Phosphine-Borane complexes: In situ deprotection and application as ligands in the Rh- or Pd-Catalysed hydroformylation reaction

Article abstract of DOI:10.1007/BF03246221

Triarylphosphine-borane complexes are directly useful in the Rh-catalysed hydroformylation reaction of 1-octene (or Pdcatalysed hydroformylation of 1-pentene). Mild reaction conditions provide similar yields and selectivities of the anticipated aldehyde products to reactions making use of the corresponding free phosphines as ligands. The mono- or bidentate P-B adducts undergo in situ CO-mediated deprotection the produce the free phosphine ligands. The results demonstrate that phosphine-borane complexes may be directly applied to carbonylation reactions without a prior deprotection step, with little to no change in the reaction outcome.

Full text of DOI:10.1007/BF03246221

J. Iran. Chem. Soc., Vol. 8, No. 1, March 2011, pp. 240-246.
JOURIrNaAnLiaOnF THE
Chemical Society
Phosphine-Borane Complexes: in situ Deprotection and Application as Ligands in the
Rh- or Pd-Catalysed Hydroformylation Reaction
D.B.G. Williams*, P.D.R. Kotze, A.C. Ferreira and C.W. Holzapfel
Department of Chemistry, University of Johannesburg, P.O. Box 524, Auckland Park, Johannesburg, South Africa
(Received 4 June 2010, Accepted 29 July 2010)
Triarylphosphine-borane complexes are directly useful in the Rh-catalysed hydroformylation reaction of 1-octene (or Pd-
catalysed hydroformylation of 1-pentene). Mild reaction conditions provide similar yields and selectivities of the anticipated
aldehyde products to reactions making use of the corresponding free phosphines as ligands. The mono- or bidentate P-B adducts
undergo in situ CO-mediated deprotection the produce the free phosphine ligands. The results demonstrate that phosphine-borane
complexes may be directly applied to carbonylation reactions without a prior deprotection step, with little to no change in the
reaction outcome.
Keywords: Carbonylation, Hydroformylation, Palladium, Phosphanes, Rhodium
INTRODUCTION
acids; this has been attributed to the fact that the P-B and B-H
bonds have low polarity and polarisability [11]. P-B
complexes cannot normally be used to modify transition metal
catalysts: a separate deprotection step to release the free P
ligand is usually called for, after which complexation to the
metal in question ensues. Deprotection is usually effected ex
situ (when the free phosphine is to be used to modify a
catalyst) by amines in a separate step. We have previously
investigated the suitability of several amines and other
deprotection agents to cause this deprotection [12], which
typically proceeds in high yield. Purification of the free
phosphine is effected by removal of the new N-BH3 complex
from the free phosphine by extraction into a second liquid
phase, which most frequently is water. The need for an
independent deprotection procedure to provide the free ligand
adds cost and an additional step to the process.
Phosphines often suffer instability towards air, water and
acids. Once oxidised, they are rendered practically useless in
reactions where free phosphines are required as metal-
modifying ligands in catalytic processes. Therefore, a number
of methods have been developed to overcome the instability of
phosphines [1,2,3], the most effective of which is probably
borane protection of the phosphine. Burg et al. [4] reported the
first synthesis of a phosphine-borane complex. This method
has been extensively utilised for the preparation of a wide
range of phosphine-borane compounds, generally to protect
the P atom in the (III) oxidation state against oxidation [5]
employing BH3.THF and BH3.DMS as reagents, amongst
others [5-9]. In such complexes, the P-B donor-acceptor bond
is unusually strong and the B-H bonds show signs of an
Umpolung effect [10]. Consequently, these complexes are
inert towards oxygen, many nucleophiles, moisture and most
It is known that diborane (B2H6) forms a BH3.CO complex
with carbon monoxide [13]. Additionally, B-CO complexes
are intermediates in the synthesis of alcohols, aldehydes and
ketones from alkylborane precursors [14]. The present study
*Corresponding author. E-mail: bwilliams@uj.ac.za

Williams et al.
