Hydroboration of an alkene by amine-boranes catalysed by a [Rh(PR 3)2]+ fragment. Mechanistic insight and tandem hydroboration/dehydrogenation

Article abstract of DOI:10.1039/c1dt10819k

The catalytic hydroboration of tert-butylethene using H ₃B·NMe₃ gives RH₂B·NMe ₃. With H₃B·NMe₂H tandem hydroboration under mild conditions/dehydrocoupling occurs that produces R ₂BNMe₂ (R = H, CH₂CH₂ ^tBu). The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c1dt10819k

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2011, 40, 7499
www.rsc.org/dalton
COMMUNICATION
Hydroboration of an alkene by amine–boranes catalysed by a [Rh(PR3)2]+
fragment. Mechanistic insight and tandem hydroboration/dehydrogenation†
Laura J. Sewell, Adrian B. Chaplin and Andrew S. Weller*
Received 3rd May 2011, Accepted 2nd June 2011
DOI: 10.1039/c1dt10819k
The catalytic hydroboration of tert-butylethene using
H3B·NMe3 gives RH2B·NMe3. With H3B·NMe2H tandem hy-
droboration under mild conditions/dehydrocoupling occurs
No intermediates were observed. 2 was characterised by NMR
spectroscopy, ESI-MS, micro-analysis and single-crystal X-ray
diffraction. The solid-state structure was poor, suffering from
extensive disorder, but it was sufficient to confirm connectivity and
that the anti-Markovnikov product had been formed (Fig. 1). In
solution the Rh–H–B 3c–2e interactions in 2 are demonstrated by
t
that produces R2B NMe2 (R = H, CH2CH2 Bu).
We have recently reported upon the dehydrocoupling1–3 of the
secondary amine–borane H3B·NMe2H to form amino–borane
products plus dihydrogen, that is catalysed by well–defined
1
a broad, relative integral 2H, signal at d -7.07 in the H NMR
spectrum that resolves into an apparent triplet on decoupling
2
11B (J = 32 Hz) similar to that observed for [Rh(PiPr3)2{h -
i
[Rh(PR3)2]+ fragments (R = Pr,4 iBu5 and chelating phosphines6).
2
(H2BNMe2)2}][BArF ].4 This h -bonding motif has been observed
4
A plausible early mechanistic step in this transformation is B–
H activation at the Rh(I) centre to form a Rh(III)-hydrido-base
stabilised boryl A (Scheme 1); as shown by H/D exchange
experiments, calculations5 and the isolation of model complexes.7
Given the precedent for the involvement of hydrido-boryl in-
termediates in the transition-metal catalysed hydroboration of
olefins8,9 we speculated that intermediates such as A would also
be productive in the hydroboration reaction. We report here that
this is the case by the catalytic hydroboration of tert-butylethene
(tbe) using H3B·NMe3 under mild conditions. We also describe a
tandem10 hydroboration/dehydrocoupling reaction that produces
(tBuCH2CH2)2B NMe2 from the combination of H3B·NMe2H
and tbe.
previously.5,13 Signals due to two methylene groups, NMe3 and
tBu are also observed. The 31P{ H} NMR spectrum shows one
1
environment d 53.8 (J(RhP) 182 Hz). 11B NMR spectroscopy
shows the coordinated borane as a broad signal at d 37.9,
shifted to high frequency compared to the free borane (vide
infra), as expected.14 Formally complex 2 contains the product
of hydroboration of tbe by H3B·NMe3. Addition of H3B·NMe3
to 2 (1 : 1 ratio) established an equilibrium between 1 and 2 that
favours the hydroborated product (K(298 K) = 1.62, DG = -1.1
kJ mol-1). Related h -sigma adducts of alkyl–substituted amine
1
boranes have been reported previously by Kawano, Shimoi and
co–workers from the direct reaction of the B–substituted borane
1
with the metal fragment, e.g. [CpMn(CO)2(h -BH2Me·NMe3)].15
Scheme 1 Interception of hydrido-boryl intermediate.
equivalent of H3B·NMe3 to [Rh(PiBu2 -
t
Addition of
1
Bu)2][BArF ] B (ArF = 3,5-C6H3(CF3)2) affords the recently
4
t
2
reported11 “Shimoi”12 sigma–borane complex [Rh(PiBu2 Bu)2(h -
H3B·NMe3)][BArF ] 1. Addition of tbe (5 equivalents) to
4
1
in 1,2-F2C6H4 solution resulted in the slow (3 days),
t
2
but clean, formation of a single product, [Rh(PiBu2 Bu)2(h -
t
Me3N·H2BCH2CH2 Bu)][BArF ] 2‡in quantitative yield (NMR).
