Triplet energy dissipation in methylenecyclopropane rearrangement

Article abstract of DOI:10.1021/jo071114x

(Chemical Equation Presented) Certain 1,1-dimethyl-2-aryl-3- methylenecyclopropanes containing carbonyl substituents undergo rearrangement when irradiated with 350 nm light. These rearrangements occur via the (n,π*) triplet state, which fragments the stra

Full text of DOI:10.1021/jo071114x

Triplet Energy Dissipation in Methylenecyclopropane Rearrangement
Xavier Creary,* Andrea Losch, and William Sullivan
Department of Chemistry and Biochemistry, UniVersity of Notre Dame, Notre Dame, Indiana 46556
creary.1@nd.edu
ReceiVed May 25, 2007
Certain 1,1-dimethyl-2-aryl-3-methylenecyclopropanes containing carbonyl substituents undergo rear-
rangement when irradiated with 350 nm light. These rearrangements occur via the (n,π*) triplet state,
which fragments the strained cyclopropane bond. Intersystem crossing followed by ring closure gives
the observed products. No photoreduction is seen in i-PrOH. Potential Norrish type II processes are also
bypassed. It is suggested that the cyclopropane bond fragmentation dissipates the triplet energy and that
the new intermediates are not energetic enough to abstract hydrogen atoms in an intramolecular fashion
or from solvent. Nitro substituted systems undergo analogous photoinitiated rearrangements. Benzophenone
sensitization of naphthyl, biphenyl, styrene, and phenylacetylene analogues also leads to rearrangement,
presumably via the sensitized generation of triplet states. When triplet states cannot be accessed by direct
irradiation or by sensitized processes, methylenecyclopropane rearrangements do not occur. An exception
is the ferrocenyl analogue, which does not photorearrange, presumably due to the very short lifetime of
the triplet intermediate.
Introduction
While the thermal methylenecyclopropane rearrangement is
a much studied reaction,2 there are only isolated examples of
photochemical variants.3,4 Thus, direct irradiation of 4 under
We have been interested in the methylenecyclopropane
rearrangement as a probe for free radical stabilizing effects.
Toward this end, we have examined the thermal rearrangement
of numerous methylenecyclopropanes of a general structure 1.1
These rearrangements proceed thermally to give 2 via the
intermediacy of biradicals 3, where radical stabilizing groups
on the aromatic ring enhance the rearrangement rate.
(1) Creary, X. Acc. Chem. Res. 2006, 39, 761.
(2) (a) Kon, G. A. R.; Nanji, H. R. J. Chem. Soc. 1932, 2557. (b)
Ettlinger, M. G. J. Am. Chem. Soc. 1952, 74, 5805. (c) Ullman, C. F. J.
Am. Chem. Soc. 1960, 82, 505. (d) Chesick, J. P. J. Am. Chem. Soc. 1963,
85, 2720. (e) Gilbert, J. C.; Butler, J. R. J. Am. Chem. Soc. 1970, 92, 2168.
(f) Gajewski, J. J. J. Am. Chem. Soc. 1968, 90, 7178. (g) Gajewski, J. J. J.
Am. Chem. Soc. 1971, 93, 4450. (h) Doering, W. v. E.; Birladeanu, L.
Tetrahedron 1973, 29, 499. (i) Gajewski, J. J.; Chou, S. K. J. Am. Chem.
Soc. 1977, 99, 5696. (j) Gajewski, J.; Benner, C. W.; Stahly, B. N.; Hall,
R. F.; Sato, R. I. Tetrahedron 1982, 38, 853.
(3) (a) Gilbert, J. C.; Butler, J. R. J. Am. Chem. Soc. 1970, 92, 7493. (b)
Gros, W. A.; Luo, T.; Gilbert, J. C. J. Am. Chem. Soc. 1976, 98, 2019.
(4) (a) Kende, A. S.; Goldschmidt, Z.; Smith, R. F. J. Am. Chem. Soc.
1970, 92, 7606. (b) Kagan, J. HelV. Chim. Acta 1972, 55, 1219. (c)
Goldschmidt, Z.; Mauda, S. Tetrahedron Lett. 1976, 4183. (d) Baum, T.;
Rossi, A.; Srinivasan, R. J. Am. Chem. Soc. 1985, 107, 4411. (e) Leigh,
W. J. Chem. ReV. 1993, 93, 487. (f) Takahashi, Y.; Mori, Y.; Nakamura,
A.; Tomioka, H. Tetrahedron Lett. 2005, 46, 8415. (g) Takahashi, Y.;
Sakakibara, T.; Tominaga, T.; Inaba, M.; Tomioka, H. J. Photochem.
Photobiol., A 2007, 185, 253. For related photochemical processes, see (h)
Gilbert, J. C.; Luo, T. J. Org. Chem. 1981, 46, 5237. (i) Maier, G.; Ju¨rgen,
D.; Tross, R.; Reisenauer, H. P.; Hess, B. A., Jr.; Schaad, L. J. Chem. Phys.
