Catalytic alkene hydroboration mediated by cationic and formally zwitterionic rhodium(I) and iridium(I) derivatives of a P,N-substituted indene

Article abstract of DOI:10.1021/om060742m

Cationic (2a,b) and formally zwitterionic (3a,b) Rh(I) and Ir(I) complexes supported by P,N-substituted indene or indenide ligands (respectively) have been employed in the selective hydroboration of substituted vinylarenes with pinacolborane (HBpin, pin = 1,2-O₂C₂Me₄). Notably, 3a,b exhibited remarkably high, but differing, selectivities in the hydroboration of 1-phenylpropene.

Full text of DOI:10.1021/om060742m

Organometallics 2006, 25, 5965-5968
5965
Notes
Catalytic Alkene Hydroboration Mediated by Cationic and Formally
Zwitterionic Rhodium(I) and Iridium(I) Derivatives of a
P,N-Substituted Indene
Judy Cipot,† Christopher M. Vogels,‡ Robert McDonald,§ Stephen A. Westcott,*,‡ and
Mark Stradiotto*,†
Department of Chemistry, Dalhousie UniVersity, Halifax, NoVa Scotia, Canada B3H 4J3,
Department of Chemistry, Mount Allison UniVersity, SackVille, New Brunswick, Canada E4L 1G8, and
X-Ray Crystallography Laboratory, Department of Chemistry, UniVersity of Alberta,
Edmonton, Alberta, Canada T6G 2G2
ReceiVed August 15, 2006
Summary: Cationic (2a,b) and formally zwitterionic (3a,b) Rh-
(I) and Ir(I) complexes supported by P,N-substituted indene or
indenide ligands (respectiVely) haVe been employed in the
selectiVe hydroboration of substituted Vinylarenes with pina-
colborane (HBpin, pin ) 1,2-O2C2Me4). Notably, 3a,b exhibited
remarkably high, but differing, selectiVities in the hydroboration
of 1-phenylpropene.
group 9 species that feature formal charge separation between
a cationic metal fragment and an associated anionic ancillary
ligand are emerging as a complementary class of catalysts.5
While the first zwitterionic Rh complexes explored in catalytic
applications involving E-H and/or E-E bond activation were
those supported by η6-tetraphenylborate and related ligands,6
other classes of Rh zwitterions in which the anionic charge
carrier is a substituted carborane,7 sulfate,8 or borate9 fragment
have been developed and in some cases have exhibited desirable
catalytic properties. Conversely, Ir zwitterions are much less
common,10 and their reactivity properties have not been explored
in a systematic manner.
With the goal of establishing new classes of metal complexes
that exhibit interesting and synthetically useful reactivity patterns
with substrate E-H and/or E-E bonds, one of our groups has
reported previously that 1a can be employed in the preparation
of cationic complexes such as 2a,b, as well as structurally related
Introduction
Cationic square-planar complexes of the heavier group 9
metals are among the most effective and widely used classes
of homogeneous catalysts for the addition of E-H and E-E
bonds (E ) main-group element) to unsaturated organic
substrates.1 While the design and construction of new cationic
catalysts of this type continues to facilitate advancements in
metal-mediated synthesis,2 such discrete salts often display poor
solubility in low-polarity media and can be rendered catalytically
inactive in strongly coordinating solvents.3 Furthermore, the
strategic design of group 9 cationic catalysts is complicated by
the fact that the accompanying outer-sphere counteranion
influences the solubility and reactivity properties of the catalyst.4
In this context, structurally related and formally zwitterionic
(5) Formally zwitterionic complexes are defined herein as those species
lacking conventional resonance structures that place an anionic charge onto
one of the donor atoms of the ancillary ligand.
(6) Selected synthetic and catalytic studies: (a) Vogels, C. M.; Decken,
A.; Westcott, S. A. Can. J. Chem. 2006, 84, 146. (b) Van den Hoven, B.
G.; Alper, H. J. Am. Chem. Soc. 2001, 123, 10214. (c) Dai, C.; Robins, E.
G.; Scott, A. J.; Clegg, W.; Yufit, D. S.; Howard, J. A. K.; Marder, T. B.
Chem. Commun. 1998, 1983. (d) Kishimoto, Y.; Itou, M.; Miyatake, T.;
Ikariya, T.; Noyori, R. Macromolecules 1995, 28, 6662. (e) Aresta, M.;
Quaranta, E.; Albinati, A. Organometallics 1993, 12, 2032. (f) Westcott,
S. A.; Blom, H. P.; Marder, T. B.; Baker, R. T. J. Am. Chem. Soc. 1992,
114, 8863. (g) Longato, B.; Pilloni, G.; Graziani, R.; Casellato, U. J.