proposed to determine whether in situ deprotection of
Synthesis of Triarylphosphine Ligands
phosphine-borane complexes in carbonylation reactions would
be possible (i.e. where the CO present serves to deprotect the
P-BH3 complex). Here, the free phosphine ligand is intended
to bind to, and therefore modify, a transition metal as the
ligand is released from the borane complex. Our approach
successfully paves the way for the direct employment of
phosphine-borane complexes in such reactions without the
need for a separate deprotection step. The investigation was
pursued by performing the Rh-catalysed hydroformylation
reaction using a series of triarylphosphine-borane complexes
and comparing the results with those obtained when making
use of the corresponding free phosphine ligands. The Pd-
catalysed hydroformylation reaction, making use of bidentate
phosphines, was also successfully investigated. No additional
reagents were added with which to effect the deprotection,
rendering this method advantageous over procedures requiring
extra reagents and separate deprotection steps.
All of the phosphines of this study are commercially
available or have been prepared and characterised [15]. The
following method is given for ease of reference (see also Ref.
[16]). The aryl halide (0.049 mol, 1.2 equiv.) was added to a
THF (20 ml) mixture containing magnesium turnings (1.0 g,
0.041 mol, 1.0 equiv.) and a few iodine crystals. The
suspension was allowed to stir under reflux for several hours.
The solution was cooled to -40 °C after which it was slowly
added to a cold (-40 °C) THF solution of the relevant P-Cl
reagent (chlorodiphenyl-phosphine, 2.5 ml, 0.021 mol, 0.5
equiv.; dichlorophenyl-phosphine, 1.86 ml, 0.014 mol, 0.33
equiv; or trichloro-phosphine, 0.72 ml, 0.008 mol, 0.2 equiv).
The resulting solution was allowed to stir overnight while
slowly warming to room temperature. The product was
extracted using DCM and H2O and the solvent was removed in
vacuo. The product was isolated by flash column
chromatography using hexane as the eluent.
EXPERIMENTAL
General Synthesis of Phosphine-Borane Complexes
(see Ref. [5])
General
The free phosphine ligand (1 equiv.) was dissolved in THF
(20 ml), unless otherwise stated, and cooled to 0 °C. To this
mixture was added a THF solution containing the BH3.THF
complex (1 M, 4 equiv.). The resulting solution was allowed
to stir overnight at room temperature. The product was
extracted with DCM and H2O and the solvent was removed in
vacuo. The product was isolated by flash column
chromatography (hexane:EtOAc in v/v ratios as indicated).
Borane complexes not mentioned below have been [17]
prepared and characterised.
Diphenyl-o-tolylphosphine-borane (4c). The title
compound was prepared as described in the general synthesis
method, using diphenyl-o-tolylphosphine (0.0018 mol),
providing the product as a white powder (0.0014 mol, 79%).
m.p.: 135-136 °C; TLC: Rf 0.51 (19:1 hexane:EtOAc); 1H
NMR (300 MHz, CDCl3) δH 7.33-7.16 (m, 10H), 7.05-7.03
(m, 2H), 6.94 (t, 1H, J = 7.5 Hz), 6.81 (dd, 1H, JH-H = 6.6 Hz,
JH-P = 6.0 Hz), 2.06 (s, 3H), 1.70-0.60 (br m, 3H, BH3); 13C
NMR (75 MHz, CDCl3) δC 142.8 (d, 1C, J = 10.6 Hz), 134.0
(d, 2C, J = 8.6 Hz), 133.1 (d, 4C, J = 8.7 Hz), 131.8 (d, 1C,
J = 8.9 Hz), 131.3 (d, 1C, J = 2.6 Hz), 131.1 (d, 2C, J = 2.3
All experiments were performed using standard Schlenk
techniques under an atmosphere of dry argon. H and 13C{H}
1
NMR spectra were recorded by means of a Varian Gemini
2000, 300 MHz spectrometer in CDCl3 and were referenced to
SiMe4. 31P{H} NMR spectra were run in CDCl3 and
referenced to external 85% aqueous phosphoric acid. Gas
chromatography (GC) was carried out on a Varian Spectrum
3400 CX gas chromatograph, using nitrogen as the carrier gas
with a flow pressure of 1 bar. A DB1 30 m column, which
separated components on the basis of boiling points, was used.
