LiAl(OC(Ph)(CF3)2)4: A hydrocarbon-soluble catalyst for carbon-carbon bond-forming reactions

Article abstract of DOI:10.1021/om9604031

The compound LiAl(OC(Ph)(CF₃)₂)₄ (1), prepared from LiAlH₄ and HOC(Ph)(CF₃)₂, is an active Lewis acid catalyst in toluene solution for the 1,4-conjugate addition of silyl ketene acetals to α,β-unsaturated carbonyl compounds and for the direct substitution of allylic acetates by silyl ketene acetals. The structure of 1 contains a rare trigonal prismatic coordination sphere around the Li⁺ cation, with two Li-O(C, Al) bonds and four Li-F(C) bonds.

Full text of DOI:10.1021/om9604031

3776
Organometallics 1996, 15, 3776-3778
LiAl(OC(P h )(CF 3)2)4: A Hyd r oca r bon -Solu ble Ca ta lyst for
Ca r bon -Ca r bon Bon d -F or m in g Rea ction s
Thomas J . Barbarich,1a Scott T. Handy,1b Susie M. Miller,1a Oren P. Anderson,1a
Paul A. Grieco,*,1b and Steven H. Strauss*,1a
Departments of Chemistry, Colorado State University, Fort Collins, Colorado 80523, and
Indiana University, Bloomington, Indiana 47405
Received May 24, 1996X
Summary: The compound LiAl(OC(Ph)(CF3)2)4 (1), pre-
pared from LiAlH4 and HOC(Ph)(CF3)2, is an active
Lewis acid catalyst in toluene solution for the 1,4-
conjugate addition of silyl ketene acetals to R,â-unsatur-
ated carbonyl compounds and for the direct substitution
of allylic acetates by silyl ketene acetals. The structure
of 1 contains a rare trigonal prismatic coordination
sphere around the Li+ cation, with two Li-O(C, Al)
bonds and four Li-F(C) bonds.
4HOC(Ph)(CF3)2 + LiAlH4 f
LiAl(OC(Ph)(CF3)2)4 + 4H2
1
The hygroscopic, volatile, white solid, which was puri-
fied by vacuum sublimation at 138 °C, is extremely
soluble in hydrocarbon solvents including hexane and
toluene. Diffraction-quality crystals were grown by
cooling a hexane solution of 1.7 The solid-state struc-
ture, shown in Figure 1, consists of discrete LiAl(OC-
(Ph)(CF3)2)4 molecules. The Al atom is tetrahedrally
coordinated by four 1,1,1,3,3,3-hexafluoro-2-phenyl-2-
propoxide ligands. The O1-Al-O2 angle of 91.8(1)° is
smaller than the five other O-Al-O angles, which
One of the most important uses of new weakly
coordinating anions (WCAs) is to enhance the catalytic
activity of metal cations.2 Two examples that have
received considerable attention recently are metal-
locene-catalyzed olefin polymerization3 and lithium-
catalyzed Diels-Alder reactions and 1,4-conjugate ad-
dition reactions.4 The best Li+-based catalysts, including
LiClO4,4a,b,d LiN(O2SCF3)2,4e,g and LiCo(C2B9H11)2,4c ex-
hibit higher activity as the coordinating ability of the
solvent decreases from, for example, diethyl ether to 1,2-
dichloroethane (DCE). Due to restrictions or hazards
associated with DCE and other weakly coordinating
halogenated solvents,5 hydrocarbon-soluble and hydro-
carbon-active catalysts would be desirable for large-scale
industrial use. However, few lithium salts of WCAs are
soluble in hydrocarbon solvents, and none that we know
of is catalytically active.