Organomet. Chem. 1991, 407, 369. (h) Schrock, R. R.; Osborn, J. A. Inorg.
Chem. 1970, 9, 2339.
* To whom correspondence should be addressed. Tel: 1-506-364-2372
(S.A.W.); 1-902-494-7190 (M.S.). Fax: 1-506-364-2313 (S.A.W.); 1-902-
494-1310 (M.S.). E-mail: swestcott@mta.ca (S.A.W.); mark.stradiotto@dal.ca
(M.S.).
† Dalhousie University.
‡ Mount Allison University.
§ University of Alberta.
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10.1021/om060742m CCC: $33.50 © 2006 American Chemical Society
Publication on Web 10/19/2006

5966 Organometallics, Vol. 25, No. 25, 2006
Notes
Scheme 1. Cationic and Zwitterionic K2-P,N Rh(I) and Ir(I)
differing from that of the uncatalyzed reaction, Rh and Ir
complexes have emerged as the most effective catalysts for the
addition of HBcat or HBpin to vinylarenes.15,17 Nevertheless,
achieving high levels of selectivity in such metal-catalyzed
hydroboration reactions can still represent a significant chal-
lenge; in addition to the typical linear and less common branched
hydroboration B-H addition products, borylated olefins and
hydrogenated products are also observed frequently in such
metal-catalyzed transformations. In the context of comparing
and contrasting the catalytic abilities of our cationic (2a,b) and
formally zwitterionic (3a,b) group 9 catalyst complexes, we
viewed vinylarene hydroboration as representing an appealing
prototype for metal-mediated E-H bond additions in which
product selectivity can be difficult to achieve.15,17,18
Our initial catalytic studies (room temperature, THF-d8, 2 mol
% catalyst) examined the hydroboration of monosubstituted
vinylarenes (4-vinylanisole and 4-fluorostyrene) with either
HBcat or HBpin, catalyzed by 2a,b or 3a,b. Catalytic reactions
were monitored by use of multinuclear magnetic resonance
techniques, which allowed for the identification of numerous
boron-containing species,6f including linear and branched hy-
droboration products and mono- and diborated species derived
from dehydrogenative borylation reaction pathways,19 as well
as the corresponding substituted ethylbenzene arising from the
hydrogenation of the vinylarene substrate. In turning our
Complexes Derived from 1a
and formally zwitterionic species, including 3a,b (Scheme 1).11
Whereas 2a,b can be compared with other more traditional
group 9 [(COD)M(κ2-P,N)]+X- salts, the zwitterions 3a,b
constitute an unusual class of substituted indenyl-metal com-
plexes,12 which are comprised of a formally cationic [(COD)M]+
fragment counterbalanced by an uncoordinated 10-π-electron
indenide unit that is incorporated into the backbone of the
κ2-P,N ancillary ligand framework. Preliminary reactivity studies
revealed the catalytic utility of 2a and 3a in alkene hydrosilyl-
ation reactions, and of 2b and 3b for alkene hydrogenation.11c,d
In an effort to assess further the ability of these Rh and Ir
complexes to mediate the addition of E-H and E-E bonds to
unsaturated substrates, we turned our attention to the hydro-
boration and diboration of alkenes. Herein we report on the
results of this catalytic survey and related reactivity studies,
which suggest that 2a,b and 3a,b can be viewed as a
complementary family of catalyst complexes for vinylarene
hydroboration, especially when pinacolborane is employed
(HBpin; pin ) 1,2-O2C2Me4). Notably, the successful applica-
tion of 3b in this context represents the first reported hydro-
boration reaction mediated by a formally zwitterionic Ir
complex.
