Photodenitrogenation of a Spiroepoxy-Substituted Azoalkane
SCHEME 1. Ster eoselectivity (Sin glet ver su s
Tr ip let) of th e Oxeta n e (d 2)-1 F or m ed in th e
P h otod en itr ogen a tion of th e Deu ter a ted
Azoa lk a n es a n ti-(d 2)-AZ1 a n d syn -(d 2)-AZ2
anti-(d2)-AZ1 and syn-(d2)-AZ2. Thus, in an aprotic
solvent (benzene), the singlet diradical generated upon
direct irradiation (λexc ) 330 nm) of the azo chromophore
gave stereoselectively the oxetane, i.e., trans-(d2)-1 from
anti-(d2)-AZ1 and cis-(d2)-1 from syn-(d2)-AZ2 (Scheme
1), by concerted rearrangement of the 2-spiroepoxy 1,3-
diradical DR2. In contrast, nonselective formation of the
oxetanes trans- and cis-(d2)-1 was observed from the
triplet state generated by benzophenone sensitization (λexc
) 370 nm) through stepwise migration in the intermedi-
ary triplet 1,4-diradical T-1,4-DR (Scheme 1). In metha-
nol, a spin-state-dependent product distribution was also
found in the formation of the MeOH adducts 2a and 3a
(Scheme 2).8a Whereas the direct photodenitrogenation
(singlet state) gave a 25/75 mixture of the MeOH adduct
2a and its regioisomer 3a (the latter derived from the
methanolysis of the oxetane 1), for the benzophenone-
sensitized reaction (triplet state) the yield of adduct 2a
decreased; i.e., the ratio 2a /3a () 1) was 10/90.
What dipoles are being trapped by methanol? Is the
structure of the dipole in the direct irradiation different
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from that in the sensitized photodenitrogenation? The
answers to these questions would clarify the mechanism
of the spin-state-dependent reactivity. The possible struc-
tures of the dipoles DP 1-3 are shown in Scheme 3, in
which DP 1 and DP 2 are the resonance forms of the
singlet state of the diradical DR2. Such mesomeric forms
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In the present study, we have investigated first the
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() 1), to elucidate the mechanism for the formation of
the trapping product 2. We have selected 2,2,2-trifluo-
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13
14
roethanol (TFE, pKa ) 12.4, NT ) -3.93) and
1,1,1,3,3,3-hexafluoro-2-propanol (HFIP, pKa13 ) 9.2, NT
14
) -5.26) as solvents, which are more acidic (pKa) and
less nucleophilic (NT) than methanol (MeOH, pKa )15.5,
NT )0.17). For such alcoholic solvents, we had already
determined the thermal reactivity of the oxetane 1.15
Thus, the TFE-induced reaction of oxetane 1 gave a 2:1
mixture of 3b (R ) CH2CF3) and cis-4b (R ) CH2CF3) in
high yield (total > 85%); the adduct cis-4c [R ) CH(CF3)2]
was observed exclusively in the HFIP-induced reaction
(Scheme 2).15 Presently, we have performed the deuterium-
labeling study for the dipole-trapping reactions by using
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J . Org. Chem, Vol. 68, No. 5, 2003 1797