Synergistic effects of hemilabile coordination and counterions in homogeneous catalysis: New tunable monophosphine ligands for hydrovinylation reactions [15]

Full text of DOI:10.1021/ja992493h

J. Am. Chem. Soc. 1999, 121, 9899-9900
9899
Table 1. Ligand Substituent Effects on Hydrovinylation of
2-Methoxy-6-vinylnaphthalenea
Synergistic Effects of Hemilabile Coordination and
Counterions in Homogeneous Catalysis: New
Tunable Monophosphine Ligands for
Hydrovinylation Reactions
yield
entry
ligand
group X in 3
(%) %ee
remarks
1
2
3
(R)-BINAP Ph2Ph
0
Malay Nandi, Jian Jin, and T. V. RajanBabu*
3b (MOP)
3c
OCH3
OCH2Ph
>98
97
87
93
69
12
0
0
96
79
62
73 -55 °C
73
80 at -70 °C
70
Department of Chemistry, The Ohio State UniVersity
100 W. 18th AVenue, Columbus, Ohio 43210
AgNTF2
4
5
6
7
8
9
10
3c
3d
3e
3f
3g
4a
4b
OCH2Ph
O-i-Pr
CH2CH3
OC(O)CH3
P(O)Ph2
OC(H)(Ph)(CH3)
OC(H)(Ph)(CH3)
ReceiVed July 15, 1999
<3
Heterodimerization of olefins is a reaction of enormous
synthetic potential, since it has been demonstrated that excellent
yields and selectivities can be achieved under exceptionally mild
conditions in many cases.1,2 For example, we have recently shown
that using catalysts derived from [(allyl)Ni(Br)]2 and a tertiary
phosphine in combination with a silver salt, a nearly quantitative
hydrovinylation of vinylarenes can be achieved (eq 1). In our
71 (R,R)
65 (R,S)
a See eq 2 for typical procedure.6 Isolated yield; ee determined by
HPLC.
dialkylphospholanes which were found to be exceptionally good
ligands for the exacting hydrovinylation reaction, if and only if
the appropriate counterions are used. The results of these studies
are reported in this paper.
initial efforts to find a broadly applicable asymmetric version of
this reaction,2 we considered the requirement of a coordination
site for ethylene on the putative cationic Ni intermediates, and
chose (R)-2-diphenylphosphino-2′-alkoxy-1,1′-binaphthyl (MOP),3
in which the 2′-methoxy group would play the role of a
‘hemilabile′ ligand4 (eq 2). With the fortuitous choice of this
The results of hydrovinylation using the 2′-modified MOP
ligands are shown in Table 1.6 Replacement of the -OMe group
of MOP with a OCH2Ph (3c) resulted in an isolated yield of 97%
(73% ee) for the hydrovinylation of 2-methoxy-6-vinylnaphthalene
(MVN) at -55 °C. The 2′-isopropoxy derivative 3d gave lower
conversions (69%) with little change in the selectivity (70% ee)