Mass spectrometry was performed on a Varian Spectrum 3400
CX mass spectrometer at an ionisation potential of 70 eV. GC-
MS analyses were performed on a Varian Spectrum 3400 CX.
High pressure IR spectroscopy was performed a high pressure
Parr reactor fitted with opposing ZnS windows. The turnover
frequencies of the catalyst systems were calculated from CO
uptake measurements using a Danfoss gas flow meter coupled
to a high-pressure ballast vessel (turnover frequency is
measured in mol product/mol cat.h) and were calculated using
the linear part of the CO uptake curve.
241

Phosphine-Borane Complexes: in situ Deprotection and Application
4H), 6.97 (t, 2H, J = 7.6 Hz), 6.82 (dd, 2H, JH-H = 7.1 Hz,
Hz), 129.0 (d, 2C, J = 57.2 Hz), 128.8 (d, 4C, J = 10.0 Hz),
127.5 (d, 2C, J = 55.3 Hz), 125.8 (d, 1C, J = 9.4 Hz), 22.4 (d,
1C, J = 4.9 Hz); 31P NMR (121 MHz, CDCl3) δP 20.8 (d, J =
90.9 Hz); IR: νmax (CHCl3)/cm-1 3067, 3009, 2389, 1438,
1105, 1063; EIMS: m/z 290 [M+], 276 [M-BH3]+.
JH-P = 4.7 Hz), 2.10 (s, 6H), 1.70-0.60 (br m, 3H, BH3); 13C
NMR (75 MHz, CDCl3) δC 143.1 (d, 2C, J = 11.0 Hz), 133.7
(d, 2C, J = 9.0 Hz), 133.5 (d, 2C, J = 8.6 Hz), 131.9 (d, 2C,
J = 9.0 Hz), 131.3 (d, 2C, J = 2.5 Hz), 131.2 (d, 1C, J = 2.6
Hz), 128.7 (d, 2C, J = 9.5 Hz), 128.6 (d, 1C, J = 56.6 Hz),
127.6 (d, 2C, J = 54.0 Hz), 126.0 (d, 2C, J = 9.0 Hz), 22.7 (d,
2C, J = 4.6 Hz); 31P NMR (121 MHz, CDCl3) δP 21.6 (d, J =
54.9 Hz); IR: νmax (CHCl3)/cm-1 3068, 3010, 2387, 1437,
1105,1059; EIMS: m/z 304 [M+], 290 [M-BH3]+.
(o-Ethylphenyl)diphenylphosphine-borane (4d). The
title compound was prepared as described in the general
procedure, starting with (o-ethylphenyl)diphenylphosphine
(0.0017 mol), which provided the product as colourless
crystals (0.0012 mol, 71%). m.p.: 129-132 °C; TLC: Rf 0.53
1
Di-(o-ethylphenyl)phenylphosphine-borane
(4g).
(19:1 hexane:EtOAc); H NMR (300 MHz, CDCl3) δH 7.30-
Phosphine-borane 7 was generated as described the general
synthesis method, using toluene (20 ml) as solvent with di-(o-
ethylphenyl)phenylphosphine (0.0016 mol) as the substrate,
providing the product as a white powder (0.0011 mol, 70%).
