Diastereomeric discrimination in the lifetimes of norrish type II triplet 1,4 biradicals and stereocontrolled partitioning of their reactivity (yang cyclization versus type II fragmentation)

Article abstract of DOI:10.1002/chem.200600880

The stereochemistry at C2 and C3 carbons controls the partitioning of triplet 1,4-biradicals of ketones 2 among various pathways. Differences in the major reaction pathways, for example, cyclization (syn) and fragmentation (anti), adopted by the diastereo-meric 1,4-radicals of ketones 2 have permitted unprecedented diastereomeric discrimination in their lifetimes to be observed by nanosecond laser flash photolysis. From quantum yield measurements and transient lifetime data, the absolute rate constants for cyclization and fragmentation of a pair of diastereomeric triplet 1,4-biradicals have been determined for the first time.

Full text of DOI:10.1002/chem.200600880

DOI: 10.1002/chem.200600880
Diastereomeric Discrimination in the Lifetimes of Norrish Type II Triplet
1,4-Biradicals and Stereocontrolled Partitioning of Their Reactivity
(Yang Cyclization versus Type II Fragmentation)
Jarugu Narasimha Moorthy,*[a] Apurba L. Koner,[b] Subhas Samanta,[a] Nidhi Singhal,[a]
Werner M. Nau,*[b] and Richard G. Weiss*[c]
Abstract: The stereochemistry at C2
and C3 carbons controls the partition-
ing of triplet 1,4-biradicals of ketones 2
among various pathways. Differences
in the major reaction pathways, for ex-
ample, cyclization (syn) and fragmenta-
tion (anti), adopted by the diastereo-
meric 1,4-radicals of ketones 2 have
permitted unprecedented diastereomer-
ic discrimination in their lifetimes to be
observed by nanosecond laser flash
photolysis. From quantum yield meas-
urements and transient lifetime data,
the absolute rate constants for cycliza-
tion and fragmentation of a pair of dia-
stereomeric triplet 1,4-biradicals have
been determined for the first time.
Keywords:
diastereoselectivity
·
ketones · photolysis · radicals
Introduction
crystals[7] and in polymer films[8] with less well-defined and
more dynamic cavities.[1d] An entirely different approach, in-
volving intramolecularly designed control, which has been
adopted by us[9] entails the imposition of steric interactions
about contiguous chiral centers, such that the resulting dia-
stereomers exhibit distinct conformational preferences; re-
actions in which the diastereomers exhibit differing reactivi-
ties are termed “diastereomer-differentiating”.[10] This strat-
egy has been exploited to demonstrate a remarkable diaster-
eomeric discrimination in triplet lifetimes of a diketone.[11]
Scheme 1is a paradigm for Norrish–Yang Type II reac-
tions.[12] Initial g-hydrogen abstraction from the triplet states
of aromatic ketones (R1 =phenyl) leads to a cisoid confor-
mation for the 1,4-biradicals. A subsequent twisting motion
Conformational control of reactivity of intermediates can be
achieved, in principle, by restricting their mobility. This has
been achieved environmentally (for example, intermolecu-
larly) in several ways. An extreme is reaction within the
neat crystalline state in which lattice forces can control the
fates of intermediates generated both thermally and photo-
chemically.[1] Usually, less specific environmental control is
exerted when reactions are conducted in “container” sys-
tems, such as cyclodextrins,[2] calixarenes,[3] cucurbiturils,[4]
etc.,[5] in zeolites[6] with well-defined cavities, or in liquid
ꢀ
about the C1 C2 bond is necessary to attain another cisoid
[a] Prof. Dr. J. N. Moorthy, S. Samanta, N. Singhal
Department of Chemistry, Indian Institute of Technology
Kanpur 208 016 (India)
conformation (shown in Scheme 1), a requisite for cycliza-
tion, in which the two singly-occupied orbitals are directed
toward each other. Although the fragmentation products
may derive from both cisoid and transoid conformations, the
fragmentation is generally ascribed to transoid conforma-
tions.[12,13] While the geometries of triplet 1,4-biradicals
(3BR) appear to control the product profiles, there is no
clear understanding until now of how they partition them-
selves among the various pathways shown in Scheme 1.[14] In
view of this, understanding of the factors that influence the
lifetimes of triplet 1,4-biradicals (tBR), which are also inter-
mediates in reactions, such as Paterno–Büchi and enone-
olefin photocycloadditions,[15] and their behavior[16] contin-
ues to be of significant contemporary interest.[17] Insofar as
Fax : (+91)512-259-7436
E-mail: moorthy@iitk.ac.in
[b] A. L. Koner, Prof. Dr. W. M. Nau
School of Engineering and Science, Campus Ring 1
International University Bremen, 28725 Bremen (Germany)
Fax : (+49)421-200-3229
E-mail: w.nau@iu-bremen.de
[c] Prof. Dr. R. G. Weiss
Department of Chemistry, Georgetown University
Washington, DC 20057-1227 (USA)
Fax : (+1)202-687-6209
E-mail: weissr@georgetown.edu
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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FULL PAPER
for the cyclization and frag-
mentation pathways of the dia-
stereomeric 3BR have been
calculated from lifetime and
quantum yield measurements.
