Synthesis and Lewis acidity of triorganosilyl and triorganostannyl esters of orthoboric, metaboric, and arylboronic acids

Article abstract of DOI:10.1016/S0022-328X(99)00352-6

The synthesis of 30 new triorganosilyl and triorganostannyl esters of orthoboric, metaboric, and arylboronic acids are described: (R₃SiO)₃B {R₃=ⁿBuMe₂, PhMe₂, [2-(3-cyclohexenyl)ethyl]Me₂, (ⁿOctadecyl)Me₂, ⁿHex₃}; (R₃MO)₃B₃O₃ {M=Si: R₃=ⁿPr₃, ⁿBu₃, ⁿHex₃, EtMe₂, ⁿBuMe₂, [2-(3-cyclohexenyl)ethyl]Me₂, [(Me₂HC)(Me)₂C]Me₂, (ⁿOctadecyl)Me₂, PhMe₂, Ph₂Me, Ph₃. M=Sn, R=Ph, Me}; (R₃MO)₂BAr {M=Si: Ar=Ph, R₃=ⁿPr₃, ⁿBu₃, Ph₂Me; Ar=C₆H₄Br-4, R=Ph, ⁿPr; Ar=C₆H₄Me-2, R=Ph, ⁿPr. M=Sn: R=Ph, Ar=Ph, C₆H₄Br-4, C₆H₄Me-2; R=ⁿBu, Ar=Ph, C₆H₄Br-4}. The esters were characterised by elemental analysis, m.p./b.p., ¹H-, ¹³C-, and ¹¹B-NMR and IR spectroscopy; solids were characterised by solid-state ¹¹B MAS NMR spectroscopy. The Lewis acidity of liquid compounds was determined by Gutmann's acceptor number (AN) method and well-defined ranges were observed for the compound classes {(R₃MO)₃B, 23-36; (R₃MO)₂BAr, 23-31; (R₃MO)₃B₃O₃, 44-81}. The effect of the R₃Si or R₃Sn group was to enhance the Lewis acidity at boron relative to organic esters. Increasing the steric bulk of the R₃M moiety significantly reduced AN values and there was little differentiation in AN values between R₃Si and R₃Sn derivatives.

Full text of DOI:10.1016/S0022-328X(99)00352-6

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 588 (1999) 107–112
Synthesis and Lewis acidity of triorganosilyl and triorganostannyl
esters of orthoboric, metaboric, and arylboronic acids
Michael A. Beckett a,*, Paul Owen a, K. Sukumar Varma b
a Chemistry Department, Uni6ersity of Wales, Bangor, Gwynedd, LL57 2UW, Wales, UK
b Pilkington Group Research, Pilkington Technology Centre-Europe, Lathom, Lancashire, L40 5UF, UK
Received 4 May 1999
Abstract
The synthesis of 30 new triorganosilyl and triorganostannyl esters of orthoboric, metaboric, and arylboronic acids are described:
(R3SiO)3B {R3=nBuMe2, PhMe2, [2-(3-cyclohexenyl)ethyl]Me2, (nOctadecyl)Me2, nHex3}; (R3MO)3B3O3 {M=Si: R3=nPr3,
nBu3, nHex3, EtMe2, nBuMe2, [2-(3-cyclohexenyl)ethyl]Me2, [(Me2HC)(Me)2C]Me2, (nOctadecyl)Me2, PhMe2, Ph2Me, Ph3. M=Sn,
n
n
R=Ph, Me}; (R3MO)2BAr {M=Si: Ar=Ph, R3=nPr3, Bu3, Ph2Me; Ar=C6H4Br-4, R=Ph, Pr; Ar=C6H4Me-2, R=Ph,
nPr. M=Sn: R=Ph, Ar=Ph, C6H4Br-4, C6H4Me-2; R=nBu, Ar=Ph, C6H4Br-4}. The esters were characterised by elemental
analysis, m.p./b.p., 1H-, 13C-, and 11B-NMR and IR spectroscopy; solids were characterised by solid-state 11B MAS NMR
spectroscopy. The Lewis acidity of liquid compounds was determined by Gutmann’s acceptor number (AN) method and
well-defined ranges were observed for the compound classes {(R3MO)3B, 23–36; (R3MO)2BAr, 23–31; (R3MO)3B3O3, 44–81}.
The effect of the R3Si or R3Sn group was to enhance the Lewis acidity at boron relative to organic esters. Increasing the steric
bulk of the R3M moiety significantly reduced AN values and there was little differentiation in AN values between R3Si and R3Sn
derivatives. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Acceptor number; Borate esters; Lewis acidity; Group 4 elements
or more B–O–M (M=Si, Sn) linkages and assessing
the effect of Si or Sn atom on Lewis acidity at B.
Compounds containing the B–O–M (M=Si, Sn) link-
age are represented in the literature [3–13] as the
triorganosilyl and triorganostannyl esters of orthoboric
(structure A), metaboric (structure B), and arylboronic
(structure C) acids; other compounds containing this
structural motif are described in Ref. [9].
1. Introduction
Lewis acidity plays an important role in stoichiomet-
ric and catalytic reactions and trivalent boron com-
pounds with a sextet of electrons around the boron are
considered archetypal Lewis acids [1]. Information con-
cerning the Lewis acidity of particular compounds and
the subtle factors that affect Lewis acidity are of inter-
est to academics and industrialists alike, since knowl-
edge of the Lewis acidity of a particular compound
allows an assessment of that compound to be made
with regard to its ability to function as a Lewis acid in
both laboratory and industrial settings. Thus, for exam-
ple, epoxide resins may be cured by the addition of BF3
or (MeO)3B3O3, and the rate of the Lewis acid initiated
polymerisation of the epoxide monomer phenylglycidyl
ether (PGE) has been correlated with the acid strength
of the initiator [2]. We have become interested in deter-
mining the Lewis acidity of compounds containing one
This paper reports on the synthesis and characterisa-
tion of series of new and previously reported derivatives
containing B–O–Si or B–O–Sn links. The Lewis acid-
ity of liquid products have been estimated by Gut-
mann’s acceptor number (AN) method [14], and the
effects of Si and Sn are noted and discussed.
* Corresponding author.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00352-6