Fig. 1 Ball and stick representation of a partially refined structure of 2.
Hydrogen atoms placed in calculated positions. Anions and most hydrogen
atoms omitted for clarity. The X-ray data is of poor quality and the
structure presented here is only to confirm bond connectivity. Selected
4
Department of Chemistry, Inorganic Chemistry Laboratories, University of
Oxford, Oxford, UK, OX1 3TA. E-mail: andrew.weller@chem.ox.ac.uk
† Electronic supplementary information (ESI) available: Synthesis and
characterisation of the new complexes and details of catalytic runs. See
DOI: 10.1039/c1dt10819k
˚
˚
crystallographic data: Triclinic, ◦P-1, a = 23.729(2◦) A, b = 25.4785(2) A, c
◦
˚
= 25.6623(3) A, a = 72.4864(4) , b = 67.6371(4) , g = 67.4747(4) , V =
3
˚
13033.0(2) A .
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7499–7501 | 7499
©

View Article Online
In these complexes, as here, the alkyl borane complexes are also
favoured over the H3B·NMe3 adducts, suggested to be due to the
electron-releasing groups raising the energy of the B–H sigma
orbital favouring overlap with the metal fragment. Compound 2
also models15 alkane complexes of Rh(I) that are only stable in
solution at low temperatures.16
Under catalytic conditions [Rh(PiBu2 Bu)2][BArF ] (5 mol%,
t
4
1.475 mM, 94 h) also promotes this reaction to form free
Me3N·H2BCH2CH2 Bu 3 [11B d -0.83, t, J(HB) 96 Hz], Scheme
t
t
2. Workup involving H2O2/KOH afforded HOCH2CH2 Bu, con-
firming the regio–chemistry of hydroboration. We did not observe
any of the branched product, as has been reported previously for
both the Rh(III)-=catalysed, and the uncatalysed, hydroboration
of tbe using H3B·THF.17 However in our system, a maximum
conversion of ~70% is only ever reached, with ~30% H3B·NMe3
unreacted. We suggest this is due to product inhibition of the
catalyst, as 3 competes effectivelywith H3B·NMe3 for coordination
to the metal, vide supra. We have not explored the use of more
co-ordinating solvents than 1,2-F2C6H4 which might promote
complete conversion.17 In line with these observations, at the
t
Scheme 3 Suggested catalytic cycle for hydroboration. PR3 = PiBu2 Bu.
Related transfer dehydrogenation/alkene hydrogenation catalysis
has been reported.20 Combination of H3B·NMe2H and tbe with B
(5 mol%) resulted in complete consumption of the starting materi-
als and a mixture of 3 major products, which in the 11B NMR spec-
trum showed as a doublet [d 43.2, J(HB) 118 Hz], a singlet [d 45.8]
and a triplet [d 5.4, J(HB) 113 Hz] suggesting the formation of the
1
start of catalysis the resting state observed is 1 (31P{ H} NMR
spectroscopy) and as the reaction proceeds 2 grows in to become
the major organometallic species present at the end. Related
t
2
hindered amino–boranes respectively: Me2N BH(CH2CH2 Bu) 4
and Me2N B(CH2CH2 Bu)2 5, alongside the cyclic amino-borane
Rh(PR3)2(h -carborane) complexes have been reported as the
t
resting states during cage B–H hydroboration reactions with
alkenes.18 Kinetic plots (298 K, 11B NMR, see ESI†) gave the
[H2BNMe2]2, a well-established product of dehydrogenation of
H3B·NMe2H.3 The 11B chemical shifts and coupling patterns of
4 and 5 are consistent with their formulation as primary and
secondary amino–boranes respectively,21 as is other analytical
data. Addition of further tbe and fresh catalyst did not convert
4 to 5, showing that they were produced by parallel routes,
and that 5 does not arise from hydroboration of tbe using
4. On the basis of this we suggest a mechanism in which
H3B·NMe2H partakes in successive hydroborations with tbe to
initial rate over the first 2100 s of reaction as (3.9 0.4) ¥ 10-6
M
s-1. After the reaction had stopped, addition of a further excess
of 20 equivalents each of H3B·NMe3/tbe restarts catalysis, but
now at a slower initial rate of (1.0 0.5) ¥ 10-6 M s-1 consistent
with inhibition by the product. Again only ~70% conversion is
achieved. Addition of an excess of tbe to B results in no reaction,
while 1 equivalent of H3B·NMe3 immediately and cleanly forms
1. Catalysis (5 mol%) with a 2–fold excess of tbe to H3B·NMe3
(40 : 20 respectively relative to B) resulted in a doubling of initial
rate, (8.1 1.0) ¥ 10-6 M s-1, while addition of a 10-fold excess of
H3B·NMe3 resulted in no change, (4.2 0.5) ¥ 10-6 M s-1. Doubling
the catalyst concentration (10 mol%) effectively doubled the initial
rate to (6.5 0.7) ¥ 10-6 M s-1.