(17) For selected studies, see: (a) Alexakis, A.; Polet, D.; Bournaud,
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Results and Discussion
The addition of B-H bonds to unsaturated organic molecules
represents an important synthetic methodology in organic
chemistry.13 While BH3 adds rapidly to alkenes at -80 °C,
alternative hydroborating agents such as catecholborane (HBcat;
cat ) 1,2-O2C6H4) and HBpin are slow to react, even at elevated
temperatures.14 However, the reluctance of these (RO)2BH
reagents to add to alkenes can allow for hydroborations to be
carried out under the control of a catalyst.15 Following a report
by Ma¨nnig and No¨th,16 who demonstrated that Rh complexes
could be used to catalyze the hydroboration of alkenes with
HBcat under mild conditions and in some cases with selectivity
(11) (a) Wile, B. M.; Burford, R. J.; McDonald, R.; Ferguson, M. J.;
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Notes
Organometallics, Vol. 25, No. 25, 2006 5967
Scheme 2. Metal-Catalyzed Hydroboration of
2-Phenylpropene (4) using HBpin
Scheme 3. Metal-Catalyzed Hydroboration of
1-Phenylpropene (6) using HBpin
Table 1. Product Distribution Data for the Metal-Catalyzed
Hydroboration of 2-Phenylpropene (4) and 1-Phenylpropene
(6) with Hbpina
presumably arises from an initial metal-mediated isomerization
of the double bond in 6 to give the corresponding R-olefin (3-
phenylpropene), followed by a regioselective hydroboration step.
While the isomerization of 3-phenylpropene to 1-phenylpropene
(6) under similar conditions is common,20 the apparent reverse
rearrangement of 6 to 3-phenylpropene (ultimately leading to
7c) has not been observed previously in metal-catalyzed
hydroboration reactions. In contrast to the selectivity achieved
in these reactions employing HBpin, complex product mixtures
were generated in analogous hydroborations employing HBcat.
In addition, in evaluating the utility of 2a,b or 3a,b in mediating
the diboration21 of the vinylarenes featured herein using either
B2cat2 or B2pin2, complex product distributions arising from
competing diboration, dehydrogenative borylation, hydrobora-
tion, and hydrogenation pathways were obtained, as has been
observed previously in reactivity studies involving some other
catalyst systems.19i
catalyst
product
2a
3a
30
70
0
2b
3b
5a
5b
5c
7a
7b
7c
0
90
10
80
20
0
0
0
60
40
>95
0
>95
0b
0b
0
0
0b
0b
>95b
>95b
a Except where noted, reactions were carried out at room temperature in
THF-d8 with 2 mol % catalyst. Product distributions were determined based
on 1H NMR spectroscopic data after 72 h, at which point quantitative
consumption of the vinylarene substrate was achieved. See the Supporting
Information for complete experimental details. b Reaction carried out in
refluxing toluene.
attention to disubstituted alkene substrates, the hydroboration
of 2-phenylpropene (R-methylstyrene, 4) mediated by 2a,b or
3a,b was investigated under similar conditions. Reactions of
HBcat with 4 once again gave a complex mixture of products
arising from competing hydroboration, dehydrogenative boryla-
tion, and hydrogenation reaction pathways. However, greatly
enhanced selectivity was achieved when the less reactive HBpin
was employed (Scheme 2), with each of the catalyst complexes
exhibiting a preference for the linear addition product, 5b (Table
1). Whereas the cationic Rh complex 2a proved to be more
selective than the structurally analogous zwitterion 3a, the
opposite trend was observed in the Ir system, with the formally
zwitterionic catalyst 3b producing 5b exclusively. Notably,
while significant quantities of the alkenyl boronate ester 5c were
generated in reactions employing the cationic catalyst complexes
2a,b, such products arising from dehydrogenative borylation
were not detected in reactions mediated by their formally
zwitterionic relatives 3a,b. In an effort to identify potential
intermediates in these metal-catalyzed hydroborations, com-
plexes 2a,b and 3a,b were treated separately with HBpin and
B2pin2 (vide infra) and the progress of each reaction was
monitored by use of 31P NMR techniques. Unfortunately, under
all experimental conditions surveyed (1 or 10 equiv HBpin or
B2pin2; room temperature or 60 °C), complex reaction mixtures
resulted, from which no pure materials could be obtained.
Encouraged by the catalytic results involving 2-phenylpropene
(4), the metal-mediated hydroboration of 1-phenylpropene (â-
methylstyrene, 6) with HBpin was examined (room temperature,
THF-d8, 2 mol % catalyst), as depicted in Scheme 3. While the
cationic Rh catalyst 2a exhibited excellent selectivity for the
hydroboration product 7a (Table 1), the structurally related
zwitterion 3a proved superior, generating 7a exclusively.