We now report that the Li+ salt of a new tetrakis-
(polyfluoroalkoxy)aluminate shows great promise as a
hydrocarbon-active catalyst for 1,4-conjugate addition
reactions and the direct substitution of allylic acetates
by silyl ketene acetals. The new hydrocarbon-soluble
compound LiAl(OC(Ph)(CF3)2)4, 1, was prepared by
adding 4 equiv of HOC(Ph)(CF3)2 to LiAlH4 in toluene
solution:6
-
range from 105.4(1) to 116.7(1)°. The Al(OC(Ph)(CF3)2)4
anion is a hexadentate chelating ligand for the Li+
cation. The O1-Al-O2 unit forms a four-membered
chelate ring with Li+, with Li-O(Al,C) bond distances
of 1.966(8) and 1.978(8) Å. Four C-F bonds from four
different CF3 groups form five-membered O-Li-F-
C-C chelate rings, with Li-F(C) bond distances of
1.984(9), 2.082(9), 2.098(11), and 2.354 (10) Å. The
overall LiO2F4 coordination sphere can best be described
(6) A toluene solution of HOC(Ph)(CF3)2 (5.00 g, 20.5 mmol) was
added to a stirred toluene slurry of LiAlH4 (0.180 g, 4.75 mmol). The
mixture was slowly heated to reflux, during which time
a gas,
presumably dihydrogen, was evolved. During 7 h of stirring under
reflux, the appearance of the supernatant changed from colorless to
blue-green to blue to purple to orange-red, at which point there were
no solids present. The volatiles were removed under vacuum to yield
a mixture of white and orange solids. The crude product was sublimed
at 138 °C. At this temperature, the mixture of solids melted to become
a brown liquid. The white solid product 1 collected on the cold finger
(yield 1.38 g, 29%). The compound can be further purified by recrys-
tallization from hexanes. Proton, 7Li, 13C, and 19F NMR spectra of 1
in methylcyclohexane-d14 were obtained (Supporting Information). The
1H spectrum is unremarkable. The 7Li spectrum shows a single broad
resonance at δ -0.35 ppm. The 13C spectrum shows a single type of
CF3 group (J (13C-19F) ) 289 Hz), and a single type of phenyl group.
The 19F spectrum shows a single environment for all 24 fluorine atoms,
indicating rapid exchange of all 24 fluorine atoms. The observed
X Abstract published in Advance ACS Abstracts, August 1, 1996.
(1) (a) Colorado State University. (b) Indiana University.
(2) (a) Bochmann, M. Angew. Chem., Int. Ed. Engl. 1992, 31, 1181.
(b) Strauss, S. H. Chem. Rev. 1993, 93, 927. (c) Strauss, S. H.
Chemtracts: Inorg. Chem. 1994, 6, 1.
(3) For examples see the following and references therein: (a)
Turner, H. W. European Patent Appl. 277,004 (assigned to Exxon),
1988. (b) Pellecchia, C.; Longo, P.; Proto, A.; Zambelli, A. Makromol.
Chem., Rapid Commun. 1992, 13, 265. (c) J ia, L.; Yang, X.; Stern, C.;
Marks, T. J . Organometallics 1994, 13, 3755. (d) Tjaden, E. B.;
Swenson, D. C.; J ordan, R. F.; Peterson, J . L. Organometallics 1995,
14, 371.
(4) For examples see the following and references therein: (a) Grieco,
P. A.; Nunes, J . J .; Gaul, M. D. J . Am. Chem. Soc. 1990, 112, 4595. (b)
Grieco, P. A.; Cooke, R. J .; Henry, K. J .; VanderRoest, J . M. Tetrahe-
dron Lett. 1991, 32, 4665. (c) DuBay, W. J .; Grieco, P. A.; Todd, L. J .
J . Org. Chem. 1994, 59, 6898. (d) Saidi, M. R.; Heydari, A.; Ipaktschi,
J . Chem. Ber. 1994, 127, 1761. (e) Kobayashi, H.; Nie, J .; Sonoda, T.