m.p.: 132-134 °C; TLC: Rf 0.51 (19:1 hexane:EtOAc); 1H
NMR (300 MHz, CDCl3) δH 7.28-7.21 (m, 5H), 7.07-7.02 (m,
4H), 6.96 (t, 2H, J = 7.5 Hz), 6.85 (dd, 2H, JH-H = 7.5 Hz, JH-P
= 4.5 Hz), 2.70 (q, 4H, J = 7.5 Hz), 0.96 (t, 6H, J = 7.2 Hz);
13C NMR (75 MHz, CDCl3) δC 149.0 (d, 2C, J = 10.5 Hz),
133.8 (d, 2C, J = 9.0 Hz), 133.6 (d, 2C, J = 8.6 Hz), 131.4 (d,
2C, J = 2.5 Hz), 131.0 (d, 1C, J = 2.0 Hz), 130.0 (d, 2C, J =
9.0 Hz), 128.9 (d, 1C, J = 52.0 Hz), 128.7 (d, 2C, J = 10.0
Hz), 127.8 (d, 2C, J = 54.6 Hz), 125.8 (d, 2C, J = 9.5 Hz),
27.9 (d, 2C, J = 5.5 Hz), 14.8 (s, 2C); 31P NMR (121 MHz,
CDCl3) δ P 20.8 (d, J = 61.1 Hz); IR: νmax (CHCl3)/cm-1 3060,
3008, 2967, 2933, 2870, 2385, 1465, 1438, 1065; EIMS: m/z
332, 318 [M-BH3]+.
7.20 (m, 10H), 7.08-7.03 (m, 2H), 7.00 (t, 1H, J = 7.4 Hz),
6.86 (dd, 1H, JH-H = 7.4 Hz, JH-P = 4.5 Hz), 2.68 (q, 2H, J =
7.5 Hz), 0.95 (t, 3H, J = 7.5 Hz); 13C NMR (75 MHz, CDCl3)
δC 148.9 (d, 1C, J = 10.3 Hz), 134.2 (d, 2C, J = 16.3 Hz),
133.2 (d, 4C, J = 9.5 Hz), 131.5 (d, 1C, J = 2.3 Hz), 131.1 (d,
2C, J = 2.3 Hz), 129.9 (d, 1C, J = 8.6 Hz), 129.6 (d, 2C, J =
58.4 Hz), 128.8 (d, 4C, J = 9.9 Hz), 127.1 (d, 1C, J = 55.5
Hz), 125.8 (d, 1C, J = 9.7 Hz), 27.8 (d, 1C, J = 5.7 Hz), 15.0
(s, 1C); 31P NMR (121 MHz, CDCl3) δP 20.9 (d, J = 64.0 Hz);
IR: νmax (CHCl3)/cm-1 3058, 3009, 2965, 2932, 2386, 1465,
1436, 1068; EIMS: m/z 304 [M+], 290 [M-BH3]+.
Phenyl-di-p-tolylphosphine-borane (4e). The title
compound was synthesised as described in the general
procedure with phenyl-di-p-tolylphosphine (0.0017 mol) as
the starting material, yielding the product as colourless
crystals (0.0017 mol, 100%). m.p.: 183-185 °C; TLC: Rf 0.51
1
(19:1 hexane:EtOAc); H NMR (300 MHz, CDCl3) δH 7.41-
7.21 (m, 9H), 7.06 (d, 4H, J = 7.8 Hz), 2.21 (s, 6H), 1.63-0.63
(br m, 3H, BH3); 13C NMR (75 MHz, CDCl3) δC 141.6 (d, 2C,
J = 2.6 Hz), 133.1 (d, 4C, J = 10.0 Hz), 133.0 (d, 2C, J = 9.7
Hz), 131.0 (d, 1C, J = 2.6 Hz), 129.5 (d, 4C, J = 10.5 Hz),
129.1 (d, 1C, J = 51.0 Hz), 128.6 (d, 2C, J = 9.9 Hz), 125.8 (d,
2C, J = 59.8 Hz), 21.4 (s, 2C); 31P NMR (121 MHz, CDCl3) δP
20.1 (d, J = 72.9 Hz); IR: νmax (CHCl3)/cm-1 3058, 3009, 2381,
1500, 1435, 1091; EIMS: m/z 304 [M+], 290 [M-BH3]+.
