Zeolite-Promoted Oxidations of 1,1-Diarylethylenes

Article abstract of DOI:10.1021/ol035927t

(Equation Presented) The intrazeolite photooxygenations of four diarylethylenes have been examined. Several intermediates, including an epoxide, have been identified by comparison to independently synthesized samples. Aldehyde intermediates were shown to undergo intrazeolite Norrish type I cleavages in competition with a novel new photooxygenation/autoxidation reaction.

Full text of DOI:10.1021/ol035927t

ORGANIC
LETTERS
2
003
Vol. 5, No. 26
979-4982
Zeolite-Promoted Oxidations of
,1-Diarylethylenes
4
1
Edward L. Clennan* and Gui-lan Pan
Chemistry Department, Department 3838, 1000 East UniVersity AVenue,
UniVersity of Wyoming, Laramie, Wyoming 82071
clennane@uwyo.edu
Received October 1, 2003
ABSTRACT
The intrazeolite photooxygenations of four diarylethylenes have been examined. Several intermediates, including an epoxide, have been identified
by comparison to independently synthesized samples. Aldehyde intermediates were shown to undergo intrazeolite Norrish type I cleavages
in competition with a novel new photooxygenation/autoxidation reaction.
Oxidations initiated by irradiation into oxygen/substrate
charge transfer (CT)1,2 bands are well-known reactions that
have been extensively investigated. Unfortunately, despite
structures of the final products has been achieved. In this
letter, we report a reinvestigation of the intrazeolite photo-
oxygenations of diarylethylenes 1a and 1b, a comparison to
the photooxygenation of diarylethylene, 1c, and the first
direct evidence for the intermediacy of an epoxide in these
reactions. In addition, we suggest a mechanism that provides
a rationale for all the experimental results, including novel
wavelength dependences observed for the photooxygenations
of 1a and 1b and the lack of a similar wavelength dependence
for photooxygenation of 1c.
The zeolite used in all of the experiments reported here is
the honeycomblike Y zeolite characterized by four 7.4 Å
windows tetrahedrally arranged around a 13 Å diameter
supercage. The zeolite is prepared for reaction by heating
under an oxygen atmosphere in a tube oven at 500 °C for
3
the economic and environmental appeal of using oxygen as
a terminal oxidant, these reactions are of limited synthetic
utility. The wavelength of light needed to initiate the
oxidation also causes decomposition of the sensitive peroxide
product. This problem has been recently and elegantly
addressed by Frei and co-workers4 who reported that
incorporation of the oxygen/substrate charge transfer complex
into the interior of a zeolite induces a dramatic red-shift of
the CT band into a region where the peroxide product does
not absorb. These intrazeolite photooxidations have been
suggested to occur by a photochemically induced electron
transfer followed by either collapse of the ion pair to a
dioxetane or abstraction of an allylic hydrogen (Scheme 1).
More recently, intrazeolite photooxidations of 1,1-diaryl
ethylenes that do not possess allylic hydrogens have been
Scheme 1
5
reported. These reactions have been suggested to proceed,
without any direct evidence, via epoxide intermediates. In
addition, no agreement on the mechanism of formation or
(
1) (a) Evans, D. F. J. Chem. Soc. 1950, 345-347. (b) Tsubomura, H.;
Mulliken, R. S. J. Am. Chem. Soc. 1960, 82, 5966-5974.
(
2) Stevens, B. Chem. Soc. Ann. Rep. A 1974, 29-47.
(3) Onodera, K.; Furusawa, G.-I.; Kojima, M.; Tsuchiya, M.; Aihara,
S.; Akaba, R.; Sakuragi, H.; Tokumaru, K. Tetrahedron 1985, 41, 2215-
220 and references therein.
4) (a) Blatter, F.; Frei, H. J. Am. Chem. Soc. 1993, 115, 7501-7502.
b) Vasenkov, S.; Frei, H. Photo-Oxidations in Zeolites; Ramamurthy, V.,
Schanze, K. S., Eds.; Marcel Dekker: New York, 2000; Vol. 5, pp 295-
23.
2
(
(
3
1
0.1021/ol035927t CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/27/2003

