C O M M U N I C A T I O N S
connected with triplet biradical behavior only, a spin-correlation
effect was expected on the endo/exo-ratio of Paterno`-Bu¨chi
photocycloadditions of aliphatic aldehydes capable of reacting from
both their singlet and their triplet excited states. Indeed, propional-
dehyde (4) as the carbonyl component shows a pronounced spin-
correlation effect.14 We have now additionally investigated the
concentration dependence of the noninduced diastereoselectivity
for the deuterated aldehyde 4-d19 and the combination of 4 with
2-d (Scheme 2).
unsubstituted cycloalkenes (as 2) gave higher effects of 1.9-3.0.
Cyclohexene, for example, gave a selectivity increase upon change
to deuterated benzaldehyde 1-d from 74:26 to 85:15. We assume
that H/D exchange does not influence the energies of the ISC-
reactive conformers A-C nor the SOC constants. In the case of
deuterated substrates, however, due to the lower HFC contribution,11
the SOC-induced stereoselectivity is expected to become strength-
ened. Because SOC is already the dominating mechanism, these
magnetic isotope effects are only moderate but clearly detectable.
Furthermore, the fact that similar MIEs were detected for the
combinations 1/2-d and 1-d/2 strongly argues for the intermediacy
of 2-oxatetramethylenes (OTM) as triplet biradicals and against a
1-OTM structure which was discussed as an alternative in recent
theoretical publications.21 A significantly weaker MIE is expected
from the 1-d/2 reaction via a 1-OTM biradical. That a MIE is
operative became also clear from the result of the photocycload-
Scheme 2. Paterno`-Bu¨chi Reactions of 2, 2-d, 4, 4-d
In the triplet region (low substrate concentration), a similar
isotope effect on the endo/exo-ratio was determined for the 2-d/4
and the 2/4-d combinations (Figure 2). An average isotope
22
dition of the pentadeuterated aldehyde 1-d5 with 2 (Scheme 4).
Scheme 4. Paterno`-Bu¨chi Reaction of d5-Benzaldehyde with 2
As compared to 1-d, a slightly weaker but still significant isotope
selectivity effect of 1.9 was determined for this system.
Acknowledgment. This study was initiated by a comment from
Peter R. Ogilby (University of Århus). He called our attention to
the pronounced difference in 1H and 2H nuclear-spin electron-spin
coupling and the potential consequences.
Supporting Information Available: Substrate preparation, NMR
data, and NMR traces of the photocycloaddition reactions (PDF). This
Figure 2. Concentration/selectivity profiles for the photocycloaddition of
EtCHO with 2 (blue) and deuterated substrate combinations.
selectivity effect (as defined above) of 1.2 resulted for both reactions
at 0.01 M. These results originate from several factors, all in
connection with intersystem crossing processes: ISC rates are
reduced, and thus singlet as well as triplet lifetimes are increased.
This effect seems to be balanced out for the 2/4 reaction: the
inversion regions are nearly identical for all combinations.
In Scheme 3, gauche and anti conformers for the intermediate
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