Lifetime of the 1,4-Biradical Derived from Alkyl Phenylglyoxylate Triplets: An Estimation Using the Cyclopropylmethyl Radical Clock

Full text of DOI:10.1021/jo961659j

J . Org. Chem. 1997, 62, 755-757
755
Sch em e 1
Lifetim e of th e 1,4-Bir a d ica l Der ived fr om
Alk yl P h en ylglyoxyla te Tr ip lets: An
Estim a tion Usin g th e Cyclop r op ylm eth yl
Ra d ica l Clock
Shengkui Hu and D. C. Neckers*
Center for Photochemical Sciences,1 Bowling Green
State University, Bowling Green, Ohio 43403
Received August 28, 1996
glyoxylate cannot be trapped by the 5-hexenyl radical is
understandable. This biradical is even shorter lived than
the biradical from phenyl ketones. Radical clocks with
higher rearrangement rates must be used in order to
accomplish this purpose.
There have been several studies on the mechanism and
synthetic utility of the photoreactions of alkyl phenyl-
glyoxylates since they were originally proposed to un-
dergo Norrish Type II photoreactions by one of us
(Scheme 1).2,3 No products resulting from cyclization of
the intermediate 1,4-biradical have been observed, how-
ever. We reasoned3 it to be because of the short lifetime
of this biradical. It is known that an oxygen atom
between the two radical sites accelerates the decay of 1,4-
biradicals.4 Nanosecond laser flash photolysis of alkyl
phenylglyoxylates3 reveals two transient decays with
relatively long lifetimes and are attributed to the triplet
excited state and mandelate radical. No transient ab-
sorption of the 1,4-biradical is detected because it may
be too short-lived and escape detection, or its signal is
blurred by that of two other transients. This study which
follows was undertaken to assess the lifetime of the 1,4-
biradical formed in phenylglyoxylate photoprocesses.
Free radical clocks capable of timing reaction inter-
mediates within a very broad lifetime range are now
available.5 If one or both of the radical centers is
attached to a group that can undergo rapid rearrange-
ment, the lifetime of the biradical can be deduced from
the product distribution and the radical rearrangement
rate if the latter is assumed to be equal to that of an
analogous monoradical. In a pioneering experiment,
Wagner et al. measured the lifetime of the 1,4-biradical
derived from a phenyl ketone triplet using the 5-hexenyl
clock.6 The trapping product, 2-phenyl-2-norbornanol
was obtained, though in low yield, see eq 1.
Cyclopropylmethyl radicals with different substituents
rearrange with rate constants ranging from 107 s-1 to 109
s-1
. These fast rearrangements are thus capable of
monitoring short-lived 1,4-biradicals.4d,9 We therefore
incorporated cyclopropylmethyl groups in phenylglyoxy-
lates in order to study the lifetime of the biradical formed
upon Norrish Type II hydrogen abstraction. R-Keto
esters 1a -c were synthesized and photolyzed. The
lifetime of the intermediate 1,4-biradical is successfully
deduced from the relative amounts of products formed
before and after the cyclopropylmethyl radical rearrange-
ment as well as from its rearrangement rate.
Resu lts
Phenylglyoxylates 1a -c were irradiated in dilute
(0.005 M) benzene solution to avoid competition from
intermolecular hydrogen abstraction processes.3 As shown
in Scheme 2, if biradical 2 undergoes no rearrangement,
normal Norrish Type II products 3-5 derive. When R1
and R2 are something other than hydrogens, two different
rearrangements can occur to biradical 2. Rearrangement
A leads to 1,7-biradical 6 while rearrangement B leads
to 1,7-biradical 7. Rearrangement A is preferred over B
since 6 is more stable than 7. It is thus expected that
the resulting lactone 8 will be the major rearrangement
product.
Compound 8c has been reported before7 as a labile
product (25% yield) when photolysis of 1c is carried out
(3) Hu, S.; Neckers, D. C. J . Org. Chem. 1996, 61, 6407-6415 and
references therein.
(4) (a) Caldwell, R. A.; Majima, T.; Pac, C. J . Am. Chem. Soc. 1982,
104, 629-630. (b) Freilich, S. C.; Peters, K. S. J . Am. Chem. Soc. 1981,
103, 6255-6257. (c) Wagner, P. J .; Meador, M. A.; Park, B.-S. J . Am.
Chem. Soc. 1990, 112, 5199-5211. (d) Wagner, P. J .; J ang, J .-S. J .
Am. Chem. Soc. 1993, 115, 7914-7915.