1994, 189, 383.
10.1021/jo071114x CCC: $37.00 © 2007 American Chemical Society
Published on Web 09/14/2007
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J. Org. Chem. 2007, 72, 7930-7938

Dissipation in Methylenecyclopropane Rearrangement
rather harsh conditions gave a competing methylenecyclopro-
pane rearrangement and photofragmentation to 1,1-diphenyl-
ethylene and vinylidene.3 These reactions are believed to occur
on the singlet energy surface. In another interesting variation,
a photosensitized reaction of 4 using chloranil or anthraquinone
as sensitizers also led to the rearranged product 5.5 This reaction
is proposed to occur via radical cation intermediates generated
by one electron oxidation of 4 by the photoexcited oxidant.
With these precedents in mind, we have now sought to carry
out analogous rearrangements under photochemical conditions
where excited state triplets are involved. The carbonyl containing
systems 8 and 9 were chosen for initial investigation since the
photochemistry of carbonyl compounds is well- understood.
FIGURE 1. Evolving 1H NMR spectra (benzylic region) during
irradiation of 8 in C6D6 at 350 nm.
(ISC) to give the triplet state of benzophenone (12). This triplet
readily abstracts a hydrogen atom from isopropyl alcohol to
generate the radical 13. Subsequent coupling of 13 gives the
observed product 11. With this photoreduction mechanism in
mind, we now report on the photochemistry of 8 and 9 and
related substrates.
It should be recognized that 8 and 9 are also substituted
benzophenones and that benzophenones are subject to photore-
duction when irradiated in certain solvents.6 For example,
irradiation of benzophenone in isopropyl alcohol leads to the
formation of benzopinacol (11).
Results and Discussion
The first experiment carried out was room-temperature
irradiation of 8 in C6D6 using light centered at 350 nm.7 A
smooth reaction occurred to give exclusively the rearranged
product 15, as illustrated by the evolving NMR spectra in Figure
1. When the photolysis was carried out in isopropyl alcohol as
the solvent, the same product was formed (Figure 2). No
photoreduction of 8 to the corresponding pinacol occurred under
conditions where benzophenone was readily reduced to ben-
zopinacol. This rearrangement of 8 to 15 was quite efficient
photochemically and proceeded with comparable efficiency to
that of the photoreduction of benzophenone to benzopinacol.
The rearranged product 15 also resisted photoreduction when
irradiated in i-PrOH.
The mechanism in Scheme 1 is suggested to account for the
facile photoinduced rearrangement of 8 as well as the lack of
photoreduction in i-PrOH. Photoexcitation of 8 followed by
intersystem crossing would give the closed (n,π*) triplet 17.
This closed triplet is suggested to fragment the strained
cyclopropane bond very rapidly to give the ring opened triplet
This classic photochemical reaction proceeds by excitation
of the benzophenone followed by rapid intersystem crossing
(5) (a) Takahashi, Y.; Miyashi, T.; Mukai, T. J. Am. Chem. Soc. 1983,
105, 6511. (b) Miyashi, T.; Takahashi, Y.; Mukai, T.; Roth, H. D.; Schilling,
M. L. M. J. Am. Chem. Soc. 1985, 107, 1079. (c) Miyashi, T.; Takahashi,
Y.; Ohaku, H.; Ikeda, H.; Morishima, S. Pure Appl. Chem. 1991, 63, 223.
(6) (a) Ciamician, G.; Silber, P. Chem. Ber. 1900, 33, 2911. (b)
Ciamician, G.; Silber, P. Chem. Ber. 1901, 34, 1641. (c) Pitts, J. N., Jr.;
Letsinger, R. L.; Taylor, R. P.; Patterson, J. M.; Recktenwald, G.; Martin,
R. B. J. Am. Chem. Soc. 1959, 81, 1068. (d) Moore, W. M.; Hammond, G.
S.; Foss, R. P. J. Am. Chem. Soc. 1961, 83, 2789. (e) Hammond, G. S.;
Baker, W. P.; Moore, W. M. J. Am. Chem. Soc. 1961, 83, 2795.
(7) All photolyses in this paper were carried out in a Rayonet Photo-
chemical Reactor (Southern New England Ultraviolet Company) using lamps
that emit over a narrow range centered at 350 nm. For details of lamp output,
see www.rayonet.org.
J. Org. Chem, Vol. 72, No. 21, 2007 7931

Creary et al.
getic enough to abstract a hydrogen atom. Therefore, no
photoreduction in i-PrOH was observed. Triplet 18, also
represented by 18a, needs only to spin invert and close the ring
to give the observed product 15. The rearranged product 15 is
also resistant to photoreduction since the excited triplet state
20 derived from the irradiation of 15 should also dissipate its
energy by cyclopropane bond fragmentation. The resultant open
triplet is identical to 18, and it should simply reconvert (after
ISC) back to 15. These suggestions are summarized in Figure
3.
