Paterno-Buechi reactions of allylic alcohols and acetates with aldehydes: Hydrogen-bond interaction in the excited singlet and triplet states? [2]

Full text of DOI:10.1021/ja015656m

J. Am. Chem. Soc. 2001, 123, 6191-6192
6191
Patern o` -B u1 chi Reactions of Allylic Alcohols and
Scheme 1. PBR of Prenol and Prenyl Acetate
Acetates with Aldehydes: Hydrogen-Bond
Interaction in the Excited Singlet and Triplet States?
Axel G. Griesbeck* and Samir Bondock
Institute of Organic Chemistry
UniVersity of Cologne, Greinstrasse 4
D-50939 K o¨ ln, Germany
ReceiVed February 10, 2001
ReVised Manuscript ReceiVed April 19, 2001
Table 1. PBR of Prenol and Prenyl Acetate
The concept of hydrogen-bonding as a tool for directing
1
R )
cis/trans-3
cis/trans-4
photochemical reactions has been reported for intermolecular
1
Me (a)
Et (b)
Bu (c)
81:19
86:14
83:17
77:23
81:19
80:20
93:7
[
2 + 2] and [4 + 4]-cycloadditions. The first example of such
a Patern o` -B u¨ chi reaction (carbonyl-ene-photocycloaddition) with
i
excellent diastereoselectivity control was described recently by
Ph (d)
>97:3
2
Adam and co-workers. In this reaction, a chiral allylic alcohol
interacts with the first excited triplet state of benzophenone.
Direction and magnitude of stereocontrol resembled the corre-
3
sponding singlet oxygen ene reactions with the same substrates.
At first glance, the first excited state of molecular oxygen and an
electronically excited nπ* state (singlet as well as triplet) of a
carbonyl compound behave similar in their reactivity, that is, both
are electrophilic species and prefer similar interaction paths with
unsaturated substrates. With respect to the state-of-the-art mecha-
nistic concepts, however, both reactions strongly differ, and the
parallelism in stereocontrol is striking. For the singlet oxygen
Figure 1. Simple diastereoselectivity: triplet biradical geometries.
well as triplet excited states were applied (all 0.1 M in benzene).
From all substrate combinations, the cis-oxetanes were formed
as the major diastereoisomers in moderate to good yields. The
relative configurations of the major diastereoisomers were
unambiguously determined from the NOE effects which were
detected for one of the gem-methyl groups by saturation of both
methine H and by NOE between the methylene H of the two
side chains. The d.r. values were nearly identical for the three
aliphatic aldehydes in the reaction with prenol but slightly
higher than with prenyl acetate. Benzaldehyde gave exclusively
(>97:3) the cis-diasteroisomer 3d from 1, with prenyl acetate a
4
ene reaction, a perepoxide mechanism is favored, Patern o` -B u¨ chi
reactions with singlet excited carbonyls proceed via conical
5
intersections, triplet carbonyl-ene reactions pass through triplet
6
1
,4-biradical intermediates. Prior to these intermediates, exci-
plexes have been postulated which might also be sensitive toward
7
hydrogen bonding interaction. On the other hand, it was shown
that the nπ* excitation of carbonyls in the presence of protic
8
solvents is blue-shifted, that is hydrogen bonding increases the
9
excitation energy, a well-known fact in Norrish chemistry. A
93:7 mixture of 4d was isolated (Scheme 1).
appropriate mechanistic concept which explains the extent and
direction of stereocontrol is still lacking. Thus, we addressed the
following questions: (a) is there a specific spin-directing effect
connected with hydrogen-bonding, (b) do hydrogen-bonding
interactions influence induced as well as noninduced (“simple”)
diastereoselectivity of the Patern o` -B u¨ chi reaction, (c) does
hydrogen-bonding effect the rate of the Patern o` -B u¨ chi reaction?
The role of hydrogen-bonding interactions in the Patern o` -
B u¨ chi reaction (PBR) of allylic alcohols can be easily tested by
comparison of the free alcohols with the O-protected substrates,
for example, the corresponding acetates. As a typical triplet
precursor, benzaldehyde was irradiated in the presence of prenol
Conclusion I. The regioselectivity of the PBR with 1 and 2 is
high (ca. 20% regioisomeric oxetanes for benzaldehyde addition,
<
5% from aliphatic aldehydes) and corresponds to the classical
7,10b
biradical stabilization concept.