108
M.A. Beckett et al. / Journal of Organometallic Chemistry 588 (1999) 107–112
2. Results and discussion
2.1. Synthesis and characterisation
(R3SiO)3B3O3; l 25.9–27.9, ArB(OSiR3)2; chemical
shifts of 15.1 (CCl4) and 19.6 (CCl4) have been reported
previously [15] for (Me3SiO)3B and (Me3SiO)3B3O3,
respectively. The increased shielding at boron of 2–3
ppm for (Me3SiO)3B relative to organic esters derived
from primary or secondary alcohols can be seen to quite
general for all the (R3SiO)3B derivatives and this has
been attributed to steric interactions inducing a higher
probabilty of favourable conformations for B–O p-
bonding [16]. Fewer R3Sn esters were prepared, but the
solution l(11B) data obtained suggest a slight deshield-
ing of the B nuclei for the orthoborate and metaborate
esters relative to the R3Si derivatives. The arylboronic
triorganostannyl acid esters (35–39) had resonances
within the range l 18.2–22.1 and the boron nuclei were
significantly more shielded (Dl ca. 7 ppm) than those in
the R3Si esters and in most other arylboronic acids
derivatives [15]. Solid state 11B-MAS-NMR data were
also obtained on most solid samples and these are
reported in Table 2. All spectra were characterised by a
broad complex signal that was simulated as described
previously [17,18]; the liso values so obtained were in
good agreement with the solution l values discussed
above and confirmed the relative upfield shifts for the B
nuclei of ArB(OSnR3)2 in relation to ArB(OSiR3)2
Silyl and stannyl esters of orthoboric, metaboric, and
arylboronic acids are conveniently prepared by the
azeotropic removal of water from the corresponding
mixtures of boron acid and triorganyosilanol or trior-
ganyostannyl derivatives [3] and the previously reported
compounds (R3MO)3B {M=Si: R3=Et3 [4–8] (1), nPr3
n
[6] (2), Bu3 [7] (3), Ph3 [4,9] (4). M=Sn: R3=nBu3 [10]
(5), Ph3 [10] (6)}, (Et3SiO)3B3O3 [8] (7), (nBu3SnO)3B3O3
[10] (8), and (Ph3SiO)2BPh [9] (9) were prepared by these
methods. Similarly, the new triorganosilyl esters of
orthoboric acid (10–14), metaboric acid (15–25), and
arylboronic acids (26–32) were prepared from the tri-
organosilanol as shown in Eqs. (1)–(3) and the tri-
organostannyl esters of metaboric acid (33, 34) and
arylboronic acids (35–39) were synthesised as shown in
Eqs. (2)–(4) with (nBu3Sn)2O being used as an alterna-
tive for nBu3SnOH (Eq. (4)) for the tributylstannyl
derivatives. Compounds 15, 19 and 23 were prepared
also by a method described for 7 [8] from a 1:1 stoichio-
metric reaction of B2O3 and the corresponding orthobo-
rate esters (2, 10, 11) at high temperature (250°C, 22 h)