In view of the proposed ability of the unpaired electron in
17 to facilitate fragmentation of the cyclopropane bond, the
m-benzoyl derivative 9 was next investigated. This substrate
also underwent facile photochemical rearrangement to give 21
in C6D6 as well as in i-PrOH. Again, no photoreduction was
observed. This observation leads to the suggestion that the m
substituted (n,π*) triplet 22 can also facilitate cyclopropane bond
fragmentation. A valence bond rationalization of this observation
is given in Scheme 2. It is suggested that ring opening of triplet
22 leads to the open triplet 23. Spins from the m situated radical
centers of 23 can be delocalized to the same three positions
(indicated by the asterisk) of the phenyl ring. Hence, these
radical centers interact, and 23a is a viable resonance contribu-
tor. Spin inversion and ring closure again give the rearrangement
product 21 with no photoreduction in i-PrOH.
FIGURE 2. Evolving 1H NMR spectra (aromatic region) during
irradiation of 8 in i-PrOH at 350 nm.
SCHEME 1
The (n,π*) triplet state of acetophenone lies 74 kcal/mol
above the ground state and is accessible by irradiation of the
weak absorption band with a maximum centered at 319 nm.9
With this in mind, the p-acetyl derivative 24 (R ) CH3) was
irradiated in C6D6 and in i-PrOH using 350 nm light. Facile
rearrangement to 25 (R ) CH3) occurs, presumably due to
energy absorption at the tail end of the n f π* band of 24 (R
) CH3). The other carbonyl derivatives 24 (R ) H, t-Bu, or
CF3) also rearrange photochemically in C6D6 and in i-PrOH
with no photoreduction in this latter solvent. These rearrange-
ments undoubtedly proceed via the (n,π*) triplet states derived
from 24.
The Norrish type II photoreaction of ketones is another well-
understood photochemical process.10 Thus, irradiation of vale-
18. Hence, triplet 17 does not live long enough to abstract a
hydrogen atom from the solvent. The triplet 18, formed after
the release of approximately 40 kcal/mol of strain energy
associated with the methylenecyclopropane ring,8 is not ener-
(9) Pitts, J. N., Jr.; Wan, J. K. S. The Chemistry of the Carbonyl Group,
Patai, S., Ed.; Wiley-Interscience: New York, 1966; Ch. 16.
(10) (a) Wagner, P. J. Acc. Chem. Res. 1971, 4, 168-177. (b) Turro, N.
J. Modern Molecular Photochemistry; The Benjamin/Cummings Publishing
Company, Inc.: Menlo Park, CA, 1978; Ch. 10.
(8) Johnson, W. T. G.; Borden, W. T. J. Am. Chem. Soc. 1997, 119,
5930.
7932 J. Org. Chem., Vol. 72, No. 21, 2007

Dissipation in Methylenecyclopropane Rearrangement
SCHEME 3
to give cyclobutanols or fragment to give an enol, the precursor
of acetophenone. With the possibility of the Norrish type II
reaction in mind, the substrate 26 was next irradiated. Rear-
rangement readily occurred to give 27 only (Scheme 3). There
were no traces of Norrish type II photoproducts 28 or 29 under
conditions where the model ketone PhCO-n-Bu readily gave
acetophenone and 1-phenyl-2-methylcyclobutanols. As in the
case of the benzophenone analogue 8, it is suggested that the
initial triplet derived from 26 readily ring opens and thereby
dissipates its energy. The resultant ring opened triplet lacks the
energy necessary for intramolecular hydrogen atom abstraction.
The photochemistry of nitro compounds is very analogous
to that of carbonyl compounds.11 Thus, irradiation of nitroben-
zenes leads to an (n,π*) singlet state, which undergoes ISC to
produce the (n,π*) triplet state. This triplet state, which has
biradical properties, can lead to both intra- and intermolecular
hydrogen atom abstraction.12 With this in mind, the p- and
m-nitro derivatives 30 and 3213 were irradiated at 350 nm. The
result was the facile rearrangement to 31 and 33, respectively.
As in the case of the carbonyl derivatives, the suggested
mechanism in Scheme 4 involves the photoexcitation of 30
followed by conversion to the triplet 34. Facile fragmentation
of the cyclopropane bond of 34 leads to the open triplet 35,
which can undergo intersystem crossing and subsequent closure
of the singlet biradical to give the observed product 31.
FIGURE 3. Photochemical pathway for the rearrangement of 8 to 15.
SCHEME 2
To further verify the ability of triplet intermediates to lead
to the methylenecyclopropane rearrangement, the naphthyl
substrates 36 and 3714 were next investigated under photo-
chemical conditions. Under irradiation in C6D6 with 350 nm
light, rearrangement was a relatively slow process (Figure 4)
since 36 and 37 do not absorb strongly at this wavelength.