Chem. Lett. 1995, 307. (f) Arai, T.; Sasai, H.; Aoe, K.; Okamura, K.;
Date, T.; Shibasaki, M. Angew. Chem., Int. Ed. Engl. 1996, 35, 104.
(g) Handy, S. T.; Grieco, P. A.; Mineur, C.; Ghosez, L. Synlett 1995,
565.
pattern at δ -75.9 is the superposition of a 1:1:1:1 quartet and a broad
3
singlet due to coupling to a single lithium atom (I )
/
for 7Li, 92.5%
2
NA; I ) 1 for 6Li, 7.5% NA). The observed separation of the central
peaks is 2.4 Hz. Therefore, if the solid-state structure is maintained
in solution, then J (7Li-19F) ) 14.4 Hz ((2.4 Hz)(24 F atoms)/(4 F
atoms)). The spectra are consistent with rapid intramolecular exchange
of the Li+ ion. Similar spectra were observed in toluene-d8. Lower
temperatures did not result in slow-exchange limit spectra. The
solubilities of 1 in benzene-d6 and methylcyclohexane-d14 are at least
41 and 32 mM, respectively.
(7) Crystals of 1 were grown by cooling a hexane solution: C36H20
-
AlF24LiO4, Mr ) 1006.4, monoclinic, space group C2/c; a ) 42.297(6),
b ) 10.641(1), c ) 19.132(2) Å; â ) 114.808(9)°; V ) 7817(2) Å3, Z ) 8,
Fcalc ) 1.710 g cm-3, F(000) ) 4000, λ(Mo KR) ) 0.710 73 Å, µ ) 2.07
cm-1, T ) -100 °C, crystal dimensions 0.60 × 0.46 × 0.12 mm; Siemens
P4 diffractometer, θ-2θ scan technique, 6883 independent reflections
measured (4.2° e 2θ e 50°); anisotropic refinement (full-matrix least-
squares on F2) for all non-hydrogen atoms; hydrogen atoms in
calculated positions; no absorption correction; final R(I > 2σ(I)) ) 0.052,
final R(all data) ) 0.105; total number of parameters ) 596.
(5) Kirschner, E. M. Environment, Health Concerns Force Shift in
Use of Organic Solvents. Chem. Eng. News 1994, J une 20, p 13.
S0276-7333(96)00403-7 CCC: $12.00 © 1996 American Chemical Society

Communications
Organometallics, Vol. 15, No. 18, 1996 3777
Ta ble 1. LiAl(OC(P h )(CF 3)2)4-Ca ta lyzed Con ju ga te
Ad d ition of
1-Meth oxy-1-((ter t-bu tyld im eth ylsilyl)oxy)eth ylen e
to r,â-Un sa tu r a ted Ca r bon yl Com p ou n d sa
F igu r e 1. Drawing of the structure of 1, LiAl(OC(Ph)-
(CF3)2)4 (hydrogen atoms omitted for clarity). Selected
interatomic distances (Å) and angles (deg): Li-O1, 1.978-
(8); Li-O2, 1.966(8); Li-F1, 1.984(9); Li-F4, 2.354(10); Li-
F7, 2.098(11); Li-F10, 2.082(9); O1-Li-O2, 79.9(3); F1-
Li-F7, 129.5(4); F4-Li-F10, 149.1(4); C5-F1, 1.376(5);
C6-F4, 1.341(5); C7-F7, 1.361(5); C8-F10, 1.384(5); other
C-F, 1.298(5)-1.342(5); Al-O1, 1.773(2); Al-O2, 1.755-
(3); Al-O3, 1.687(3); Al-O4, 1.706(3); O1-Al-O2, 91.8-
(1); O1-Al-O3, 115.8(1); O1-Al-O4, 112.5(1); O2-Al-
O3, 114.8(1); O2-Al-O4, 116.7(1); O3-Al-O4, 105.4(1).
a
All reactions were conducted at ambient temperature and
pressure employing a 0.1 M solution of substrate in toluene, 10
mol % LiAl(OC(Ph)(CF3)2)4, and 2.0 equiv of silyl ketene acetal.
