Notes and references
1 For books see: R. H. Crabtree, The Organometallic Chemistry of
the Transition Metals, Wiley, New-York, 3rd edn, 2001; P. W. N.
M. van Leeuwen, Homogeneous catalysis, Kluwer Academic
Publisher, Dordrecht, 2004; M. Beller and C. Bolm, Transition
metals for organic synthesis: building blocks and fine chemicals,
Wiley-VCH, Weinheim, 1998; For specific p-coordination with gold,
see: H. Schmidbaur and A. Schier, Organometallics, 2010, 29, 2.
2 For selected recent reviews on gold chemistry, see: T. C. Boorman
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Angew. Chem., Int. Ed., 2010, 49, 5232; P. Belmont and E. Parker,
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Scheme 3 Evidences for the stereochemical course and pathway of the
Au-catalyzed rearrangement of 2-(benzyloxyprop-1-ynyl)aziridines.
It is worth mentioning that when possible, the allenylidene-
or spiro-isochromans were obtained as mixtures of diastereo-
isomers in variable ratios (entries 3 and 6–8). In our attempts
to establish the structure and stereochemistry of the products,
we were fortunate in the transformation of 1h to 3h to isolate
small amount of allenes by quenching the reaction after only
10 min, then to separate both diastereoisomers and to get
crystals suitable for X-ray diffraction from the major one
(Scheme 3). This allene 3h-maj was then re-submitted to the
A. Arcadi, Chem. Rev., 2008, 108, 3266; E. Jimenez-Nunez and
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A. M. Echavarren, Chem. Rev., 2008, 108, 3326; D. J. Gorin,
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3 For a recent review on silver chemistry, see: J.-M. Weibel, A. Blanc
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4 D. J. Gorin and F. D. Toste, Nature, 2007, 446, 395.
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Au-catalyzed cyclization conditions, producing
a single
spiroisochroman, the structure of which corresponded to the
major isomer 2h observed in the direct reaction from 1h
(Scheme 3).
6 A. Blanc, K. Tenbrink, J.-M. Weibel and P. Pale, J. Org. Chem.,
2009, 74, 4360; A. Blanc, K. Tenbrink, J.-M. Weibel and P. Pale,
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These results and their stereochemical outcomes confirmed
the proposed mechanism (Schemes 2 and 4). The major
product clearly resulted from a concerted anti nucleophilic
addition of the arene to the alkynylaziridine 1 moiety (step 2 in
Scheme 4), probably activated by dual coordination (step 1).
The proton released upon re-aromatization could then be
transferred to the amine, liberating the aminoallene 3 and
the metal ion (step 3). The latter could then activate the allene
(step 4), initiating thus the next cyclization15 (step 5), ending
with the formation of the spiro derivative 2 (step 6).
In conclusion, we have developed a novel and efficient
rearrangement of 2-(3-arylmethyloxyprop-1-ynyl) aziridines
catalyzed by gold(I) giving spiro[isochroman-4,20-pyrroline]
compounds in high yields. The isolation of an allenyl inter-
mediate and the determination of the stereochemical course of
the reaction confirmed our mechanistic hypothesis based on
the dual as s and p activation of alkynylaziridines by gold(I).
The latter induces a cascade reaction via the formation of
transient aminoallenylidene isochromans, which cyclized into
the spiro derivates. Further investigations of the rearrange-
ment of alkynylaziridines bearing various nucleophiles are
ongoing in our laboratory.
9 A. G. Perez, C. S. Lopez, J. Marco-Contelles, O. N. Faza,
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E. Soriano and A. R. de Lera, J. Org. Chem., 2009, 74, 2982.
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13 Analogous spirochromans have been described as serotonin
analogs exhibiting high affinity for 5-HT1A subtype of serotonin
receptors, see: C. Comoy, C. Marot, T. Podona, M.-L. Baudin,
L. Morin-Allory, G. Guillaumet, B. Pfeiffer, D.-H. Caignard,
P. Renard, M.-C. Rettori, G. Adam and B. Guardiola-Lemaıtre,
J. Med. Chem., 1996, 39, 4285; S. Usse, G. Guillaumet and
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N. Krause, Org. Lett., 2004, 6, 4121; N. Morita and N. Krause,
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Scheme 4 Mechanism for the Au(I)-catalyzed rearrangement of aryl
alkynylaziridines.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6665–6667 6667