Y. Kayaki et al. / Tetrahedron Letters 50 (2009) 6491–6493
6493
A possible catalytic mechanism of the urethane formation is
Acknowledgments
shown in Scheme 2. The catalytic reaction is likely initiated by
the oxidative addition of carbamic acid generated from the allenic
amine and CO2, which is responsible for the formation of hydrid-
opalladium species. The allene moiety inserts into the Pd–H bond
This work is supported by a Grant-in-Aid for Scientific Research
on Priority Areas (No. 18065007; ‘Chemistry of Concerto Catalysis’)
and partially supported by GCOE program from the Ministry of
Education, Culture, Sports, Science, and Technology of Japan.
to give an
g
3-allylpalladium intermediate, and the following
reductive elimination would furnish the cyclization product.14
A
similar mechanistic sequence has been expected for the Pd-cata-
lyzed intramolecular hydroamination of allenes in which the
presence of acetic acid enhances the rates and yields.15 In the light
of the reaction conducted in dense CO2 favoring the carbamic acid
formation from secondary amines, the hydropalladation mecha-
nism seems to be feasible, while an alternative mechanism involv-
ing nucleophilic attack on the allene bound to a Pd(II) center16 may
be operative in the case of the 5-endo cyclization producing 3a.17
References and notes
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Although 1,3-dienes may also be accessible to the
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In summary, we have first accomplished the Pd-catalyzed cyclic
urethane synthesis via intramolecular addition of carbamic acids
across the carbon–carbon double bond in allenic substrates. The
dense CO2 conditions proved to be crucial for smooth CO2 fixation.
Although further improvement of the catalyst performance is
needed for the reaction, the present direct addition system
provides a straightforward method without forming salt waste,
which is thus of great promise for achieving environmentally
benign processes.
·
R
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·
R
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Pd
N R
O
R
·
N
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O C
H Pd
O
O
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17. The cyclic urethane product 2a containing an allylic carbon–oxygen bond
proved to be stable even in the presence of Pd catalysts such as Pd(OCOCH3)2,
Pd(dba)2, and Pd[P(OC6H5)3]4 under the reaction conditions that will preclude
the possibility of catalytic access from 2a to 3a.
Scheme 2. A plausible mechanism for the palladium-catalyzed carboxylative
cyclization of allenic amines.