Isolated yield.
as trigonal prismatic: the O1-F1-F4 and O2-F7-F10
triangular planes make a dihedral angle of only 12°.
There are several unique and noteworthy features
about the structural environment of the Li+ cation in
1. Lithium(I) is rarely six-coordinate, and on those rare
occasions it is generally octahedral (e.g., LiF, LiClO4‚
3H2O, LiSbF6), not trigonal prismatic. In addition, this
is the first example of a compound with more than two
Li-F(C) bonds for a given Li+ ion and the first example
in which a majority of atoms coordinated to Li+ are
organofluorine atoms. Furthermore, the 1.984(9) Å Li-
F1 distance is shorter than any previously reported Li-
F(C) distances.8,9 For comparison, the Li-F(C) dis-
tances in [Li(FN2O3)][ClO4],8 Li(2,4,6-C6H2(CF3)3)(Et2O),10
b
of silyl ketene acetal 2 gave rise to a 97% isolated yield
of the 1,4-addition product 3 after 4 h (rxn 1). In the
11
absence of catalyst there was no reaction. When 10 mol
% lithium trifluoromethanesulfonimide or lithium per-
chlorate was used instead of 1, no reaction was ob-
served.13
and Li(C6F5NSiF(t-Bu)2)(THF)2 are 2.035(5), 2.25(1)
(average of two distances), and 2.27(1) Å, respectively,
and the Li-F distance in crystalline LiF is 2.009 Å.12
The Li-F bonds in 1 cannot be considered secondary
bonds: the Li-F1 distance is the same, to within (3σ,
as the two Li-O distances. Finally, the Li-F(C) bonds
are sufficiently strong to significantly lengthen the
C-F(Li) bonds (1.341(5)-1.384(5) Å) relative to the
other C-F bonds (1.298(5)-1.342(5) Å) in 1. Neverthe-
less, since carbon-halogen bonds are very poor donor
ligands,9 we anticipated that 1 might function as a
hydrocarbon-soluble Lewis acid.
To probe the catalytic activity of 1, the conjugate
addition of silyl ketene acetals to R,â-unsaturated
carbonyl substrates was studied in toluene at ambient
temperature. Addition of 10 mol % of 1 to a 0.1 M
toluene solution of cyclohexenone containing 2.0 equiv
A number of other substrates were investigated using
1-methoxy-1-((tert-butyldimethylsilyl)oxy)ethylene (4) as
the nucleophile (Table 1). In the case of the sterically
encumbered substrate 5, the major product was the
result of 1,2-addition: a 0.1 M solution of 5 in toluene
containing excess 4 and 10 mol % 1 afforded a 93%
isolated yield of a 9:1 mixture of the 1,2- to 1,4-addition
products after 40 min. However, addition of 10 mol %
N,N,N′,N′-tetramethylethylenediamine (TMEDA) gave
rise, after 12 h, to a 94% isolated yield of exclusively
the 1,4-addition product 6 (rxn 2).14 Similarly, when a
toluene solution of 4 and enone 7 was treated with 10
mol % 1/TMEDA for 2.5 h, a 93% yield of 8 was isolated
(8) Plenio, H.; Diodone, R. J . Am. Chem. Soc. 1996, 118, 356. FN2O3
) 23-fluoro-4,7,20-trioxa-1,10-diazatricyclo[8.7.5.112,16]tricosa-12,14,-
16(23)-triene.
(9) Kulawiec, R. J .; Crabtree R. H. Coord. Chem. Rev. 1990, 99, 89.
(10) Stalke, D.; Whitmire, K. H. J . Chem. Soc., Chem. Commun.
1990, 833.
(11) Stalke, D.; Klingebiel, U.; Sheldrick, G. M. Chem. Ber. 1988,
121, 1457.
