Rhodium catalysed dehydrogenative borylation of vinylarenes and 1,1-disubstituted alkenes without sacrificial hydrogenation - A route to 1,1-disubstituted vinylboronates

Article abstract of DOI:10.1039/b211789d

The complex trans-[Rh(Cl)(CO)(PPh₃)₂] (1) is an efficient catalyst precursor for the dehydrogenative borylation of alkenes without consumption of half the alkene substrate by hydrogenation, giving useful vinylboronate esters including 1,1-disubstituted derviatives that cannot be made by alkyne hydroboration.

Full text of DOI:10.1039/b211789d

Rhodium catalysed dehydrogenative borylation of vinylarenes and
1,1-disubstituted alkenes without sacrificial hydrogenation—a route to
1,1-disubstituted vinylboronates
R. Benjamin Coapes,a Fabio E. S. Souza,b Rhodri Ll. Thomas,a Jonathan J. Halla and Todd B.
Marder*a
a Department of Chemistry, University of Durham, Durham, UK DH1 3LE.
E-mail: todd.marder@durham.ac.uk; Fax: 0191 384 4737; Tel: 0191 374 3137
b Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
Received (in Cambridge, UK) 28th November 2002, Accepted 20th January 2003
First published as an Advance Article on the web 4th February 2003
The complex trans-[Rh(Cl)(CO)(PPh3)2] (1) is an efficient
catalyst precursor for the dehydrogenative borylation of
alkenes without consumption of half the alkene substrate by
hydrogenation, giving useful vinylboronate esters including
1,1-disubstituted derviatives that cannot be made by alkyne
hydroboration.
hydroboration using the catalyst precursor trans-
[RhCl(CO)(PPh3)2] (1) and the diboron reagents B2pin2 or
B2neop2 (pin = OCMe2CMe2O; neop = OCH2CMe2CH2O),
all of which are commercially available.
Vinylboronate esters (VBEs) are useful synthetic intermediates
in many reactions, including C–C bond formation via Pd-
catalysed Suzuki-Miyaura cross-coupling.1 They can be pre-
pared by uncatalysed2 or metal catalysed3 hydroboration of
alkynes. Recently, VBE’s have been synthesised from alkenyl
halides via catalysed4 or stoichiometric reactions,5 the latter
requiring lithiation at 2110 °C. An exciting alternative is the
catalytic dehydrogenative borylation of alkenes3a,6 eqn. (1),
In the course of our studies on alkene diboration,7 we
examined the reaction of 4-vinylanisole (VA) with B2pin2
catalysed by 1 in a variety of solvents. Toluene, THF and
1,4-dioxane all gave complicated mixtures containing dehy-
drogenative borylation, diboration, hydroboration and hydro-
genation products and, in some cases, vinyl-bis(boronate) esters
(VBBEs). In contrast, reaction in CH3CN was clean giving 93%
VBE but the rate was much slower than that in e.g. toluene. We
therefore examined the reaction using 3 mol% of 1 and 0.67
equiv. (a slight excess) of B2pin2 in 3+1 toluene+acetonitrile
(3+1 T+A) which proved an excellent compromise between
selectivity and rate, giving 88% selectivity towards VBE, with
12% hydroboration and 100% conversion in 2 days (Table 1,
Entry 1). Importantly, the stoichiometry demonstrates the
potential to use both boron atoms of the diboron reagent.
Reaction with B2neop2 was less selective (Entry 2). Reaction of
VA with 2 equiv. of B2pin2 in the presence of 5 mol% of 1 in
3+1 T+A (Entry 3) gave 85% selectivity for VBBE,†
(1)
which would allow the direct synthesis of 1,1-disubstituted
VBEs that cannot be made by hydroboration of alkynes.
