Phosphine promoted substituent redistribution reactions of B-chlorocatechol borane: Molecular structures of ClBcat, BrBcat and L·ClBcat (cat = 1,2-O2C6H4; L = PMe3, PEt3, PBut3,

Article abstract of DOI:10.1039/b010025k

In order to evaluate the potential for side reactions when using B-chlorocatechol borane (ClBcat) in stoichiometric or catalytic transformations involving metal phosphine complexes, we examined the interaction between ClBcat and a series of PR₃

Full text of DOI:10.1039/b010025k

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Phosphine promoted substituent redistribution reactions of
B-chlorocatechol borane: molecular structures of ClBcat, BrBcat
and LؒClBcat (cat ؍ 1,2-O2C6H4; L ؍ PMe3, PEt3, PBut3, PCy3,
NEt3)†
R. Benjamin Coapes,a Fabio E. S. Souza,b Mark A. Fox,a Andrei S. Batsanov,a
Andrés E. Goeta,a Dimitrii S. Yufit,a Michael A. Leech,a Judith A. K. Howard,a
Andrew J. Scott,c William Cleggc and Todd B. Marder*ab
a Department of Chemistry, University of Durham, Durham, UK DH1 3LE.
E-mail: todd.marder@durham.ac.uk
b Department of Chemistry, University of Waterloo, Waterloo, Ontario, N2L 3G1, Canada
c Department of Chemistry, University of Newcastle upon Tyne, Newcastle upon Tyne,
UK NE1 7RU
Received 15th December 2000, Accepted 19th February 2001
First published as an Advance Article on the web 28th March 2001
In order to evaluate the potential for side reactions when using B-chlorocatechol borane (ClBcat) in stoichiometric
or catalytic transformations involving metal phosphine complexes, we examined the interaction between ClBcat and
a series of PR3 compounds. Reactions of ClBcat with the basic phosphines PMe3, PEt3, PMe2Ph and PBut3, in exactly
1 : 1 stoichiometry, all afforded crystalline adducts of the form R3PؒClBcat, which have been characterised
spectroscopically. All display broad singlets at room temperature in both 11B{1H} and 31P{1H} NMR spectra with no
B–P coupling observed. The low temperature 11B{1H} and 31P{1H} NMR spectra, however, do show B–P coupling,
suggesting rapid dissociation at room temperature. The compounds ClBcat, BrBcat, and the adducts between ClBcat
and PMe3, PEt3 and PBut3 have been characterised by single crystal X-ray diffraction. In ClBcat and BrBcat, the
halogen contributes little π-bonding to boron as the B–O bond lengths are identical in both compounds. The reaction
of ClBcat with a small excess of PCy3 yielded Cy3PؒClBcat, which has also been characterised by single crystal X-ray
diffraction and shows considerable distortion. The reaction between ClBcat and the less basic phosphines PPh3 and
PPh2Me yielded only the redistribution products R3PؒBCl3 and B2cat3. In fact, reaction of all of the phosphines with
an excess of ClBcat gave only redistribution products, although there was evidence for an intermediate in the 11B{1H}
NMR spectra, with PMe3 and PEt3. The isolated R3PؒClBcat adducts proved unstable, even at low temperature in the
solid state, eventually leading to R3PؒBCl3 and B2cat3. Reaction of ClBcat with NEt3 afforded Et3NؒClBcat, which
was characterised spectroscopically and by single crystal X-ray diffraction. This adduct is stable both in solution and
in the solid state.
Introduction
There are now many examples of the transition metal catalysed
addition of boron compounds to various unsaturated sub-
strates, in reactions such as hydroboration,1–3 diboration,4,5
dehydrogenative borylation of alkenes,6–16 silylboration,17,18
thioboration,19–21 stannylboration,22,23 reactions of aryl or vinyl
halides or triflates with B2(OR)4 or HB(OR)2,24–34 and direct
activation of C–H bonds in alkanes and arenes.35–41 These reac-
tions all involve an EBcat or EBpin species (E = H, Bcat, Bpin,
SR, SiR3, SnR3, cat = 1,2-O2C6H4, pin = OCMe2CMe2O), and
several reviews highlight studies of the intermediate metal–
boryl species.42–45 Whilst dehydrogenative borylation of alkenes
to vinylboronate esters using HB(OR)2 or B2(OR)4 is known,6–16
a more attractive and less expensive variant would be the use of
a commercially available ClB(OR)2 reagent, such as ClBcat, and
base in a boron analogue of the Heck reaction (Scheme 1).
Such a reaction would likely entail the oxidative addition of
ClBcat to a metal centre,46 the use of tertiary amines as base,
and the use of phosphine ligands on a late transition
metal catalyst. Hence, there is a need to ascertain what, if any,
reactivity there is between ClBcat and R3P or R3N, as it is
known that nucleophiles,47–49 including phosphines,49 can
† B-chlorocatechol borane = 2-chloro-1,3,2-benzodioxaborole.
Scheme 1 Catalysed routes to vinylboronate esters from alkenes.
