Enhanced regioselectivity of rhodium-catalysed alkene hydroboration in supercritical carbon dioxide

Article abstract of DOI:10.1039/a909636a

Catalysed alkene hydroboration proceeds in supercritical CO₂ with several rhodium(I) complexes using tunable fluorinated ligands and shows higher regioselectivity relative to tetrahydrofuran or perfluoromethylcyclohexane.

Full text of DOI:10.1039/a909636a

Enhanced regioselectivity of rhodium-catalysed alkene hydroboration in
supercritical carbon dioxide
Charles A. G. Carter,a R. Thomas Baker,*a Steven P. Nolanb and William Tumas*a
a Los Alamos Catalysis Initiative, Chemical Science and Technology Division, Los Alamos National Laboratory,
MS J514, Los Alamos, NM 87545 USA. E-mail: weg@lanl.gov; tumas@lanl.gov
b Department of Chemistry, University of New Orleans, New Orleans, LA 70148 USA
Received (in Cambridge, UK) 2nd December 1999, Accepted 26th January 2000
Table 1 Ligand effects on catalysed hydroboration of vinylanisole in
scCO2
Catalysed alkene hydroboration proceeds in supercritical
a
CO2 with several rhodium( ) complexes using tunable
I
fluorinated ligands and shows higher regioselectivity rela-
tive to tetrahydrofuran or perfluoromethylcyclohexane.
Selectivity (%)
Conversion
(%)
Entry L
A
B
C
D
The power of homogeneous transition metal catalysis rests in
chemists’ ability to fine-tune the steric and electronic properties
of the metal coordination environment and to optimize activity
and selectivity by judicious choice of the reaction medium.1
1
2
3
4
5
6
7
8
No added ligand
PPh3 (2)
89
92
14
71
75
82
88
90
89
100
14
13
—
—
—
—
—
—
31
13
19
17
12
—
11
—
41
3
6
P[3,5-(CF3)2C6H3]3 (3) 94
P(RF)3 (4)b
94
81
89
88
100
1
While a number of studies have employed fluorinated lig-
Ph2PORF (5)b
Cy2PORF (6)b,c
Ph2PRF (7)b
Cy2PRF (8)b
—
—
—
—
2–4
ands
to conduct a variety of metal-catalysed reactions5 in
environmentally benign supercritical carbon dioxide (scCO2),
little work has been reported on catalyst tunability. Recently
3,6,7
reported phosphines PR2RF
and phosphinites PR2(ORF)6,7
a (hfacac)Rh(coe)2 1 (0.02 mmol), ligand L (0.04 mmol) and substrate (1.0
(RF = CH2CH2C6F13) are easily prepared and offer the ability
not only to enhance catalyst solubility, but to control the
stereoelectronic environment of catalysts in scCO2. We chose to
investigate RhI-catalysed alkene hydroboration8,9 due to the
importance of boronate ester products in synthetic organic
chemistry, medicinal chemistry, materials science and molec-
ular recognition. The catalyzed reaction exhibits higher rates
and complementary regioselectivity compared to the un-
catalysed reaction, and efficient chirality transfer has been
demonstrated in the asymmetric variant.9 We now report the
first demonstration of catalysed alkene hydroboration in scCO2,
the ability to tune both regio- and chemo-selectivity, and the
discovery of much higher selectivity in scCO2 compared to
perfluoromethylcyclohexane (CF3C6F11).
mmol). Conversion and product selectivity were determined by 1H and 11
B
NMR using hexamethyldisiloxane standard. b RF = CH2CH2C6F13
.
c Some
polymeric material was observed in this reaction.
Catalyzed hydroboration of styrene derivatives with cate-
cholatoborane (HBcat) was carried out in scCO2 at 40 °C and
2800 psi for 5 h with a rhodium catalyst precursor and added
phosphorus ligands using magnetically-stirred, high-pressure
reactors with sapphire view windows that have been described
elsewhere.10 Addition of HBcat to vinylanisole using only the
Scheme 2
boration. Triphenylphosphine, its 3,5-bis(trifluoromethyl)phe-
nyl analog, and P(RF)3 all greatly increased hydroboration
regiochemistry, but chemoselectivity was improved only mar-
ginally. Alkylboronate ester A was formed exclusively with
cyclohexyl-substituted phosphine 8 (L+Rh ratio = 2+1),
demonstrating the ability to control regioselectivity and chem-
oselectivity by tuning the solubility and stereoelectronic
properties of ligands and metal catalysts in scCO2.
In several studies of rhodium-catalyzed hydroboration of
disubstituted alkenes, such as a- and b-methylstyrenes, mono-
dentate ligands have been found to give poor selectivity to the
desired Markovnikov addition product.8 While conversions
were low in some cases, the ability to control regiochemistry
through tunable fluorinated ligands in scCO2 is further
illustrated in Scheme 2 and Table 2. For b-methylstyrene, the
tris[3,5-bis(trifluoromethyl)phenyl]phosphine 3 and cyclo-
hexyl-substituted ligands 6, 8 gave the highest activity, but with
only fair regiocontrol. Ligands 4, 5 and 7 gave alkylboronate
ester E exclusively. The advantage of ligand tunability is also
demonstrated for the reaction of a-methylstyrene and HBcat, as
changing from cyclohexyl 6 to phenylphosphinite ligand 5
yields only the Markovnikov addition product EA.
hexafluoroacetylacetonate
(hfacac)
rhodium
complex
(hfacac)Rh(coe)2 1 (coe = cyclooctene) as catalyst precursor2
led to a homogeneous solution and, after 5 h, high conversion to
a mixture of alkylboronate esters A and B, alkenylboronate ester
C, and 4-ethylanisole D (Scheme 1, Table 1). Products C and D
result from a competing dehydrogenative borylation pathway
which is favoured by phosphine-free rhodium catalysts.11
Addition of triphenylphosphine to 1 gave turbid solutions due to
poor ligand and/or catalyst solubility in scCO2. The fluorinated
ligands, including 5–8 led to homogeneous solutions demon-
strating that a single fluorinated substituent is sufficient to
impart high solubility in scCO2. As shown in Table 1, added
ligands dramatically influence the regioselectivity of hydro-
Solvent effects play an important role in determining the
Scheme 1
selectivity of catalytic hydroboration reactions.12 Gladysz and
DOI: 10.1039/a909636a
Chem. Commun., 2000, 347–348
This journal is © The Royal Society of Chemistry 2000
347

