Stoichiometric and catalytic B-C bond formation from unactivated hydrocarbons and boranes [1]

Full text of DOI:10.1021/ja991258w

7696
J. Am. Chem. Soc. 1999, 121, 7696-7697
Communications to the Editor
the alkylborane derivatives, CyBX2 and PhBX2 (X2 ) (C6F5)2,16
Stoichiometric and Catalytic B-C Bond Formation
2-
2-
(CH3)2CO-CO(CH3)2 ≡ Pin, O2C6H4 ≡ Cat). The rates for
B-C bond formation and Ir product distributions were borane
dependent. For example, the reaction between [HB(C6F5)2]2 and
compound 1 gave Cp*Ir(PMe3)H2 (3) and CyB(C6F5)2 (eq 2).17
from Unactivated Hydrocarbons and Boranes
Carl N. Iverson and Milton R. Smith, III*
Department of Chemistry, Michigan State UniVersity
East Lansing, Michigan 48824
ReceiVed April 20, 1999
The catalytic functionalization of hydrocarbons represents a
long-standing challenge in homogeneous and heterogeneous
catalysis.1 Over the past 20 years a detailed picture of metal-
mediated C-H activation has emerged from intensive study by
several research groups.2-6 While an activation step is requisite
for generating species with M-C bonds, further elaboration of
the M-C bond must be carefully designed to engender catalytic
viability. Lewis acidic reagents are logical choices for converting
activated alkyl or aryl groups to functionalized organic products
since the reactive complexes and intermediates responsible for
C-H activation can be coordinatively unsaturated or cationic
reagents, whose reactivity may be hampered by Lewis basic
substrates. For borane reagents, thermochemical and computa-
tional data establish that the reaction in eq 1 is essentially
thermoneutral.7 Hence, catalysis is thermodynamically viable. For
these reasons, we have been examining metathetic B-C bond
forming reactions of B-X and M-C bonds.8-13 In this com-
munication, we describe B-C bond forming chemistry for the
archetypal C-H activation products, Cp*Ir(PMe3)(H)(R),14 and
demonstrate catalytic viability of eq 1 for the first time.15 In
Conversions were quantitative, as judged by 1H NMR, and
complete within 5 min at ambient temperature. Alkane elimination
from compound 1 was not observed, and Ir products from solvent
activation were not detected. Characterization of CyB(C6F5)2 was
1
established by comparing H and 11B NMR data to an authentic
sample, prepared from [HB(C6F5)2]2 and cyclohexene. The
reaction between [HB(C6F5)2]2 and compound 2 gave a mixture
of products.
For oxygen-substituted boranes, B-C bond formation required
more forcing conditions as illustrated by the reactivity of
pinacolborane (HBPin) with compound 1. In benzene-d6 solutions,
this reaction yielded Cp*Ir(PMe3)(H)(BPin) (4) and 2-d6 as the
major Ir products.18 The boryl compound, 4, was identified by
1
comparing H, 11B, and 31P NMR data to those for compound 4
generated from compound 3 and HBPin.19 The fate of the
cyclohexyl group is revealed by the formation of CyBPin and
cyclohexane. A typical product distribution obtained for the
reaction of compound 1 with 6 molar equiv of HBPin at 95 °C is
shown in eq 3. Although rigorous kinetic analysis was precluded
particular, the borane substituents have significant impact on rates
for B-C bond formation and are critical for effecting catalytic
conversion.
Cp*Ir(PMe3)(H)(Cy) (1, Cy ) c-C6H11) and Cp*Ir(PMe3)(H)-
(Ph) (2, Ph ) C6H5) reacted with HBX2 reagents in C6D6 to yield
by irreproducible rate data, the qualitative rates for cyclohexane
formation were much higher than expected for thermal elimination
of cyclohexane from compound 1.20 Two potential mechanisms
that could account for enhanced cyclohexane formation include
the reaction of CyBPin and compound 3 to generate 4 and
cyclohexane or pathways that involved metathesis between
compound 1 and C6D6.21 Control experiments indicated that (i)
CyBPin and Cp*Ir(PMe3)H2 do not produce cyclohexane and
* Corresponding author. E-mail: smithmil@pilot.msu.edu.
(1) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Acc.
Chem. Res. 1995, 28, 154-162.
(2) For recent reviews on C-H activation see refs 3-6.
(3) Bengali, A. A.; Arndtsen, B. A.; Burger, P. M.; Schultz, R. H.; Weiller,
B. H.; Kyle, K. R.; Moore, C. B.; Bergman, R. G. Pure Appl. Chem. 1995,
67, 281-288.
(4) Crabtree, R. H. Chem. ReV. 1995, 95, 2599-2599.