DOI: 10.1039/b010025k
J. Chem. Soc., Dalton Trans., 2001, 1201–1209
This journal is © The Royal Society of Chemistry 2001
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Table 1 Selected bond lengths (Å) and bond angles (Њ) for ClBcat, BrBcat and the R3P and Et3N adducts of ClBcat
Me3PؒClBcat 3ab
Et3PؒClBcat
But3PؒClBcat Cy3PؒClBcat
Et3NؒClBcat
3h
ClBat 1a
BrBcat 2a
Molecule A
Molecule B
3b
3d
3e
B–X (X = Cl, Br)
B–O(1)
B–O(2)
1.744(3)
1.381(2)
1.381(2)
1.898(3)
1.381(2)
1.381(2)
1.9105(17)
1.4565(18)
1.4578(18)
1.9646(17)
1.3706(17)
1.3719(17)
1.393(2)
1.375(2)
1.405(2)
1.380(3)
1.402(2)
1.9178(17)
1.4631(19)
1.4559(18)
1.9691(17)
1.3725(17)
1.3699(17)
1.396(2)
1.377(2)
1.404(2)
1.387(2)
1.402(2)
1.9254(18)
1.459(2)
1.463(2)
2.0079(18)
1.3817(18)
1.3776(19)
1.394(2)
1.387(2)
1.405(3)
1.391(3)
1.410(3)
1.914(2)
1.459(2)
1.470(2)
2.048(2)
1.372(2)
1.372(2)
1.398(2)
1.383(2)
1.402(2)
1.382(2)
1.405(2)
1.380(2)
1.900(2)
1.903(2)
1.908(2)
0.1123
1.9946(17)
1.4721(18)
1.4083(16)
1.9905(16)
1.3646(16)
1.3971(17)
1.567(2)
1.379(2)
1.346(2)
1.421(2)
1.3519(19)
1.3539(18)
1.8835(14)
1.7680(13)
1.8698(19)
0.2537
1.8758(10)
1.4527(11)
1.4576(13)
1.6433(13)
1.3717(10)
1.3748(10)
1.3905(13)
1.3785(13)
1.4114(15)
1.3829(17)
1.4065(15)
1.3821(12)
1.5120(12)
1.5261(13)
1.5198(13)
0.0363
B–E (E = N, P)
C(1)–O(1)
C(2)–O(2)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(1)–C(6)
E–C(10)
1.395(2)
1.395(2)
1.394(3)
1.381(2)
1.400(2)
1.400(3)
1.400(2)
1.381(2)
1.395(2)
1.395(2)
1.396(3)
1.402(3)
1.379(2)
1.402(3)
1.398(3)
1.379(2)
1.377(2)
1.378(2)
1.381(2)
1.7991(16)
1.7917(16)
1.8007(16)
0.1539
1.799(16)
1.7931(15)
1.7928(16)
0.2006
1.8210(18)
1.8215(17)
1.8247(18)
0.1435
E–C(20)
E–C(30)
B–Planec
O(1)–B–O(2)
O(1)–B–X
O(2)–B–X
O(1)–B–E
O(2)–B–E
114.0(2)
123.00(10)
123.00(10)
113.9(2)
123.1(1)
123.1(1)
107.74(11)
111.86(10)
110.79(10)
109.77(10)
112.05(10)
104.68(8)
107.40(8)
109.44(8)
107.71(8)
109.93(8)
112.35(8)
109.87(8)
105.28(11)
104.97(11)
107.25(12)
110.24(10)
110.71(10)
113.75(10)
110.29(10)
104.64(8)
108.07(8)
107.81(8)
108.17(8)
110.68(7)
113.05(7)
108.91(7)
104.94(11)
105.21(11)
107.82(13)
109.58(11)
110.90(11)
109.69(11)
111.67(11)
107.17(8)
105.98(9)
107.11(8)
107.99(8)
115.37(8)
110.05(8)
109.97(8)
105.13(12)
105.10(12)
107.10(11)
109.19(9)
108.98(9)
111.40(9)
110.51(9)
109.59(7)
110.95(6)
110.30(6)
110.23(6)
110.75(6)
107.74(6)
106.75(6)
105.71(10)
105.39(10)
105.04(11)
108.69(6)
110.64(6)
107.79(6)
113.34(6)
110.01(6)
106.31(6)
108.69(6)
110.64(6)
107.79(6)
113.34(6)
110.01(6)
107.52(10)
105.33(10)
107.32(7)
111.16(7)
111.43(6)
109.73(7)
110.35(7)
106.86(6)
110.97(7)
105.01(8)
110.26(8)
106.72(7)
111.20(8)
112.76(7)
106.10(7)
105.94(7)
Cl–B–E
C(10)–E–C(20)
C(20)–E–C(30)
C(10)–E–C(30)
C(10)–E–B
C(20)–E–B
C(30)–E–B
B–O(1)–C(1)
B–O(2)–C(2)
103.71(13)
103.70(13)
103.9(1)
103.9(1)
a Symmetry related data included for ease of comparison. Atom numbering is as follows: C(2) = C(1A), C(3) = C(2A), C(4) = C(3A), C(5) = C(3),
C(6) = C(2), O(2) = O(1A). b Compound 3a contains two independent molecules in the asymmetric unit. Molecule A: C(10) = C(7), C20 = C(8),
C(30) = C(9); molecule B: E = P(2), B = B(2), C(1)–C(6) = C(11)–C(16); C(10) = C(17), C(20) = C(18), C(30) = C(19). c Distance from B(1) to least-
squares plane defined by O(1), C(1), C(6), C(5), C(4), C(3), C(2), O(2).
Fig. 2 Structure of BrBcat 2. Atoms related by the two-fold axis are
Fig. 1 Structure of ClBcat 1. Atoms related by the two-fold axis are
marked ‘A’.
marked ‘A’.
promote the degradation of HBcat, and it has been suggested48
space group C2/c, passing through the halogen and boron
that PMe3 can promote degradation of ClBcat.
atoms. The boron atom exhibits trigonal planar geometry with
the sum of the angles around B being 360.0(2)Њ for 1 and
360.1(2)Њ for 2. The B–O bond lengths, of 1.381(2) Å in both
cases, are unaffected by the change of halogen, showing that the
halogens contribute equally, and hence probably minimally, to
any π-bonding to the boron atom. An 11B{1H} NMR study of
ClBcat in dry CDCl3 solution at room temperature showed no
evidence of decomposition over a period of at least 8 weeks.
The reaction of 1 with 1.0 equivalents of each of the phos-
phines PMe3, PEt3, PMe2Ph and PBut3, yielded the R3PؒClBcat
adducts 3a, 3b, 3c and 3d respectively. The ambient temperature
31P{1H} NMR spectra of the adducts each displays a broad
peak, at a frequency considerably shifted from the position of
We have discussed elsewhere50 the adducts between BCl3 and
phosphines. Here, we present studies of the reaction of ClBcat
with a variety of phosphines, Et3N, and pyridines in order to
assess the stability of any adducts that form, and the nature and
occurrence of boron substituent redistribution processes.
Results and discussion
Single crystal structures of ClBcat 1 (Fig. 1) and BrBcat 2 (Fig.
2) have been obtained and the relevant bond distances and
angles are listed in Table 1. Crystals of 1 and 2 are isomorphous
and in each case the molecule is located on a two-fold axis in
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Table 2 Ambient and low temperature (Ϫ40 ЊC) 31P and 11B NMR data for 1 and 3a–e
31P shift (ppm) at
ambient temperature
31P shift (ppm)
at Ϫ40 ЊC
31P shift (ppm) of
free phosphine
11B shift (ppm) at
ambient temperature
11B shift (ppm) at
Ϫ40 ЊC
Compound
1
28.4
11.1
11.4
11.8
15.8
12.2
3a
3b
3c
3d
3e
Ϫ27.9
Ϫ24.9
Ϫ61.4
11.0
11.3
11.6
12.1
11.8
1.4
Ϫ29.8
25.5
6.8
Ϫ22.8
15.3
Ϫ19.9
Ϫ45.4
61.8
Ϫ3.3
Ϫ6.6
10.9
that of free phosphine (higher frequency for 3a, 3b and 3c,
lower for 3d), with no evidence of any coupling to boron
(Table 2). The absence of observed coupling, and the width of
the peak, compared to the sharp signals displayed by the free
phosphines, both point to the adduct being in equilibrium with
free ClBcat and phosphine at this temperature (Scheme 2).
