Exploiting η5- to η3-Indenyl Ring Slippage to Access a Directed Reaction: Ether-Directed, Rhodium-Catalyzed Olefin Hydroboration

Full text of DOI:10.1021/jo9720647

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J . Org. Chem. 1998, 63, 1370-1371
Exp loitin g η5- to η3-In d en yl Rin g Slip p a ge to
Access a Dir ected Rea ction : Eth er -Dir ected ,
Rh od iu m -Ca ta lyzed Olefin Hyd r obor a tion
Sch em e 1
Christine E. Garrett and Gregory C. Fu*
Department of Chemistry, Massachusetts Institute of
Technology, Cambridge, Massachusetts 02139
Received November 11, 1997
Directed reactions, which comprise an important subset
of synthetically useful metal-catalyzed processes, are de-
manding with respect to coordinative unsaturation at the
metal.1 As Crabtree has noted in the context of hydrogena-
tion reactions, access to a 12ε (or less) metal complex is often
required for a directed olefin addition process (eq 1), due to
the need to simultaneously accommodate the directing group
(B; 2ε), the olefin (2ε), and the addend (X2; 2ε).2 This
that we established is accelerated by (Ind)Rh(C2H4)2 (turn-
overs numbers as high as 1000).6,7 One functional group
that is known to direct hydrogenations catalyzed by 12ε
[(bisphosphine)Rh]+ is the alkyl ether,1,8 and we have
determined that this functional group also directs hydrobo-
rations catalyzed by (Ind)Rh(C2H4)2 (eq 2).9-11 This obser-
analysis adequately rationalizes the observation that com-
plexes such as “[(bisphosphine)Rh]+” (12ε fragment) are
effective catalysts for directed hydrogenation processes,
whereas “(phosphine)2RhCl” complexes (14ε fragment; e.g.,
from Wilkinson’s catalyst) are not.
vation of an ether-directed hydroboration suggests that the
ring-slippage strategy outlined above is in fact viable
(Scheme 1). As a control experiment, we investigated the
hydroboration of the corresponding silyl ether;12 silyl ethers
do not direct [(bisphosphine)Rh]+-catalyzed hydrogenations,
and we found that the (Ind)Rh(C2H4)2-catalyzed hydrobo-
ration of this substrate proceeds with relatively poor ster-
eoselectivity (eq 2).
To furnish additional evidence that the alkyl ether is
directing the (Ind)Rh(C2H4)2-catalyzed hydroboration of
4-(benzyloxy)cyclohexene, we compared the diastereoselec-
tion in solvents of high and low Lewis basicity. Consistent
The design of catalysts that provide ready access to the
requisite coordinative unsaturation should facilitate the
development of directed reactions. Toward that end, we
have initiated an investigation of complexes that bear
ligands prone to coordinative isomerization, a process that
can afford a low-energy pathway to an unsaturated metal
center. The family of ligands best known for displaying this
mode of behavior are cyclopentadienyl derivatives, which can
readily interconvert between η5- and η3-complexation to a
metal (ring slippage).3 To the best of our knowledge,
coordinative isomerization has never been exploited as a
means for effecting a directed reaction. In this report, we
furnish evidence, in the context of ether-directed rhodium-
catalyzed hydroboration, that this strategy is indeed viable.
To validate our approach, we chose to examine the
chemistry of (η5-Ind)Rh (Ind ) indenyl), a 14ε fragment;4
we selected the indenyl, rather than the cyclopentadienyl,
ligand due to the greater propensity of indenyl to ring slip
from η5 to η3.3b,5 According to the Crabtree analysis, if ring
slippage does not occur, then 14ε (η5-Ind)Rh should not effect
a directed olefin addition reaction. Conversely, if a directed
process is observed, then it is reasonable to conclude that
η5 to η3 ring slippage provided the necessary coordinative
unsaturation (Scheme 1).
(6) (a) For the first report of transition metal-catalyzed hydroboration
of olefins with catecholborane, see: Ma¨nnig, D.; No¨th, H. Angew. Chem.,
Int. Ed. Engl. 1985, 24, 878-879. (b) For a review, see: Fu, G. C.; Evans,
D. A.; Muci, A. R. In Advances in Catalytic Processes; Doyle, M. P., Ed.;
J AI: Greenwich, CT, 1995; Vol. 1, pp 95-121.
(7) For a report of amide-directed, metal-catalyzed olefin hydroboration,
see: Evans, D. A.; Fu, G. C. J . Am. Chem. Soc. 1991, 113, 4042-4043.
(8) Brown, J . M.; Hall, S. A. Tetrahedron 1985, 41, 4639-4646.
(9) The cis-1,3-diol derivative, rather than the cis-1,4 isomer, should be
the predominant product in a directed reaction wherein the directing group
is bound to the metal in the regioselectivity-determining step.1
(10) (a) In the absence of (Ind)Rh(C2H4)2 under otherwise identical
conditions, less than 10% conversion is observed. The product that is
generated is an approximately equimolar mixture of four isomeric alcohols.
(b) We have established through 11B NMR studies that catecholborane does
not disproportionate in the presence of alkyl ethers and that the preoxida-
tion product of the (Ind)Rh(C2H4)2-catalyzed hydroboration is an alkylbo-
ronate ester. For leading references to potential complications in hydrobo-
rations of olefins with catecholborane, see: Garrett, C. E.; Fu, G. C. J . Org.
Chem. 1996, 61, 3224-3225. (c) We have found that commercially available
catecholborane is sometimes contaminated with SMe2-derived impurities
that can affect catalyst activity and reaction diastereoselectivity. (d) This
(Ind)Rh(C2H4)2-catalyzed hydroboration proceeds with comparable rate and
diastereoselectivity in the presence of mercury (a test for homogeneous
catalysis: Foley, P.; DiCosimo, R.; Whitesides, G. M. J . Am. Chem. Soc.
1980, 102, 6713-6725).
With respect to choice of reaction, we elected to explore
the hydroboration of olefins with catecholborane, a process
(1) For a review, see: Hoveyda, A. H.; Evans, D. A.; Fu, G. C. Chem.
Rev. 1993, 93, 1307-1370.
(2) Crabtree, R. H.; Davis, M. W. J . Org. Chem. 1986, 51, 2655-2661.
(3) (a) For the original observation, see: Schuster-Woldan, H. G.; Basolo,
F. J . Am. Chem. Soc. 1966, 88, 1657-1663. (b) For a review, see: O’Connor,
J . M.; Casey, C. P. Chem. Rev. 1987, 87, 307-318.
(4) CpRhL2 complexes serve as catalysts for a variety of olefin addition
reactions. See: Dictionary of Organometallic Compounds; Chapman and
Hall: London, 1995.
(11) In contrast, hydroborations of 4-(benzyloxy)cyclohexene in the
presence of catecholborane/ClRh(PPh3)3 or BH3-THF are not selective
(approximately statistical mixture of products).
(5) (a) Hart-Davis, A. J .; Mawby, R. J . J . Chem. Soc. A 1969, 2403-
2407. (b) Rerek, M. E.; J i, L.-N.; Basolo, F. J . Chem. Soc., Chem. Commun.
1983, 1208-1209.
(12) For a discussion of the low Lewis basicity of silyl ethers, see:
Shambayati, S.; Blake, J . F.; Wierschke, S. G.; J orgensen, W. L.; Schreiber,
S. L. J . Am. Chem. Soc. 1990, 112, 697-703.
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Published on Web 02/06/1998

