analysis of allylcopper compounds and develop a catalytic
aldehyde allylation process based on these intermediates.
The allylation of aldehydes is an important reaction that
allows the formation of homoallylic alcohols. Catalytic
allylation utilizes allylmetalloids such as those of boron,
Scheme 1. Synthesis of Allylcopper Species
7
silicon, and tin. Mechanistically, most catalytic processes
8
involve aldehyde activation. Alternatively, Lewis base
9
methods for allylsilane activation have been described. A
final mechanistic possibility, transmetalation to a new
reactive organometallic, is also possible but has generally
1
0
been less studied beyond stoichiometric examples. Pal-
ladium complexes with pincer ligands have been shown to
catalyze allylation of allylstannanes by a transmetalation
1
1
mechanism. Other selective allylation reactions have been
reported, including those of copper and silver, for which there
12
exists at least some evidence of transmetalation pathways.
The fluoride complex, (IPr)CuF (IPr ) 1,3-bis(2′,6′-
diisopropylphenyl)imidazol-2-ylidene), is a stable complex
that allows access to organocopper compounds upon treat-
ment with organosiloxanes containing transferable groups,
affording complexes that are challenging to access by
alternative methods.
It is possible to employ this methodology in the synthesis
of allylcopper species. When a stoichiometric amount of an
8
allylsilane is added to (IPr)CuF in THF-d (Scheme 1), the
2
.14 (4H) and δ 1.09 (3H) ppm. On the basis of subsequent
1
X-ray structural data (see below), it is likely that the η
structure is the lower-energy isomer in solution. A previous
NMR study of allylic cuprates reported data consistent with
an η complex, rather than the dynamic structures observed
here with neutral allylcopper species. Finally, the crotylsilane
reagent reacts with (IPr)CuF, affording an organocopper
1
13
formation of an allylcopper species 2 can be observed in
1
less than 5 min by H NMR, where peaks at δ 6.43 (quint,
1
H) and δ 3.09 (d, 4H) ppm correspond to an allyl unit in
1
complex 4 with a H NMR spectrum containing four peaks
for an allylcopper complex at δ 5.39 (1H), 3.93 (1H), 0.99
1
3
fast exchange between η and η coordination modes. In
variable-temperature NMR experiments with complex 2,
significant line broadening occurred below -60 °C, but no
static structure was observed upon cooling to -90 °C.
The NMR spectrum of the 2-methallylcopper complex 3
(3H), and 0.59 (2H) ppm.
Although stable for hours in solution (THF) at room
temperature, the substituted complexes 3 and 4 are signifi-
cantly less stable than the parent allyl complex 2, and we
have been unable to isolate complexes 3 or 4 in reasonable
yield. However, it is possible to grow single crystals of the
complexes 2 and 3 suitable for X-ray diffraction by diffu-
sional recrystallization in THF/pentane at -35 °C. The X-ray
structures of compounds 2 and 3 (Figure 1) show that both
1
3
is also consistent with fast exchange between η and η
coordination modes, exhibiting two singlet resonances at δ
(
6) For examples of NHC-copper complexes and their use in catalysis,
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417–2420. (b) Mankad, N. P.; Laitar, D. S.; Sadighi, J. P. Organometallics
2
2
004, 23, 3369–3371. (c) Kaur, H.; Zinn, F. K.; Stevens, E. D.; Nolan,
S. P. Organometallics 2004, 23, 1157–1160. (d) Laitar, D. S.; Tsui, E. Y.;
Sadighi, J. P. J. Am. Chem. Soc. 2006, 128, 11036–11037. (e) Blue, E. D.;
Davis, A.; Conner, D.; Gunnoe, T. B.; Boyle, P. D.; White, P. S. J. Am.
Chem. Soc. 2003, 125, 9435–9441. (f) Goj, L. A.; Blue, E. D.; Delp, S. A.;
Gunnoe, T. B.; Cundari, T. R.; Petersen, J. L. Organometallics 2006, 25,
4
097–4104. (g) Delp, S. A.; Munro-Leighton, C.; Goj, L. A.; Ramirez,
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365–2367. (h) Lebel, H.; Davi, M.; Diez-Gonzalez, S.; Nolan, S. P. J.
Org. Chem. 2006, 72, 144–149.
7) Denmark, S. E.; Almstead, N. G. In Modern Carbonyl Chemistry;
Otera, J., Ed.; Wiley-VCH: Weinheim, 2000.
8) (a) Denmark, E. S.; Wilson, T.; Willson, T. M. J. Am. Chem. Soc.
988, 110, 984–986. (b) Denmark, E. S.; Almstead, N. G. J. Am. Chem.
Soc. 1993, 115, 3133–3139.
9) (a) Denmark, S. E.; Coe, D. M.; Pratt, N. E.; Griedel, B. D. J. Org.
Chem. 1994, 59, 6161–6163. (b) Denmark, S. E.; Fu, J. J. Am. Chem. Soc.
2
(
(
1
(
2
2
2
000, 122, 12021–12022. (c) Denmark, S. E.; Wynn, T. J. Am. Chem. Soc.
001, 123, 6199–6200. (d) Denmark, S. E.; Fu, J. Chem. ReV. 2003, 103,
763–2793.
Figure 1
.
X-ray structure of complexes 2 and 3.
(
10) Lipshutz, B. H.; Crow, R.; Dimock, S. H.; Ellsworth, E. L. J. Am.
Chem. Soc. 1990, 112, 4063–4064.
1
(
11) (a) Solin, N.; Kjellgren, J.; Szab o´ , K. J. Angew. Chem., Int. Ed.
complexes crystallize in the η coordination mode, without
significant interaction between the metal and the π system.
2
003, 42, 3656–3658. (b) Wallner, O. A.; Szab o´ , K. J. Chem.sEur. J. 2006,
1
2, 6976–6983.
(
12) (a) Wadamoto, M.; Yamamoto, H. J. Am. Chem. Soc. 2005, 127,
14556–14557. (b) Yamasaki, S.; Fujii, K.; Wada, R.; Kanai, M.; Shibasaki,
(13) Lipshutz, B. H.; Ellsworth, E. L.; Dimock, S. H.; Smith, R. A. J.
M. J. Am. Chem. Soc. 2002, 124, 6536–6537.
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