Conversely, poor substrate conversions were achieved under
analogous conditions employing 2b and 3b. However, both of
these Ir catalyst complexes performed well at elevated temper-
atures (refluxing toluene, 2 mol % catalyst), affording the
terminally substituted borane 7c quantitatively. Product 7c
Control experiments conducted in the course of the afore-
mentioned alkene hydroboration studies confirmed the stability
of 2a,b and 3a,b in either THF or toluene at room temperature
for a minimum of 72 h. However, the remarkably similar
catalytic performance exhibited by 2b and 3b in the selective
hydroboration of 1-phenylpropene (6) with HBpin at elevated
temperatures prompted us to investigate the extent to which these
Ir species might access a common reactive intermediate upon
heating in solution. In this regard, we observed that heating of
the formally zwitterionic 3b in THF at 60 °C for 72 h resulted
in the partial conversion (ca. 25%) to a single new phosphorus-
containing species (8; δ(31P) 14). Significant quantities of 8,
along with another as yet unidentified phosphorus-containing
species (δ(31P) 61), were also produced in toluene under similar
conditions. Although we have thus far been unsuccessful in our
efforts to isolate sufficient amounts of analytically pure 8 to
allow for the comprehensive characterization of this complex,
in one instance we were able to obtain a minute quantity of
(20) Competing isomerization reactions have been reported previously
in catalyzed hydroborations: (a) Morrill, T. C.; D’Souza, C. A. Organo-
metallics 2003, 22, 1626. (b) Morrill, T. C.; D’Souza, C. A.; Yang, L.;
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G.; Westcott, S. A. Can. J. Chem. 2001, 79, 1898. (d) Reference 6f.
(21) For examples of catalyzed diboration reactions, see: (a) Laitar, D.
S.; Tsui, E. Y.; Sadighi, J. P. J. Am. Chem. Soc. 2006, 128, 11036. (b)
Corbe´ran, R.; Ram´ırez, J.; Poyatos, M.; Peris, E.; Ferna´ndez, E. Tetrahedron:
Asymmetry 2006, 17, 1759. (c) Sieber, J. D.; Morken, J. P. J. Am. Chem.
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Tetrahedron: Asymmetry 2005, 16, 1289. (e) Kabalka, G. W.; Das, B. C.;
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1998, 98, 5137. (m) Reference 19e.

5968 Organometallics, Vol. 25, No. 25, 2006
Notes
the Ir starting complex, along with the formation of 8 as the
major product. Nonetheless, all of our efforts to isolate pure 8
from such reactions have met with only limited success, and as
such we are unable to comment regarding the catalytic
competence of 8. Unlike 2b and 3b, both of the Rh complexes
2a and 3a exhibited excellent thermal stability under similar
experimental conditions, with no decomposition observed for
2a and <5% decomposition detected in the case of 3a (31P
NMR). As well, no reaction was observed upon exposure of
2a or 3a to NEt3 for 12 h at room temperature in THF.
Summary and Conclusions
The cationic Rh and Ir complexes 2a,b, as well as their
formally zwitterionic relatives 3a,b, have proven capable of
mediating the selective hydroboration of disubstituted vinyl-
arenes with pinacolborane (HBpin). To the best of our knowl-
edge, the successful application of 3b in this context represents
the first reported hydroboration reaction mediated by a formally
zwitterionic Ir complex. Despite the structurally similar metal
coordination environments that are found in these cationic and
formally zwitterionic complexes, some notable reactivity dif-
ferences were observed both within and between the Rh and Ir
cation/zwitterion pairs 2a/3a and 2b/3b. Particularly interesting
was the selectivity observed in the hydroboration of 1-phenyl-
propene (6) with HBpin; while the branched product (7a) was
favored when using one of the Rh catalysts (2a or 3a), the linear
product (7c) was formed exclusively in reactions mediated by
either of the Ir catalysts (2b or 3b). The observation that the
formally zwitterionic catalysts (3a,b) provided either comparable
or improved catalytic performance versus their cationic relatives
(2a,b) in this transformation provides further evidence that such
zwitterions represent an effective class of neutral catalyst
complexes for the addition of E-H bonds to unsaturated
substrates, whose reactivity properties are in some cases
complementary to those of more traditional group 9 [(COD)M-
(κ2-P,N)]+X- salts.