Phenyl-di-o-tolylphosphine-borane (4f). The borane
complex was synthesised as described in the general synthesis
using toluene (20 ml) as solvent with phenyl-di-o-
tolylphosphine (0.0017 mol) as the starting material, affording
the product as a white amorphous solid (0.0016 mol, 95%).
m.p.: 155-157 °C; TLC: Rf 0.52 (19:1 hexane:EtOAc); 1H
NMR (300 MHz, CDCl3) δH 7.35-7.19 (m, 5H), 7.04-7.00 (m,
Rh-Catalysed Hydroformylation of 1-Octene
Acetylacetonatodicarbonylrhodium(I) (1.1 mg, 0.0000043
mol, 0.0091 mol% vs. 1-octene) and the triarylphosphine-BH3
complex or the corresponding free phosphine ligand (0.043
mol, 10 equiv. with respect to Rh) were dissolved in toluene (5
ml) in a high pressure autoclave reactor. 1-Octene (7.4 ml,
0.047 mol) was added after which the reactor was sealed and
pressurised with 2500 kPa of syngas (H2:CO = 1:1, constant
pressure). The reaction mixture was allowed to stir at 100 °C
for 5 h using a stirrer speed of 1000 rpm. The reaction mixture
was filtered through silica and analysed using GC-FID (30 m
Zebron ZB1 analytical column with a 0.2 mm internal
diameter and 0.5 µm film thickness and He as the carrier gas,
with the following temperature programme: 50 °C starting
242

Williams et al.
THF, 0 °C
oven temperature, hold 4 min, 10 °C min-1 ramp to 300 °C,
total run time 29 min). Products were compared against
authentic samples of the product aldehydes.
ArMgX
+
Ph
PCl
Ph
Ph
PAr
x
(3-x)
x
(3-x)
1
2
3
BH THF
3
BH
3
Pd-Catalysed Hydroformylation of 1-Pentene
PAr
(3-x)
x
In a glovebox, Pd(OC(O)CF3)2 (112 mg, 0.337 mmol), 1,3-
bis(diphenylphosphino)propane.2BH3 (148 mg, 0.337 mmol)
[17] and trifluoroacetic acid (282 mg, 2.47 mmol) were
dissolved in THF (10 ml). THF (29 ml), 1-pentene (39.0 ml,
25.0 g, 0.356 mol) and the catalyst solution were transferred to
the autoclave under inert conditions. The reactor was sealed,
pressurised to 5000 kPa with 1:1 H2/CO, heated to 100 °C and
the contents were stirred at 1000 rpm. Readings on the flow
meter were initiated after the system had stabilised
(approximately 10 min) and the reaction was allowed to
proceed for 3 h. The reaction mixture was filtered through
silica and analysed as for the Rh-catalysed reactions.
4
x = 1, 2, 3
Ar = 2-MePh, 2-EtPh, 4-MePh
Scheme 1. Preparation of phosphine-borane complexes
Ph3P
3a
Ph3P BH3
4a
BH3
Ph2P
Ph2P
Ph2P
3b
3c
3d
3e
4b
4c
4d
4e
BH3
Ph2P
RESULTS AND DISCUSSION
H3B
Ph2P
PhP
Ph2P
With the exception of triphenylphosphine (obtained from a
commercial source), all phosphines used in this study were
prepared according to the method of Steube [16], using aryl-
Grignard reagents 1 with various P-Cl electrophiles 2 in THF
as solvent (Scheme 1). The phosphines 3 (Fig. 1) were borane-
protected [5] by treatment thereof with four equivalents of
H3B
PhP
2
2
H3B
PhP
PhP
BH3·THF at
0 °C for several hours, affording the
2
2
3f
4f
corresponding phosphine-borane complexes 4 in good yields
(Table 1).
H3B
PhP
PhP
P
A diagnostic change in the 31P NMR signals was observed
upon phosphine-borane complex formation. The P-B complex
afforded a line-broadened doublet resonance with a significant
down-field shift (Table 1) vs. the higher-field sharp singlet
peaks observed for the free phosphines. This shift provides
ready proof of complexation. IR spectroscopy using the
borane complexes showed a broad absorption peak in the area
of 2385 cm-1 with a shoulder at about 2350 cm-1, which was
indicative of the presence of the borane complexed to the
phosphine [5].