However, conditions must be found wherein the H2 produced is
not consumed via hydrogenation of half of the alkene substrate,
a problem which has plagued this approach from the earliest
reports. We present herein a novel, high yield, highly selective
catalytic synthesis of VBEs, including 1,1-disubstituted VBEs,
directly from alkenes without significant hydrogenation or
a
Table 1 Product distribution for the dehydrogenative borylation of alkenes with B2pin2 and B2neop2
Hydroboration
(hydrogenation)
(%
Total VBE
(Maj. isomer)
(%)
Total VBBE
(Maj. isomer)
(%)
Boron
reagent
Time/
days
Conversion
(%)
Entry Substrate
BBEb (%)
Solventc
1
2
3
4
5
6
7
8
9
4-Vinyl anisole
4-Vinyl anisole
4-Vinyl anisoled
B2pin2
B2neop2
B2pin2
B2pin2
B2pin2
B2pin2
B2pin2
B2pin2
B2neop2
B2pin2
B2pin2
12 (trace)
5 (22)
1
1
9
88
73
14
2
1
5
4
2
6
3
2
1
4
3
3
3+1 T+A
3+1 T+A
3+1 T+A
3+1 T+A
T
100
62
100
68
72
70
90
100
49
85 (83)
a-Methyl styrene
a-Methyl styrene
a-Methyl styrene
a-Methyl styrenee
a-Methyl styrenef
a-Methyl styrene
1,1-Diphenylethylene
1,1-Diphenylethylenef
99 (97)
87 (74)
100 (98)
100 (97)
100 (98)
91 + trace
98
4
trace
A
3+1 T+A
3+1 T+A
3+1 T+A
3+1 T+A
3+1 T+A
3+1 T+A
9
2
1
10
11
12
48
100
100g
99
Methylene cyclopentane B2pin2
100 (92, 4, 4)
[3 isomers]
92
13
Methylene cyclohexane B2pin2
8
5
3+1 T+A
80g
a Typical reaction conditions: in a nitrogen-filled glove box, a mixture of boron reagent (0.4 mmol total boron) and alkene (0.3 mmol) in 1 ml of solvent was
added to a solution of trans-[Rh(Cl)(CO)(PPh3)2] (3 mol%) in 1 ml of solvent (2 ml total solvent volume). The mixture was shaken vigorously to ensure
complete mixing, transferred to ampoules sealed with a Teflon Young’s tap, removed from the glove box, and then heated to 80 °C. The reaction was
monitored by either GC-MS or a combination of GC-MS and NMR spectroscopy. b BBE = saturated bis(boronate)ester. c T = toluene, A = acetonitrile.
d 2 equiv. of B2pin2 and 5 mol% of catalyst used. e 5 mol% of catalyst used. f 1 equiv. of B2pin2 used. g Conversion determined by 1H NMR
spectroscopy.
614
CHEM. COMMUN., 2003, 614–615
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4-MeOC6H4CHNC(Bpin)2. Thus, both H’s of the NCH2 group
were replaced by Bpin in a single catalytic reaction [eqn. (2)].
formation to a cis-boryl ligand on Rh (Scheme 1) would provide
an alternative to a planar agostic Rh–C–C–H intermediate
typically expected for b-H elimination, and would lead to an H–
Bpin s-complex.9
In conclusion, with appropriate choice of solvent, mono-
substituted and 1,1-disubstituted alkenes can be converted
directly into useful vinylboronates or even vinyl bis(boronate)
esters in high yield and with excellent selectivity via catalytic
borylation of C–H bonds employing commercially available
catalyst and diboron reagents.
We thank EPSRC for a postgraduate studentship (R.B.C.)
and for research grant GR/M23038 (T.B.M.). T.B.M. also
thanks NSERC (Canada) for research support, Professor Z. Lin
(Hong Kong University of Science and Technology) for helpful
discussions, and Frontier Scientific Inc. for a donation of B2pin2
and B2neop2.