2
4 h. The zeolite is then cooled and the 1,1-diarylethylene
in our system. In addition, this reduction product was
unreactive under the reaction conditions and does not serve
as a precursor to any of the observed products in Scheme 2.
On the other hand, the previously reported effect of
irradiation wavelength on the product ratios during photo-
oxidations of 1a and 1b was corroborated in our studies. In
particular, as the wavelength of irradiation decreased from
420 to 254 nm, the diarylmethanes, 2a and 2b, dramatically
increased at the expense of the aldehydes, 4a and 4b (Table
1).
loaded by absorption from a hexane solution. In all cases,
intercalation into the zeolite was 100% complete as deter-
mined by gas chromatographic monitoring of the hexane
prior to irradiation. The reactions were conducted in oxygen-
saturated solvent slurries by irradiation through appropriate
filters. The products were either extracted with THF or CH
Cl from the intact zeolite or from solution after digestion
of the zeolite with 10% HCl. The mass balances were greater
than 90% in all cases. The products of the irradiations are
depicted in Scheme 2 although the precise product ratios were
5
a
2
-
2
These results, and experiments described below that were
designed to test the viability/competency of several of the
potential intermediates, are consistent with the mechanism
depicted in Scheme 3.
Scheme 2
Scheme 3
very sensitive to the experimental conditions as shown in
Table 1.
Table 1. Product Distributions in the Intrazeolite
Photooxygenations of 1a-ca
product ratios (%)
〈
s〉b
λ (nm)c
2
3
4
5
6
1
1
1
a
b
c
0.27
>420
366
254
>420
>420
366
254
254
>420
>420
366
66
42
36
61
47
54
34 ( 4
43
40 ( 2
49
37 ( 2
27
34
58
37
14
53
25
11 ( 6
7
59 ( 5
51
62 ( 3
63
0
0
0
.27
.27
.27d
27
The viability of epoxide 6 as a competent intermediate
was demonstrated by irradiation of diarylethylenes 1a and
25
0.27
1
c in zeolites that had been pretreated with pyridine to
0
0
0
.27
.27
.27e
21
55 ( 3
51
remove any residual Br o¨ nsted acid sites. At loading levels
of 1 molecule of pyridine per every 100 supercages,
substantial amounts of the epoxides, 6a and 6c, were isolated
at the expense of 4a and 4c, respectively (Table 1). In
addition, doping of non-pyridine-treated zeolite samples with
independently synthesized samples of epoxides 6a and 6c
resulted in thermal rearrangements to carbonyl compounds,
0.27f
0
0
0
0
0
.54
.27f
.27
.54
.27d
254
254
>420
10
20
29
42
51
22
35
4a and 4c, respectively (Scheme 4). Presumably the carbonyl
a
All photooxygenations were conducted in hexane slurries except where
b
c
d
noted. Molecules per supercage. Irradiation wavelength. Conducted in
NaY doped with pyridine 〈s〉 ) 0.01. Conducted in perfluorohexane slurry.
Trace amounts of another product tentatively assigned to Ph2C(CH3)CHO.
e
f
Scheme 4
1
,1-(4-Methoxyphenyl)ethane, previously suggested as a
5a
product of the intrazeolite oxidation of 1b, was not formed
(5) (a) Lakshminarasimhan, P.; Thomas, K. J.; Johnston, L. J.; Rama-
murthy, V. Langmuir 2000, 16, 9360-9367. (b) Kojima, M.; Nakajoh, M.;
Matsubara, C.; Hashimoto, S. J. Chem. Soc., Perkin Trans. 2 2002, 1894-
1
901.
4980
Org. Lett., Vol. 5, No. 26, 2003