However, it is known that the triplet state of naphthalene, which
lies 61 kcal/mol above the ground state, can be efficiently
accessed by a photosensitized reaction with triplet benzophenone
(69 kcal/mol above the ground state).15 With this in mind,
benzophenone was added to solutions of 36 and 37 in C6D6.
(11) Morrison, H. A. The Chemistry of the Nitro and Nitroso Groups;
Feuer, H., Ed.; Wiley-Interscience: New York, 1969; pp 165-213.
(12) (a) Morrison, H.; Migdalof, B. J. Org. Chem. 1965, 30, 3996. (b)
Kaneko, C.; Yamada, S.; Yokoe, I. Tetrahedron Lett. 1966, 4729. (c) Hurley,
R.; Testa, A. J. Am. Chem. Soc. 1966, 88, 4330.
(13) Creary, X.; Mehrsheikh-Mohammadi, M. E.; McDonald, S. J. Org.
Chem. 1987, 52, 3254.
rophenone, PhCO-n-Bu, gives 1-phenyl-2-methylcyclobutanols
as well as acetophenone. This reaction occurs via the triplet
state, which readily abstracts a hydrogen atom from the γ-carbon
of the butyl group. The resultant 1,4-biradical can either cyclize
(14) Creary, X.; Mehrsheikh-Mohammadi, M. E.; McDonald, S. J. Org.
Chem. 1989, 54, 2904.
J. Org. Chem, Vol. 72, No. 21, 2007 7933

Creary et al.
are expected to meet this criterion.16 Indeed, these substrates
all underwent facile rearrangement when irradiated in a C6D6
solution containing benzophenone. In the absence of benzophe-
none, these substrates gave no photoreaction. In addition,
fluorenone, which has a triplet energy of only 53 kcal/mol, was
an ineffective sensitizer. Hence, rearrangements of the naphthyl
derivative 36 and the biphenyl analogue 42 were not sensitized
by added fluorenone.
SCHEME 4
The azo derivative 46 is another substrate of interest17 whose
photochemical behavior on irradiation using 350 nm light is
illustrated in Figure 5. Direct irradiation led to a relatively
inefficient methylenecyclopropane rearrangement (only about
15% in 2 h). However, there was a much more efficient
photochemical process involving E- to Z-isomerization. Ulti-
mately, a photostationary state consisting of 47% 46 and 53%
47 was produced. Fluorescent laboratory light also promoted
this isomerization, although a different photostationary state was
attained. The Z-isomer 47 was a labile substance, and as shown
in Figure 5, it reverted back to 46 on standing in the dark (half-
life of 5.9 h at 24 °C). This E- to Z-photoisomerization of azo
compounds is a well-established process18 and has been studied
for many compounds, including the related substrate (t-
butylazo)benzene, PhsNdNst-Bu.19 The photochemistry of 46
appears to be very analogous to that of PhsNdNst-Bu except
for the superimposition of a slow methylenecyclopropane
rearrangement. It is suggested that a triplet state of 46 is
responsible for this inefficient methylenecyclopropane rear-
rangement.
Irradiation led to the facile rearrangement to 37 and 39,
respectively. A comparison of this benzophenone sensitized
reaction of 36 with the reaction in the absence of benzophenone
is shown in Figure 4.
To account for the facile rearrangement in the presence of
benzophenone, it is suggested (Scheme 5) that the triplet energy
transfer from benzophenone to 36 leads to a closed (π,π*) triplet
represented by 40. Triplet 40 readily fragments the strained
cyclopropane bond to give the open triplet 41, which lies
approximately 40 kcal/mol lower in energy. Subsequent ISC
and ring closure leads to 37. Analogous processes occur in the
photosensitized rearrangement of the 3-naphthyl derivative 38.
What about the possibility of other triplet photosynthesized
methylenecyclopropane rearrangements of substrates of a gen-
eral structure 1? To achieve benzophenone sensitization, it will
be necessary to have substrates with triplet energies lower than
69 kcal/mol. Substrates 42-45 were next examined since they
Lest the impression be created that all of our methylenecy-
clopropanes undergo photoinitiated methylenecyclopropane
(16) Triplet energies of biphenyl, styrene, and phenylacetylene are 66,
62, and 72 kcal/mol, respectively. See: Murov, S. L.; Carmichael, I.; Hug,
G. L. Handbook of Photochemistry, 2nd ed.; Marcel Dekker: New York,
1993. The cyclopropyl groups in 42-45 should further lower the triplet
energies of 42-45, making the triplet states of these substrates accessible
from triplet benzophenone.
(17) Creary, X.; Engel, P. S.; Kavaluskas, N.; Pan, L.; Wolf, A. J. Org.
Chem. 1999, 64, 5634.
(18) (a) Engel, P. A.; Bodager, G. A. J. Org. Chem. 1988, 53, 4748. (b)
Engel, P. A.; Melaugh, R. A.; Page, M. A.; Szilagyi, S.; Timberlake, J. W.
