5
6
P (RNCH
2
CH
2
)
3
N: Ca ta lysts for th e
nonmetallic catalysts, or as nonmetallic cocatalysts in
a variety of useful organic transformations. The unex-
pectedly potent catalytic properties of 3 are associated
with its flexible Nax f P transannular interaction that
can adjust the electron density at phosphorus in response
to substrates that polarize the phosphorus lone pair of
electrons.5 Here, we disclose that 3c and 3e are superior
phosphine catalysts for the dimerization of 2, which
function at room temperature.
Hea d -to-Ta il Dim er iza tion of Meth yl
Acr yla te
Weiping Su, Dale McLeod, and J ohn G. Verkade*
,6
Department of Chemistry, Iowa State University,
Ames, Iowa 50011
Received J une 12, 2003
Abstr a ct: The dimerization of methyl acrylate to the head-
to-tail 2-methylene-pentanedioic acid dimethyl ester product
was realized in 82 and 85% yield in only 4 h at room
temperature in THF in the presence of catalytic amounts of
P(RNCH2CH2)3N (R ) i-Bu and Bn, respectively). These
phosphines are to our knowledge the best nonmetallic
catalysts so far reported for this reaction. In contrast, less
sterically hindered P(MeNCH2CH2)3N failed to catalyze this
dimerization, giving oligomer or polymer instead.
Initially, we monitored room-temperature dimerization
6 6
reactions of 2 in the presence of 10 mol % 3c in C D ,
1
8 8
THF-d , and dioxane-d by H NMR spectroscopy. It was
observed that 3c catalyzed the reaction, affording the
head-to-tail dimer 1 as well as about 10% trimer. In polar
2
-Methylene-pentanedioic acid dimethyl ester (1 in eq
1
) has been found to be a useful monomer for the
synthesis of polymers and as a building block for
construction of larger molecules.
1
solvents, such as THF-d
proceeded much faster than in a nonpolar solvent such
as C . In the former two solvents, the reactions were
8 8
or dioxane-d , dimerization
2
6
D
6
complete after 4 h, while in the latter solvent, it took 12
h for complete conversion.
We then screened catalysts 3a -e with 10 and 1 mol
catalyst loadings in three solvents as shown in Table
%
Compound 1 is generally made by head-to-tail dimeriza-
1
. With 10 mol % 3c, the reaction in THF and dioxane
tion of methyl acrylate (2) at elevated temperatures (50-
produced dimer 1 in 82 and 83% yield (entries 1 and 2),
respectively. Under the same conditions, pentane as the
solvent gave a significantly lower yield (77.3% entry 3).
3
1
10 °C) using phosphines [e.g., P(C
6
H
5
)
3
and P(C
4
H
9
)
3
]
4
or transition metal trialkylphosphine complexes as
3 3
catalysts. With the exception of the Cp*Ru(PCy )H -cata-
3
Considering that the substituents on each PN nitrogen
lyzed dimerization of 2 at 80 °C in 91%,4 the aforemen-
tioned syntheses suffer from relatively low selectivity and
yields (ranging from 10 to 79%), with the highest yield
a
in 3 can affect the electronic and steric nature of the
phosphorus, several other proazaphosphatranes were
also examined as dimerization catalysts. It was found
that 3b was capable of catalyzing the dimerization of 2,
although less efficiently than 3c (entries 4-6). Use of
bulkier 3d and 3e led to an increased yield of dimer 1 in
THF and dioxane (entries 7-10) compared with those
facilitated by 3b in these solvents (entries 4 and 5). The
highest yield (85%) was realized with 3e (10 mol %) in
THF. However, in the cases of 3c and 3e, increasing the
catalyst loading from 10 to 15% did not give rise to a
higher yield (entries 11 and 12, respectively).
of 79% arising from the use of (c-C
6
H
11
)
3
2
P‚CS in refluxing
pyridine for 16 h.2b
We have demonstrated in recent years that proaza-
5
phosphatranes 3 function as potent nonionic bases,
(
1) (a) Trumbo, D. C.; Zander, R. A. J . Polym. Sci. Polym. Chem.
1
991, 29, 1053. (b) Nguyen, H. A.; Marechal, E. J . Macromol. Sci.-
Rev. Macromol. Chem. Phys. 1998, 187, C27. (c) Gegnas, L. D.; Waddell,
S. T.; Chabin, R. M.; Reddy, S.; Wong, K. K. Bioorg. Med. Chem. Lett.
1
998, 8, 1643.
2) (a) Tanne, M. E.; Vaganay, S.; Heijenoort, J . V.; Blanot, D. J .
(
Org. Chem. 1996, 61, 1756. (b) Cookson, R. C.; Smith, S. A. J . Chem.
Soc., Perkin. Trans. 1 1979, 2447.
In contrast to 3b-e, the less sterically hindered 3a
failed to catalyze the dimerization of 2, instead giving
polymers or oligomers as dominant products (entries 13
and 14). Thus, upon addition of a pentane solution of 3a
to a solution of 2 in pentane, a sticky oil precipitated from
(3) (a) McClure, J . D. J . Org. Chem. 1970, 35, 3045. (b) Baizer, M.
M.; Anderson, J . D. J . Org. Chem. 1965, 30, 1357. (c) Tembe, G. L.;
Bandyopadhyay, A. R.; Ganeshpure, P. A.; Satish, S. Catal. Rev. Sci.
Eng. 1996, 38, 299. (d) White, D. A. Synth. React. Inorg. Met. Org.
Chem. 1977, 7, 433. (e) Rauhut, M. M.; Currier, H. U.S. Patent 3,-
0
74,999, 1963. (f) J enner, G. Tetrahedron Lett. 2000, 41, 3091.
4) (a) Yi, C. S.; Liu, N. J . Organomet. Chem. 1998, 553, 157. (b)
Grenoullet, P.; Nelbecker, D.; Tkatchenko, I. Organometallic 1984, 3,
130. (c) McKinney, R. J .; Cotton, M. C. Organometallics 1986, 5, 1080.
d) Aresta, M.; Dibenedetto, A.; Quaranta, E. Organometallics 2000,
(
(6) (a) Urgaonkar, S.; Nagarajan, M.; Verkade, J . G. Org. Lett. 2003,
5, 815. (b) Urgaonkar, S.; Nagarajan, M.; Verkade, J . G. Tetrahedron
Lett. 2002, 43, 8921. (c) Urgaonkar, S.; Verkade, J . G. J . Org. Chem.
2003, 68, 452.
1
(
1
9, 4199.
(7) (a) Schmidt, H.; Lensink, C.; Xi, S. K.; Verkade, J . G. Z. Anorg.
Allg. Chem. 1989, 578, 75. (b) Kisanga, P. B.; Verkade, J . G.
Tetrahedron 2001, 57, 467.
(5) For a recent review, see: Verkade, J . G. Top. Curr. Chem. 2003,
2
33, 1.
1
0.1021/jo034815c CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/06/2003
J . Org. Chem. 2003, 68, 9499-9501
9499