as compared to 3c. The ligand with an the (R)-phenethyl side
chain, 4a, prepared by the Mitsunobu reaction of 3a, gave an
excellent reaction (96% isolated yield, 71% ee) in the hydrovi-
nylation of MVN, whereas the use of the ligand, 4b ((S)-phenethyl
side chain), under otherwise identical conditions resulted in only
a moderate yield (entries 9 and 10). The first clear indication of
the role of hemilabile chelation came from the hydrovinylation
experiments using the ligand 3e, in which the alkoxy group of
MOP was replaced by an ethyl group. Under the standard
conditions (eq 2) using the BARF as the counterion, the
corresponding Ni-complex gave only 17 and 12% conversion in
the hydrovinylation of styrene and MVN, respectively. Most
strikingly, the ee of the isolated product was nearly zero in both
cases! If the hemilabile ligation is important, one should expect
different reactivities from ligands with varying donor properties.4b,c
Accordingly, the allyl(Ni)-complexes of 2′-acetoxy (3f) and the
2′-diphenylphosphinoxy (3g)7 analogues of MOP failed to produce
any hydrovinylation products from styrene and MVN under the
standard conditions (entries 7 and 8). Phosphineoxide is known
to be a strongly coordinating group,4c and it is not surprising that
ethylene is unable to displace this group from the Ni coordination
sphere. Also, carbonyl groups are known to be better hemilabile
ligands than ether oxygens.8a
ligand, we also realized that counterions played a key role in the
success of this reaction. For example, we found that AgOTf, which
gave in many cases isolated yields >95% with Ph3P as a ligand,
gave low yields in the hydrovinylation reactions using the MOP
ligands. In sharp contrast, the use of Na+ B[(3,5-(CF3)2C6H3)]4
-
(NaBARF) in conjunction with MOP fully restored the activity
of the catalyst (eq 2).5 Since this original discovery, we have
sought to clarify the role of hemilabile chelation and of the effect
of various counterions on the efficiency and selectivity of the
Ni-and Pd-catalysts for this reaction. With this goal, variation of
the 2′ substituent of the MOP ligand was studied. In addition,
we synthesized a new class of “tunable” hemilabile 1-aryl-2,5-
(1) (a) Wilke, G. Angew. Chem., Int. Ed. Engl. 1988, 27, 185. (b) Keim,
W. Angew. Chem., Int. Ed. Engl. 1990, 29, 235. (c) Jolly, P. W.; Wilke, G.
In Applied Homogeneous Catalysis with Organometallic Compounds; Cornils,
B., Herrmann, W. A., Eds.; VCH: New York, 1996; p 1024 and references
therein. (d) Muller, G.; Ordinas, J. I. J. Mol. Catal. A: Chemical 1997, 125,
97.
(2) (a) Nomura, N.; Jin, J.; Park, H.; RajanBabu, T. V. J. Am. Chem. Soc.
1998, 120, 459. (b) RajanBabu, T. V.; Nomura, N.; Jin, J.; Radetich, B.; Park,
H.; Nandi, M. Chem. Eur. J. 1999, 5, 1963.
(3) Uozumi, Y.; Tanahashi, A.; S.-Y. Lee.; Hayashi, T. J. Org. Chem. 1993,
58, 1945.
(4) Other recent applications of hemilabile ligands in synthesis see: (a)
Britovsek, G. J. P.; Keim, W.; Mecking, S.; Sainz, D.; Wagner, T. J. Chem.
Soc., Chem. Commun. 1993, 1632. (b) Britovsek, G. J. P.; Cavell, K. J.; Keim,
W. J. Mol. Catal. A: Chemical 1996, 110, 77. (c) Meking, S.; Keim, W.
Organometallics 1996, 15, 2650. (c) Tye, H.; Smyth, D.; Eldred, C.; Wills,
M. J. Chem. Soc., Chem. Commun. 1997, 1053. For recent reviews of
coordination chemistry of hemilabile ligands, see: Bader, A.; Lindner, E.
Coord. Chem. ReV. 1991, 108, 27.; Slone, C. S.; Weinberger, D. A.; Mirkin,
C. A. In Progress in Inorganic Chemistry; Karlin, K. D., Ed.; Wiley: New
York, 1999; p 233.
Next we turned our attention to 1-aryl-2,5-dialkylphospholanes,
a ligand scaffolding that appears to hold great potential in
asymmetric catalysis.9 A series of simple phospholane derivatives
(6) See Supporting Information for details.
(5) (a) For the use of Ar4B- in olefin dimerization reactions, see: DiRenzo,
G. M.; White, P. S.; Brookhart, M. J. Am. Chem. Soc. 1996, 118, 8, 6225. (b)
For reviews on weakly coordinating anions, see: Beck, W.; Su¨nkel, K. Chem.
ReV. 1988, 88, 1405; Strauss, S. H. Chem. ReV. 1993, 93, 927.
(7) As with other chelating phosphines, BINAP gave no reaction. The
monophenoxy BINAP ligand 3g was prepared by a low-temperature oxidation
of BINAP with Davis’ oxaziridine (H. Park, unpublished results). Davis, F.
A.; Chen, B. C. Chem. ReV. 1992, 92, 919.