The subtle differences among
the rates provide unprecedent-
ed insights into the relation-
ships among the competing
processes.
Scheme 1. A representative mechanistic picture for Norrish–Yang Type II reactions.
Results and Discussion
the Norrish Type II reaction is concerned, its potential as
The diastereomers of ketone 2
were synthesized by 1,4-addition of MeMgI to PhCOC-
(CH3)=CHCH2Ph. Because the separation of the two dia-
applied to photorelease processes,[18] photocyclization reac-
tions,[19] asymmetric synthesis,[20] etc. has begun to unfold
only recently.
AHCTREUNG
stereomers could not be achieved by silica-gel chromatogra-
phy using a variety of solvent systems, an indirect route was
followed. The diastereomeric mixture was first reduced with
NaBH4 to afford a mixture of four diastereomeric alcohols,
which could be separated fractionally by TLC using diethyl
ether/petroleum ether 25:75 as eluent. Gravity column chro-
matography of the mixture afforded three alcohols (one as a
mixture), which upon oxidation yielded pure syn and anti
isomers of ketone 2. Configurational assignment of the dia-
We have recently shown that different triplet lifetimes,
caused by conformational preferences intrinsic to the dia-
stereomers of a,b-disubstituted ketones, such as 1, can lead
to reactivity differences.[9,21] In this instance, rapid Yang cyc-
lization and Norrish Type II fragmentation, accentuated by
the formation of thermodynamically stable styrene/stilbene
products (R=Me/Ph), precluded direct observation of the
intermediary triplet 1,4-biradicals by nanosecond transient
absorption spectroscopy and, hence, incontrovertible evi-
dence for the hypothesized link between diastereomeric dis-
crimination and biradical lifetimes. In pursuit of the latter,
we designed ketones 2 based on the following rationale:
1) the 3BR of 2 are bisbenzylic and hence should be relative-
ly longer lived than those of 1 and 2) they should be less
prone to fragment, further increasing the tBR of 2 with re-
spect to those of 1 with R = Ph (in which fragmentation
leads to a thermodynamically more stable alkene).
1
stereomers was based on comparisons of H NMR spectro-
scopic chemical shifts of diagnostic methyl signals with those
of analogous ketones reported in the literature.[9,23] Further-
more, the a-methyl carbon atoms of the syn and anti diaster-
eomers of analogous ketones have been noted in the litera-
ture to exhibit distinct 13C NMR spectroscopic chemical
shifts:[24] the signal from the a-methyl carbon of the analo-
gous syn diastereomer appears approximately 2–5 ppm fur-
ther upfield than that of the anti. Thus, the 13C NMR signal
from the a-methyl carbon of
the diastereomer of 2 assigned
as syn based upon the
1H NMR spectroscopic chemi-
cal shifts appears at 15.13 ppm,
while that of the anti diaster-
eomer resonates at 17.8 ppm.