(5) (a) Griller, D., Ingold, K. U. Acc. Chem. Res. 1980, 13, 317-323.
(b) Newcomb, M.; Glenn, A. G. J . Am. Chem. Soc. 1989, 111, 275-
277. (c) Maillard, B.; Forrest, D.; Ingold, K. U. J . Am. Chem. Soc. 1976,
98, 7024-7026. (d) Mathew, L.; Warkentin, J . J . Am. Chem. Soc. 1986,
108, 7981-7984. (e) Engel, P. S.; Keys, D. E. J . Am. Chem. Soc. 1982,
104, 6860-6861. (f) Engel, P. S.; Keys, D. E.; Kitamura, A. J . Am.
Chem. Soc. 1985, 107, 4964-4875. (g) Bowry, V. W.; Lusztyk, J .;
Ingold, K. U. J . Am. Chem. Soc. 1989, 111, 1927-1928. (h) Adam,
W.; Grabowski, S.; Scherhag, F. Tetrahedron Lett. 1988, 29, 5637-
5640.
A similar attempt using 5-hexenyl radical clock to trap
the 1,4-biradical derived from an alkyl phenylglyoxylate
for synthetic purposes was unsuccessful. Instead, the
Norrish Type II reaction proceeds normally, eq 2.7 The
5-hexenylradical cyclization has a reported rate constant
(6) Wagner, P. J .; Liu, K.-C. J . Am. Chem. Soc. 1974, 96, 5952-
5953.
(7) Kraus, G. A.; Wu, Y. J . Am. Chem. Soc. 1992, 114, 8705-8707.
If alkenes of proper electron density are situated properly, the
intramolecular Paterno´-Bu¨chi reaction dominants. For a timely paper
on the interplay of Norrish Type II and Paterno´-Bu¨chi reactions of
phenylglyoxylate, see Hu, S.; Neckers, D. C. J . Org. Chem., in press.
(8) Carlsson, D. J .; Ingold, K. U. J . Am. Chem. Soc. 1968, 90, 7047-
7055.
of 1 × 105 s-1 5,8
,
so that a 1,4-biradical from a phenyl-
(1) Contribution No. 297 from the Center for Photochemical Sci-
ences.
(2) Huyser, E. S.; Neckers, D. C. J . Org. Chem. 1964, 29, 276-278.
(9) Wagner, P. J .; Liu, K.-C.; Noguchi, Y. J . Am. Chem. Soc. 1981,
103, 3837-3841.
S0022-3263(96)01659-3 CCC: $14.00 © 1997 American Chemical Society

756 J . Org. Chem., Vol. 62, No. 3, 1997
Notes
Sch em e 2
resulting alkene radical being 2.2.15 So the rate constant
of 2a rearranging to cis-6(7)a is 2.9 × 107 s-1 and only
the cis-1,7-biradical can cyclize to the corresponding
seven-membered ring lactone. Considering the 3a /8(9)-
a ratio of 0.79:1, the total rate constant for the decay of
2a to products is 5.4 × 107 s-1. Taking this number and
the quantum yield of disappearance of 1a as 0.88, i.e.,
12% of 2a disproportionates to 1a , the total biradical
Ta ble 1. Qu a n tu m Yield s of Disa p p ea r a n ce of 1 a n d
Rela tive Am ou n t of P r od u cts
1
Φa
3
8
9
1a
1b
1c
0.88
0.87
0.79
0.49
0.04
1b
0.57
0.62
0.39
0.38
a
b
Quantum yield of the disappearance of 1. 8a and 9a are the
same.
decay rate of 2a can be calculated to be 6.1 × 107 s-1
,
to about 70% completion.10 In addition to 8c, we also
observed 9c in the photolysis mixture obtained from of
1c.11 The relative amounts of 3c, 8c, and 9c are obtained
by repeated integration of the representative peaks
(peaks of H*s in Scheme 2) in the 1H NMR spectrum from
a reaction mixture obtained in C6D6. The relative
amounts of products from the photolysis of 1a and 1b12
are similarly obtained, see Table 1. In all cases, NMR
spectra of the reaction mixtures give material balances
better than 95%. We were able to purify 8(9)a through
repeated short, quick silica gel chromatography13 even
though all seven-membered ring lactones such as 8 and
9 have proven to be quite labile under chromatographic
conditions.
which reflects a lifetime of 16 ns for 2a . The cis- and
trans-2-methylcyclopropylmethyl radicals rearrange to
primary and secondary radicals with rate constants
ranging from 2.8 × 108 s-1 to 1.0 × 109 s-1 16
.