This comparison showed that
hydrogen-bonding is not responsible for controling product
regiochemistry, at least not in these cases. The simple diastereo-
selectivity is moderate with aliphatic aldehydes and high for
benzaldehyde. Comparing prenol with its acetate shows, that
hydrogen bonding might slightly increase the cis/trans-selectivity,
but also other reasons such as steric effects can explain the
marginal differences. From the numbers in Table 1, one can
unambiguously derive that the triplet excited carbonyl state reacts
highly cis-selective with prenol (to give 3d) and prenyl acetate
(1) and prenyl acetate (2), respectively. Additionally, three
aliphatic aldehydes which can react either from their singlet as
(
to give 4d). This selectivity can be rationalized by the assumption
(
1) (a) Yokoyama, A.; Mizuno, K. Org. Lett. 2000, 2, 3457-3459. (b)
of spin-orbit coupling (SOC)-controlled intersystem crossing
Bach, T.; Bergmann, H.; Harms, K. J. Am. Chem. Soc. 1999, 121, 10650-
10
(
ISC) geometries at the stage of the triplet 1,4-biradical.
1
1
0651. (c) Sieburth, S. McN.; McGee, Jr., K. F. Org. Lett. 1999, 1, 1775-
777. (d) Sydnes, L. K.; Hansen, K. I.; Oldroyd, D. L.; Weedon, A. C.;
Optimal φ-values for strong SOC are expected in the 90 (
Jorgensen, E. Acta Chem. Scan. 1993, 47, 916-924.
11
1
(
0° range. This model presupposes conformational flexibility
(
2) Adam, W.; Peters, K.; Peters, E.-M.; Stegmann, V. R. J. Am. Chem.
Soc. 2000, 122, 2958-2959.
3) Adam, W.; Prein, M. Angew. Chem., Int. Ed. Engl. 1996, 35, 477-
and mobility) at this intermediate triplet stage in contrast to the
(
corresponding singlet reactions where conical intersections guide
4
94.
5
the substrate to the products nearly barrier-free. A view along
(
4) Stratakis, M.; Orfanopoulos, M. Tetrahedron 2000, 56, 1595-1615.
3
the C3-C4 bond (Figure 1) of the intermediate 1,4- BR shows
(
5) Palmer, I. J.; Ragazos, I. N.; Bernardi, F.; Olivucci, M.; Robb, M. A.
J. Am. Chem. Soc. 1994, 116, 2121-2132.
the relevant contribution to the high degree of stereoselectivity:
increasing gauche interactions were expected for biradical ap-
proach from the same half space as the hydroxymethyl or
(
6) (a) Freilich, S. C.; Peters, K. S. J. Am. Chem. Soc. 1985, 107, 3819-
3
6
822. (b) Freilich, S. C.; Peters, K. S. J. Am. Chem. Soc. 1981, 103, 6255-
257.
(
7) Turro, N. J. Modern Molecular Photochemistry; University Science
Books: Sausalito, 1991.
(10) (a) Griesbeck, A. G.; Mauder, H.; Stadtm u¨ ller, S. Acc. Chem. Res.
1994, 27, 70-76. (b) Griesbeck, A. G.; Buhr, S.; Fiege, M.; Lex, J.;
Schmickler, H. J. Org. Chem. 1998, 63, 3847-3854.
(11) Carlacci, L.; Doubleday: C., Jr.; Furlani, T. R.; King, H. F.; McIver,
J. W., Jr. J. Am. Chem. Soc. 1987, 109, 5323-5329.
(
8) Liao, D.-W.; Mebel, A. M.; Chen, Y.-T.; Lin, S.-H. J. Phys. Chem. A
997, 101, 9925-9934.
9) Wagner, P. J.; Park, B.-S. Org. Photochem. (Padwa, A., ed.) 1991, 11,
27-366.
1
(
2
1
0.1021/ja015656m CCC: $20.00 © 2001 American Chemical Society
Published on Web 06/02/2001

6192 J. Am. Chem. Soc., Vol. 123, No. 25, 2001
Communications to the Editor
Scheme 2. Competition Experiments
Table 2. Results of Competition Experiment
1
2
entry
R )
R )
concentration, M
3,4:5
A
B
C
D
Ph
Et
Et
Et
H
H
H
Ac
1.0
1.0
0.01
1.0
47:53
55:45
60:40
13:87
Scheme 3. PBR of RCHO with Chiral Allylic Alcohol/
Acetate 6 and 8
Figure 2. Concentration dependence of propionaldehyde PBR with
and 2.
1
acetoxymethyl group, respectively. An additional internal hydro-
2
gen bond between the hydroxy group (R ) H) and the oxygen
3
atom at position 2 of the 1,4- BR slightly increases this confor-
mational preference.12
To evaluate the differences in simple diastereoselectivity of
the PBR in the triplet versus the singlet channel, we investigated
the concentration dependence of the reaction of propionaldehyde
with 1 and 2. As already described for analogous reactions with
15
prenyl acetate is approximately 3 which can originate either from
hydrogen-bonding activation or simply from electronic deactiva-
tion of the alkene.