Cyclization and fragmenta-
tion photoproducts accounted
for >90% of consumed
ketone when the diastereomers
of 2 were irradiated (lirrad ca.
350 nm)
at
approximately
258C in solvents, such as
chloroform, cyclohexane, etc.
Here, we report the results of our investigation involving
the unprecedented diastereomeric discrimination in tBR gen-
erated by irradiation of the diastereomeric a,b-dimethyl-g-
phenyl-butyrophenones 2 and stereocontrolled partitioning
of their reactivity between Yang cyclization and Norrish
Type II fragmentation.[22] Furthermore, and perhaps of even
greater mechanistic importance, the absolute rate constants
Whereas irradiation of the syn diastereomer led mostly to
cyclization products (approximately 66% relative yield)
with >90% diastereoselectivity as indicated by 1H NMR
spectroscopy, the anti diastereomer afforded predominantly
fragmentation products (>85% relative yield, Scheme 2).
The cyclobutanol (CB) from the syn diastereomer was iso-
lated from a preparative-scale irradiation, characterized by
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8745

J. N. Moorthy, W. M. Nau, R. G. Weiss et al.
Scheme 2. Mechanistic rationalization of distinct photochemical fates observed for syn and anti diastereomers of ketone 2.
1
IR and H and 13C NMR spectroscopies, and its stereochem-
Furthermore, the disappearance quantum yields (F) for the
syn and anti diastereomers upon irradiation at 313 nm in cy-
clohexane[25] were determined to be 0.52ꢁ0.02 and 0.31ꢁ
0.02, respectively.
istry was deduced from NOESY experiments (see Support-
ing Information).
Irradiations of both diastereomers in cyclohexane were
examined in detail in view of their very different tBR in this
solvent (vide infra). Thus, photoproduct distributions from
approximately 0.03m cyclohexane solutions of each diaster-
eomer in NMR tubes under nitrogen atmospheres were de-
Laser flash photolyses (XeCl excimer laser, 308 nm and
10 ns pulses of 80 mJ energy) of solutions of the diastereom-
ers of 2 in a number of solvents led to transient absorption
spectra, such as those in Figure 1. The transient spectra are
virtually identical in position and shape, as expected. Each
has the attributes of a benzylic/ketyl radical:[26] a strong ab-
sorption (OD ca. 0.05–0.10) at 310–320 nm and a weak,
broad absorption with a shoulder in the region around 400–
470 nm. Also, the absorption was efficiently quenched by
oxygen, and the decay times of the transients are similar to
those reported for a 1,4-biradical from g-phenylbutyrophe-
none in MeOH (146ꢁ19 ns) and in a low polarity solvent,
such as heptane (55ꢁ8 ns).[27] Based upon these observa-
tions, the transient absorptions in Figure 1are attributed to
3BR from the diastereomers of 2.[28]
1
termined directly by H NMR spectroscopy using the homo-
nuclear gated decoupling (hmg) technique. The cyclization/
fragmentation product ratio was approximately 2:1from the
syn diastereomer and 15:85 from the anti (Scheme 2). The
cyclobutanol shown in Scheme 2 from the anti isomer was
detected in very low yield (approximately 6–7%), and its
isolation from a preparative photolysis of approximately
1mmol of the ketone was complicated by formation of a
nearly equal amount of an inseparable second cyclobutanol
derived by means of hydrogen abstraction from the b-
methyl group. The major fragmentation pathway for the anti
does not involve the “minor” 3BR resulting from the ab-
straction of hydrogen from the b-methyl group, because b-
methylstyrene (along with propiophenone) is the almost ex-
clusively detected fragmentation alkene (Scheme 2); very
little (if any) 3-phenylpropene, the olefin expected from
fragmentation of the “minor” 1,4-biradical, was detectable
The decays monitored at 320 and 420–440 nm were clearly
monoexponential and had the same decay constants. The
3
lifetimes thus determined for the diastereomeric BR of 2 in
a variety of solvents are collected in Table 1. Small differen-
ces in the lifetimes for the diastereomeric biradicals were
observed in a number of polar and low-polarity protic and
aprotic solvents. However, an unprecedented discrimination
in the tBR was found in cyclohexane, a relatively viscous and
low-polarity solvent.[29] In at least six independent determi-
nations, the tBR from the two diastereomers reproducibly ex-
1
by H NMR spectroscopic analyses. Thus, with a maximum
limit of 7% relative yield for the cyclobutanol from the
“major” 1,4-biradical and 85–87% yield for fragmentation,
the cyclization/fragmentation ratio is estimated to be 1:10.