(2)
Whilst 1,1,2-trisubstituted alkenes proved unreactive, reac-
tion of 0.67 equiv. of B2pin2 with a-methylstyrene, a 1,1-dis-
ubstituted alkene, in the presence of 3 mol% of 1 in 3+1 T:A at
80 °C gave 97% (E)-Ph(Me)CNCH(Bpin)† 2% (Z)-
Ph(Me)CNCH(Bpin) and 1% Ph(Me)(H)C–CH2Bpin (Entry 4)
with 68% conversion. Selectivity was lower using toluene alone
(Entry 5) but the reaction was faster; the opposite was true using
neat acetonitrile (Entry 6). Conversion was increased to 90%
using 5 mol% of 1 in 3+1 T+A, and to 100% using 3 mol% 1
with 1 equiv. of B2pin2, with excellent selectivity in both cases
(Entries 7,8). Again, B2neop2 was less selective and with this
substrate also less reactive (Entry 9). With 0.67 equiv. of
B2pin2, 1,1-diphenylethylene was less reactive than a-methyl-
styrene, most likely due to increased steric bulk, but highly
selective, giving 98% VBE (Entry 10); again, using 1 equiv. of
B2pin2 increased conversion to 100% giving 99% VBE (Entry
11). Interestingly, methylenecyclopentane gave 92% VBE + 4%
each of two isomeric derivatives (Entry 12), whereas methyle-
necyclohexane gave 92% VBE (Entry 13), demonstrating that
1,1-disubstituted alkenes other than styrene derivatives are
suitable substrates for the reaction.
Whilst the detailed mechanism of the reaction is not yet
known, the following points are worth considering. The
simplest pathway (Scheme 1) would involve B–B oxidative
addition, and then alkene insertion into Rh–B followed by b-
hydride elimination. Interestingly, the boryl ligand migrates
preferentially to the least substituted carbon centre with all
substrates we have examined. Reductive elimination processes,
which can compete with b-hydride elimination, and lead to
saturated diboration or hydroboration products, are effectively
inhibited when acetonitrile is present. Reductive elimination
may be aided by ring strain induced by coordination of a boryl
oxygen atom to Rh in the b-borylalkyl intermediate8 (see
Scheme 1). The presence of strongly coordinating acetonitrile
may inhibit this chelation, making b-hydride elimination faster
than reductive elimination. Finally, the possibility of a b-
hydride elimination pathway involving direct B–H bond
Notes and references
† Selected characterisation data: 4-MeO-C6H4CHNC(Bpin)2: 1H NMR
(300 MHz, CDCl3): d 1.24 (s, 24H, (Bpin)2), 3.79 (s, 3H, CH3O), 6.81 (m,
2H, C6H4), 7.43 (m, 2H, C6H4), 7.64 (s, 1H, ArCHN); 11B{1H} NMR (96
MHz, CDCl3): 30.7 (s, br); MS (EI): m/z (rel. int.): 386 (33) [M+], 371 (3)
[M+ 2 Me]. The NOESY NMR spectrum shows a correlation between
ortho CH on arene ring and CH = of alkene. (E)-PhC(Me)NCH(Bpin): 1H
NMR (300 MHz, CDCl3) d 1.3 (s, 12H; Bpin), 2.41 (d, 3J(H,H) = 1 Hz, 3H,
CH3PhCN), 5.77 (q, 3J(H,H) = 1 Hz, 1H; NCHBpin), 7.31 (m, 2H, C6H5),
7.48 (m, 3H C6H5); 13C{1H} NMR (126 MHz, CDCl3) d 20.1 (s,
CH3PhC = ), 24.8 (s, BO2C2(CH3)4), 82.9 (s, BO2C2(CH3)4), 115.5 (s, br,
NCHBpin), 125.8 (s, C6H5), 128.0 (s, C6H5), 128.2 (s, C6H5), 143.8 (s,
C6H5), 157.8 (MePhCN); 11B{1H} NMR (96 MHz, CDCl3) d 29.0 (s, br);
Elemental analysis calcd. (%) for C15H21O2B: C 73.79, H 8.67; found C
73.21, H 8.67; MS (EI): m/z (rel. int.): 244 (89) [M+], 229 (24) [M+ 2 Me].
The NOESY NMR spectrum shows correlations consistent with this
molecular geometry.
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Scheme 1 Some mechanistic possibilities for the dehydrogenative boryla-
tion reaction.
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