(12) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Oxford
University Press: Oxford, England, 1984; p 444.
(13) Shibasaka et al. have recently reported a heterobimetallic
catalyst (LiAl(R-BINOL)2) for the 1,4-conjugate addition of malonates
to cyclohexenones.4f When 10 mol % of LiAl(R-BINOL)2 was employed
in the reaction of cyclohexenone with silyl ketene acetal 2 in toluene,
no reaction was observed.
(14) Chelation of Li+ by TMEDA would produce a sterically more
congested lithium Lewis acid. Presumably, 1,4-addition is favored over
1,2-addition when the enone is coordinated to the more congested Lewis
acid. As expected, the weaker Lewis acidity of the Li(TMEDA)+ complex
results in decreased reaction rates.

3778 Organometallics, Vol. 15, No. 18, 1996
Communications
stituted silyl ketene acetal 4 was employed in the above
reaction, a >90% yield of the corresponding methyl
cyclohexenyl acetate was isolated. This type of direct
substitution has previously been observed for substrates
such as 9 in the presence of 4, but concentrated solutions
of lithium perchlorate in diethyl ether (e.g., 5.0 M
LiClO4-Et2O) were required to effect the transforma-
tion.17 Even more striking was the result with cyclo-
hexenyl acetate 11. Upon exposure of 11 to 2 in toluene
containing 10 mol % 1, a 92% yield of 12 was isolated
after 10 h (rxn 5). The mildness and efficiency of this
(rxn 3).15 The ability of 1 to catalyze sterically demand-
ing 1,4-conjugate addition reactions in toluene obviates
the necessity of using either highly polar media (5.0 M
LiClO4-Et2O)4b or ultrahigh pressure.16
new carbon-carbon bond-forming reaction will be of
considerable interest to those engaged in organic syn-
thesis. Further studies are underway to prepare more
effective catalysts and to determine the scope and
limitation of this novel process.
Ack n ow led gm en t. This research was supported by
grants from the National Science Foundation (CHE-
9308583, CHE-8103011, and CHE-9107593) and the
National Institutes of Health (GM 33605). We also
thank Akzo Nobel, Inc., for financial support and
Central Glass Co. (Ube, J apan) for a gift of H(OC(Ph)-
(CF3)2). T.J .B. thanks the U.S. Department of Educa-
tion for fellowship support under the Graduate Assis-
tance in Areas of National Need Program (Grant No.
P200A10210).
During the course of this study, we made the remark-
able observation that allylic acetates undergo facile
carbon-carbon bond formation when treated with silyl
ketene acetals in toluene in the presence of 1. Treat-
ment of a 0.1 M toluene solution of allylic acetate 9
containing 2.0 equiv 2 with 10 mol % 1 afforded an 82%
isolated yield of 10 after 2 h (rxn 4). In the absence of
catalyst, no reaction was observed. When the unsub-
Su p p or tin g In for m a tion Ava ila ble: NMR spectra and
tables of atomic coordinates, bond distances and angles,
thermal parameters, and crystal data and ORTEP diagrams
for 1 (16 pages). Ordering information is given on any current
masthead page.
(15) When hexamethylphosphoramide (HMPA) was employed in
place of TMEDA in rxns 2 and 3, dramatically lower rates were
observed: for example, substrate 5 required ca. 50 h in order to realize
an 87% yield of 6.
OM9604031
(16) (a) Matsumoto, K. Angew. Chem., Int. Ed. Engl. 1980, 19, 1013.
(b) Bunce, R. A.; Schlecht, M. F.; Dauben, W. G.; Heathcock, C. H.
Tetrahedron Lett. 1983, 24, 4943.
(17) (a) Pearson, W. H.; Schkeryantz, J . M. J . Org. Chem. 1992, 57,
2986. (b) Grieco, P. A.; Collins, J . L.; Henry, K. J . Tetrahedron Lett.
1992, 33, 4735.