Fig. 3 31P{1H} NMR spectra of 3a in toluene–10% C6D6 at ambient
temperature (top) and at Ϫ40 ЊC.
Scheme 2 Adduct formation from reaction of ClBcat and LR3.
The low temperature 31P{1H} NMR spectra of 3a, 3b and 3c,
all show a further shift to higher frequency with evidence of
P–B coupling, at Ϫ20 ЊC, and display an approximate
1 : 1 : 1 : 1 quartet at Ϫ40 ЊC, as expected for coupling to 11B
(I = 3/2), with JB–P = 175, 169 and 166 Hz for 3a, 3b and 3c,
respectively (Fig. 3). Coupling to 10B is not observed. Adduct
3d, however, shows no P–B coupling at Ϫ20 ЊC, but shifts
further to lower frequency and shows coupling at Ϫ40 ЊC
and below, with JB–P = 160 Hz, and with a splitting pattern
which is, approximately, a 1 : 2 : 2 : 1 quartet (Fig. 4). This,
it seems likely, is due to the partial thermal decoupling of the
boron quadrupole which results in the quartet collapsing towards
a singlet, with the two outside peaks decreasing in size, and the
two central peaks growing in intensity. The spectrum taken at
Ϫ60 ЊC shows a more exaggerated distortion from 1 : 1 : 1 : 1
and is consistent with the thermal decoupling of 11B.
Fig. 4 31P{1H} NMR spectrum of 3d in toluene–10% C6D6 at Ϫ40 ЊC.
The ambient temperature 11B{1H} NMR spectra of 3a, 3b, 3c
and 3d all display a peak considerably shifted to lower fre-
quency (Table 2) from the position of free ClBcat (δ 28.4) with,
as expected, no evidence of coupling to 31P. The low temper-
ature 11B{1H} NMR spectra at Ϫ40 ЊC, however, all show broad
doublets at a frequency shifted slightly further away from free
ClBcat (Fig. 5) and give B–P coupling constants of 178, 167,
166 and 158 Hz for 3a, 3b, 3c and 3d respectively.
Fig. 5 11B{1H} NMR spectra of 3a (bottom), 3b, 3c and 3d (top) in
toluene–10% C6D6 at Ϫ40 ЊC.
J. Chem. Soc., Dalton Trans., 2001, 1201–1209
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Fig. 6 Structure of one of the two crystallographically independent
molecules of Me3PؒClBcat, 3a.
Fig. 9 Structure of Cy3PؒClBcat 3e. Hydrogens on the Cy rings
omitted for clarity. Hollow bonds highlight distances which deviate
significantly from those in other adducts.
The increase in B–P bond lengths correlates with the decrease
in B–P coupling constants and the apparent stability of the
adducts. These trends are probably due to a compromise
between basicity,51 measured as the acidity of the conjugate
acid [HPR3]ϩ, and cone angle52 of the phosphine. For example,
the pKa of [HPBut3]ϩ is 11.4 vs. 8.65 for [HPMe3]ϩ. The cone
angle of PBut3, however, is 182 vs. 118Њ for PMe3, and so
Me3PؒClBcat contains the stronger B–P interaction. Similarly,
[HPEt3]ϩ has a pKa of 8.69, very close to that of [HPMe3]ϩ, but
a cone angle of 132Њ and so forms a weaker adduct than PMe3.
Although purely empirical, this does provide a good guide to
explaining the adduct stabilities, structural and spectroscopic
parameters.
Fig. 7 Structure of Et3PؒClBcat 3b.
As expected, the increase in coordination number at boron
from three to four with change in hybridisation from sp2 to sp3,
and the concomitant increase in steric repulsion, cause a
lengthening of the B–Cl and B–O bonds from those in 1. The
B–Cl bonds lengthen by ca. 10% from 1.744(3) Å in 1 to
1.914(2), 1.925(2) and 1.914(2) Å in 3a, 3b and 3d, respectively,
and the B–O bonds lengthen by ca. 6% [an average of 1.461(2)
Å]. Note also that O–B(pz) π-bonding should be minimal in the
four-coordinate adducts as B(pz) is now tied up in σ-bonding
to P. The boron atom is also ‘tipped’ out of the plane of the
catechol ring, by between ca. 0.11 and 0.20 Å (Table 1). The
B–P bond lengths in Table 1 are comparable with those in
the analogous R3PؒBCl3 adducts; for example the B–P bond
lengths in Me3PؒBCl3, Et3PؒBCl3, and But3PؒBCl3 are 1.957(5),
1.981(3) and 2.055(2) Å respectively.50
Fig. 8 Structure of But3PؒClBcat 3d. Hydrogens on the But groups are
omitted for clarity.
The 1 : 1 reaction of ClBcat with PCy3 yielded a mixture of
products, as indicated by 11B{1H} NMR spectroscopy, includ-
ing Cy3PؒClBcat 3e (δ 12.1), the redistribution products
Cy3PؒBCl3 (δ 3.7, d, JB–P = 147 Hz) and B2cat3 (δ 23.4) and
additional peaks at δ 21.9 and 11.1. In contrast, the 31P{1H}
NMR spectrum showed only a broad peak due to 3e at δ Ϫ3.3
which presumably obscures the signal due to the Cy3PؒBCl3
adduct (δ 3.2, q, JP–B = 147 Hz).50 A 1.1 : 1 reaction of PCy3 and
ClBcat 1 gave a small amount of a white precipitate of 3e,
recrystallisation of which yielded single crystals. The 11B{1H}
NMR spectrum showed that the crystals were 3e, but an
additional peak was observed at δ 22.2. The level of this
impurity changes, however, depending on concentration of the
sample and on the time in solution, so the formation of
the impurity would seem to take place in solution. The low
temperature 31P{1H} NMR spectrum of 3e showed some
coupling at Ϫ20 ЊC, and, at Ϫ40 ЊC, displayed a broad
1 : 2 : 2 : 1 quartet, similar to 3d, at δ Ϫ6.6, with JP–B = 159 Hz.
The low temperature 11B{1H} NMR spectrum at Ϫ40 ЊC
showed a doublet at δ 11.8 with JB–P = 160 Hz.
