434
J . Org. Chem. 1998, 63, 434-435
1
of the H NMR spectrum of the crude reaction mixture,
using the signals previously reported and assigned to 2,
the yield of the photoisomer was 9%. This compound was
then partially purified by preparative chromatography.11
The original assignment of 2 was made only on the basis
Th e Role of Non a r om a tic Isom er s in th e
P h otoch em istr y of 3,5-Dim eth oxyben zyl
Aceta te
Frances L. Cozens, Alexandra L. Pincock,
J ames A. Pincock,* and Richard Smith
1
of 100 MHz H NMR spectra, and two of the reported
features were puzzling. First, the signal for Ha was at δ
3.58, a very low value for a doubly allylic methine
hydrogen at an ester carbon (an approximate value of δ
6.6 can be predicted using the corresponding CH2 in
o-isotoluene (C7H8) at δ 3.17 (C6H6)12 and δ 3.32 (CCl4)13
and the expected substituent effect of an ester oxygen of
δ 3.414) and, second, the two methoxy groups, although
in very similar chemical environments, gave quite dif-
ferent chemical shifts (δ 3.24 and δ 3.62). When 1H-
13C-correlated spectra15 demonstrated that both Ha and
He were on sp3-hybridized carbons at δ 76.6 and δ 52.6,
respectively, assignment of the structure 2 to this com-
pound became untenable. In contrast, the NMR spectra
are consistent with structure 5, particularly when the
1H and 13C chemical shifts, after correcting for substitu-
ent effects, are compared with those of the previously
reported13 unsubstituted compound, C7H8, another isomer
of toluene. Moreover, the structure of 5 makes chemical
sense, because an obvious mechanism for its formation
is the allowed disrotatory butadiene to cyclobutene ring
closure by secondary photochemistry of 2.16
As expected, freshly isolated samples of 5 were trans-
parent at 320 nm; however, 5 slowly isomerized to 2, and
this first-order conversion could be monitored by either
1H NMR or the increasing absorbance at 320 nm (t1/2
approximately 10 h at room temperature).17 The conver-
sion of 5 to 2 was cleanly first order in hexanes at 50 °C
(t1/2 ) 89 min)18 and 2 could then be isolated, spectra19
taken, and its reactivity studied. NMR samples of 2 in
CDCl3 were relatively unstable as 2 rearranged to the
toluene derivative 6; this process was quantitative after
2 h at 50 °C.20 In acetonitrile, 2 rearranges back to
Department of Chemistry, Dalhousie University, Halifax, Nova
Scotia B3H 4J 3, Canada
Received December 15, 1997
More than 20 years ago, J aeger reported1,2 that pho-
tolysis of 3,5-dimethoxybenzyl acetate (DMBA), 1, in
hexane, gave a 17% yield of the isomer 2 along with
radical-derived coupling products, the in-cage one, 3
(47%), being the major one (Scheme 1). We were in-
trigued by the possibility that 2 might also be a primary
photoproduct in polar, nucleophilic solvents, like metha-
nol, where photolysis of DMBA gives, as the major
product, the ether 4 (56%),3 clearly derived from an
intermediate arylmethyl cation. If 2 is formed in metha-
nol, it might be expected to undergo rapid ground-state
solvolysis to 4 complicating mechanistic arguments4,5
centered on the yield of ion-derived products in this
“photosolvolysis” reaction, i.e., is 4 formed entirely in the
primary photochemistry of DMBA or, at least in part, in
a secondary ground-state process from 2? This is an
important question because DMBA, and other multiple
methoxy substituted benzyl acetates, has been shown3
to give a higher yield of ion-derived products than
expected on the basis of a mechanism that emphasizes
formation of ion pairs from radical pairs by electron
transfer. Moreover, on the basis of the pioneering study
by Zimmerman6 on the influence of meta methoxy sub-
stituents on benzene excited state reactivity, the 3,5-
dimethoxybenzyl chromophore in benzoin derivatives has
recently been advocated in the design of practical ex-
amples of photo labile protecting groups.7,8
We therefore decided to isolate and examine the
reactivity of 2 and now report that (1) the compound
previously reported as 2 is, in fact, the bicyclic isomer 5;
(2) 2 can be prepared from 5 by pyrolysis in hexane; (3)
2 does undergo rapid ground-state solvolysis in methanol;
and (4) 2 is formed as a primary photoproduct of DMBA
in methanol.