J. Am. Chem. Soc. 1976, 98, 1971. (c) Drewer, R. J. The Chemistry of
Hydrazo, Azo, and Azoxy Groups; Patai, S., Ed.; Wiley-Interscience: New
York, 1975; pp 935-1015.
(15) (a) Terenin, A.; Ermolaev, V. J. Chem. Soc., Faraday Trans. 2 1956,
52, 1042. For related intramolecular processes, see: (b) Engel, P. A.; Horsey,
D. W.; Scholz, J. N.; Karatsu, T.; Kitamura, A. J. Phys. Chem. 1992, 96,
7524.
(19) Porter, N. A.; Funk, M. O. Chem. Commun. 1973, 263.
7934 J. Org. Chem., Vol. 72, No. 21, 2007

Dissipation in Methylenecyclopropane Rearrangement
FIGURE 4. Plot of the amount of 36 remaining after irradiation in C6D6 vs irradiation time. About 10% of 37 was present at the beginning of
irradiation.
SCHEME 5
derived from fluorenone (53 kcal/mol) at a diffusion controlled
rate. With this in mind, the ferrocenyl derivative 5514 was
investigated. We were initially surprised when direct irradiation
of 55 resulted in no rearrangement. Substrate 55 also resisted
photosensitized rearrangement when fluorenone or benzophe-
none were added as sensitizers. The lifetime of the triplet state
of ferrocene, which is only 0.6 ns,21 offers a clue as to why the
triplet state of 55 fails to lead to rearrangement. Benzophenone
has a triplet lifetime22 of 10 µs (∼105 longer than that of
ferrocene), and if triplet lifetimes of methylenecyclopropanes
such as 8 and 9 are comparable, then this is apparently long
enough to permit fragmentation of the cyclopropane bond. It is
suggested that the triplet state of 55 does not live long enough
to fragment the cyclopropane bond. Instead, as in the case of
ferrocene itself, rapid return to the ground state occurs via
radiationless decay, and 55 remains photochemically inert.
rearrangement, unsuccessful reactions must be addressed. The
ester 49 and the amide 50 resisted photochemical rearrangement.
This behavior of 49 and 50 contrasts with that of other carbonyl
derivatives and is undoubtedly due to the inability to access
the (n,π*) triplet states of 49 and 50 using 350 nm light. The
simple unsubstituted system 51 (R ) p-H), with an expected
triplet energy in excess of 80 kcal/mol,16 did not undergo
rearrangement under direct irradiation with 350 nm light or with
attempted benzophenone photosensitization. Neither the p-OCH3
and p-SCH3 derivatives 52 and 53 nor the p-cyano derivative
54 photorearranged. The key to successful photorearrangement
undoubtedly lies with the accessibility of the triplet excited state.
Finally, the O-methyl oxime derivative 5617 gave no reaction
on direct irradiation. Benzophenone sensitization also gave no
methylenecyclopropane rearrangement. However, 56 was not
inert under photosensitized conditions. Benzophenone sensitized
irradiation led to isomerization of the E-oxime 56 to the
Z-isomer 57, and ultimately a mixture consisting of 42% 56
and 58% 57 was produced. Fluorenone, with a triplet energy
The triplet energy of ferrocene is approximately 40 kcal/mol
above the ground state and is readily accessible from a variety
of sensitizers.20 For example, ferrocene quenches the triplet
(21) Maciejewski, A.; Jaworska-Augustyniak, A.; Szeluga, Z.; Wojtczak,
J.; Karolczak, J. Chem. Phys. Lett. 1988, 153, 227.
(20) Herkstroeter, W. G. J. Am. Chem. Soc. 1975, 97, 4161.
(22) Bell, J. A.; Linschitz, H. J. Am. Chem. Soc. 1963, 85, 528.
J. Org. Chem, Vol. 72, No. 21, 2007 7935

Creary et al.
1
FIGURE 5. Evolving H NMR spectra (olefinic and benzylic regions) during and after irradiation of 46 in C6D6.
of only 53 kcal/mol, was also an effective photosensitizer, and
use of this sensitizer led to almost complete conversion of 56
to the Z-isomer 57. The fact that direct irradiation with 350 nm
light gave no isomerization, while benzophenone or fluorenone
sensitization led to 57, suggests that a triplet state of 56 is
involved in this transformation. Yet, there is no methylenecy-
clopropane rearrangement from this triplet state. The reason for
the lack of cyclopropane bond fragmentation in this triplet state
is not understood. Theoretical studies are in progress to
determine the precise nature of the lowest triplet state of
O-methyl oximes.
established photochemical processes are bypassed due to
fragmentation of the cyclopropane bond and subsequent dis-
sipation of the triplet energy. The new intermediates are no
longer energetic enough to abstract hydrogen atoms. Triplets
derived from benzophenone sensitization of naphthyl derivatives
36 and 38, as well as hydrocarbons 42-45, also undergo the
methylenecyclopropane rearrangement. When the triplet states
cannot be accessed photochemically, rearrangements do not
occur. An exception is the ferrocenyl analogue 55, which does
not photorearrange due to the short triplet lifetime.
