Trifluoromethyl-substituted indenyl rhodium and iridium complexes are highly selective catalysts for directed hydroboration reactions

Article abstract of DOI:10.1021/ol005653z

(equation presented) Rhodium and iridium catalysts containing trifluoromethyl-substituted indenyl ligands (Ind′MCod, Ind′ = C₉H₇, (1-CF₃)C₉H₆, (2-CF₃)C₉H₆, (1,3-CF

Full text of DOI:10.1021/ol005653z

ORGANIC
LETTERS
2000
Vol. 2, No. 7
981-983
Trifluoromethyl-Substituted Indenyl
Rhodium and Iridium Complexes Are
Highly Selective Catalysts for Directed
Hydroboration Reactions
John A. Brinkman, Tam T. Nguyen, and John R. Sowa, Jr.*
Department of Chemistry, Seton Hall UniVersity, South Orange, New Jersey 07079
sowajohn@shu.edu
Received February 11, 2000
ABSTRACT
Rhodium and iridium catalysts containing trifluoromethyl-substituted indenyl ligands (Ind′MCod, Ind′ ) C H , (1-CF3)C H , (2-CF3)C H , (1,3-
9
7
9
6
9 6
CF3)2C H ) have been developed for the directed hydroboration of 4-(benzyloxy)cyclohexene to cis-3-(benzyloxy)cyclohexanol. Compared to
9
5
unsubstituted complexes, trifluoromethyl substitution yields a 3−10% increase in selectivity which is attributed to the strong electron-withdrawing
effect of the trifluoromethyl group. Rhodium complexes give selectivities of 74−84%, and iridium complexes give high levels of selectivity
(93−98%).
Directed reactions are an important class of stereoselective,
transition-metal-mediated processes.1 In these reactions,
donor groups (D) direct reactions to occur on the same face
of the substrate as the donor group (eq 1).2 A requirement
degree of coordinative unsaturation by slipping from a 6ꢀ
donor (η5-Ind) to a 4ꢀ donor (η3-Ind).15 The indenyl ligand
was chosen because of its propensity to undergo coordinative
isomerization from η5- to η3-coordination.4 This process, also
known as ring slippage, allows the metal to accommodate
the 2ꢀ donor ligand (D) and dramatically increases rates of
ligand substitution reactions.5
With IndRh(C2H4)2, Fu and Garrett obtained 75% selectiv-
ity for the directed hydroboration of 4-(benzyloxy)cyclo-
hexene (1) to cis-(3-benzyloxy)cyclohexanol (2) (eq 2).2 We
of the transition metal is that it must achieve the requisite
degree of coordinative unsaturation to facilitate the directed
reaction. Crabtree proposed that the metal must achieve a
12ꢀ count or less in order to simultaneously accommodate
the directing group, the addend (X2), and the target functional
group of the substrate which is often an olefin (eq 1).3
In 1998, Garrett and Fu2 demonstrated that the indenyl
ligand could enable a transition metal to obtain the required
chose to further examine this reaction using rhodium and
iridium complexes containing trifluoromethyl-substituted
indenyl ligands. The syntheses of indenes 3-5 are previously
reported.6 The corresponding rhodium and iridium complexes
(1) Review: Hoveyda, A. H.; Evans, D. A.; Fu, G. C. Chem. ReV. 1993,
93, 1307-1370.
(2) Garrett, C. E.; Fu. G. C. J. Org. Chem. 1998, 63, 1370-1371.
10.1021/ol005653z CCC: $19.00 © 2000 American Chemical Society
Published on Web 03/16/2000