(5) Shilov, A. E.; Shul’pin, G. B. Chem. ReV. 1997, 97, 2879-2932.
(6) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed. Engl.
1998, 37, 2181-2192.
(7) Rablen, P. R.; Hartwig, J. F. J. Am. Chem. Soc. 1996, 118, 4648-
4653.
(16) Parks, D. J.; von h. Spence, R. E.; Piers, W. E. Angew. Chem., Int.
Ed. Engl. 1995, 34, 809-811.
(8) Some aspects of this chemistry have been highlighted,11 and some
preliminary findings have been presented: Iverson, C. N.; Smith, M. R., III:
216th National Meeting of the American Chemical Society, Boston, MA,
August 23-27, 1998; INOR 518.
(17) The yields in eqs 2-5 were determined from integration of NMR
spectra.
(18) Unless otherwise noted, satisfactory combustion analyses were obtained
for all new Ir complexes.
(9) Iverson, C. N.; Smith, M. R., III J. Am. Chem. Soc. 1995, 117, 4403-
4404.
(19) Compound 4 is an oil at room temperature, but satisfactory combustion
(10) For recent reviews of boryl complexe chemistry see refs 11-13.
(11) Smith, M. R., III Prog. Inorg. Chem. 1999, 48, 505-567.
(12) Braunschweig, H. Angew. Chem., Int. Ed. Engl. 1998, 37, 1787-1801.
(13) Irvine, G. J.; Lesley, M. J. G.; Marder, T. B.; Norman, N. C.; Rice,
C. R.; Robins, E. G.; Roper, W. R.; Whittell, G. R.; Wright, L. J. Chem. ReV.
1998, 98, 2685-2722.
analysis was obtained. Relevant spectroscopic data include the following: 1H
2
NMR (C6D6) δ -17.64 (d, 1 H, Ir-H, | JH-P| ) 29 Hz), 1.17 (12 H,
2
BO2C6H12), 1.50 (d, 9 H, PMe3, | JH-P| ) 10 Hz), 2.10 (d, 15 H, C5Me5,
4
| JH-P| ) 2 Hz). 11B NMR (C6D6) δ 33. 31P{1H} NMR (C6D6) δ -42.8.
(20) Expected yields of CyH from thermal elimination from compound 1
were determined from the extrapolated rate constant at 95 °C from previously
reported activation parameters: Buchanan, J. M.; Stryker, J. M.; Bergman,
R. G. J. Am. Chem. Soc. 1986, 108, 1537-1550.
(14) Janowicz, A. H.; Bergman, R. G. J. Am. Chem. Soc. 1983, 105, 3929-
3939.
(15) For leading references to related stoichiometric C-H activation by
metal boryl complexes see: Waltz, K. M.; Hartwig, J. F. Science 1997, 277,
211-213.
(21) Trialkylaluminum reagents react with compound 3 to yield alkanes
and Ir-Al bonds: Golden, J. T.; Peterson, T. H.; Holland, P. L.; Bergman,
R. G.; Andersen, R. A. J. Am. Chem. Soc. 1998, 120, 223-224.
10.1021/ja991258w CCC: $18.00 © 1999 American Chemical Society
Published on Web 08/06/1999

Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 33, 1999 7697
compound 4 and (ii) deuterium incorporation in compound 4 was
insignificant.22
Thermal B-H or B-B elimination from compounds 4 and 6
was assessed by heating C6D6 solutions of the pure compounds.
After several days at 200 °C, decomposition was not evident and
there was no evidence for elimination. Thus, photochemical
reactivity of mono- or diboryl derivatives seemed necessary for
catalytic activity. Although the diboryl compound, 6, was
photochemically inert, the phenylhydride compound, 2, was
regenerated when a benzene solution of the monoboryl compound,
4, was photolyzed. Although less efficient, the regeneration of 1
also occurred when a cyclohexane solution of 4 was photolyzed.
Thus, thermal B-C and photochemical Ir-C bond-forming events
constitute a catalytic cycle for HBPin.
The reaction between compound 1 and HBCat gave a ratio of
CyBCat to cyclohexane similar to that in eq 3. In addition to
Cp*Ir(PMe3)(H)(BCat) (5) and compound 2, the diboryl com-
pound, Cp*Ir(PMe3)(BCat)2 (6), was also formed in this reaction.
The boryl products were identified by comparing spectroscopic
data to authentic samples prepared from compound 3 and HBCat.
Reactions of compound 2 with HBPin and HBCat required
elevated temperatures. In both cases PhBX2 was the major
organoboron product. For HBCat, the diboryl compound, 6, was
the predominant Ir product (eq 4), while the reaction with HBPin
To evaluate the viability of photocatalytic benzene activation,
a thick-walled flask was charged with compound 3 (0.15 mol)
and HBPin (0.89 mmol) and the contents were dissolved in C6H6.
