C O M M U N I C A T I O N S
Table 2. Enantioselective Hydrovinylation of R-Alkyl Vinylarenes
high correlation between the enantioselectivity of the reaction and
the size of R-alkyl group in the substrates 9. When the ethyl group
in substrate 9g was n-Pr (9h), i-Bu (9i), and i-Pr (9a), the
enantiomeric excess values of the corresponding hydrovinylation
products increased successively from 70 to 82, 88, and 99% (Table
a
Catalyzed by [Ni(allyl)Br(Sa,R,R)-8]
2
, entries 1 and 7-9). The substrates with a R-isopropyl and
Substrate (9)
crude
selectivity
ee of
R-cyclohexyl gave excellent enantioselectivities (94-99% ee),
showing that a bulky alkyl group at the R-position of vinylarenes
is definitely necessary for obtaining chiral all-carbon quaternary
centers in high enantioselectivity in the hydrovinylation reaction.
In summary, we have developed a highly efficient catalyst system
for the asymmetric hydrovinylation of R-alkyl vinylarenes by using
spiro phosphoramidite ligands, which provided a new approach to
construct all-carbon quaternary stereocenters in excellent enantio-
selectivity. The hydrovinylation products, bearing a chiral all-carbon
quaternary center, are potentially useful intermediates in the
synthesis of versatile optically enriched molecules, such as chiral
carboxylic acids, aldehydes, etc. The scope and the applications of
this discovery are being investigated in our laboratory.
entry
Ar
R
yield (%)b
of 10 (%)
10 (%)c
1
2
3
4
5
6
7
8
9
Ph
i-Pr
89
96
96
95
94
76
80
92
92
94
92
84 (10a)
89 (10b)
89 (10c)
84 (10d)
86 (10e)
85 (10f)
89 (10g)
84 (10h)
86 (10i)
80 (10j)
86 (10k)
99 (S)
97
98
98
98
94
70 (S)
82
88
99
p-MePh
m-MePh
m-MeOPh
p-MeOPh
p-ClPh
Ph
i-Pr
i-Pr
i-Pr
i-Pr
i-Pr
Et
Ph
Ph
n-Pr
i-Bu
i-Pr
1
1
0
1
2-naphthyl
2-naphthyl
c-C6H11
99
a
b
Conditions: see Table 1; T ) 35 °C. Purified by a short silica gel
c
column chromatography. Conversions were 95-100%. Determined by
chiral GC and HPLC (for details, see Supporting Information).
Acknowledgment. We thank the National Natural Science
Foundation of China, the Major Basic Research Development
Program (Grant No. G2000077506), and the Ministry of Education
of China for financial support.
styrene with good to excellent conversions, but in only moderate
chemo- and enantioselectivities (entries 3-8). It was remarkable
that the spiro phosphoramidite ligand (S
0a in 99% ee with 96% conversion and 82% chemoselectivity
entry 9). In a contrast, the ligand (R ,R,R)-8, with mismatched
chiralities, gave very low conversion and chemoselectivity (entry
1). The superiority of ligand 8 over 7 on the activity and
a
,R,R)-8 gave the product
1
(
Supporting Information Available: Experimental procedures, the
characterizations of substrates and products, and the analyses of
enantiomeric excess values of hydrovinylation products (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
a
1
enantioselectivity showed that the interaction of the pheny ring of
the ligand with a Ni center in the catalyst might be crucial.6b The
reaction is sensitive to the temperature. When the reaction was
carried out at 10 or 0 °C, the chemoselectivity of product 10a was
improved significantly to 97 and 96%, respectively, with the
enantioselectivity remaining at 98%, but the conversion of substrate
References
(
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(
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a was lowered to 63% (entries 12 and 13). As the temperature
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2
2
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(
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2
Et O and other coordinating solvents were employed. The activator
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(
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6 6 4 4
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1
85.
With optimized reaction conditions, a variety of R-alkyl vinyl-
arenes can be successfully hydrovinylated to form new arene
compounds bearing an all-carbon quaternary stereocenter. As
illustrated in Table 2, the hydrovinylations of the R-alkyl styrenes
without an electron-deficient group at a para or meta position on
phenyl rings gave almost quantitative conversion of substrates
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1
2
998, 120, 459-460. (b) Park, H.; RajanBabu, T. V. J. Am. Chem. Soc.
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(
7
2
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5
c, and (a) Monteiro, A. L.; Seferin, M.; Dupont, J.; Souza, R. F.
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R-alkyl vinylarenes were in the range of 80-89%. There was a
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(
(
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(
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JA057654Y
J. AM. CHEM. SOC.
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