2
2
3g
3h
4g
4h
H3B
P
3
3
Fig. 1. Phosphines and their corresponding borane complexes.
loading of only 0.0091 mol% (ca. 11 000:1 ratio of 1-
octene/Rh) and a rhodium to ligand ratio of 1:10. These
reactions were intentionally limited to 5 h in order to
determine ligand effects, especially on the conversions of the
substrate to product. (Higher (quantitative) conversions to
aldehyde products are readily attainable using higher
The Rh-catalysed hydroformylation reaction of 1-octene
(Scheme 2) was performed with the phosphine-borane
complexes and their corresponding free phosphine ligands,
respectively. The reactions were performed using a catalyst
243

Phosphine-Borane Complexes: in situ Deprotection and Application
Table 1. Phosphine-Borane Complexes and Their 31P NMR Data
Complex
Yield (%)
31P NMR (ppm)
31P NMR (ppm)
free phosphine
4a
4b
4c
4d
4e
4f
100a
100
79a
21.0
19.9
21.1
20.9
20.1
21.6
20.8
19.5
-5.1
-5.9
-13.4
-14.8
-6.2
71a
100a
95b
70b
100a
-20.4
-24.2
-7.5
4g
4h
aBorane protection performed in THF. bBorane protection performed in toluene.
Table 2. Hydroformylation Results with Free Ligands and Phosphine-Borane Complexes
Entry
Ligand
Yielda
aldehydes
l/ba,b
TOFc
Entry
P-B
complex
Yielda
aldehydes
l/ba,b
TOFc
×102
×102
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3f
77
60
59
58
58
44
39
54
1.7
1.9
1.6
1.5
1.8
1.5
1.4
1.8
16.9
13.2
13.0
12.7
12.7
9.7
9
4a
4b
4c
4d
4e
4f
79
71
95
94
65
42
34
61
1.7
1.9
1.6
1.6
1.5
1.5
1.8
1.4
17.4
15.6
13.9
13.2
14.3
9.2
10
11
12
13
14
15
16
3g
3h
8.6
4g
4h
7.5
11.9
13.4
aAverage of four independent experiments with individual yield deviations less than 3% from the average.
Selectivity to aldehydes > 98%. Reactions limited to 5 h. bl/b = linear/branched ratio. cTOF = turnover frequency
in mol product/mol Rh.h.
temperatures, pressures and/or longer reaction times). The
resulting reaction mixtures were analysed by GC-FID (Table
H
2) against authentic samples of the aldehyde products.
linear
O
Rh(CO)2(acac)
PAr3, Toluene
The hydroformylation reactions provided similar results
(Table 2) for the phosphine-borane and the free phosphine
ligand systems in terms of yields and ratios of the aldehydes.
Decreasing turnover frequencies were observed, as a trend,
along the series mono-, di- and tri-substituted triarylphosphine
free ligands. The phosphine-borane adducts followed a similar
+
1-Octene
H2/CO (1:1)
2500 kPa
100 °C
branched
O
H
Scheme 2. Hydroformylation of 1-octene
244

Williams et al.
trend. This trend is anticipated [18] for phosphine ligands
IR spectroscopy was performed in an effort to investigate
the reaction between CO and the triphenylphosphine-borane
complex. This was carried out by pressurising a solution of
triphenylphosphine-borane in toluene in a high pressure Parr
reactor fitted with opposing ZnS windows. The solution was
subjected to conditions similar to those of the Rh-catalysed
hydroformylation reaction detailed above. IR spectroscopy
was performed on the mixture at regular intervals. The peak
indicative of the phosphine-borane complex at around 2385
cm-1 disappeared fully within 45 min, clearly demonstrating
the efficiency of the high pressure CO at effecting this in situ
deprotection step. Additionally, NMR studies in a sapphire
high-pressure NMR tube showed an interesting solvent effect.
Attempted deprotection reactions under CO pressure (4000
kPa CO, 120 °C) in toluene failed to demonstrate any
deprotection at all during the course of one hour. In contrast,
those performed in THF (3000 kPa CO, 80 °C) showed 100%
deprotection within one hour, but failed to show any
deprotection under milder conditions (100 kPa CO, 25 °C).
This effect is ascribed to solvent participation in the
deprotection reaction in the case of THF. (Ethers have been
shown to catalyse the reaction of diborane with Lewis bases-a
similar effect may account for the present observations [22].