After the solution was photolyzed to generate compound 2
(confirmed by 31P NMR), the flask was heated in an oil bath to
convert compound 2 to compound 4 and C6H5BPin. After the
thermal conversion was complete, the solution was photolyzed
to regenerate compound 2. From the initial Ir and borane
concentrations, full conversion should have required five thermal/
photolytic cycles; however, 11B NMR indicated full conversion
of the borane after the third thermolysis. This observation clearly
implicated a thermal catalytic process. Although we had deter-
mined that solutions of compound 4 do not react with C6H6,
reexamination of the NMR spectra for the “stoichiometric”
reaction in eq 5 indicated that the intensity of the pinacolate
resonance for CyBPin is greater than anticipated. When a solution
of compound 4 and 5 molar equiv of HBPin was heated in C6D6,
11B NMR indicated exchange between the boron hydride and
solvent deuterons after 18 h at 150 °C. Conversion to C6D5BPin
yielded the monoboryl compound, 4 (eq 5).23
The stoichiometric chemistry of compounds 1 and 2 suggested
potential catalytic conversion of unactivated hydrocarbons to
organoboranes in this system. Since B-C bond formation was
most facile for reactions of compounds 1 and 2 with [HB(C6F5)2]2,
catalytic viability of this borane reagent was assessed first. The
dihydride product in eq 2 reacts with [HB(C6F5)2]2 to generate a
borohydride complex, Cp*Ir(PMe3)(H)(HBH(C6F5)2) (7). At
elevated temperatures, a complex mixture formed when benzene
or cyclohexane solutions of compound 7 were heated with excess
[HB(C6F5)2]2, eliminating the possibility of a clean thermal
catalytic process. Since photolyses of compound 3 in the
appropriate solvent generate compounds 1 and 2, similar photo-
chemical generation of compounds 1 and 2 from compound 7
would complete a photocatalytic cycle. Unfortunately, compound
7 was photochemically inert.
1
was quantitative after 60 h at 150 °C, as judged by H and 11B
NMR.24,25 The reaction was repeated on a larger scale (4, 0.15
mmol; HBPin, 0.75 mmol) in C6H6, and C6H5BPin was isolated
in 53% yield (eq 6).26,27
Aside from methane-to-methanol conversion,28,29 catalytic C-H
functionalizations for unactivated hydrocarbons are extremely
rare.30 Since applications of boronate esters in cross-coupling
chemistry are expansive,31 the demonstration of catalytic viability
of eq 1 is significant. Although the nature of the borane reagent
is critical to catalytic viability, the role of added borane in the
hydrocarbon activation by compound 4 is more important. We
hope to answer this question and extend our findings to catalytic
activation of hydrocarbons in related systems.
(22) The isotopic constitution for cyclohexane produced in reaction between
compound 1 and HBPin in toluene-d8 was examined by GC/MS. Deuterium
incorporation was not detected.
(23) The yield for the reaction in eq 5 cannot be determined due to catalytic
B-C bond formation from C6D6 and HBPin. It is noteworthy that some C6H6
elimination is observed under the reaction condition, while pure compound 2
does not exchange with C6D6 after extended thermolysis at 190 °C.
(24) Although hydroboration of benzene has been proposed to account for
isotopic scrambling between B2H6 and C6D6,25 we see evidence (11B NMR)
for exchange between HBPin and C6D6. Moreover, B-C bond formation was
not observed when solutions of HBPin and C6H6 were heated at 150 °C.
(25) Gaines, D. F.; Heppert, J. A.; Kunz, J. C. Inorg. Chem. 1985, 24,
621-624.
(26) Conversion was judged to be nearly quantitative by 11B NMR.
(27) Although photolysis of compound 4 in cyclohexane produces com-
pound 1 and HBPin, thermolysis of the resulting mixture does not afford the
CyBPin. We currently have no explanation for this apparent solvent effect.
(28) Periana, R. A.; Taube, D. J.; Evitt, E. R.; Loffler, D. G.; Wentrcek, P.
R.; Voss, G.; Masuda, T. Science 1993, 259, 340-343.
Acknowledgment. We appreciate support from the National Science
Foundation (CHE-9817230). C.N.I. gratefully acknowledges support from
Carl H. Brubaker and Dow Fellowships.
Supporting Information Available: Synthetic and spectroscopic
details for all new compounds as well as 1H, 31P{1H}, and 11B NMR
spectra for the crude reaction mixture corresponding to eq 1 (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
(29) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii,
H. Science 1998, 280, 560-564.
(30) Jones, W. D.; Kosar, W. P. J. Am. Chem. Soc. 1986, 108, 5640-
5641.
(31) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457-2483.
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