10.1021/ja992493h CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/13/1999

9900 J. Am. Chem. Soc., Vol. 121, No. 42, 1999
Communications to the Editor
Scheme 1. Synthesis of 1-Aryl-2,5-dialkylphospholanes
6
as the solution is cooled to -70 °C, except for small changes in
chemical shifts (<1 ppm). The catalytically active triflate complex
((allyl)Ni[5a](OTf)) shows the same peaks (at 27 °C, δ 43.89
and 43.31), but the peaks in both 31P and 1H NMR are
considerably broadened at temperatures below -20 °C. Upon
cooling the complex new peaks begin to appear, and at -70 °C,
a complex spectrum consisting of at least eight lines is ob-
tained.6,11,12 As compared to the stable neutral bromide complex,
(allyl)Ni[5a](Br), partial decomposition of the triflate complex,
(allyl)Ni[5a](OTf), can be observed in the NMR tube after several
hours. A similar behavior is observed for the SbF6 and BARF
complexes. In particular, the (allyl)Ni[5a](BARF) undergoes rapid
decomposition even at room temperature as it is being prepared,
showing a multitude of broad peaks (δ 43.73, 43.03, 42.34, and
41.06) at 0 °C. The temperature-dependent changes of all
complexes except that of the BARF complex are completely
reversible, which leads to one to suspect that in the absence of
hemilabile ligation, the highly dissociated BARF-complex is very
unstable (vide infra). In sharp contrast, the (allyl)Ni[5c](BARF),
is a remarkably stable species showing no sign of decomposition
upon storage at room temperature for several days.6 It exhibits
1H NMR spectrum between temperatures 27 and -70 °C,
indicative of highly fluxional behavior. The room temperature
31P NMR spectrum consists of a broad singlet at δ 23.63, which
remains unchanged even at -70 °C, except for some line
broadening and minor changes in the chemical shift. Similar
behavior (31P NMR δ 23.71) is seen for the other actiVe complex
(allyl)Ni[5c](SbF6). The neutral, catalytically inactiVe complex
(allyl)Ni[5c](Br), on the other hand shows two sharp peaks in
the 31P NMR (at 0 °C 37.04 and 36.44, ratio ∼3:2) at temperatures
between 0 and -40 °C, suggesting stable exo- and endo-isomers
similar to (allyl)Ni[5a](Br). As expected8b hemilabile coordination
of an ether oxygen results in an upfield shift of the 31P signal
(∆δ ) ∼12 ppm). The corresponding (allyl)Ni[5c](OTf) [major
31P δ ) 31.3] was found to undergo rapid decomposition in the
NMR tube. One plausible explanation why the triflate salt (allyl)-
Ni[5c](OTf) is catalytically inactive might be that with the
hemilabile oxygen and a coordinating counterion there is little
chance of ethylene incorporation to initiate the reaction. Also, in
the absence of an internal hemilabile ligand, catalysts with highly
dissociated counterions (BARF, SbF6) appear to have only
transient existence, which might explain why there is no reactions
when they are involved. Studies to clarify such mechanistic details
are underway.
Table 2. Effect of Counterions on the Hydrovinylation of Styrene
Using “Hemilabile” Ligandsa
yield of product (%)
entry additive
5a
5c
remarks
1
2
3
4
5
AgOTf
AgClO4
AgNTf2
AgSbF6
NaBAr4
94
95
<2
<2
<2
<4
<2
48
94
97
37%ee (S) with 5a
29% isom. with 5a
47%ee (S), 9% isom. with 5c
48%ee (S) with 5c
b
50%ee (S) with 5c
a For reaction conditions see Supporting Information. b Ar ) 3,5-
(CF3)2-C6H3; ee determined by HPLC.
5a-d, prepared according to Scheme 1,6 were tested as ligands
in the Ni-catalyzed asymmetric hydrovinylation reactions. Our
initial investigations started with ligands 5a, and a close analogue,
5b, with a potential hemilabile group at the ortho-position. While
we found that 5a to be an excellent ligand for the Ni-catalyzed
hydrovinylation of vinylarenes, especially with OTf as the
counterion (vide infra Table 2), 5b led to significant isomerization
of the initially formed product 1 (to 2) under the standard reaction
conditions (eq 2) even at -55 °C. One of the principal differences
between 5b and the ligand 3b, we conjectured, was the placement
of the hemilabile alkoxy group with respect to the phosphorus.
