C O M M U N I C A T I O N S
Table 2. Hydroformylation of MCPs and ACPs
Scheme 3. Formation of Enantioenriched Aldehyde Possessing
Quaternary Carbon Center
Entry
R1
R2
R3
R4
Product
E/Za
Yieldb
1 (2a)
2 (2b)
3 (2c)
4 (2d)
5 (2e)
6 (2f)
7 (2g)
8 (2h)
9 (2i)
Me
Me
Me
Me
Ph
Bu
Me
Ph
Et
Bu
Et
Ph
Et
Hex
Ph
Et
Ph
Tol
CH(Ph)2
(CH2)2Ph
Me
H
H
Me
Ph
H
H
H
H
H
H
H
Me
Ph
Me
Me
2a
2b
2c
2d
2e
2f
2g
2h
2i
100/0
100/0
96/4
60/40
60/40
-
85
87
90
85
88
93
91
81
83
91
90
In conclusion, we have reported the first example of a rhodium-
catalyzed hydroformylation reaction of methylene- and alky-
lidenecyclopropane derivatives under mild conditions and low
catalyst loading. One of the most remarkable features of this reaction
is the high selectivity of the catalytic process that results in the
exclusive formation of the expected linear aldehydes, and the
stereointegrity of the quaternary carbon center remains unaffected
in the process.
-
-
-
50/50
50/50
Me
Me
Me
Et
Et
Et
10 (2j)
11 (2k)
Naphtyl
Ph
2j
2k
a E/Z ratios were determined by 1H NMR b Isolated yields after
purification by column chromatography.
Acknowledgment. This research was financially supported by
the State of Lower-Saxony and the Volkswagen Foundation,
Hannover, Germany and by a grant from the Israel Science
Foundation administrated by the Israel Academy of Sciences and
Humanities (70/08).
A postulated reaction pathway for the regioselective hydroformy-
lation reaction of an alkylidenecyclopropane is described in Scheme
2.9 A coordinatively unsaturated HRh(CO)L2 4 in situ generated
forms a π-olefin-Rh complex 5 with 1 that then becomes the Rh
complex 6 Via hydrometalation reaction. A selective ring-opening
proceeds (selective cleavage of the C1-C2 bond without any trace
of C1-C3 fragmentation). Carbon monoxide coordinates to 7 to
form the saturated alkyl rhodium complex 8, and a migratory
insertion takes place to give the unsaturated acyl-Rh 9. Oxidative
addition of molecular hydrogen to 9 gives the acyl-Rh dihydride
10, and after reductive elimination, the aldehyde 2 is produced and
the active catalyst 4 is regenerated. It is worth mentioning that in
the presence of bppf, the selective splitting of the C-C bond is
faster as compared to CO/H2 insertion and reductive elimination,
since no cyclopropylcarboxaldehydes 3 were detected in the crude
reaction mixture.
Supporting Information Available: Experimental procedures and
spectral data for all products. This material is available free of charge
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To further probe the utility of the hydroformylation reaction of
alkylidenecyclopropanes in organic synthesis, the transformation
of easily obtained enantiomerically pure alkylidenecyclopropane10
1a into a linear aldehyde possessing the challenging enantiomeric
enriched quaternary stereogenic carbon center11 was achieved as
described in Scheme 3.
JA100544C
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J. AM. CHEM. SOC. VOL. 132, NO. 12, 2010 4067