Taking an
average rate constant of 4.0 × 108 s-1 for the model
2-methylcyclopropylmethyl radical rearrangement and
the product ratio of 3b/(8b+9b), the lifetime of biradical
2b is estimated to be 5 ns. The rate constant of the model
2-phenylcyclopropylmethyl radical rearrangement is re-
ported to be greater than 1 × 1011 s-1 17
.
The lifetime of
2c was not estimated because of the lack of a definite
rate constant for this model, though the very small ratio
of 3c/(8c+9c) agrees with the expectation of a fast
rearrangement of 2c.
It has previously been demonstrated that one can
safely assume that 1,4-biradicals undergo radical rear-
rangement at rates similar to their analogous model
radicals.8 We thus assume 2a rearranges to 6(7)a with
the rate of the cyclopropylmethyl radical rearrangement.
The latest data for the rate constant of such a rearrange-
ment is 9.4 × 107 s-1 14 with the trans/cis ratio of the
No product was observed from the disproportionation
of the biradicals 6 and 7. The triplet decay rate for
1a was measured by nanosecond laser flash photolysis
and showed no significant change when compared with
that of other alkyl phenylglyoxylates,3 indicating no
significant change in the facility of triplet γ-hydrogen
abstraction results from the introduction of a cyclopropyl
group.
(10) Wu, Y. Ph D. Dissertation, Iowa State University.
(11) Besides the H* peak for 8c at 5.74 ppm (dt, J 1 ) 8 Hz, J 2 ) 2
Hz), there is a distinct H* peak from 9c at 5.91 ppm (ddd, J 1 ) 8 Hz,
J 2 ) 1.6 Hz, J 3 ) 1.2 Hz). The trans relationship between the two
phenyl groups is derived from an X-ray structure determination in an
earlier study.7
As expected, the lifetime of biradical 2 is shorter than
are those of 1,4-biradicals without oxygen in their
skeleton (τ ) 35-40 ns18). We find the lifetimes of 2 are
(12) H* peaks for 8b (mixture of diastereoisomers): 6.16 ppm (ddd,
(15) Beckwith, A. L. J .; Moad, G. J . Chem. Soc., Perkin Trans. 2
1980, 1473-1482.
J 1 ) 8 Hz, J 2 ) 2.8 Hz, J 3 ) 2 Hz), 6.26 ppm (ddd, J 1 ) 7.6 Hz, J 2
)
2.4 Hz, J 3 ) 2 Hz). H* peaks for 9b (mixture of diastereoisomers):
(16) (a) Beckwith, A. L. J .; Bowry, V. W. J . Org. Chem. 1989, 54,
2681-2688. (b) Newcomb, M.; Glenn, A. G.; Williams, W. G. J . Org.
Chem. 1989, 54, 2675-2681.
5.88 ppm (dd, J 1 ) 6 Hz, J 2 ) 2.4 Hz), 6.08 ppm (dd, J 1 ) 8 Hz, J 2
2.4 Hz).
)
(13) Column chromatography was performed under a pressure of
air. The elution solvent is hexanes/ethyl acetate ) 10:1. The longer
the compound is in contact with silica gel, the more it decomposes.
(14) Engel, P. S.; Culotta, A. M. J . Am. Chem. Soc. 1991, 113, 2686-
2696.
(17) Catellino, A. J .; Bruice, T. C. J . Am. Chem. Soc. 1988, 110,
7512-7519.
(18) Newcomb, M.; J ohnson, C. C.; Manek, M. B.; Varik, T. R. J .
Am. Chem. Soc. 1992, 114, 10915-10921. Small, R. D., J r.; Scaiano,
J . C. J . Phys. Chem. 1977, 81, 2126-2131.

Notes
J . Org. Chem., Vol. 62, No. 3, 1997 757
MHz, CDCl3) δ 0.41 (m, 1H), 0.55 (m, 1H), 0.81 (m, 1H), 0.99
(m, 1H), 1.01 (d, J ) 6 Hz, 3H), 4.23 (m, 2H), 7.52 (m, 2H), 7.66
(m, 1H), 8.02 (m, 2H). 13C NMR (100 MHz, CDCl3) δ 11.85,
12.11, 18.08, 18.16, 70.63, 128.83, 129.94, 132.49, 134.81, 164.07,
186.56. IR (neat) 3071.62, 3006.02, 2955.85, 2870.96, 1728.75,
1694.02, 1597.55, 1296.56, 1200.09, 984.00. MS 41 (43.9), 69
(100), 77 (40.3), 105 (74.7), 173 (M+ - 45, 0.1). HRMS m/ e
calculated: 218.0943, measured: 218.0942.