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Norrish Type II Triplet 1,4-Biradicals
FULL PAPER
tions between the a,b-dimethyl substituents should cause
conformational preferences of the ketones such that their
3
respective BR, generated subsequent to photolysis, are also
similarly influenced. Furthermore, steric repulsions between
the C2 and C3 methyl substituents must disfavor the attain-
3
ment of a cisoid conformation of the anti BR; the confor-
mational preference and the source of most reactivity
should be from the transoid conformation. The opposite
conformational preference (and source of reaction) is pre-
3
dicted by the same steric arguments for the syn BR; the
cisoid conformation must be favored energetically. Thus, the
observed predominant fragmentation from the anti diaster-
eomer and high cyclization yields from the syn diaster-
eoisomer are in accord with expectations based on simple
arguments for stereochemical preferences of the diastereom-
ers of 2 and their 1,4-biradicals. Although the cisoid con-
former may, in principle, lead to fragmentation, it is less
likely because of the stringent stereoelectronic require-
ments,[13] involving the overlap of the singly-occupied orbi-
tals at C1and C4 with the C2–C3 sigma orbitals and the
steric crowding that this geometry entails.
Figure 1. Transient absorption spectra attributed to 3BR derived from
&
*
syn-2 ( ) and anti-2 ( ) in cyclohexane after a pulse delay time of 35 and
65 ns, respectively. Inset shows the decay profiles monitored at 320 nm
(slow for anti and fast for syn) of the diastereomeric 3BR; the relatively
higher OD after the decay of the biradical in the case of the anti diaster-
eomer is unquestionably due to stronger absorbance of the derived frag-
mentation photoproducts.
Inherent to the stereocontrolled partitioning of the dia-
stereomeric biradical reactivity discussed above is the possi-
bility to observe differential rates for their decay. The two
tBR, especially in cyclohexane, demonstrate that their very
different chemical fates (for example, predominantly frag-
mentation or cyclization) occur at distinctly different rates;
although the observed differences in the tBR are small such
a discrimination has not been heretofore scrutinized. By
using a simplified form of the mechanism in Schemes 1
and 2, in which kE refers to fragmentation from both the
3
Table 1. Lifetimes of BR (ns) derived from the diastereomers of 2.[a,b]
Entry
Solvent
syn-2
anti-2
1
2
3
4
5
6
7
8
acetonitrile
benzene
methanol
chloroform
n-hexane
cyclohexane
tBuOH/ethylene glycol 3:2
benzene/water 95:5
134
65
158
59
47
46
142
71
161
59
50
61
173
74
194
93
3
transoid and cisoid BR precursors, the experimentally-de-
3
termined BR decay rate constants and absolute rate con-
[a] Monitoring wavelength=320 nm. [b] Error=ꢁ5%.
stants for each of the reactive pathways followed by the dia-
stereomeric triplet 1,4-biradicals can be calculated from dis-
appearance quantum yields and the relative cyclization and
fragmentation photoproduct yields.
hibited a lifetime difference of 15 ns, representing approxi-
mately 30% of the average. A similar trend, albeit less pro-
nounced, was observed in two mixed solvent systems,
tBuOH/ethylene glycol 3:2 and benzene/water 95:5. More
specifically, the tBR from the syn diastereomer (which
decays to cyclobutanol as the major product) were consis-
tently shorter than those from the anti (which fragments
principally).
