Azidohydroperoxidation of pinenes: stereoselectivity pattern and the first
X-ray structure of a 2-azidohydroperoxide
Axel G. Griesbeck,* Johann Lex, Kurt M. Saygin and Jörg Steinwascher
Institute of Organic Chemistry, University of Cologne, Greinstr. 4, D-50939 Köln, Germany.
E-mail: griesbeck@uni-koeln.de
Received (in Liverpool, UK) 17th July 2000, Accepted 2nd October 2000
First published as an Advance Article on the web
The photoinduced electron transfer of azide anions in the
presence of an excited organic dyestuff, oxygen, and a- or b-
pinene, respectively, gave 2-azidohydroperoxides in ex-
cellent regio- and good diastereoselectivity.
Photooxygenation in the presence of azide anions leads to
difunctionalized products which can derive either from an azide
trapping reaction of cyclic peroxidic intermediates or from an
oxygen trapping reaction of intermediary a-azido carbon
radicals.1–3 The resulting 2-azidohydroperoxides have seldom
been isolated and are usually reduced to the corresponding
azido- or aminoalcohols.4 Recent investigation by Workentin et
al. showed that azidyl radicals are produced under photo-
induced electron transfer (PET) conditions and these highly
reactive azidyl radicals readily add to nucleophilic alkenes.5
In order to evaluate the facial selectivity of the azidyl radical
addition in combination with simple diastereoselectivity origi-
nating from the secondary attack of molecular oxygen, we
Fig. 1 Fluorescence quenching of rhodamine B by azide anions.
investigated the enantiomerically pure a- and b-pinene as
substrates. The trisubstituted a-pinene (1) was among the most
reactive alkenes when irradiated with visible light in the
presence of azide anions, oxygen and an organic dye (Scheme
1). The crude reaction mixture consisted of a mixture of
diastereoisomeric azidohydroperoxides 2a–c (d.r. 90+8+2) and
the corresponding azidoalcohols 3 which could be separated
chromatographically (total yield 58%). The hydroperoxide–
alcohol ratio could be altered by use of different PET-sensitizers
and variation of the water content and was optimal (79+21) for
rhodamine B in a 95+5 MeOH–H2O mixture. Stern–Volmer
analysis of the fluorescence quenching gave a rate constant for
with the literature-known compound.8 The relative configura-
tion of the two minor components were determined from the
3JHH coupling constants of the hydrogen proximal to the azido
group. Thus, the primary attack of the azidyl radical proceeds
with a 11+1 endo-selectivity and the subsequent oxygen attack
with a 49+1 endo-selectivity.
Unexpectedly, b-pinene (5) also showed noticeable reactivity
under standard reaction conditions (Scheme 2): the azido-
hydroperoxides 6a,b† (Fig. 2) were formed with a dr of 76+24
(72% based on converted 5b). The addition of molecular
oxygen was again directed by the shielding effect of the
dimethylmethylene bridge in 5. From both the azidohydroper-
oxides 6a,b as well as the azidoalcohol 7a the aminoalcohol 8a,
was generated by LAH-reduction. Crystallization from hexane–
ethyl acetate gave single crystals of the azidohydroperoxide 6a.
X-Ray structure analysis of 6a revealed that the molecules are
connected by intermolecular hydrogen bonds between the OOH
group and the internal nitrogen of the azido function.
PET from N3 to rhodamine B of kET = 1.6 3 109 M21 s21
2
(Fig. 1) comparable with the rate constants determined for
9,10-dicyanoanthracene6 and thioxanthone.7
The configuration of the major diastereoisomer 2a was
determined by NOE-spectroscopy and by comparison of the
aminoalcohol 4a which resulted from the LAH-reduction of 2a
Thus, this procedure serves as a simple synthetic approach to
aminoalcohols from unsaturated substrates from the ‘chiral
pool’. Additionally, the pinene skeleton offered the opportunity
to study the rate of oxygen addition due to the highly reactive
cyclobutylcarbenium radicals (radical clocks) formed after
azidyl radical addition. The corresponding ring-opening proc-
esses have been studied in free radical additions to a- and b-
Scheme 1 Reagents and conditions: (i) NaN3, O2, Rh-B, hn, MeOH–H2O,
Scheme 2 Reagents and conditions: (i) NaN3, O2, Rh-B, hn, MeOH–H2O,
(ii) NaBH4, MeOH, (iii) LiAlH4, Et2O.
(ii) NaBH4, MeOH, (iii) LiAlH4, Et2O.
DOI: 10.1039/b005834n
Chem. Commun., 2000, 2205–2206
This journal is © The Royal Society of Chemistry 2000
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