t
finally afford HMe2N·BH(CH2CH2 Bu)2 which then undergoes
dehydrogenation (Scheme 4). Dehydrogenation is competitive at
each step, thus accounting for the product distributions. Although
the relative ratio of 5 could be increased at the expense of 4
and [H2BNMe2]2 by using a 200 fold-excess of tbe relative to B
(final relative ratios 1.5 : 1.5: 1 respectively), we were never able to
isolate pure material for 4 or 5. Multiple hydroboration of 3 is not
observed, presumably for steric reasons.
Scheme 2
These results suggest a first order process in catalyst and tbe only,
in which fast binding of H3B·NMe3 forms 1, that then proceeds
via B–H activation, olefin insertion and reductive elimination
(Scheme 3). These results are in–line with those suggested for
rhodium-catalysed hydroboration of olefins using substrates such
as catechol borane, although in these cases there can be multiple
pathways operating.8 The control reaction of tbe with H3B·NMe3
resulted in no reaction after 2 days.
Scheme 4 Tandem catalysis using B. Hydroboration = blue arrows,
dehydrogenation = red arrows.
Having successfully delivered a catalysed hydroboration using
a tertiary amine–borane we reasoned that use of a secondary
amine–borane would allow for an additional tandem10 dehydro-
genation reaction to give B-substituted amino–boranes, given that
[Rh(PR3)2]+ fragments are well-established to facilitate this.2–6,19
In conclusion we report a metal-catalysed hydroboration of a
terminal alkene using a tertiary amine–borane. When using a sec-
ondary amine–borane tandem hydroboration/dehydrogenation
occurs. Although the scope of this particular catalyst system
is limited due to product inhibition and competition between
7500 | Dalton Trans., 2011, 40, 7499–7501
This journal is
The Royal Society of Chemistry 2011
©

View Article Online
hydroboration and dehydrogenation, the mechanistic insight from
the system revealed here will guide future developments in the
area.
9 D. Mannig and H. Noth, Angew. Chem., Int. Ed. Engl., 1985, 24, 878–
879.
10 C. J. Chapman and C. G. Frost, Synthesis, 2007, 2007, 1–21; D. E.
Fogg and E. N. dos Santos, Coord. Chem. Rev., 2004, 248, 2365–
2379.
11 L. J. Sewell, A. B. Chaplin, J. A. B. Abdalla and A. S. Weller, Dalton
Notes and references
Trans., 2010, 39, 7437–7439.
‡ Compound 2: (1,2-C6H4F2, 298 K); 1H (500 MHz) d 8.34 (s, 8H, BArF4),
7.69 (s, 4H, BArF4), 2.94 (s, 9H, NMe3), 2.18 (br, 4H, iBu{CH}), 1.97–1.79
(m, 8H, iBu{CH2}), 1.73–1.70 (m, 2H, CH2), 1.34–1.20 (m, 44H, iBu{Me},
12 G. J. Kubas, Metal Dihydrogen and o-Bond Complexes, Kluwer
Academic/Plenum Publishers, New York, 2001.