As the triplet states of phenyl alkyl ketones and the trip-
let 1,4-biradicals are formed with unit efficiency,[12,13] the
quantum yields for fragmentation (FE) and cyclization (FC)
kE
kC
are given by: FE =
and FC =
in which kE, kC,
kEþkCþkR
kEþkCþkR
and kR are the rate constants for fragmentation, cyclization,
3
The BR may decay by three possible unimolecular path-
and reversion of the 3BR to 2, respectively. Accordingly,
E
C
ways, denoted by kC, kE, and kR in Scheme 1, which involve
intersystem crossing.[12,15] As mentioned earlier, there is still
no clear understanding of how biradical structure affects its
partitioning among the three pathways,[14] although certain
structural features of biradical reactivity are established.[12,13]
We have recently shown that the diastereomer-differentiat-
ing photochemical reactions of 1 can be rationalized based
on 1) stereocontrolled stabilization of the geometry (cisoid/
kE = F and kC = F
.
tBR
tBR
Based on the relative product ratios for the two diaster-
eomers, the total quantum yield for ketone disappearance
(FE +FC), the decay constants in cyclohexane (1/tBR), and
the absolute rate constants for cyclization and fragmentation
of the diastereomeric BR are calculated to be: kCanti =0.46”
3
syn
106 sꢀ1, kEanti =4.6”106 sꢀ1[30] and kCsyn =7.5”106 sꢀ1, kE
=
3.8”106 sꢀ1. As 1 t/BR
(kC+kE)=kR, its values are calculated
ꢀ
A
3
transoid) of the BR and 2) the premise that cyclization will
be observed only if a BR assumes a cisoid geometry while
fragmentation occurs predominantly from the transoid ge-
to be 11.5”106 sꢀ1 (anti) and 11.3”106 sꢀ1 (syn). Clearly, the
difference in the overall quantum yields for consumption of
the two diastereomers of 2 is governed by their rates of cycli-
zation; the values of kE and kR are nearly the same for the two
diastereomers! These data demonstrate the importance of
diastereocontrol over the conformations of the 1,4-biradicals.
3
ometry.[9]
We apply similar considerations to 2 as well (Scheme 2,
double arrows signal steric congestion). The steric interac-
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8747

J. N. Moorthy, W. M. Nau, R. G. Weiss et al.
5.7 Hz, 2H; arom. H), 7.17–7.33 (m, 5H; arom. H), 3.36–3.42 (m, 1H;
CH), 2.61–2.68 (m, 1H; CH), 2.50–2.56 (m, 1H; CH), 2.22–2.31 (m, 1H;
CH), 1.15 (d, 3H, J=5.1Hz; CH 3), 0.83 (d, 3H, J=5.1Hz; CH 3);
13C NMR (75 MHz, CDCl3): d=204.1(PhCO), 140.8 (C ar), 136.7 (Car),
132.8 (CarH), 129.3 (CarH), 128.6 (CarH), 128.4 (CarH), 128.3 (CarH), 126.2
(CarH), 43.4 (CH), 41.8 (CH2), 37.4 (CH), 15.1 (CH3), 10.8 (CH3); IR
Conclusion
We have shown that the partitioning of the 3BR between
cyclization (cisoid) and fragmentation (transoid) pathways
can be controlled by means of steric interactions built
around substituents at C2 and C3 of ketone 2. The two dia-
(neat): n˜ =1680 cmꢀ1
.
3
Photolysis procedure: Solutions of a diastereomer of ketone 2 (approxi-
mately 6–8 mg) in 0.6 mL of CDCl3 in NMR tubes were purged with ni-
trogen gas and irradiated in a Luzchem photoreactor fitted with 350 nm
lamps. Disappearance of the ketone was monitored periodically by
1H NMR spectroscopic analyses. To quantify yields of products, a small
amount of methyl benzoate was added to each tube as an internal stand-
ard.
stereomeric BR derived from irradiation of 2 collapse by
distinct pathways—cyclization, fragmentation and return to
2—that can be discriminated kinetically to correlate struc-
ture and reactivity with unprecedented detail. To the best of
our knowledge, these are the first absolute rate constant de-
terminations for cyclization and fragmentation of a pair of
diastereomeric triplet 1,4-biradicals. These results offer ex-
tremely detailed insights into the motions that triplet 1,4-bi-
radicals must undergo to decay to singlet ground-state prod-
ucts. Clearly, other substitution patterns will expand our
knowledge of the motions of 1,4-biradicals.