compounds are formed by acid-catalyzed openings of the
epoxides followed by hydride shifts to the benzylic cationic
centers (step 1 in Scheme 3).
The competencies of carbonyl compounds, 4a-c, as
precursors for the diarylmethanes, 2, and phenones, 3, were
also verified by examining intrazeolite reactions of authentic
samples (Scheme 5). These results prompt us to suggest that
served during intrazeolite photooxidations of 1a and 1b
(Table 1) can be traced to the unusual behavior of aldehydes
4a and 4b; the diarylmethanes, 2, dominate at short
wavelengths, but the phenones, 3, become increasingly
important at long wavelengths. We suggest that at short
wavelengths, Norrish type I cleavage is an important
contributor to the photochemical behavior of 4a and 4b.
However, at long wavelengths, a new reaction initiated by
photochemically induced electron transfer in an aldehyde/
oxygen charge-transfer complex (A in Scheme 3) becomes
the predominant reaction pathway. Photochemically initiated
oxidations of aldehydes are well-established reactions that
exhibit quantum yields much greater than 1, consistent with
Scheme 5a
9
a radical mechanism for this process. This pathway to give
phenone 3 can also be initiated thermally, albeit at a rate
diminished in comparison to the photochemical reaction by
approximately a factor of 4. Parenthetically, this thermal
process is also responsible for the reduced yield of 4a
observed in the reaction of 6a depicted in Scheme 4.
The direct observation of the oxygen CT complex A by
diffuse reflectance UV-vis spectroscopy (Figure 1) provides
a
Average number of molecules of starting material per supercage.
b
c
Irradiation wavelength. Irradiation time
the diarylmethanes, 2a and 2b, are formed by Norrish type
I cleavages of the aldehyde precursors as shown in Scheme
3
(step 2). A substantial body of literature precedent supports
6
this suggestion. Decarbonylations of aliphatic and R,â-
unsaturated aldehydes are well-established reactions. The
7
absence of any cleavage product during intrazeolite photo-
oxidation of 4c also supports the suggestion of a Norrish
type I mechanism; photodecarbonylation is only observed
when the acyl radical has an adjacent stabilizing substituent
•
present. The acetyl radical (H
3
C- CdO) formed in the
cleavage of 4c has a substantial activation barrier for loss of
CO, and recombination to reform the ketone rather than
decarbonylation is the anticipated and observed result.
Finally, intrazeolite irradiation of 1b in a perfluorohexane
slurry (Table 1) was not accompanied by a reduced yield of
2
b, supporting the suggestion (Scheme 3) of an intra-
molecular hydrogen transfer of the aldehyde proton to the
diarylmethyl radical intermediate rather than by hydrogen
abstraction from the slurry solvent. Intrazeolite recombina-
tions of radicals generated by Norrish type I cleavage
reactions of dibenzyl ketones have previously been reported
by Turro and co-workers.8
Figure 1. Diffuse reflectance UV-Vis Spectra of 4a in the absence
and presence of O
2 2
. top; in the presence of O . bottom; a
comparison.
strong support for this aldehyde/oxygen CT-mediated path-
way. In addition, thermal generation of an acyl radical has
been implicated during the aldehyde-induced oxidation of
olefins to epoxides in the presence of oxygen.10 In these
epoxidation reactions, the acyl radical is converted to an acyl
Examination of the results in Scheme 5 suggests that the
5a
previously reported dramatic wavelength dependence ob-
(6) (a) Gilbert, A.; Baggott, J. Essentials of Molecular Photochemistry;
CRC Press: Boca Raton, FL, 1991; Chapter 7, pp 288-302. (b) Bohne, C.
In CRC Handbook of Organic Photochemistry and Photobiology; Horspool,
W. M., Song, P.-S., Eds.; CRC Press: Boca Raton, FL, 1995.
(9) Niclause, M.; Lemaire, J.; Letort, M. In AdVances in Photochemistry;
Noyes, W. A., Jr., Hammond, G. S., Eds.; John Wiley & Sons: New York,
1966; pp 25-48.
(
7) Armesto, D.; Ortiz, M. J.; Romano, S.; Agarrabeitia, A. R.; Gallego,
M. G.; Ramos, A. J. Org. Chem. 1996, 61, 1459-1466.
8) Turro, N. J. Acc. Chem. Res. 2000, 9, 637-646.
(
(10) Jarboe, S. G.; Beak, P. Org. Lett. 2000, 2, 357-360.
Org. Lett., Vol. 5, No. 26, 2003
4981

point we cannot rigorously exclude contribution from Hock
1
1
cleavage of a transient allylic hydroperoxide.
Scheme 6
Remarkably diarylethylene 7 is inert under these reaction
conditions. It is tempting to suggest that back electron
transfer (Scheme 6) in the photochemically generated su-
peroxide/ethylene radical cation ion pair that precedes
formation of the epoxide competitively inhibits its formation.
+
•
-•
If the thermodynamically more favorable BET in 1a /O
were in the Marcus inverted region, it would be slower than
2
+
•
-•
BET in 7 /O
2
allowing epoxide formation to compete. In
support of this speculation, literature precedent demonstrates
that back electron-transfer rates often become slower with
increasing driving force (i.e., they are indeed in the Marcus
peroxy radical by reaction with oxygen. The acyl radical in
our reactions (B in Scheme 3) rapidly decarbonylate because
of the incipient stability of the diarylmethyl radical.
The lack of reactivity of ketone 4c by this oxygen CT-
mediated oxidation is also consistent with the acyl radical
pathway. However, this observation also requires a second
pathway to form 3a in the reaction of 1c (Scheme 2). As an
alternative, we suggest formation and decomposition of a
dioxetane as shown in Scheme 3 (step 3), although at this
1
2
inverted region).
Additional work to study these back electron transfer and
other novel aspects of these intrazeolite photoxygenations
are in progress and will be reported in due course.
Acknowledgment. We thank the National Science Foun-
dation for their generous support of this research.
Supporting Information Available: Experimental pro-
cedures. This material is available free of charge via the
Internet at http://pubs.acs.org.
(
11) Frimer, A. A. Chem. ReV. 1979, 79, 359-387.
(12) Vitale, M.; Castagnola, N. B.; Ortins, N. J.; Brooke, J. A.;
Vaidyalingam, A.; Dutta, P. K. J. Phys. Chem. B 1999, 103, 2408-2416.
OL035927T
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Org. Lett., Vol. 5, No. 26, 2003