It is clear from the temperature dependence of the 31P and 11
B
shifts in Table 2, that the adducts 3a–e are mostly bound
in solution at ambient temperature. However, 3d shows
substantial shifts in both 31P and 11B resonances as a function
of temperature suggesting more dissociation than the others,
consistent with PBut3 having a particularly large cone angle
(vide infra).
3
It was noticed that the JH–P coupling constant in 3b was 16
Hz whereas that for 3d was 11 Hz. A low temperature 1H NMR
spectrum of 3d at Ϫ40 ЊC, however, gave 3JH–P = 15 Hz, with the
peak shifted by ca. 0.1 ppm to δ 1.65, further indicating that the
adducts are undergoing rapid exchange at room temperature.
There is also a change in coupling constant from that in the
free phosphine (3JH–P = 10 Hz), indicating that the increase in
coupling constant gives a measure of the association between
the phosphine and 1.
The single crystal structures of 3a, 3b and 3d have been
determined by X-ray diffraction; molecular structures are
shown in Figs. 6–8, with bond distances and angles provided in
Table 1. The B–P bond lengths for 3a, 3b and 3d are 1.967(2)
(average for the two independent molecules), 2.008(2) and
2.048(2) Å, respectively.
The molecular structure of 3e has been obtained (Fig. 9) and
comparison of the structures of the four adducts 3a, 3b, 3d and
3e, determined by X-ray crystallography, reveals that of 3e to be
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Table 3 Comparison of selected bond lengths (Å) and energies between experimental and optimised (HF/6-31G*) geometries of R3PؒClBcat
adducts
R
B–O
B–Cl
B–P
P–C
Eh/hartrees
∆Ea/kJ molϪ1
Me
Et
Exptl.
Theory
Exptl.
Theory
Exptl.
Theory
Exptl.
Theory
1.46, 1.46
1.44, 1.44
1.46, 1.46
1.44, 1.44
1.46, 1.47
1.44, 1.44
1.47, 1.41
1.44, 1.44
1.91
1.89
1.93
1.89
1.91
1.91
1.99
1.91
1.97
2.03
2.01
2.04
2.05
2.08
1.99
2.05
1.80, 1.80, 1.79
1.82, 1.82, 1.82
1.82, 1.82, 1.82
1.84, 1.83, 1.84
1.90, 1.90, 1.91
1.92, 1.92, 1.93
1.77, 1.87, 1.88
1.86, 1.86, 1.87
Ϫ1323.14176
Ϫ1323.14556
Ϫ1440.23448
Ϫ1440.24085
Ϫ1674.38484
Ϫ1674.38936
Ϫ1905.12893
Ϫ1905.16231
10.0 (7.5)
16.7 (11.3)
11.7 (6.7)
87.4 (46.0)
But
Cy
a Values (× 10Ϫ3) in parentheses are generated by ∆E/E(theory) for a more realistic comparison.
quite different from the others. Compared to the analogous
bond distances found in 3a, 3b and 3d, the most obvious dif-
ferences in the geometry of 3e are the B–O(2) bond, shortened
by 0.05 Å to 1.408(2) Å, the B–Cl bond, lengthened by 0.06 Å
to 1.995(2) Å, and the P–B bond length of 1.991(2) Å being
shorter than all but that in 3a. Additionally, P–C(20), at
1.768(2) Å, is over 0.1 Å shorter than P–C(10), at 1.884(2) Å,
and P–C(30), at 1.870(2) Å. There is also further distortion
throughout the catechol group, most significantly C(2)–O(2)
and C(1)–C(2) when compared to values obtained from the
other adducts, and the boron is further ‘tipped’ out of the plane
of the catechol group by 0.25 Å.
In an attempt to understand why 3e has a distorted molecular
structure in the solid state relative to the other adducts, geom-
etry optimisations (at the HF/6-31G* level with Gaussian 9453)
from the X-ray structural data of all four adducts were carried
out. Table 3 contains selected bond lengths of experimental and
optimised ‘gas-phase’ geometries for these adducts, and these
values show very good agreement in all cases except 3e. Energy
differences between experimental (with their hydrogens opti-
mised as they are not located accurately by X-ray crystal-
lography) and optimised geometries in these adducts reveal a
relatively large value of 87 kJ molϪ1 for 3e compared with much
smaller values of ca. 10–17 kJ molϪ1 for 3a, 3b and 3d. Even
allowing for the fact that the total energy of 3e is larger than
those of the other adducts, the energy difference for 3e is
substantial. We find no obvious intramolecular reasons for the
distortions observed for 3e in the solid state.
Crystal packing forces must therefore account for a large part
Scheme 3 Equation and proposed mechanism for boron substituent
redistribution of R3PؒClBcat in the presence of an excess of ClBcat.
of the distortion and the associated energy difference computed
theoretically between the optimised and observed geometries.
Examination of the crystal structure of 3e revealed close
Cl ؒ ؒ ؒ H intermolecular distances of 2.9 Å involving cyclohexyl
hydrogens. A Cambridge Structural Database Search54 for
B–Cl ؒ ؒ ؒ H–C interactions gave an average distance of 3.4 Å
from 166 compounds, very few being shorter than 2.9 Å. Thus,
intermolecular packing forces appear to be responsible for the
observed differences in the molecular structure in the crystal of
the PCy3 adduct compared to its optimised ‘gas-phase’ geom-
etry. The experimental solid state structures of the other
adducts are largely unaffected by packing forces, as they cor-
respond very well with their optimised ‘gas-phase’ geometries,
obtained from the ab initio molecular orbital calculations.
The 1 : 1 reactions of 1 with the phosphines PPh2Me and
PPh3 both yielded the redistribution products B2cat3 4 and
R3PؒBCl3 5f and 5g, as shown in Scheme 3. The reactions were
not instantaneous, however, and the 11B{1H} NMR spectrum of
each, when taken after 1 h, showed B2cat3, a doublet due to 5f
or 5g, and another peak due to unreacted 1. This peak was
shifted by ca. 0.5 ppm to lower frequency from the expected
position of free 1, most likely indicating a very weak interaction
between phosphine and 1. When left for 24 h, the 11B{1H}
NMR spectra showed only peaks due to B2cat3 4 and 5f or 5g.
In these two cases, it seems likely that R3PؒClBcat adducts do
form but that the interactions are very weak due predominantly
to low phosphine basicity (PPh2Me, pKa = 4.57, PPh3, pKa =
2.73), and the large cone angle (PPh2Me = 136Њ, PPh3 = 145Њ).