In 5b it is on the â carbon and in 3b it is on the δ carbon, resulting
in 5- vs 7-membered Ni-chelate intermediates in the respective
cases. To probe the effect of the relative positioning of the
hemilabile group, the o-benzyloxymethyl analogue 5c was
prepared, and most gratifyingly, this ligand proved to be one of
the best for highly selective hydrovinylation reactions.
The results of hydrovinylation of styrene using 5a and 5c are
shown in Table 2. For the simple phospholane ligand 5a, with
no possibility of hemilabile coordination, the reaction did not
proceed unless a weakly coordinating anions such a OTf is used
(entries 1 and 2). Incidentally, ClO4- was also acceptable, except
that significant isomerization of the primary product was observed.
Additives such as AgBF4, NaBPh4, AgNTf2, AgSbF6, and
NaBARF gave practically no reaction under the standard condi-
tions (entries 3-5). In sharp contrast, when ligand 5c (or 5d)
with the o-alkoxymethylphenyl substituent were used, best results
were obtained with noncoordinating counteranions BARF and
-
SbF6 (entries 4 and 5). Not surprisingly, AgOTf, AgClO4,
Acknowledgment. We acknowledge the financial assistance by the
U.S. National Science Foundation (CHE- 9706766), U.S. Environmental
Protection Agency (R826120-01-0), and the Petroleum Research Fund
of the American Chemical Society.
AgBF4, and NaBPh4 were found to be ineffective in these cases.10
Some indication of the effect of counterions on the relative
stability/reactivity of the 1,5-phospholane complexes can be
obtained by a variable temperature 31P and 1H NMR study of the
(allyl)Ni[P]X (P ) 5a, 5c, X ) Br, OTf, SbF6 and BARF).6 The
proton spectra between 27 and -70 °C are broad and generally
unintelligible. At 27 °C, the catalytically inactiVe complex (allyl)-
Ni[5a](Br), shows two peaks (1:1) in the 31P spectrum at δ 43.10
and 42.08, characteristic of the square planar endo- and exo-
diastereomers shown below. There are no changes in the spectrum
Supporting Information Available: Experimental procedures for the
1
synthesis of ligands 3f, 3g, 4a, 4b, 5a-c, their H, 13C, and 31P NMR
spectra, procedures for their use in hydrovinylation reactions. Selected
variable-temperature 1H and 31P NMR spectra of complexes of the general
structure (allyl)Ni(ligand)(counterion) (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
JA992493H
(8) (a) Braunstein, P.; Chauvin, Y.; Na¨hring, J.; DeCian, A.; Fischer, J.;
Tiripicchio, A.; Ugozzoli, F. Organometallics 1996, 15, 5551. (b) Reddy, V.
V. S.; Varshney, A.; Gray, G. M. J. Organomet. Chem. 1990, 391, 259.
(9) (a) Burk, M. J.; Feaster, J. E.; Harlow, R. L. Tetrahedron: Asymmetry
1991, 2, 569. (b) Burk, M. J.; Gross, M. F.; Harper, T. G. P.; Kalberg, C. S.;
Lee, J. R.; Martinez, J. P. Pure Appl. Chem. 1996, 68, 37.
(11) (a) Brandes, H.; Goddard, R.; Jolly, P. W.; KrU¨ ger, C.; Mynott, R.;
Wilke, G. Z. Naturforsch. Teil B. 1984, 39, 1139. (b) Barnett, B. L.; Kru¨ger,
C. J. Organomet. Chem. 1974, 77, 407.
(12) Triflate ion is known to coordinate to nickel in a bidentate fashion at
low temperature. Po¨rshke, K.-R.; Krause, J.; Haack, K.-J.; Nickel, T.; Proft,
B. Proceedings of the Ninth International Symposium on Homogeneous
Catalysis, Jerusalem, 1994; p 55. In addition, at -70 °C, some restricted
rotation around the P-Ni bond can also be expected.11a
(10) For a related result, see: Bayersdo¨rfer, R.; Ganter, B.; Englert, U.;
Keim, W.; Vogt, D. J. Organomet. Chem. 1998, 552, 187.