Experimental Procedures
General. Most of the methylenecyclopropanes used in this study
were prepared as previously described. See the text for appropriate
references. The procedures given next are for those not previously
1
described. H NMR spectra were recorded at 600 MHz, and 13C
NMR spectra were recorded at 151 MHz.
Conclusion
Photolyses of carbonyl substituted 1,1-dimethyl-2-aryl-3-
methylenecyclopropanes leads to (n,π*) triplets and subsequent
methylenecyclopropane rearrangement. Under conditions where
benzophenone in i-PrOH is readily photoreduced, the methyl-
enecyclopropane analogues 8 and 9 are not reduced. Neither is
the Norrish type II photoreaction observed in 26. These well-
7936 J. Org. Chem., Vol. 72, No. 21, 2007

Dissipation in Methylenecyclopropane Rearrangement
Preparation of Methylenecyclopropane 8. A solution of 751
mg of bromide i23 in 8 mL of THF was cooled to -78 °C, and 2.5
mL of 1.6 M n-BuLi in hexanes was added dropwise. After 30
min at -78 °C, a solution of 566 mg of N,N-dimethylbenzamide
in 5 mL of THF was added dropwise. The mixture was allowed to
warm to room temperature, and water was then added. The mixture
was transferred to a separatory funnel with ether, and the ether
extract was washed with water and a saturated NaCl solution and
dried over a mixture of Na2SO4 and MgSO4. After filtration, the
solvents were removed using a rotary evaporator, and the residue
was chromatographed on 10 g of silica gel. The column was eluted
with 5-10% ether in hexanes. A total of 705 mg of product (85%
yield) was obtained in two fractions. The first fraction (440 mg)
was rechromatographed on 9 g of silica gel and eluted with 0-4%
ether in hexanes. A 241 mg sample that contained 94% 8 along
with 6% 15 eluted with 2% ether in hexanes and was used for
using a rotary evaporator, and the residue was chromatographed
on 15 g of silica gel. A total of 460 mg of product (76% yield;
24/25 ratio ) 3.1:1) was obtained. This mixture was used for
photochemical studies. 1H NMR of 24 (R ) CF3) (600 MHz,
CDCl3) δ 7.97 (d, J ) 8.2 Hz, 2 H), 7.33 (d, J ) 8.3 Hz, 2 H),
5.63 (d of d, J ) 2.4, 0.8 Hz, 1 H), 5.56 (d, J ) 1.6 Hz, 1 H), 2.54
(t, J ) 2.0 Hz, 1 H), 1.39 (s, 3 H), 0.89 (s, 3 H). 13C NMR of 24
(R ) CF3) (151 MHz, CDCl3) δ 180.1 (q, J ) 35 Hz), 148.0, 144.4,
129.9 (q, J ) 2 Hz), 129.5, 127.6, 116.8 (q, J ) 292 Hz), 104.3,
32.7, 26.3, 26.0, 18.3. Exact mass (FAB) calcd for C14H13F3O
254.0918, found 254.0921.
1
photochemical studies. H NMR of 8 (600 MHz, CDCl3) δ 7.79
(d, J ) 8.1 Hz, 2 H), 7.73 (d, J ) 8.1 Hz, 2 H), 7.57 (t, J ) 7.4
Hz, 1 H), 7.47 (t, J ) 7.8 Hz, 2 H), 7.28 (d, J ) 8.0 Hz, 2 H), 5.62
(d of d, J ) 2.4, 0.8 Hz, 1 H), 5.57 (d, J ) 1.2 Hz, 1 H), 2.54 (t,
J ) 1.8 Hz, 1 H), 1.38 (s, 3 H), 0.90 (s, 3 H). 13C NMR of 8 (151
MHz, CDCl3) δ 196.5, 145.0, 144.0, 137.9, 135.1, 132.2, 130.01,
129.97, 128.7, 128.2, 103.9, 32.4, 26.2, 25.0, 18.4. Exact mass
(FAB) calcd for C19H18O 262.1358, found 262.1342.
Preparation of Methylenecyclopropane 26. A solution of 60
mg of aldehyde 24 (R ) H)17 in 2 mL of ether was added dropwise
to 0.5 mL of 1.6 M n-BuLi dissolved in 2 mL of ether at -78 °C.
The mixture was warmed to 0 °C, and water was then added. The
ether phase was separated, washed with water and saturated NaCl
solution and dried over a mixture of Na2SO4 and MgSO4. After
filtration, the solvents were removed using a rotary evaporator, and
the crude alcohol product was used in the next step without further
purification.