Interestingly, even for those R3PؒClBcat adducts that were
isolated, boron substituent redistribution took place over a
period of several weeks in the solid state at Ϫ30 ЊC. It is clear,
therefore, that the thermodynamic products are 4 and
R3PؒBCl3, with the phosphine adducts of 1 being only kinetic-
ally stable. The mechanism shown in Scheme 3 is proposed for
the redistribution reaction. In this mechanism, a molecule of
1 with coordinated phosphine reacts with an uncoordinated
molecule of 1, first forming an intermediate, 6, which then
reacts with a further uncoordinated molecule of 1 to form 4 and
the R3PؒBCl3 adduct 5.
This mechanism is based on the fact that once a phosphine
is bound to ClBcat, the oxygen atoms, having lost the ability to
π-bond to boron, become nucleophilic. Attack at the electro-
philic boron of an uncoordinated ClBcat leads to ring opening
of the original BO2 moiety and subsequent transfer of a ClϪ
between the two boron centres. Note that the electrophilic
attack of ClBcat on an oxygen of the R3PؒClBcat adduct with
ring opening is analogous to the known ability of ClBcat to
cleave C–O bonds in ethers. 55
As the mechanism depends on there being an excess of 1 with
respect to PR3, we presumed that the reaction of all of the
J. Chem. Soc., Dalton Trans., 2001, 1201–1209
1205

View Article Online
phosphines with an excess of 1 would cause the redistribution
to take place. In order to investigate this, each of the phos-
phines was reacted with a five-fold excess of 1. The reactions
were monitored by in situ 11B{1H} NMR spectroscopy and
all showed peaks due to 1 and 4, and doublets due to 5a–g.
Additionally, 1 h after treatment of ClBcat with PMe3 and
PEt3, doublets were observed at δ 4.0 (JB–P = 183 Hz) and δ 4.1
(JB–P = 175 Hz) respectively. We believe that these additional
low frequency resonances, which disappear after 24 h, may be
due to the four-coordinate boron intermediates 6a and 6b,
shown in Scheme 3. The resonance for the three-coordinate
boron is likely to be superimposed on that for B2cat3. The
enhanced stability for such a species as an intermediate for
PMe3 and PEt3 may be due to the smaller size of these
phosphines.
Since the boron substituent redistribution yields B2cat3, we
postulated that the complicated 11B{1H} NMR spectrum
obtained for the reaction of 1 with PCy3 was the result of some
interaction between PCy3 and B2cat3 4. Six reactions were con-
ducted between B2cat3 and 0.1, 1, and 2 equivalents of either
PEt3 or PCy3. The in situ 11B{1H} NMR spectra of the reactions
using PEt3 showed a shift of approximately 2 ppm to lower
frequency from the expected position of free 4, indicating that
some interaction between PEt3 and 4 is taking place. This is
confirmed by the 31P{1H} NMR spectra, which show broad
singlets shifted by ca. 20 ppm to higher frequency from the
position of free PEt3. The NMR spectra of the reactions with
PCy3, however, clearly show that a reaction is taking place. The
11B{1H} NMR spectra show the formation of a sharp peak at
δ 15.0, consistent with the [Bcat2]Ϫ anion, a sharp peak at
δ Ϫ14.8, and also other, as yet unassigned peaks, which are
consistent with those observed for the reaction between PCy3
and 4. Removal of the solvent from the 2 : 1 PCy3 : B2cat3
reaction followed by recrystallisation yielded single crystals, the
X-ray diffraction analysis of which, though there was disorder,
clearly showed the [Bcat2]Ϫ anion and the [HPCy3]ϩ phos-
phonium cation.56 No other products were isolated from any of
the reactions with PCy3. The behaviour of PCy3 is unique both
in terms of reactivity and structure of the ClBcat adduct; the
reasons for this are still elusive. However, it would appear that
PCy3 is sufficiently nucleophilic to displace [Bcat2]Ϫ from B2cat3.
To investigate the effect of nitrogen σ-donors, reactions of 1
with triethylamine, pyridine, 2-picoline and 4-picoline were car-
ried out. The in situ 11B{1H} NMR spectra all showed broad
peaks, shifted to lower frequency from the position of free 1.
The reactions all proceed cleanly, and no evidence of any
decomposition was observed. Isolation of the pyridine and
picoline adducts as solids proved difficult, as they tend to form
oils, but removal of the solvent from the reaction with triethyl-
amine and recrystallisation yielded single crystals of Et3NؒClB-
cat 3h suitable for X-ray structural analysis.
Fig. 10 Structure of Et3NؒClBcat 3h. Hydrogens on the Et groups
omitted for clarity.
free 1 in solution. The less associated adducts derived from
the less basic phosphines provide a larger concentration of 1
through dissociation leading to more rapid redistribution. Even
those phosphine adducts that were observed or isolated were
only kinetically stable, as redistribution took place slowly even
in the solid state at Ϫ30 ЊC. In the case of PCy3, the phosphine
reacts with the B2cat3 formed by the redistribution reaction.
The presence of an excess of ClBcat relative to any of the
phosphines causes the rapid boron substituent redistribution.
The nitrogen σ-donors formed stable adducts with ClBcat
with no redistribution observed, either in the solid state or in
solution, over an extended period of time.
Oxidative addition of ClBcat to low-valent, late transition
metal phosphine complexes is known.46 However, if labile
phosphine ligands are employed in a catalytic process, reaction
according to Scheme 3 is to be expected. This may, in fact, be
valuable, in so far as it would inhibit re-coordination of the
phosphine to the metal centre, with BCl3 serving as an in situ
phosphine sponge. It will be important to show that B2cat3 and
R3PؒBCl3 will not otherwise interfere with any catalytic process.
Strong binding of amine bases to ClBcat may serve to inhibit
phosphine-promoted redistribution reactions. Examination of
the oxidative addition of the B–Cl bond in amine adducts such
as 3h to low-valent late transition metals will be the basis for
subsequent reports.
Experimental
Air-sensitive compounds were manipulated in a nitrogen
atmosphere using Schlenk techniques or in Innovative Tech-
nology, Inc. System 1 glove boxes. NMR spectra (at room
temperature except where stated) were recorded on Bruker AC
200 (13C), Varian Mercury 200 (1H, 31P), Varian Unity 300 (1H,
11B) and Varian Unity Inova 500 (11B and VT 11B) instruments.