are prepared by deprotonation of the indene followed by
methyl-substituted Ind′Rh(Cod) catalysts. The first complex,
(1-CF3Ind)Rh(Cod), showed a 7% increase in selectivity to
81% for the cis-1,3 isomer (2). The run was repeated in
triplicate, and the selectivity was remarkably reproducible
with a variation of (1%. With the trifluoromethyl group at
the 2-position of the indenyl ligand, (2-CF3Ind)Rh(Cod),
there was no change in selectivity (81%). However, the
catalyst with two trifluoromethyl groups, (1,3-(CF3)2Ind)-
Rh(Cod), resulted in a further increase in selectivity to 84%
for the cis-1,3 isomer.
reaction with [ClM(Cod)]2, M ) Rh or Ir (Scheme 1).7
Scheme 1. Trifluoromethyl-Substituted Indenes and Catalyst
Preparation6,7
Although the selectivities are markedly increased with the
trifluoromethyl-substituted catalysts, the yields of the reac-
tions are ∼10% lower than the unsubstituted catalysts.
Furthermore, the reactions (substituted or unsubstituted
catalysts) are extremely air sensitive and failure to exclude
air significantly reduces selectivity or completely inhibits the
reaction. In agreement with Garrett and Fu, we find that best
results are obtained with freshly prepared and distilled
catecholborane.12
Previous studies have shown that Crabtree’s iridium
complex, [Ir(cod)(PCy3)(py)]PF6, is a more selective catalyst
than [Rh(nbd)(dppb)]BF4 in amide-directed hydroborations.13
Thus, it seemed likely that a switch from rhodium to iridium
in our system would also result in an increase in selectivity.
This insight proved to be valuable as selectivity for the cis-
1,3 isomer increased to practical levels (93-98%) with the
iridium-based catalysts (Table 2). For example, a 19%
Our previous work examined the effect of trifluoromethyl-
substituted indenyl ligands in the rhodium-catalyzed hy-
droboration of styrene.8 In that study, there was no difference
in yield or selectivity between the unsubstituted and trifluo-
romethyl-substituted indenyl catalysts, suggesting that the
indenyl ligand was being removed from the catalyst. The
lability of anionic olefin ligands (e.g., η3-allyl) under
conditions of hydroboration is well known.9 However, the
study by Garrett and Fu2 indicates that the indenyl ligand
remains intact during the course of the directed hydroboration
reaction. Moreover, they found that electron-donating methyl
groups decrease selectivity compared to the unsubstituted
ligand. We reasoned that if indeed the indenyl ligand
remained intact, then electron-withdrawing groups will
increase selectivity. However, indenyl complexes with
electron-withdrawing substituents have not been widely
explored.10 The catalysts that we have chosen contain one
or two trifluoromethyl groups attached to various positions
on the indenyl ring as shown in Scheme 1.
Table 2. Yield and Selectivity of Iridium-Catalyzed Directed
Hydroborations
catalyst
cis-
1,3a 1,4a
cis-
trans- trans- yield,b
Ind′Ir(Cod)
1,3a
1,4a
%
IndIr(Cod)
93
96
96
98
<1
<1
2
5
2
2
2
2
<1
<1
72
60
62
60
(1-CF3Ind)Ir(Cod)
(2-CF3Ind)Ir(Cod)
(1,3-(CF3)2-Ind)Ir(Cod)
The proposed mechanism for the directed hydroboration
reaction involves initial loss of the ethylene ligands to form
a 14ꢀ intermediate.2 Since the catalysts we have developed
contain the 1,5-cyclooctadiene ligand (Cod) instead of bis-
ethylene, we first compared the selectivity and yield of
IndRh(Cod) to that of IndRh(C2H4)2.11 The results indicate
that these are equivalent catalysts (Table 1).
2
<1
a Selectivity determined by HPLC with authentic standards. b Isolated
yield of the four isomers.
increase in selectivity is observed on going from the
unsubstituted IndRh(Cod) (74%) complex to IndIr(Cod)
(93%). Furthermore, the diastereoselective trend of increasing
After confirming that the Cod ligand does not affect
reactivity and product ratios, we studied a series of trifluoro-
(3) (a) Crabtree, R. H.; Davis, M. W. J. Org. Chem. 1986, 51, 2655-
2661. (b) Crabtree, R. H. The Organometallic Chemistry of the Transition
Metals, 2nd ed.; Wiley: New York, 1994.
(4) Review: O’Connor, J. M.; Casey, C. P. Chem. ReV. 1987, 87, 307-
318.
(5) Rerek, M. E.; Ji, L. N.; Basolo, F. J. Chem Soc., Chem. Commun.
1983, 1208-1209.
(6) Gassman, P. G.; Ray, J. A.; Wenthold, P. G.; Mickelson, J. W. J.
Org. Chem. 1991, 56, 5143-5146.
Table 1. Selectivity and Yield of Rhodium-Catalyzed Directed
Hydroborations
catalyst
cis-
1,3a
cis- trans- trans- yield,c
1,4a 1,3a 1,4a
Ind′RhL2
%
IndRh(C2H4)2
73 (75)b 10 (7) 11 (8) 7 (11) 71 (78)
(7) (a) Mickelson, J. W. Ph.D. Thesis, University of Minnesota, 1991.
(b) Sowa, J. R., Jr.; Mickelson, J. W.; Ye, Z.; Albietz, P.; Kodah, J.;
Whitener, M. A. Manuscript in preparation, Seton Hall University, South
Orange, NJ.
IndRh(Cod)
(1-CF3Ind)Rh(Cod)
(2-CF3Ind)Rh(Cod)
74
81
81
11
5
6
9
10
9
6
4
4
5
70
59
58
59
(1,3-(CF3)2-Ind)Rh(Cod) 84
3
8
(8) Brinkman, J. A.; Sowa, J. R., Jr. In Catalysis of Organic Reactions;
Herkes, F., Ed.; Marcel Dekker: New York, 1998; p 543.
(9) Westcott, S. A.; Blom, H. P.; Marder, T. B.; Baker, R. T. J. Am.
Chem. Soc. 1992, 114, 8863-8869.
a Selectivity determined by HPLC with authentic standards. b Previously
reported selectivities and yield, ref 2. c Isolated yield of the four isomers.
(10) Deck, P. A.; Fronczek, F. R. Organometallics 2000, 19, 327-333.
982
Org. Lett., Vol. 2, No. 7, 2000