2
,3-dihydrofuran as alkene component,13 the spin profile of a
bimolecular photochemical reaction can be mapped by variation
of substrate concentration, provided that the lifetime of the excited
singlet state allows diffusion-controlled processes. This is the case
As recently published, chiral allylic alcohols react with triplet
excited benzophenone with high threo-selectivity. This behavior
was also observed for other triplet excited aromatic carbonyl
compounds. Aliphatic aldehydes, which showed moderate cis/
trans-selectivity with prenol, gave high simple and induced
diastereoselectivity, for example propionaldehyde added to 6 to
give only one diastereoisomer 7b in >95% selectivity.
When the corresponding allylic acetate 8 was reacted with
aliphatic aldehydes, a mixture of all four diastereoisomers resulted,
3
14
for aliphatic aldehydes which have lifetimes in the 1-2 ns range.
The results are shown in Figure 2.
16
At higher alkene concentration (alkene and aldehyde were used
in equimolar concentrations) the simple cis/trans-stereoselectivity
is identical (ca. 2:1) with both substrates 1 and 2. In the low
concentration region, the selectivity increases constantly to values
>
9:1 always with the prenol reactions being slightly more
selective.
Conclusion II. The curvature of the spin map indicated, that
and the threo/erythro-selectivity for the cis-isomers dropped
17
(Scheme 3).
Conclusion III. In light of the mechanistic picture shown in
very high selectivities are expected for “pure” triplet reactions,
wheras the singlet excited carbonyls only give moderate (if any)
selectivity. This perfectly corresponds to our recent results on
the spin-directed PBR with cyclic alkenes and clearly shows that
the SOC-determined ISC-geometry model is a powerful rationale
for explaining simple diastereoselectivities originating from 1,n-
triplet biradical combinations.12
Further competition experiments were performed to examine
the difference in reactivity comparing free and O-protected allylic
alcohols. As standard olefin, we used 2,3-dihydrofuran in equimo-
lar concentrations (Scheme 2). First, benzaldehyde was applied
as triplet state precursor (entry A, Table 2).
From the ratio of prenol-cycloadduct 3d and the dihydrofuran-
product 5, it was concluded that these alkenes have similar
reactivities with triplet nπ* carbonyls. The results with propion-
aldehyde corroborated this result for the singlet (high concentrated
region, entry B) and the triplet (low concentrated region, entry
C) nπ* state of an aliphatic aldehyde. Prenyl acetate (2) gave
the adduct 4b in minor quantities and the dihydrofuran adduct 5
prevailed. Thus, the reactivity difference between prenol and
Figure 1, an increase in bulk of the hydroxyalkyl sidechain is
expected to lead to an increase in simple diastereoselectivity due
to increasing steric interactions in one half-space of the ISC-
reactive conformer. This effect was also apparent for the
corresponding allylic acetate and thus is not coupled with
hydrogen-bonding interactions and also vanishes in the singlet
manifold (Figure 2). When allylic 1,3-strain operates (as in
substrates 6 and 8), an additional selectivity increase is observed
which is connected with hydrogen-bonding interaction with singlet
as well as triplet excited carbonyl states prior to bond formation.
Thus, a hydroxy group directing effect can be postulated for the
interaction of electronically excited nπ* states with appropriate
allylic substrates which accounts for the build-up of charge-
1
0b
transfer character during the approach of the reactants.
Acknowledgment. We thank the Fonds der Chemischen Industrie
and the Deutsche Forschungsgemeinschaft for financial support. S.B.
thanks the Egyptian government for a Ph.D. grant.
Supporting Information Available: Details of the photocycloaddition
reactions, synthesis of starting materials, NMR-spectroscopic data of the
oxetanes 3a-3d, 4a-4d, and threo-7a-7c as well as the oxetanes from
(
12) One of the reviewers informed us that the energy difference (B3LYP/
8; NOE spectra of 3b (PDF). This material is available free of charge
6
-31G*) between the two diastereoisomeric 1,4-triplet biradicals from
formaldehyde and 6 is near to zero. This is in account with our assumption
that the asymmetric induction (Scheme 3) in chiral allylic alcohols originates
from hydrogen-bonding interactions (prior to ring formation), whereas the
simple diastereoselectiVity (Scheme 1) originates from ISC-reactive conformers
via the Internet at http://pubs.acs.org.
JA015656M
(
Figure 1).
1 2
(15) Also similar to the singlet oxygen ene reaction (k /k ) 5): Griesbeck,
(
13) Griesbeck, A. G.; Fiege, M.; Bondock, S.; Gudipati, M. S. Org. Lett.
A. G.; Adam, W. Unpublished results.
2
1
000, 2, 3623-3625.
(16) Adam, W.; Stegmann, V. R. Synthesis 2001. In print.
(
14) Kossanyi, J.; Sabbah, S.; Chaquin, P.; Ronfart-Haret, J. C. Tetrahedron
(17) From 8 and acetaldehyde a 56:25:12:7 diastereoisomeric mixture was
981, 37, 3307-3315.
isolated.