Figure 1. Formation of 8 from 2b or 3b. The ORTEP diagram
for 8 is shown with 50% displacement ellipsoids; selected hydrogen
atoms have been omitted for clarity.
pure 8 in crystalline form from pentane, which allowed for the
spectroscopic (1H and 31P NMR) and X-ray crystallographic
identification of 8 as the cyclometalation product depicted in
Figure 1.22 The generation of 8 from 3b can be viewed as
proceeding by way of an initial C-H activation step involving
the NMe2 fragment, followed by net transfer of a proton (either
intra- or intermolecular) from the formally cationic Ir center to
the indenide backbone of the ancillary ligand. We have observed
similar cyclometalation processes in our examination of Pt and
Ru coordination complexes of P,N-substituted indenes.11b,23
Whereas the cationic complex 2b proved to be more thermally
robust than 3b, with only 10% decomposition observed after
72 h at 60 °C in THF or toluene (31P NMR), once again 8 was
identified as one of the thermolysis products. In this case, 8
can be viewed as arising from an initial insertion of Ir into a
C-H bond of the NMe2 unit in 2b, followed by net loss of
HPF6. Notably, treatment of either 2b or 3b with NEt3 for 12
h at room temperature in THF resulted in the consumption of
(22) (a) Spectroscopic data for 8: 1H NMR (CD2Cl2) δ 7.24 (d, 3JHH
)
Acknowledgment. Thanks are gratefully extended to the
Canada Research Chairs Programme, the Canada Foundation
for Innovation-Atlantic Innovation Fund, the Nova Scotia
Research and Innovation Trust Fund, the Natural Sciences and
Engineering Research Council of Canada, Mount Allison
University, and Dalhousie University for financial support. We
also thank Roger Smith and Dan Durant (MtA) for expert
technical assistance and Dr. Michael Lumsden (Atlantic Region
Magnetic Resonance Center, Dalhousie) for his assistance in
the acquisition of NMR data.
3
8.0 Hz, 1H, C4-H or C7-H), 7.18 (d, JHH ) 6.5 Hz, 1H, C7-H or C4-H),
3
3
7.02 (t, JHH ) 7.8 Hz, 1H, C5-H or C6-H), 6.76 (t, JHH ) 7.3 Hz, 1H,
C6-H or C5-H), 4.11 (br m, 2H, COD), 3.91 (br m, 2H, COD), 3.51 (s,
2H, C1(H)2 or N-CH2-Ir), 3.47 (s, 2H, NCH2-Ir or C1(H)2), 3.18-2.98
(m, 5H, NMe and P(CHMeaMeb)2), 2.13-2.05 (m, 4H, COD), 1.85 (m,
3
3
2H, COD), 1.74 (m, 2H, COD), 1.27 (d of d, 6H, JHH ) 7.0 Hz, JPH
)
3
3
15.0 Hz, P(CHMeaMeb)2), 1.14 (d of d, 6H, JHH ) 7.0 Hz, JPH ) 15.0
Hz, P(CHMeaMeb)2); 31P{1H} NMR (CD2Cl2) δ 13.7. (b) Selected crystal-
lographic data for 8: empirical formula, C25H37NPIr; formula weight,
574.73; crystal dimensions, 0.47 × 0.40 × 0.39; crystal system, monoclinic;
space group, P21/n (No. 14); a ) 11.0069(8) Å; b ) 9.2904(7) Å; c )
21.893(2) Å; â ) 97.828(1)°; V ) 2217.8(3) Å3; Z ) 4; Fcalcd ) 1.721 g
cm-3; µ ) 6.103 mm-1; 2θ limit ) 52.82° with -13 e h e 13, -11 e k
e 11, -27 e l e 27; total no. of data collected, 16 692; no. of independent
reflections, 4539; no. of observed reflections, 4167; absorption correction,
multiscan (SADABS); range of transmission, 0.1994-0.1616; no. of data/
Supporting Information Available: Text giving complete
experimental details and a CIF file giving single-crystal X-ray
diffraction data for 8. This material is available free of charge via
the Internet at http://pubs.acs.org.
2
2
restraints/parameters, 4539/0/254; R1 (Fo2 g 2σ(Fo )) ) 0.0193; wR2 (Fo
2
g 3σ(Fo )) ) 0.0505; goodness of fit, 1.037; largest peak, hole 1.193,
-0.672 e Å-3. Selected interatomic distances (Å) for 8: Ir-P, 2.3314(7);
Ir-C27, 2.088(3); Ir‚‚‚N, 3.064; Ir-C11, 2.168(3); Ir-C12, 2.156(3); Ir-
C15, 2.200(3); Ir-C16, 2.181(3); N-C2, 1.341(4); N-C27, 1.471(4);
N-C28, 1.444(4); C1-C2, 1.519(4); C2-C3, 1.383(4).
OM060742M
(23) Wile, B. M.; McDonald, R.; Ferguson, M. J.; Stradiotto, M.
Organometallics 2005, 24, 1959.