The cyclobutanol product from the syn diastereomer (syn-CB-2) was iso-
lated by irradiating the ketone (approximatley 150 mg) in chloroform
(100 mL). The photolysate was monitored periodically by TLC analysis
for disappearance of the ketone. After complete conversion, the solvent
was removed under reduced pressure and the residue was purified by
column chromatography on silica gel (EtOAc/hexane 20:80).
syn-CB-2: Colorless liquid; 1H NMR (300 MHz, CDCl3): d=7.20–7.45
(m, 10H; arom. H), 3.39 (d, J=7.5 Hz, 1H; CH), 2.51–2.53 (m, 1H; CH),
2.18–2.20 (m, 1H; CH), 1.23 (d, J=5.1Hz, 3H; CH 3), 1.13 (d, J=5.1Hz,
3H; CH3); 13C NMR (75 MHz, CDCl3): d=146.3 (Car), 137.4 (Car), 128.6
(x2) (CarH), 128.5 (CarH), 128.4 (CarH), 127.0 (CarH), 125.2 (CarH), 78.8
(Cal), 56.3 (CH), 46.7 (CH), 37.3 (CH), 18.8 (CH3), 11.2 (CH3); IR (neat):
Experimental Section
Preparation of a,b-dimethyl-g-phenylbutyrophenones 2: The required
precursor a,b-unsaturated ketone, namely 1,4-diphenyl-2-methyl-2-bute-
none, was prepared according to the procedure already reported by us.[9]
n˜ =3364 cmꢀ1 (br), 2926 cmꢀ1
.
Determination of quantum yields: The quantum yields were determined
by using valerophenone as an actinometer (F313nm =0.33 for the forma-
tion of acetophenone).[25] For quantum yield measurements, a solution of
a diastereomer of 2 in cyclohexane (approximatley 0.04m) and a small
amount of methyl benzoate as an internal standard were irradiated
(313 nm) by using a high-pressure Hg lamp (Applied Photophysics)
equipped with a monochromator. Conversion of the ketones was limited
to 15–16% and analyses were performed by gas chromatography.
Methyl iodide (10.09 g, 71.16 mmol) was added to the Mg turnings (1.7 g,
71.16 mmol) suspended in dry ether (50 mL) followed by a catalytic
amount of iodine. The reaction mixture was stirred at room temperature
until the Mg turnings disappeared. The Grignard reagent was added to a
solution of cuprous iodide (6.77 g, 35.58 mmol) in THF (25 mL) at
ꢀ408C through a cannula. The resulting heterogeneous mixture was stir-
red at ꢀ408C for 2 h. After this time, 1,4-diphenyl-2-methyl-2-butenone
(2.8 g, 11.86 mmol) in THF (25 mL) was added to this mixture at the
same temperature and stirred for 2 h. Subsequently, the reaction mixture
was warmed up and stirred overnight at room temperature. The reaction
was quenched with saturated NH4Cl and extracted with EtOAc. The or-
ganic layer was washed with brine solution, dried over anhydrous
Na2SO4, and the solvent removed in vacuo. The crude compound was pu-
rified by silica gel column chromatography (2.5% EtOAc/petroleum
ether), yielding the diastereomeric mixture (syn and anti) in approximate-
ly 32% yield.
Laser flash photolyses: Experiments were carried out with an LKS.60/S
nanosecond laser flash photolysis spectrometer (Applied Photophysics)
with a GSI Lumonics Pulsemaster PM-846 excimer laser running on
XeCl for excitation (308 nm, approximatley 80 mJ pulse energy, 10 ns
pulse width). The transient data were recorded with a 54830B 600 MHz
Infinium oscilloscope (Agilent Technologies) and processed with the in-
strument-supplied software. Transient spectra were recorded in a step-
scan mode in 10 nm intervals. Transient decay traces were recorded near
the transient absorption maxima at approximately 320 and 450 nm. Sam-
ples (approximately 3 mL) were prepared in fused long-neck quartz cuv-
ettes to allow bubbling with dry nitrogen gas for 15 min to remove
oxygen. The optical densities of all samples were adjusted to 0.4ꢁ0.1at
308 nm by using a Cary50 UV spectrophotometer (Varian).