Variable temperature 31P NMR experiments were conducted on
a Varian VXR-400 instrument. Proton and 13C NMR spectra
were referenced to external SiMe4 via residual protons in the
deuterated solvents or solvent resonances respectively; 11B and
31P shifts were referenced to external BF3ؒOEt2 and 85% H3PO4,
respectively. Elemental analyses were conducted in the Depart-
ment of Chemistry at the University of Durham using an
Exeter Analytical Inc. CE-440 Elemental Analyzer. Toluene
was dried and deoxygenated by passage through columns of
activated alumina and BASF-R311 catalyst under argon
pressure using a modified version of the Innovative Technology
Inc. SPS-400 solvent purification system.57 Chloroform and
CDCl3 were dried over calcium hydride, C6D6 was dried over
potassium metal, and all were distilled under nitrogen. The
B-chlorocatechol borane (Aldrich) was sublimed under vacuum
before use. Phosphines were purchased from Aldrich or Strem
Chemicals Inc. and were checked for purity by 1H and 31P{1H}
The molecular structure of 3h (Fig. 10) shows a B–Cl bond
length of 1.876(1) Å which is shorter than the B–Cl bond
lengths in any of the phosphine adducts, the shortest of which
is 1.911(2) Å in 3a. This is most probably due to a combination
of the smaller size of nitrogen vs. phosphorus, reducing steric
interactions, and the larger electronegativity of nitrogen. The
B–N bond of 1.643(1) Å is considerably shorter than the B–P
bond lengths in any of the adducts, as expected, again due to
the smaller size of nitrogen.
In the case of 3h, NMR analysis of the adduct kept both in
solution and in the solid state showed no redistribution to
Et3NؒBCl3 and 4, so this adduct appears to be thermally stable.
Conclusion
Phosphine adducts of ClBcat can be isolated only if the phos-
phine is of sufficient basicity, the less basic phosphines used
causing the rapid redistribution of ClBcat to B2cat3 and the
R3PؒBCl3 adduct. The redistribution depends on there being
1206
J. Chem. Soc., Dalton Trans., 2001, 1201–1209

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NMR spectroscopy before use. Triethylamine, pyridine and
2- and 4-picoline were purchased from Lancaster and were
distilled from calcium hydride.
filtration and was recrystallised from toluene layered with
hexane to yield a white solid (0.129 g). 11B{1H} NMR (PhMe–
10% C6D6): δ 23.4 (B2cat3), 21.4, 12.1 (Cy3PؒClBcat), 11.1, 3.7
(d, JB–P = 147 Hz, Cy3PؒBCl3); 31P{1H} NMR (PhMe–10%
C6D6): δ Ϫ3.3 (Cy3PؒClBcat).
Preparation of Me3PؒClBcat 3a
A solution of PMe3 (0.030 g, 0.389 mmol) in toluene (1 cm3)
was added to a solution of ClBcat (0.061 g, 0.389 mmol) in
toluene (1 cm3) and the mixture was stirred for 1 h. The solvent
was removed in vacuo to yield 0.081 g of a white powder. The
product was recrystallised from a minimum amount of toluene
Synthesis of Cy3PؒClBcat 3e
A 2 mol% excess of PCy3 (0.093 g, 0.330 mmol) in toluene
(1 cm3) was added to a solution of ClBcat (0.050 g, 0.324 mmol)
in toluene (1 cm3), giving a white precipitate which was col-
lected by filtration and recrystallised from toluene layered with
hexane, yielding white crystals (0.038 g) from which the single
crystal for X-ray analysis was obtained. 11B{1H} NMR (PhMe–
10% C6D6): δ 22.2, 12.2 (Cy3PؒClBcat); (at Ϫ40 ЊC): δ 11.8 (d,
JB–P = 160 Hz); 31P{1H} NMR (PhMe–10% C6D6): δ Ϫ3.3
(Cy3PؒClBcat); (at Ϫ40 ЊC): δ Ϫ6.6, (q, JP–B = 159 Hz); 13C{1H}
NMR (C6D6): δ 149.8 (s, 1,2-O2C6H4), 119.8 (s, 1,2-O2C6H4),
110.4 (s, 1,2-O2C6H4), 30.1 (d, P(C6H11)3, JC–P = 26 Hz), 28.0
(s, P(C6H11)3), 27.2 (d, P(C6H11)3, JC–P = 11 Hz), 25.8 (s,
P(C6H11)3).
1
to yield 0.078 g (87%) of a white, crystalline solid. H NMR
(CDCl3): δ 6.79 (m, 4H, 1,2-O2C6H4), 1.51 (d, 9H, P(CH3)3,
JH–P = 11 Hz); 11B{1H} NMR (PhMe–10% C6D6): δ 11.1 (s); (at
Ϫ40 ЊC): δ 11.0 (d, JB–P = 178 Hz); 31P{1H} NMR (PhMe–10%
C6D6): δ Ϫ27.9 (s); (at Ϫ40 ЊC): δ Ϫ24.9 (q, JP–B = 175 Hz);
13C{1H} NMR (CDCl3): δ 150.5 (s, 1,2-O2C6H4), 119.9 (s,
1,2-O2C6H4), 110.3 (s, 1,2-O2C6H4), 9.4 (s, br, P(CH3)3) (Found:
C, 46.89; H, 5.78. C9H13O2ClBP requires C, 46.91; H, 5.69%).
Preparation of Et3PؒClBcat 3b
As for 3a using PEt3 (0.038 g, 0.324 mmol) and ClBcat (0.050 g,
0.324 mmol). Yield 0.068 g (78%). H NMR (CDCl3): δ 6.78
1
Reaction of ClBcat with PPh2Me
A solution of ClBcat (0.050 g, 0.324 mmol) in toluene (1 cm3)
was added to a solution of PPh2Me (0.065 g, 0.324 mmol) in
toluene (1 cm3) which was then stirred for 1 h. Removal of
solvent yielded a white precipitate (0.094 g). 11B{1H} NMR
(PhMe–10% C6D6): δ 27.9 (Ph2MePؒClBcat), 23.2 (B2cat3), 4.3
(d, JB–P = 153 Hz, Ph2MePؒBCl3); 31P{1H} NMR (PhMe–10%
C6D6): δ Ϫ7.3 (q, JP–B = 155 Hz, Ph2MePؒBCl3).