Table 2 Hydroboration of a- and b-methylstyrenea in scCO2
Conversion (selectivity) (%)
3
It is also possible that a Rh–h -benzyl intermediate17 that would
1
lead to A could be stabilized in scCO2 relative to the h -
regioisomer that would yield B.
In summary, we have demonstrated that catalysed alkene
hydroboration can proceed in supercritical CO2 without any
difficulty from B–H reactivity with the solvent. Regiocontrol
can be achieved using tunable ligands of the form R2PRF and
R2PORF. Furthermore, significantly higher regioselectivities
can be obtained for these ligands in scCO2, relative to
fluorocarbons and even THF. Given further advancements in
catalyst separations and recovery in scCO2,18 catalysis in this
medium can complement fluorous phase approaches. We are
currently assessing the effects of pressure (solvent density) on
selectivity, monitoring reactive species by in situ NMR
spectroscopy, and studying stoichiometric reactions of isolable
16 electron rhodium–boryl complexes.19
Entry
L
E+F+G
EA+FA
1
2
3
4
5
6
P[3,5-(CF3)2C6H3]3 (3)
P(RF)3 (4)
Ph2PORF (5)
Cy2PORF (6)
Ph2PRF (7)
100 (0+24+76)
56 (55+45+0)
13 (100+0+0)
3 (50+50+0)
19 (96+4+0)
38 (36+64+0)
90 (78+22)
13 (100+0)
8 (100+0)
71 (89+11)
16 (100+0)
77 (71+29)
Cy2PRF (8)
a Conversion and selectivity were determined after 24 h.
Table 3 Effect of solvent on the regioselectivity of vinylanisole hydro-
borationa
This work was supported as part of the Los Alamos Catalysis
Initiative by the Department of Energy through Laboratory
Directed Research and Development (LDRD) funding and by
the Department of Energy (DE-FG02-98ER45732) for work
performed at UNO. We would like to thank Drs Chris Haar and
Dale Smith (UNO) for providing ligands and Professor John
Gladysz for a generous donation of P(CH2CH2C6F13)3.
Selectivity (%)
Conversion
Entry
1
L
Solvent
(%)
A
B
C
D
P(RF)3 (4)
THF
CF3C6F11
scCO2
82
84
66 15 13
57 16 19
6
8
1
94
82
—
17
2
3
4
5
Ph2PORF (5) THF
CF3C6F11
scCO2
Cy2PORF (6) THF
100
98
52 21 16 11
37 23 23 17
81
88
—
12
—
8
Notes and references
† Triphenylphosphine and catalyst precursor 1 (3+1) or Wilkinson’s
catalyst (ref. 9) lead to high selectivity > 95% to A in THF; however, these
catalysts are not effective for methylstyrenes (ref. 19).
94
20 37 35
CF3C6F11
89
24 26 37 13
b
scCO2
89
93
90
88
90
84
80
89
—
—
4
—
12
12
11
—
4
4
Ph2PRF (7)
Cy2PRF (8)
THF
CF3C6F11
scCO2
THF
CF3C6F11
scCO2
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25 41 17 17
100
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—
—
—
a Reactions in organic solvents were run in NMR tubes using 0.002 mmol
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observed in this reaction.
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Horvath13 reported an elegant study of alkene hydroboration in
fluorous biphasic media using RhCl[P(RF)3]3 derived from 4;
however, the regioselectivity for styrene derivatives was low.
Comparing the reaction of vinylanisole and HBcat in scCO2 to
that in perfluoromethylcyclohexane led to some surprising
results. Catalyst precursor 1 and ligand 4 showed significantly
greater regiocontrol in scCO2 (Table 3, entry 1). This trend held
for ligands 5–8 and was particularly striking for Cy2PRF (entry
5) which afforded a single product in scCO2. Remarkably, the
selectivity in scCO2 for 4–8 was also considerably higher than
that observed in THF. Ligand 4 is not soluble in THF; however,
partially fluorinated ligands 5–8 and their rhodium complexes
are fully soluble under the reaction conditions. In THF, product
selectivity using the nonfluorinated analog of ligand 8 (i.e.
Cy2PC8H17) was found to be similar to that observed for ligand
8, confirming the insulating effect of the two methylene spacers
in the RF group.† There are several reports4,5,14,15 of enhanced
selectivity of catalysed reactions in scCO2 compared to
conventional organic solvents. Except for a few cases where
density is controlled through pressure changes,16 we are
unaware of any examples presenting such a dramatic effect as
that shown here. The origin of the higher selectivities for
hydroborations is not clear. Since ligand-free rhodium com-
plexes result in poor hydroboration selectivity,11 stability and
lability of the catalysts (i.e. keeping the ligand on the metal),
which are likely solvent dependent, may play an important role.
Communication a909636a
348
Chem. Commun., 2000, 347–348