13 S. A. Westcott, T. B. Marder, R. T. Baker, R. L. Harlow, J. C. Calabrese,
K. C. Lam and Z. Y. Lin, Polyhedron, 2004, 23, 2665–2677.
14 N. Merle, G. Koicok-Kohn, M. F. Mahon, C. G. Frost, G. D. Ruggerio,
A. S. Weller and M. C. Willis, Dalton Trans., 2004, 3883–3892; G.
Alcaraz and S. Sabo-Etienne, Coord. Chem. Rev., 2008, 252, 2395–2409.
15 Y. Kawano, K. Yamaguchi, S. Y. Miyake, T. Kakizawa and M. Shimoi,
Chem.–Eur. J., 2007, 13, 6920–6931.
16 W. H. Bernskoetter, C. K. Schauer, K. I. Goldberg and M. Brookhart,
Science, 2009, 326, 553–556.
17 T. C. Morrill, C. A. D’Souza, L. Yang and A. J. Sampognaro, J. Org.
Chem., 2002, 67, 2481–2484.
1
tBu{Me}, CH2), 1.02 (s, 9H, C{Me}), -7.07 (br, 2H, BH2). 31P { H} (202
1
MHz) d 53.8 (d, JRhP = 182). 11B (160 MHz) d 37.9 (br, BH2), -6.1 (s,
BArF4). ESI-MS (1,2-C6H4F2, 60 ◦C): m/z 664.4840 [M+] (calc. 664.4758).
Anal. Calcd for C65H90B2F24NP2Rh (1527.85 g mol-1): C, 51.10; H, 5.94;
N, 0.92. Found: C, 51.51; H, 5.86; N, 0.97.
1 C. A. Jaska, K. Temple, A. J. Lough and I. Manners, J. Am. Chem.
Soc., 2003, 125, 9424–9434.
2 M. E. Sloan, T. J. Clark and I. Manners, Inorg. Chem., 2009, 48, 2429–
2435.
3 A. Staubitz, A. P. M. Robertson, M. E. Sloan and I. Manners, Chem.
Rev., 2010, 110, 4023–4078.
4 A. B. Chaplin and A. S. Weller, Inorg. Chem., 2010, 49, 1111–1121.
5 T. M. Douglas, A. B. Chaplin, A. S. Weller, X. Yang and M. B. Hall, J.
Am. Chem. Soc., 2009, 131, 15440–15456.
6 R. Dallanegra, A. P. M. Robertson, A. B. Chaplin, I. Manners and A.
S. Weller, Chem. Commun., 2011, 47, 3763–3765.
7 A. B. Chaplin and A. S. Weller, Angew. Chem. Int. Ed., 2010, 49, 581–
584.
18 J. D. Hewes, C. W. Kreimendahl, T. B. Marder and M. F. Hawthorne,
J. Am. Chem. Soc., 1984, 106, 5757–5759; E. Molinos, S. K. Brayshaw,
G. Kociok-Ko¨hn and A. S. Weller, Organometallics, 2007, 26, 2370–
2382.
19 C. Y. Tang, A. L. Thompson and S. Aldridge, J. Am. Chem. Soc., 2010,
132, 10578–10591.
20 M. E. Sloan, A. Staubitz, K. Lee and I. Manners, Eur. J. Org. Chem.,
2011, 2011, 672–675; Y. Jiang, J. Hess, T. Fox and H. Berke, J. Am.
Chem. Soc., 2010, 132, 18233–18247.
8 K. Burgess, W. A. van der Donk, S. A. Westcott, T. B. Marder, R. T.
Baker and J. C. Calabrese, J. Am. Chem. Soc., 1992, 114, 9350–9359; D.
A. Evans, G. C. Fu and B. A. Anderson, J. Am. Chem. Soc., 1992, 114,
6679–6685; J. M. Brown and G. C. Lloyd-Jones, J. Am. Chem. Soc.,
1994, 116, 866–878.
21 H. Feulner, N. Metzler and H. No¨th, J. Organomet. Chem., 1995, 489,
51–62; L. Euzenat, D. Horhant, Y. Ribourdouille, C. Duriez, G. Alcaraz
and M. Vaultier, Chem. Commun., 2003, 2280–2281; T. Miyazaki, Y.
Tanabe, M. Yuki, Y. Miyake and Y. Nishibayashi, Organometallics,
2011, 30, 2394–2404.
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 7499–7501 | 7501
©