Preparation of Methylenecyclopropane 24 (R ) CH3). A
solution of 537 mg of bromide i in 5 mL of THF was cooled to
-78 °C, and 1.5 mL of 1.6 M n-BuLi in hexanes was added
dropwise. After 40 min at -78 °C, this solution was transferred
via cannula to a solution of 347 mg of N,N-dimethylacetamide in
10 mL of ether at -78 °C. The mixture was allowed to warm to
room temperature, and water was then added. The mixture was
transferred to a separatory funnel with ether, and the ether extract
was washed with water and saturated NaCl solution and dried over
a mixture of Na2SO4 and MgSO4. After filtration, the solvents were
removed using a rotary evaporator, and the residue was chromato-
graphed on 15 g of silica gel. The column was eluted with 5-15%
ether in hexanes. A total of 230 mg of product (51% yield; 24/25
ratio ) 4.9:1) was obtained. This mixture was used for photochemi-
A mixture of 145 mg of pyridinium chlorochromate in 3 mL of
methylene chloride was stirred at room temperature as a solution
of the crude alcohol prepared as stated previously in 0.5 mL of
CH2Cl2 was added in one portion. The mixture was stirred for 10
h at room temperature, and then 5 mL of pentane was added. The
pentane/CH2Cl2 mixture was filtered through a small amount of
silica gel in a pipet, and the solvents were then removed using a
rotary evaporator. The residue was taken up into 5% ether in pentane
and filtered through 0.3 g of silica gel. Solvent removal gave 61
1
cal studies. H NMR of 24 (R ) CH3) (600 MHz, CDCl3) δ 7.86
1
(d, J ) 8.4 Hz, 2 H), 7.26 (d, J ) 8.4 Hz, 2 H), 5.61 (d of d, J )
2.4, 0.8 Hz, 1 H), 5.56 (d, J ) 1.4 Hz, 1 H), 2.575 (s, 3 H), 2.51
(t, J ) 2.0 Hz, 1 H), 1.37 (s, 3 H), 0.86 (s, 3 H). 13C NMR of 24
(R ) CH3) (151 MHz, CDCl3) δ 197.8, 144.9, 144.6, 134.9, 129.0,
128.1, 103.9, 32.3, 26.5, 26.2, 24.9, 18.3. Exact mass (FAB) calcd
for C14H16O 200.1201, found 200.1194.
Preparation of Methylenecyclopropane 24 (R ) CF3). A
solution of 568 mg of bromide i in 5 mL of THF was cooled to
-78 °C, and 1.6 mL of 1.6 M n-BuLi in hexanes was added
dropwise. After 20 min at -78 °C, this solution was transferred
via cannula to a solution of 520 mg of ethyl trifluoroacetate in 9
mL of ether at -78 °C. The mixture was allowed to warm to room
temperature, and water was then added. The mixture was transferred
to a separatory funnel with ether, and the ether extract was washed
with water and saturated NaCl solution and dried over a mixture
of Na2SO4 and MgSO4. After filtration, the solvents were removed
mg (78% yield) of ketone 26. H NMR of 26 (600 MHz, CDCl3)
δ 7.86 (d, J ) 8.4 Hz, 2 H), 7.25 (d, J ) 8.4 Hz, 2 H), 5.61 (d of
d, J ) 2.4, 0.8 Hz, 1 H), 5.55 (d, J ) 1.4 Hz, 1 H), 2.93 (t, J ) 7.5
Hz, 2 H), 2.50 (t, J ) 2.0 Hz, 1 H), 1.71 (quintet, J ) 7.5 Hz, 2
H), 1.41 (sextet, J ) 7.5 Hz, 2 H), 1.37 (s, 3 H), 0.95 (t, J ) 7.5
Hz, 3 H), 0.86 (s, 3 H). 13C NMR of 26 (151 MHz, CDCl3) δ
200.3, 145.0, 144.3, 134.9, 129.0, 127.9, 103.8, 38.3, 32.4, 26.6,
26.2, 24.9, 22.6, 18.3, 14.0. Exact mass (FAB) calcd for C17H22O
242.1671, found 242.1658.
Photolyses in C6D6. General Procedures. The following
procedure is representative. Benzene-d6 was deoxygenated by briefly
bubbling N2 through the stock sample. A solution of 5.6 mg of 8
in 330 mg of C6D6 was placed in an NMR tube under N2. The
NMR tube was sealed under N2 and placed in a Rayonet
Photochemical Reactor fitted with 350 nm lamps.7 The tube was
irradiated for various time periods at ambient temperature (22 °C)
using the air-cooling provided by the reactor fan. The tube was
1
(23) Creary, X. J. Org. Chem. 1978, 43, 1777.
periodically analyzed by 600 MHz H NMR spectroscopy, and
J. Org. Chem, Vol. 72, No. 21, 2007 7937

Creary et al.
typical data are given in Figure 1. In a separate experiment, a sample
under N2 in an NMR tube was cooled to -78 °C, evacuated at 0.1
mm, and sealed under vacuum before irradiation. There was no
difference in reactivity between the sample sealed under vacuum
and the sample sealed under N2. Most experiments were therefore
carried out in sealed tubes under N2 rather than under vacuum.