(11) Silica gel dry-flash chromatography using 5% ethyl acetate in
hexanes as eluant gave fractions with purity as high as 90% but always
contaminated with DMBA.
(12) Kopecky, K. R.; Lau, M.-P. J . Org. Chem. 1978, 43, 525-526.
(13) Hasselmann, D.; Loosen, K. Angew. Chem., Int. Ed. Engl. 1978, 17,
606-608.
(14) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric
Identification of Organic Compounds, 5th ed.; J ohn Wiley & Sons: New
York, 1991; p 213.
Photolysis9 in hexanes results, as previously reported,1
in the rapid10 disappearance of DMBA. By integration
(15) NMR spectra for 5: 1H NMR (CDCl3 at 250 mHz) δ 5.66 (Ha), 5.24
(Hc or Hd), 5.16 (Hc or Hd), 4.74 (Hb), 3.71 (3H, s), 3.66 (He), 3.33 (3H, s) and
2.09 (3H, s); the observed multiplets could be simulated using the program
NMRSIM (Dr. T. P. Forrest, Chemistry Department, Dalhousie) to give the
following coupling constants, J (Hz): eb (1.22), ec (0.91), ed (1.83), ca. (2.44),
* To whom correspondence should be addressed. Tel.: (902) 494-3324.
Fax: (902) 494-1310. E-mail: PINCOCK@CHEM1.CHEM.DAL.CA.
(1) J aeger, D. A. J . Am. Chem. Soc. 1974, 96, 6216-6217.
(2) We thank Professor J aeger for sending us his original laboratory
notebooks, including spectra. This material was of invaluable help. Although
we are reinterpreting his observations, the quality of his experiments and
the clarity of the notebooks would serve as models for anyone. Without the
visual incentive of his spectra, we would have had even more difficulty
completing these experiments.
(3) Pincock, J . A.; Wedge, P. J . J . Org. Chem. 1994, 59, 5587-5595.
(4) Pincock, J . A. Acc. Chem. Res 1997, 30, 43-49.
(5) Zimmerman, H. E. J . Am. Chem. Soc., 1995, 117, 8988-8991.
(6) Zimmerman, H. E.; Sandel, V. R. J . Am. Chem. Soc. 1963, 85, 915-
922.
(7) Pirrung, M. C.; Bradley, J .-C. J . Org. Chem. 1995, 60, 1116-1117.
(8) Cameron, J . F.; Wilson, C. G.; Frechet, J . M. J . J . Am. Chem. Soc.
1996, 118, 12925-12937.
(9) Vycor-filtered, 450 W, medium-pressure Hanovia mercury lamp.
(10) Greater than 90% conversion of 400 mg of 1 in 280 mL of hexanes
after 30 min as determined by HPLC on silica gel with 5% ethyl acetate in
hexanes as eluant.
and da (2.14); 13C NMR (CDCl3 at 250 mHz)
δ 170.5 (CdO), 158.3
(dCOCH3), 144.6 (CdCH2), 112.3 (CdCH2), 95.7 (CHb), 81.0 (COCH3), 76.6
(CHa), 56.9 (CH3O), 52.6 (CHe), 52.3 (CH3O), and 21.1 (CH3). Assignments
were made on the basis of 1H-1H and 13C-1H correlated spectra.
(16) Although we have no evidence to support the assignment, the acetate
functional group is assumed to be in the more stable exo position.
(17) This observation clarified several experimental observations. First,
the initial report1 on the isolation of 5 (thought to be 2) included a UV
absorption band at 315 nm with ꢀ ) 400 M-1 cm-1; the wavelength agrees
with expectations for 2 but the ꢀ value is too low for a conjugated triene.
Our estimate of ꢀ ) 5500 M-1 cm-1 at 320 nm is more reasonable,
particularly when compared to the value of 4400 reported12 for the
unsubstituted compound, o-isotoluene (C7H8). Second, in our attempts to
isolate the photoisomer, we monitored chromatography fractions by HPLC
with UV detection at 320 nm. Those fractions that contained 5, when first
obtained, were transparent but, after standing overnight at room temper-
ature, were strongly absorbant. Both of these observations result from
increasing amounts of 2 contaminating 5 with time.
(18) For the unsubstituted compound, a value of t1/2 ) 210 min in THF
can be calculated from the Arrhenius values reported.13
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