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Communication
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Scheme 1 Proposed reaction pathways.
give carbazate 10. Similar reaction of [Pd(allyl)Cl]2 with carbazate 10
gives carbazate 11. These displacement–elimination sequences allow
the generation of Pd(0), which is converted to PdCl2 through copper-
mediated oxidation.13–15 Displacement of PdCl2 with carbazate 2
followed by b-hydride elimination gives diazene 9 and releases
HPdCl. The attack of diazene 9 to PdCl2 followed by extrusion
of molecular nitrogen leads to the formation of alkoxycarbonyl-
palladium 17, which undergoes regioselective alkene insertion
followed by b-hydride elimination to give a,b-unsaturated ester 3
and release HPdCl.6 Reductive elimination of HPdCl regenerates
Pd(0) to continue the catalytic cycle.
In summary, we have developed an unprecedented oxidative
alkoxycarbonylation reaction of terminal alkenes with carbazates
for the synthesis of a,b-unsaturated esters in a highly regio- and
stereoselective manner. In the presence of [Pd(allyl)Cl]2 (5 mol%),
CuCl (2 equiv.), and 5 Å molecular sieves, a range of terminal
alkenes smoothly underwent oxidative alkoxycarbonylation with
carbazates under an oxygen atmosphere to afford structurally
diverse a,b-unsaturated esters in moderate to good yields with
excellent regioselectivity and E selectivity. The reaction tolerated a
variety of functional groups such as fluoro, chloro, bromo, acetoxy,
sulfonate, sulfone, nitro, and nitrile. Moreover, plausible reaction
pathways have been proposed for the palladium-catalyzed oxidative
alkoxycarbonylation of terminal alkenes with carbazates.
10 T. W. Greene and P. G. M. Wuts, Protective Groups in Organic
Synthesis, Wiley, New York, 3rd edn, 1999.
11 T. Taniguchi, Y. Sugiura, H. Zaimoku and H. Ishibashi, Angew.
Chem., Int. Ed., 2010, 49, 10154.
12 For details, see the ESI†.
13 A trace amount of the terminal alkene was consumed by the Wacker
oxidation, which was significantly suppressed by the addition of
molecular sieves. For reviews of the Wacker oxidation, see:
(a) J. M. Takacs and X. T. Jiang, Curr. Org. Chem., 2003, 7, 369;
(b) J. A. Keith and P. M. Henry, Angew. Chem., Int. Ed., 2009, 48, 9038.
14 In situ generation of PdCl2 is speculated to minimize aggregation
after being reduced to Pd(0). In contrast, direct use of PdCl2 gave a
much lower yield (Table 1, entry 18).
15 The reaction mixture turned pale green indicating the generation of
Cu(II) from Cu(I). Cu(II) was found to decompose carbazates,
and consequently, excess CuCl and carbazates were employed
for the oxidative alkoxycarbonylation of terminal alkenes. For
similar decomposition of acyl hydrazides with Cu(II), see: J. Tsuji,
T. Nagashima, N. T. Qui and H. Takayanagi, Tetrahedron, 1980,
36, 1311.
We are grateful for the financial support from the National
Natural Science Foundation of China (21232007 and 21172206),
the National Basic Research Program of China (973 Program
2010CB833300), and the Program for Changjiang Scholars and
Innovative Research Team in University (IRT1189).
Notes and references
1 F. A. Carey and R. J. Sundberg, Advanced Organic Chemistry Part B:
Reactions and Synthesis, Springer, New York, 5th edn, 2007.
c
6530 Chem. Commun., 2013, 49, 6528--6530
This journal is The Royal Society of Chemistry 2013