NaBH4 reduction of the mixture of diastereomers of 2 and reoxidation of
separated alcohols: To the mixture of diastereomers (0.930 g, 3.69 mmol)
in EtOH (40 mL) was added NaBH4 (0.084 g, 2.21mmol). After refluxing
for 2 h, the reaction mixture was quenched with 10% aqueous HCl
(10 mL) and the volume was reduced to approximately 20 mL. The resul-
tant mixture was extracted with EtOAc and the organic layer was
washed with brine solution, dried over anhydrous Na2SO4, and concen-
trated in vacuo. The crude diastereomeric mixture of the alcohols, isolat-
ed in near quantitative yields (>96%) was purified by silica gel column
chromatography (diethyl ether/petroleum ether 6:94) to obtain three
fractions of alcohols. PCC (pyridinium chlorochromate) oxidation of
each alcohol led to pure syn and anti diastereomers as revealed by
1H NMR spectroscopy in a quantitative yield.
Acknowledgements
J.N.M. is grateful to Department of Science and Technology (DST, India)
and the Alexander von Humboldt (AvH) Foundation (Germany) for sup-
porting a collaborative effort at the International University Bremen,
Bremen. The US National Science Foundation is thanked for its support,
allowing R.G.W. to participate in this project.
1
anti-2: Colorless viscous oil; H NMR (300 MHz, CDCl3): d=7.91(d, J=
5.4 Hz, 2H; arom. H), 7.56 (t, J=5.3 Hz, 1H; arom. H), 7.46 (t, J=
5.7 Hz, 2H; arom. H), 7.13–7.22 (m, 3H; arom. H), 7.04 (d, J=5.7 Hz,
2H; arom. H), 3.42–3.49 (m, 1H; CH), 2.82–2.89 (m, 1H; CH), 2.16–2.27
(m, 2H; CH2), 1.26 (d, J=5.1Hz, 3H; CH 3), 0.86 (d, J=4.5 Hz, 3H;
CH3); 13C NMR (75 MHz, CDCl3): d=204.6 (PhCO), 140.9 (Car), 137.5
(Car), 132.9 (CarH), 129.2 (CarH), 128.7 (CarH), 128.3 (CarH), 128.26
(CarH), 126.0 (CarH), 45.9 (CH), 39.2 (CH2), 38.0 (CH), 17.8 (CH3), 13.8
[1] a) J. R. Scheffer, M. A. Garcia-Garibay, O. Nalamasu, Org. Photo-
chem. 1987, 8, 249; b) A. E. Keating, M. A. Garcia-Garibay in Mo-
lecular and Supramolecular Photochemistry,Vol. 2 (Eds.: V. Rama-
murthy, K. S. Schanze), Marcel Dekker, New York, 1998, Chapter 5;
c) M. A. Garcia-Garibay, Acc. Chem. Res. 2003, 36, 491; d) R. G.
Weiss, V. Ramamurthy, G. S. Hammond, Acc. Chem. Res. 1993, 26,
530.
(CH3); IR (neat): n˜ =1680 cmꢀ1
.
syn-2: Colorless viscous oil; 1H NMR (300 MHz, CDCl3): d=7.69 (d, J=
5.4 Hz, 2H; arom. H), 7.50 (t, J=5.6 Hz, 1H; arom. H), 7.37 (t, J=
8748
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Chem. Eur. J. 2006, 12, 8744 – 8749

Norrish Type II Triplet 1,4-Biradicals
FULL PAPER
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Received: June 22, 2006
Published online: August 25, 2006
Chem. Eur. J. 2006, 12, 8744 – 8749
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8749