selectivity with increasing trifluoromethyl substitution seen
with the rhodium complexes is also observed with iridium.
Using mono-trifluoromethyl indenyl iridium complexes
increases the selectivity to 96%. Furthermore, using the (1,3-
(CF3)2Ind)Ir(Cod) complex yields a catalyst that generates
cis-(3-benzyloxy)cyclohexanol (2) in 98% selectivity.
Compared to unsubstituted systems, catalysts with one
trifluoromethyl group give a 3-7% increase in selectivity
and an additional 2-3% increase is obtained with catalysts
containing two trifluoromethyl groups. This is attributed to
the electron-withdrawing effect of the trifluoromethyl group
which enhances the ability of the catalysts to accommodate
the donor group (OBn). However, it is interesting to consider
whether the selectivity is determined by the inherent ability
of the indenyl ligand to ring slip or the enhanced electro-
philicity of the transition-metal center. In all examples, the
trifluoromethyl substituents are located on the allylic portion
of the indenyl ligand. Since this is also the anionic fragment
of the ligand, the electron-withdrawing trifluoromethyl
substituent may stabilize this fragment. This should increase
the ability of the indenyl ligand to slip from η5- to
η3-coordination. It is also well known that trifluoromethyl
substituents in cyclopentadienyl complexes increase the
electrophilicity of the metal center;1414-15 this is also most
certainly true for indenyl complexes. The more electrophilic
metal center will enhance the coordination of the OBn donor
group. Thus, it is likely that both effects work in combination
to accommodate the donor ligand and enhance its directing
influence.
The results of this study supports Garrett and Fu’s theory
that coordinative isomerism of the indenyl ligand, i.e., ring
slippage, provides the necessary coordinative unsaturation
to facilitate a directed reaction. However, we also show that
selectivity is enhanced when electron-withdrawing groups
are present on the indenyl ligand. Furthermore, switching
from rhodium- to iridium-based systems provides a series
of catalysts that give high levels (93-98%) of selectivity
for the directed hydroboration reaction of 4-(benzyloxy)-
cyclohexene. We are currently exploring the scope of this
reaction toward a wider array of substrates and completing
an extensive X-ray crystallographic study7b of the structures
of the rhodium and iridium catalysts used in this study.
Acknowledgment. We thank BP-Amoco for a grant that
supported preliminary stages of this project. We also thank
Merck for a donation of an NMR spectrometer. J.A.B. is
grateful to the Novartis Pharmaceutical Co. for a doctoral
educational fellowship. We also thank Prof. Greg Fu for
helpful discussions. Finally, we are grateful to Dr. John
Mickelson, Paul Albietz, and Jason Kodah for the preparation
of some of the catalysts used in this study.
(11) (a) General experimental procedure: Manipulations for the
hydroboration procedure were performed under an argon atmosphere.
Hydroboration: a solution of 4-(benzyloxy)cyclohexene11b,c (100 mg, 0.53
mmol) in anhydrous hexane (1 mL) was added by syringe to a solution of
IndRh(Cod) (17 mg, 0.053 mmol) in anhydrous hexane (4 mL). The light
yellow solution was cooled in an ice bath, and catecholborane (0.150 mL,
1.38 mmol) was added to give a pale yellow solution. The reaction mixture
was allowed to warm to room temperature and stirred for 24 h. Oxidation:
the reaction mixture was then cooled in an ice bath and MeOH (2 mL),
NaOH (3 N, 2 mL), and 30% H2O2 (2 mL) were added. The mixture was
stirred, with slow warming to room temperature, for 1 h. The reaction
mixture was extracted with ether, dried over MgSO4 (anhydrous), and
concentrated. The selectivity was determined by HPLC analysis with a
reverse phase column (Waters Nova-Pak C-18, 3.9 × 150 mm) using a
solvent gradient of 10% acetonitrile/water to 100% acetonitrile over a 20
min period. See Supporting Information in ref 2 for preparation of authentic
standards. (b) Godek, C. J.; Moir, R. Y.; Purves, C. B. Can J. Chem. 1951,
29, 946, (c) Schulz, J.; Gani, D.; J. Chem. Soc., Perkins Trans. 1 1997,
657-670.
Supporting Information Available: General experimen-
tal conditions, preparation of catalysts, and general catalytic
procedure sections. This material is available free of charge
via the Internet at http://pubs.acs.org.
OL005653Z
(14) (a) Gassman, P. G.; Mickelson, J. W.; Sowa, J. R., Jr. J. Am. Chem.
Soc. 1992, 114, 6942-6944. (b) Gassman, P. G.; Sowa, J. R., Jr.; Hill, M.
G.; Mann, K. R. Organometallics 1995, 14, 4879-4885.
(15) Abbreviations: Bn ) benzyl, Cod ) 1,5-cyclooctadiene, Cy )
cyclohexyl, dppb ) 1,4-bis(diphenylphosphino)butane, Ind ) indenyl ligand
(C9H7), Ind′ ) substituted indenyl ligand, nbd ) norbornadiene, py )
pyridine.
(12) Brown, H. C.; Gupta, S. K. J. Am. Chem. Soc. 1975, 97, 5249-
5255.
(13) Evans, D. A.; Fu, G. C.; Hoveyda, A. H. J. Am. Chem. Soc. 1992,
114, 6671-6679.
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