(m, 4H, 1,2-O2C6H4), 1.89 (dq, 6H, P(CH2CH3)3, JH–P = 11 Hz,
JH–H = 8 Hz,), 1.22 (dt, 9H, P(CH2CH3)3, JH–P = 16 Hz, JH–H = 8
Hz); 11B{1H} (PhMe–10% C6D6) δ 11.4 (s); (at Ϫ40 ЊC): δ 11.3
(d, JB–P = 167 Hz); 31P{1H} NMR (PhMe–10% C6D6): δ 1.4 (s);
(at Ϫ40 ЊC) δ 6.8 (q, JP–B = 169 Hz); 13C{1H} NMR (CDCl3):
δ 150.2 (s, 1,2-O2C6H4), 119.8 (s, 1,2-C6H4), 110.3 (s, 1,2-O2C6-
H4), 10.5 (d, P(CH2CH3)3, JC–P = 34 Hz), 6.4 (d, P(CH2CH3)3,
JC–P = 5 Hz) (Found: C, 52.60; H, 7.09. C12H19O2ClBP requires
C, 52.89; H, 7.03%).
Reaction of ClBcat with PPh3
Preparation of Me2PhPؒClBcat 3c
A solution of ClBcat (0.050 g, 0.324 mmol) in toluene (1 cm3)
was added to a solution of PPh3 (0.084 g, 0.324 mmol) in tolu-
ene (1 cm3) which was then stirred for 1 h. Removal of solvent
yielded a white precipitate (0.118 g). 11B{1H} NMR (PhMe–
10% C6D6): δ 27.9 (s, Ph3PؒClBcat), 23.1 (br s, B2cat3), 4.4 (d,
JB–P = 152 Hz, Ph3PؒBCl3); 31P{1H} NMR (PhMe–10% C6D6):
δ 2.0 (q, JP–B = 152 Hz, Ph3PؒBCl3).
As for 3a using PMe2Ph (0.045 g, 0.324 mmol) and ClBcat
(0.050 g, 0.324 mmol). Yield 0.075 g (79%). 1H NMR (CDCl3):
δ 7.65 (m, 2H, PMe2(C6H5)), 7.46 (m, 3H, PMe2(C6H5)), 6.83
(m, 2H, 1,2-O2C6H4), δ 6.74 (m, 2H, 1,2-O2C6H4) δ 1.51 (d, 6H,
P(CH3)2Ph, JP–H = 9 Hz); 11B{1H} NMR (PhMe–10% C6D6):
δ 11.8 (s); (at Ϫ40 ЊC): δ 11.6 (d, JB–P = 166 Hz); 31P{1H} NMR
(PhMe–10% C6D6) δ Ϫ29.8 (s), (at Ϫ40 ЊC): δ Ϫ22.8 (q,
JP–B = 166 Hz); 13C NMR (CDCl3): δ 150.5 (s, 1,2-O2C6H5),
131.5 (s, P(CH3)2Ph), 131.1 (d, P(CH3)2(C6H5), JC–P = 11 Hz),
129.1 (d, P(CH3)2(C6H5), JC–P = 11 Hz), 120.0 (s, 1,2-O2C6H4),
110.4 (s, 1,2-O2C6H4), 7.9 (d, P(CH3)2Ph, JC–P = 29 Hz), the
resonances of the ipso carbon of P(CH3)2(C6H5) were not
observed (Found: C, 57.27; H, 5.39. C14H15O2ClBP requires C,
57.49; H, 5.17%).
Preparation of Et3NؒClBcat 3h
A solution of NEt3 (0.033 g, 0.324 mmol) in CHCl3 (1 cm3) was
reacted with ClBcat (0.050 g, 0.324 mmol) in CHCl3 (1 cm3)
and the mixture was stirred for 1 h. The solvent was removed
in vacuo leaving a white powder which was recrystallised
from a minimum amount of CHCl3 to yield 0.072 g (87%) of a
1
crystalline white solid. H NMR (CDCl3): δ 6.89 (m, 4H, 1,2-
O2C6H4), 2.56 (q, 6H, N(CH2CH3)3, JH–H = 8 Hz), 0.71 (t, 9H,
N(CH2CH3)3, JH–H = 8 Hz); 11B{1H) NMR (CDCl3): δ 13.3 (s)
(Found: C, 56.69; H, 7.59; N, 5.56. C12H19BClNO2 requires C,
56.40; H, 7.49, N, 5.48%).
Preparation of But3PؒClBcat 3d
As for 3a using PBut3 (0.066 g, 0.324 mmol) and ClBcat (0.050
g, 0.324 mmol). The product was recrystallised from toluene
layered with hexane to yield 0.095 g (82%) of a crystalline white
solid. 1H NMR (C6D6): δ 6.87 (m, 4H, 1,2-O2C6H4), 1.29
(d, 27H, P(C(CH3)3), JH–P = 11 Hz); (at Ϫ40 ЊC): δ 1.65 (d,
JH–P = 15 Hz); 11B{1H} NMR (PhMe–10% C6D6): δ 15.8 (s); (at
Ϫ40 ЊC) δ 12.1 (d, JB–P = 158 Hz); 31P{1H} NMR (PhMe–10%
C6D6): δ 25.5 (s); (at Ϫ40 ЊC) δ 15.3 (q, JP–B = 160 Hz); 13C{1H}
NMR (CDCl3): δ 120.3 (s, 1,2-O2C6H4), 110.8 (s, 1,2-O2C6H4),
36.9 (d, P(C(CH3)3)3, JC–P = 27 Hz), 30.1 (s, P(C(CH3)3)3), the
resonances of C1 and C2 of 1,2-O2C6H4 were not observed)
(Found: C, 60.34; H, 8.78. C18H31O2ClBP requires C, 60.61;
H, 8.76%).
Preparation of C5H5NؒClBcat
A solution of C6H5N (0.025 g, 0.324 mmol) in CHCl3 (1 cm3)
was reacted with ClBcat (0.050 g, 0.324 mmol) in CHCl3 (1 cm3)
and the mixture was stirred for 1 h. 11B{1H} NMR (in situ
CHCl3–CDCl3): δ 11.8 (C5H5NؒClBcat).
Preparation of 2-MeC5H4NؒClBcat
As for C5H5NؒClBcat using 2-MeC5H4N (0.030 g, 0.324 mmol)
in CHCl3 (1 cm3). 11B{1H} NMR (in situ CHCl3–CDCl3): δ 11.7
(2-CH3C5H4NؒClBcat).
Reaction of ClBcat with PCy3
Preparation of 4-MeC5H4NؒClBcat
A solution of PCy3 (0.091 g, 0.321 mmol) in toluene (1 cm3) was
added to a solution of ClBcat (0.050 g, 0.321 mmol) in toluene
(1 cm3) and the mixture was stirred for 1 h, by which time a
white precipitate had formed. The precipitate was collected by
As for C5H5NؒClBcat using 4-MeC5H4N (0.030 g, 0.324 mmol)
in CHCl3 (1 cm3). 11B{1H} NMR (in situ CHCl3–CDCl3): δ 11.7
(4-MeC5H4NؒClBcat).