Photolyses in i-PrOH. General Procedures. The following
procedure is representative. A solution of 5.9 mg of 8 in 270 mg
of nitrogen purged i-PrOH was sealed in a 3 mm NMR tube under
nitrogen. The NMR tube was irradiated using 350 nm lamps for
various time periods at ambient temperature (22 °C) using the air-
cooling provided by the reactor fan. The tube was periodically
analyzed by 600 MHz 1H NMR spectroscopy, and typical spectral
data are given in Figure 2. On completion of the irradiation, i-PrOH
was removed using a rotary evaporator with the last traces of solvent
being removed at 0.1 mm pressure. The product 15 was identified
i-PrOH (0.082 M) was prepared, and a portion of this solution was
sealed in an NMR tube under nitrogen. The tube was irradiated
using 350 nm lamps for 3.0 min at ambient temperature (22 °C).
Analysis by 600 MHz 1H NMR showed that 30.9% of the
benzophenone had been converted to benzopinacol.
In a similar fashion, a solution of 21.9 mg of 8 in 1.0 mL of
i-PrOH (0.082 M) was prepared and sealed in an NMR tube. The
sample was irradiated for 3.0 min, and this was followed by analysis
by 1H NMR, which showed that 20.5% 8 had converted to 15. The
quantum yield for the disappearance of benzophenone is therefore
1.5 times greater than for conversion of 8 to 15.24
Benzophenone Sensitized Reactions. The following procedure
is representative. A solution of 8.6 mg of 36 in 625 mg of
deoxygenated C6D6 was prepared, and 273 mg of this solution was
placed in a 3 mm NMR tube. The first tube was then sealed under
nitrogen. Benzophenone (4.2 mg) was then added to the remaining
solution, and a second 3 mm NMR tube was prepared from this
solution and sealed under nitrogen. These two samples were
irradiated for various time periods using 360 nm light at ambient
temperature (22 °C). The samples were periodically analyzed by
600 MHz 1H NMR spectroscopy. Typical spectra are given in the
Supporting Information, and quantitative results are shown graphi-
cally in Figure 4.
1
by H NMR spectral comparison with an authentic sample.
Product Analyses. In a separate experiment, a solution of 32
mg of 8 in 1.40 g of C6D6 was irradiated for 100 min using 350
nm lamps. The solvent was removed using a rotary evaporator,
and the residue was placed on a column made from 0.50 g of silica
gel and eluted with 8% ether in hexanes. The solvent was removed
using a rotary evaporator, and the product 15 (30 mg, 94% yield)
was characterized by NMR spectroscopy. 1H NMR of 15 (600 MHz,
CDCl3) δ 7.78 (d, J ) 8 Hz, 2 H), 7.71 (d, J ) 8 Hz, 2 H), 7.57
(t, J ) 8 Hz, 1 H), 7.47 (t, J ) 8 Hz, 2 H), 7.16 (d, J ) 8 Hz, 2
H), 2.63 (m, 1 H), 1.93 (m, 3 H), 1.79 (m, 3 H), 1.78 (m, 3 H),
1.19 (m, 1 H). 13C NMR of 15 (151 MHz, CDCl3) δ 196.4, 148.8,
138.0, 134.8, 132.1, 130.5, 129.9, 128.2, 125.9, 123.5, 119.7, 22.3,
22.2, 20.6. 16.1. Exact mass (FAB) calcd for C19H18O 262.1358,
found 262.1367.
Supporting Information Available: 1H and 13C NMR of 8,
1
15, 24 (R ) CH3), 24 (R ) CF3), and 26, as well as evolving H
NMR spectra during irradiation of 24 (R ) CH3), 26, 30, 36, and
56. This material is available free of charge via the Internet at
http://pubs.acs.org.
JO071114X
All of the other products formed in photochemical studies in
this paper (21, 25, 27, 31, 33, 37, 39, and 48) were characterized
by NMR spectral comparison with samples previously prepared
by thermal rearrangement of the corresponding 1,1-dimethyl-2-aryl-
3-methylenecyclopropanes.13,14,17,23
(24) Previously determined quantum yields for the disappearance of
benzophenone at 335 nm in nitrogen flushed i-PrOH ranged from 0.66 to
1.37.6c In completely degassed i-PrOH, the quantum yield was 1.8-1.9.
Since the theoretical quantum yield for benzophenone disappearance in
i-PrOH is 2.0, the efficiency of conversion of 8 to 15 actually exceeds the
quantum efficiency of benzophenone photoreduction in nitrogen flushed
i-PrOH.
Photolysis of 8 in i-PrOH. Quantum Yield Estimation. A
solution of 15.1 mg of benzophenone in 1.0 mL of nitrogen purged
7938 J. Org. Chem., Vol. 72, No. 21, 2007