 29.85, 31.85, 37.88, 45.19, 55.93,
128.49, 128.56, 128.75, 129.05, 129.72, 133.44, 136.41, 139.38,
196.87, 205.24. FTIR (thin film) cm-1: 1728, 1669. EI HRMS:
calculated for C28H34O2 402.2559, found 402.2561.
(m, 12H), 1.53-1.83 (m, 10H), 3.26 (t, J ) 7.3 Hz, 1H), 4.48
(s, 2H), 7.44 (d, J ) 8.2 Hz, 2H), 7.91 (d, J ) 8.2 Hz, 2H). 13C
NMR (CDCl3, 90 MHz) δ: 26.39, 26.47, 26.73, 29.82, 31.85,
32.23, 37.88, 56.09, 128.60, 129.15, 139.86, 142.13, 204.88.
1-(2-P h en yl-2-oxoeth ylh yd r a zon e)-4-(2,2-d icycloh exyl-
1-oxoeth yl)-ben zen e. Diketone 14 (0.05 g, 0.12 mmol) was
suspended in 5 mL of dry ethanol under nitrogen. The reaction
mixture was heated slightly until the diketone dissolved, 0.04
mL (1.27 mmol) of hydrazine was added, and the solution was
refluxed for 4-8 h. The reaction mixture was cooled to 0 °C
with ice for several hours, and the product crystallized out of
solution. Filtration and washing with cold ethanol of two crops
followed by drying under vacuum yielded 0.047 g (90%) of off-
white crystals, mp 147-150 °C. 1H NMR (CDCl3, 200 MHz) δ:
0.67-1.18 (m, 12H), 1.54-1.78 (m, 10H), 3.15 (t, J ) 7.3 Hz,
1H), 3.99 (s, 2H), 5.30 (broad s, 2H), 7.30 (m, 5H), 7.61 (m,
2H), 7.81 (d, J ) 8.3 Hz, 2H). 13C NMR (CDCl3, 90 MHz) δ:
26.40, 26.47, 26.73, 29.83, 31.85, 32.16, 37.86, 55.95, 125.55,
128.09, 128.33, 128.50, 128.94, 138.74, 138.95, 140.41, 147.76,
204.95. FTIR (thin film) cm-1: 3393, 1669. EI HRMS: calcu-
lated for C28H36ON2 416.2828, found 416.2840.
1-(2-P h en yl-2-d ia zoet h yl)-4-(2,2-d icycloh exyl-1-oxo-
eth yl)-ben zen e (9b). Hydrazone (0.04 g, 0.096 mmol) was
suspended in 25 mL of pentane, along with 0.06 g (0.5 mmol)
of MgSO4 and 1.0 g (4.62 mmol) of HgO. Several drops of
concentrated KOH/EtOH were added, and the reaction was
allowed to stir for 2 h. Then, 2 µL of hydrazine was added to
catalyze the reaction. The reaction was monitored by IR and
GC, and when no hydrazone remained the pale pink pentane
solution was filtered through a glass microfiber NMR filter to
remove all traces of mercury. Solvent was removed to give dark
pink crystals. A quantitative yield is assumed. 1H NMR
(CDCl3, 360 MHz) δ: 0.67-1.15 (m, 12H), 1.45-1.80 (m, 10H),
3.13 (t, J ) 7.3 Hz, 1H), 3.78 (s, 1H), 6.81 (m, 2H), 7.20 (m,
5H), 7.78 (d, J ) 8.3 Hz, 2H). 13C NMR (CDCl3, 90 MHz) δ:
26.41, 26.48, 26.74, 29.83, 30.99, 31.84, 37.86, 56.01, 121.38,
123.65, 128.32, 128.34, 128.66, 130.98, 139.06, 139.40, 142.44,
205.06. IR (thin film) cm-1: 2041, 1673, 1605.0, 1446.8, 1234.6,
993.5.
FTIR (thin film) cm-1: 1674. EI LRMS: calculated for C21H29
-
OBr 376.1, found 294.0 (M+ - C5H11).
1-(2-P h en yl-2-oxoeth ylth ioa ceta l)-4-(2,2-d icycloh exyl-
1-oxoeth yl)-ben zen e. A solution of 2-phenyl-1,3-dithiane
(0.74 g, 3.79 mmol) in 10 mL of dry THF under nitrogen was
cooled to -40 °C. A solution of n-butyllithium in hexane (1.60
mL, 4.17 mmol) was added slowly. After 1 h of stirring at this
temperature, a solution of crude (65% by GC) bromoketone
21 (0.65 g, 1.72 mmol) in dry THF was added, and the reaction
continued stirring at -40 °C for 2 h. Butyllithium was
quenched by slowly pouring into several volumes of water. The
organic layer was extracted three times with ether and dried
over MgSO4. Solvent was removed to give 1.3 g of yellow oil,
which by GC was 60% pure (0.78 g, 92%). Flash chromatog-
raphy was used to purify the mixture, but 2-phenyl-1,3-
dithiane remained as an impurity. (This accounts for the
1
additional signals in the NMR spectra). H NMR (CDCl3, 200
MHz) δ: 0.76-1.24 (m, 12H), 1.58-1.91 (m, 10H), 2.65 (m,
5H), 3.18 (t, J ) 7.3 Hz, 1H), 3.27 (s, 2H), 6.78 (d, J ) 8.2 Hz,
2H), 7.30 (m, 4H), 7.67 (m, 3H). 13C NMR (CDCl3, 90 MHz) δ:
24.92, 26.42, 26.49, 26.77, 27.52, 29.80, 331.78, 32.09, 37.76,
51.29, 55.95, 59.44, 127.14, 127.18, 127.73, 128.33, 128.72,
129.34, 130.95, 138.79, 139.42, 140.13, 205.30. EI HRMS:
calculated for C31H40OS2 492.2521, found 492.2520.
1-(2-P h en yl-2-oxoeth yl)-4-(2,2-dicycloh exyl-1-oxoeth yl)-
ben zen e (14). The crude thioacetal (0.60 g, 1.05 mmol, 14%
2-phenyl-1,3-dithiane by GC) was suspended in 15 mL of a
9:1 (v/v) solution of methanol/water and heated slightly to
improve solubility. A solution of 0.50 g (2.1 mmol) of HgCl2 in
5 mL of the same solvent mixture and 0.20 g (0.94 mmol) of
HgO were added, and the mixture was refluxed under nitrogen
for 4-6 h. The solution was filtered through a glass microfiber
filter to remove solids, which were then washed with acetone
to remove residual organic products. The methanol/water
solution was condensed, left to stand for several hours, cooled
with ice, and filtered. The diketone (0.26 g, 63%) was obtained
Ack n ow led gm en t. Support by the National Science
Foundation and the Petroleum Research Fund is grate-
fully acknowledged.
1
in two crops, mp 154-157 °C. H NMR (CDCl3, 200 MHz) δ:
0.80-1.24 (m, 12H, H), 1.56-1.86 (m, 10H), 3.26 (t, J ) 7.3
Hz, 1H), 4.32 (s, 2H), 7.33 (d, J ) 8.3 Hz, 2H), 7.54, (m, 3H),
7.91 (d, J ) 8.3 Hz, 2H), 8.00 (m, 2H). 13C NMR (CDCl3, 90
J O990172I