tr a n s-(2′-P h en ylcyclopr opyl)m eth yl ben zoylfor m ate (1c)
is a known compound.10 trans-(2-Phenylcyclopropyl)methanol
was synthesized by LiAlH4 reduction of trans-2-phenyl-1-cyclo-
propanecarboxylic acid. Esterification of benzoylformic acid
produces 1c in 80% yield.3
close to those of other biradicals with oxygen in the
skeleton,19 such as the Paterno´-Bu¨chi (pre-oxetane)
biradicals (τ ) 1.5-4 ns4b).
Exp er im en ta l Section
Benzene (Aldrich) was dried over sodium under argon. Other
chemicals were obtained from Aldrich and used as received.
NMR spectra were recorded on a Varian Unity Plus 400 NMR
spectrometer. Chemical shifts are in ppm with TMS as an
internal standard. GC/MS were taken on Hewlett-Packard 5988
mass spectrometer coupled to a HP 5880A GC. Infrared spec-
tra were taken with a Galaxy Series 6020 FTIR spectrometer.
Thin layer chromatography was performed with Whatman
silica gel-coated TLC plates. Products are isolated by flash
column chromatography using Aldrich silica gel (60 Å, 70-270
mesh). HRMS were obtained from the University of Illinois at
UrbanasChampaign. General procedures for irradiating samples,
isolating photoreaction products, quantum yield measurement,
and nanosecond laser flash photolysis were reported earlier.3
Cyclop r op ylm eth yl ben zoylfor m a te (1a ) is obtained by
esterification of benzoylformic acid using cyclopropylmethanol
and DCC in 91% yield. The synthesis and purification procedure
has been described earlier.3 1a : 1H NMR (400 MHz, CDCl3) δ
0.40 (m, 2H), 0.65 (m, 2H), 0.88 (m, 1H), 4.23 (d, J ) 7.2 Hz,
2H), 7.53 (m, 2H), 7.65 (m, 1H), 8.01 (m, 2H). 13C NMR (100
MHz, CDCl3) δ 3.61, 9.73, 71.08, 128.84, 129.97, 132.49, 134.82,
163.98, 186.47. IR (neat) 3087.05, 3009.88, 2955.85, 1736.47,
1690.15, 1597.55, 1200.09, 1176.94, 984.00. MS 55 (82.9), 77
(47.4), 105 (100), 204 (M+, 0.03). HRMS m/ e calculated:
204.0786, measured: 204.0787.
3-Hyd r oxy-3-p h en ylcycloh ep ten e-6-la cton e (8(9)a ): 1H
NMR (400 MHz, CDCl3) δ 1.85 (m, 1H), 2.28 (m, 1H), 2.38 (m,
1H), 2.58 (m, 1H), 4.34 (s, 1H), 4.85 (m, 1H), 6.13 (dt, J 1 ) 7.6
Hz, J 2 ) 2 Hz, 1H), 7.25 (m, 5H). 13C NMR (100 MHz, CDCl3)
δ 22.99, 33.71, 71.56, 111.87, 125.75, 128.55, 128.64, 136.03,
137.91, 172.26. MS 77 (48.8), 105 (100), 120 (88.6), 158 (17.1),
176 (M+ - 28, 12.5). HRMS m/ e calculated: 204.0786, mea-
sured: 204.0787.
3-Hyd r oxy-3,4-d ip h en ylcycloh ep ten e 6-la cton e (8c) is
obtained in the photolysis of 3c in agreement with an earlier
study.10
Ack n ow led gm en t. We thank the National Science
Foundation (DMR-9013109) for financial support of this
work. Private correspondence with Dr. G. A. Kraus is
gratefully acknowledged.
Su p p or tin g In for m a tion Ava ila ble: 1H and/or 13C NMR
(APT) spectra for all new compounds 1a , 1b, 8(9)a (5 pages).
This material is contained in libraries on microfiche, im-
mediately follows this article in the microfilm version of the
journal, and can be ordered from the ACS; see any current
masthead page for ordering information.
cis,tr a n s-(2′-Meth ylcyclop r op yl)m eth yl ben zoylfor m a te
(1b) is obtained by DCC esterification procedure using cis,trans-
(2-methylcyclopropyl)methanol in 90% yield.3 1b: 1H NMR (400
(19) For an explanation of the short lifetime of biradicals with
oxygen in their skeleton, see ref 4a.
J O961659J