Indeed, the requirement of performing the borane protection
reaction of ligands 3f and 3g to form complexes 4f and 4g
(Table 1) in toluene where reactions in THF were less
successful also alludes to solvent participation). Mass transfer
of CO into solution is slow in 5 mm NMR tubes given the
small surface-to-volume ratio, explaining the differences
observed between the high pressure IR reactions performed in
a well-stirred Parr reactor vessel and the NMR results in
toluene as solvent. While THF is not typically employed as a
solvent for hydroformylation reactions, the current
observations may prove useful in other carbonylation reactions
where THF is more frequently relevant as a solvent.
bearing increasing electron density on the P atom, arising from
the positive inductive effects of the alkyl substituents. The
electronic effect of alkyl substituents on arylphosphines is
readily demonstrated by investigating the CO stretching
frequencies of the Rh-Vaska’s type complexes of the ligands
[19]. The good correlation between the reactions performed
with a given ligand and its corresponding borane complex
provided convincing indirect evidence that the complexes
underwent deprotection under the reaction conditions. What
may be further noted from Table 2 is that the linear/branched
ratio of aldehyde products remained fairly constant across
most of the series, suggesting that the presence of the borane
had no significant effect on the alkene co-ordination and
hydride migration processes (which determine the ratio of
aldehyde products obtained) [20].
The in situ deprotection technique was also applied to the
borane
complex
of
a
bidentate
ligand
(1,3-
bis(diphenylphosphino)propane.2BH3; dppp.2BH3) [17] in a
Pd(OC(O)CF3)2-catalysed hydroformylation reaction of 1-
pentene. The benchmark reactions performed in the presence
of the free ligand afforded a linear/branched ratio of aldehyde
products of 3.3:1 while those performed with the dppp.2BH3
provided a ratio of 3.2:1. The turnover frequency of the
catalyst system calculated from CO uptake measurements was
450 for the reaction performed with free dppp vs. 400 for the
reaction with the dppp.2BH3 complex, showing little
difference between the two reactions. Selectivity for aldehyde
products was determined to be greater than 98% in all
instances, by GC-FID. These two successful outcomes (with
Rh and Pd) hint that the in situ deprotection protocol is not
limited to only one type of ligand/catalyst precursor
combination. This method may well hold the potential to
simplify a variety of reactions outside of the hydroformylation
transformation making use of CO, eliminating one step in the
series of events leading up to the catalytic transformation. For
example, a supported pyridylphosphine ligand has recently
been prepared and applied in the Pd-catalysed
alkoxycarbonylation of alkenes [21]. The ligand synthesis
involved borane protection/deprotection steps, while the
catalysed reaction makes use of CO pressure and the process
may therefore benefit from the current approach.
CONCLUSIONS
A
series of triarylphosphine-borane complexes was
prepared and employed without prior deprotection in the
hydroformylation reaction with results that largely mimicked
those obtained with the corresponding free phosphine ligands.
245

Phosphine-Borane Complexes: in situ Deprotection and Application
1525.
CO used in this reaction as a reagent readily causes
deprotection of the phosphine-borane complex, allowing
ligand-modified Rh and Pd catalysts to be generated. We show
for the first time that mono- and bidentate phosphine-borane
complexes can be directly utilised in hydroformylation
reactions without the need for an independent deprotection
step. Pleasingly and importantly, the outcomes demonstrate
that there is no need for the employment of conventional
deprotection agents such as amines that produce additional
waste and that the putative BH3.CO complex does not inhibit
the reaction in any way. It is highly likely that other
carbonylation reactions will benefit from this in situ
deprotection protocol, an aspect that we are investigating,
especially since phosphine-borane complexes are very
commonly used as part of the P-atom protection strategy in
ligand synthesis.
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ACKNOWLEDGEMENTS
We thank Sasol, THRIP and the University of
Johannesburg for generous financial support.
Supplementary Data
Supplementary data associated with this article can be
found at doi:
13C NMR spectra of all new phosphine-borane complexes.
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