Communications
J . Org. Chem., Vol. 63, No. 5, 1998 1371
with the behavior expected for a directed process, we
observed higher selectivity in solvents that cannot compete
effectively with the benzyl ether for binding to rhodium (eq
3). Thus, hydroborations in CH2Cl2 and hexane preferen-
tially afford the cis-1,3 isomer, whereas the reaction in THF
is relatively unselective.
To the best of our knowledge, this study provides the first
report that an alkyl ether can direct a metal-catalyzed olefin
hydroboration. This process relies upon the capacity of an
indenyl ligand to transiently ring slip, thereby affording
access to the necessary level of coordinative unsaturation
at the metal. We anticipate that this ring-slippage approach
to directed reactions may be extendible to other systems.14
Our investigation of the catalyzed hydroboration of 4-(ben-
zyloxy)cyclohexene by a family of CpXRh catalysts (CpX ) a
cyclopentadienyl-derived ligand) provides support for the
suggestion that ring slippage plays a pivotal role in making
a directed reaction possible. Literature data establish the
following order for the relative propensity of cyclopentadi-
enyl-derived ligands to ring slip:13
Ack n ow led gm en t. Support has been provided by the
Alfred P. Sloan Foundation, the American Cancer Society,
the Camille and Henry Dreyfus Foundation, Eli Lilly,
Firmenich, Glaxo Wellcome, the National Science Founda-
tion (predoctoral fellowship to C.E.G.; Young Investigator
Award to G.C.F., with funding from Procter & Gamble,
Merck, Bristol-Myers Squibb, DuPont, Pfizer, Rohm &
Haas, Pharmacia & Upjohn, and Bayer), and the Research
Corporation. Acknowledgment is made to the donors of the
Petroleum Research Fund, administered by the ACS, for
partial support of this research.
Ind > 1,2,3-trimethylindenyl . Cp > Cp*
We have determined that each of the rhodium complexes
that bear these ligands serves as a catalyst for olefin
hydroboration, and we have observed a correlation between
the propensity of the ligand to ring slip and the preference
for generating the product of a directed reaction (the cis-1,3
isomer, eq 4).
Su p p or tin g In for m a tion Ava ila ble: Experimental proce-
dures and compound characterization data (9 pages).
J O9720647
(13) Kakkar, A. K.; Taylor, N. J .; Marder, T. B.; Shen, J . K.; Hallinan,
N.; Basolo, F. Inorg. Chim. Acta 1992, 198-200, 219-231 and references
therein.
(14) Preliminary results indicate that esters also direct the IndRh(C2H4)2-
catalyzed hydroboration of olefins.