J. Chem. Soc., Dalton Trans., 2001, 1201–1209
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Table 4 Crystal data and structure refinement for ClBcat, BrBcat and the R3P and Et3N adducts of ClBcat
ClBcat
1
BrBCat
2
Me3PؒClBcat
3a
Et3PؒClBcat
3b
But3PؒClBcat
3d
Cy3PؒClBcat
3e
Et3NؒClBcat
3h
Formula
M
T/K
C6H4BClO2 C6H4BBrO2
C9H13BClO2P C12H19BClO2P C18H31BClO2P C24H37BClO2P
C12H19BClO2N
255.54
100(2)
154.35
120(2)
198.81
120(2)
230.42
272.50
200(2)
356.66
120(2)
434.77
150(2)
160(2)
Crystal system
Space group
a/Å
b/Å
Monoclinic Monoclinic
Monoclinic
P21/c
Orthorhombic Orthorhombic Monoclinic
Orthorhombic
P212121
6.6037(3)
13.3458(7)
15.0557(7)
90
1326.88(11)
4
0.277
C2/c
C2/c
Pbca
Pna21
16.664(1)
13.455(5)
8.546(1)
90
1916.1(8)
4
P21/n
10.5584(8)
9.8180(8)
7.1370(5)
118.75(1)
648.62(9)
4
10.7250(4)
10.0682(4)
7.2078(3)
119.53(1)
677.2(2)
4
12.2244(10)
16.3667(14)
12.7944(11)
117.195(2)
2276.8(3)
8
13.4416(12)
14.0419(12)
14.6040(12)
90
2756.4(4)
8
11.541(2)
13.426(3)
15.070(3)
92.226(3)
2333.3(8)
4
c/Å
β/Њ
U/Å3
Z
µ/mmϪ1
0.506
5.992
0.447
0.380
0.289
0.250
Reflections measured
Unique reflections
Rint
2888
753
0.0319
0.0950
0.0345
3032
914
0.0250
0.0637
0.0243
14114
5258
0.0349
0.0842
27702
3169
0.1318
0.1059
0.0393
14595
4570
0.0242
0.0598
0.0257
28034
6574
0.0403
0.1055
0.0395
15865
3762
0.0152
0.0640
wR2 (all data, on F2)
R[data with F2 > 2σ(F2)]
0.0317
0.0234
2 A. H. Hoveyda, D. A. Evans and G. C. Fu, Chem. Rev., 1993, 93,
1307.
3 I. Beletskaya and A. Pelter, Tetrahedron, 1997, 53, 4957.
4 T. B. Marder and N. C. Norman, Top. Catal., 1998, 5, 63.
5 T. Ishiyama and N. Miyaura, J. Organomet. Chem., 2000, 611,
392.
6 J. M. Brown and G. C. Lloyd-Jones, J. Chem. Soc., Chem. Commun.,
1992, 710.
7 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.
8 S. A. Westcott, T. B. Marder and R. T. Baker, Organometallics, 1993,
12, 975.
9 J. M. Brown and G. C. Lloyd-Jones, J. Am. Chem. Soc., 1994, 116,
866.
10 R. T. Baker, J. C. Calabrese, S. A. Westcott and T. B. Marder, J. Am.
Chem. Soc., 1995, 117, 8777.
11 D. H. Motry and M. R. Smith, III, J. Am. Chem. Soc., 1995, 117,
6615.
12 D. R. Lantero, D. L. Ward and M. R. Smith, III, J. Am. Chem. Soc.,
1997, 119, 9699.
Computational study
Ab initio computations on R3PؒClBcat adducts were carried out
with the Gaussian 94 package.53 The experimental geometries
were fully optimised initially at the HF/3-21G level and then at
the HF/6-31G* level with no symmetry constraints. Frequency
calculations were computed on these optimised geometries at
the HF/6-31G* level and no imaginary frequencies were found.
For single point energies of the experimental geometries in
Table 3, the hydrogens were first optimised at the HF/6-31G*
level.
Crystallography
X-Ray diffraction experiments were carried out on a Bruker
SMART three-circle diffractometer with a 1K CCD area
detector, using graphite-monochromated Mo-Kα radiation
¯
(λ = 0.71073 Å) and a Cryostream (Oxford Cryosystems)
open-flow N2 gas cryostat. At least a hemisphere of data was
collected in each case. Semi-empirical absorption corrections,
by comparison of Laue equivalents, were applied (except for 3b)
using SADABS or XPREP programs.58,59 The structures were
solved by direct methods and refined by full-matrix least
squares against F2 of all data, using SHELXTL software.59
All non-H atoms were refined in anisotropic, H atoms in iso-
tropic approximation. Compound 3a crystallised with two
independent molecules in the asymmetric unit which differ little
in their geometries. Crystal data and experimental details are
summarised in Table 4.
13 D. H. Motry, A. G. Brazil and M. R. Smith, III, J. Am. Chem. Soc.,
1997, 119, 2743.
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7508.
CCDC reference numbers 154920–154926.
See http://www.rsc.org/suppdata/dt/b0/b010025k/ for crys-
tallographic data in CIF or other electronic format.
Acknowledgements
We acknowledge EPSRC for research support (T. B. M. and
J. A. K. H.), a Senior Research Fellowship (J. A. K. H.), an
Advanced Research Fellowship (M. A. F.), an equipment grant
(W. C.), and postgraduate studentships (R. B. C. and A. J. S.).
We also acknowledge NSERC (Canada) for research support
(T. B. M.) and a Postgraduate Fellowship (F. E. S. S.). We thank
Prof. R. K. Harris and Dr A. M. Kenwright for helpful discus-
sions on interpretation of some of the NMR spectra, Mr I. H.
McKeag and Mrs C. F. Heffernan for assistance with the
acquisition of low temperature NMR spectra, and Mrs J.
Dostal for the elemental analyses.
26 M. Murata, S. Watanabe and Y. Masuda, J. Org. Chem., 1997, 62,
6458.
27 S. R. Piettre and S. Baltzer, Tetrahedron Lett., 1997, 38, 1197.
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29 A. Suzuki, J. Organomet. Chem., 1999, 576, 147.
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33 M. Murata, T. Oyama, S. Watanabe and Y. Masuda, J. Org. Chem.,
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