in a sealed reaction vessel, in yields of ꢀ85%.
1
derivatives. H- and 13C-NMR data were obtained on
all new compounds and these are presented in Section 3;
observed spectra had chemical shifts, multiplicities, and
relative intensities as expected with nJ(HH) coupling
constants of 7–10 Hz.
3R3SiOH+H3BO3“(R3SiO)3B+3H2O
(1)
{R3=nBuMe2 (10), PhMe2 (11), [2-(3-cyclohex-
enyl)ethyl]Me2 (12), (nOctadecyl)Me2 (13), Hex3 (14)}
n
IR spectra were obtained for all samples and the
strongest bands in all samples were in the 1393–1300
cm−1 region and due to BꢀO stretches (Table 2). The
orthoborate (R3SiO)3B derivatives (10–14) had wmax in
the range 1336–1325 cm−1, consistent with 1 [8], and
the metaborate (R3SiO)3B3O3 derivatives (15–25) had
maxima at higher wavenumber and in the range 1393–
1350 cm−1 in accord with 7 [8]. Absorption bands at
732 and 720 cm−1 [8] or 742 and 735 cm−1 [10],
described as diagnostic for the B3O3 ring system, were
observable in our systems but this area of the spectrum
generally appeared more as a medium intensity singlet
at 730 cm−1 with a shoulder at 745 cm−1, or less
frequently vice versa, rather than as a well resolved
doublet. The arylboronate esters ArB(OSiR3)2 (26–32)
3R3MOH+3H3BO3“(R3MO)3B3O3+6H2O
(2)
n
n
{M=Si, R3=nPr3 (15), Bu3 (16), Hex3 (17), EtMe2
(18), nBuMe2 (19), [2-(3-cyclohexenyl)ethyl]Me2 (20)
[(Me2HC)(Me)2C]Me2 (21), (nOctadecyl)Me2 (22),
PhMe2 (23), Ph2Me (24), Ph3 (25); M=Sn, R=Ph (33),
Me (34)}
2R3MOH+ArB(OH)2“(R3MO)2BAr+2H2O
(3)
{M=Si, Ar=Ph, R3=nPr3 (26), nBu3 (27), Ph2Me (28);
n
Ar=C6H4Br-4, R=Ph (29), Pr (30); Ar=C6H4Me-2,
n
R=Ph (31), Pr (32); M=Sn, R=Ph, Ar=Ph (36),
C6H4Br-4 (37), C6H4Me-2 (39)}
(nBu3Sn)2O+ArB(OH)2“(nBu3SnO)2BAr+H2O (4)
{Ar=Ph (35), C6H4Br-4 (38)}
had their BꢀO stretches in the range 1335–1315 cm−1
,
in agreement with 9 [9] whilst the ArB(OSnR3)2 esters
(35–39) had diagnostic bands at ca.1375 and 1301 cm−1
for R=nBu (35, 38) and Ph (36, 37, 39), respectively.
Additionally, the R3Sn esters displayed a strong band at
1290–1280 cm−1 (R=nBu) or 1270 cm−1 (R=Ph)
which has been previously assigned to the
w(asym)(BꢀOꢀSn) stretch [10]. This band was always
weaker than that of the BꢀO band shown in Table 2
except for 5 where it was the strongest band in the
spectrum.
All new compounds were prepared in high yield as
colourless high boiling liquids or as colourless crys-
talline solids and were air-stable but generally water-
sensitive. Yields, m.p./b.p., and elemental analysis data
for the new compounds 10–39 are reported in Table 1.
Solution 11B-NMR spectra (CDCl3, RT) were ob-
tained on all compounds (Table 2) and the signals for
the R3Si derivatives were observed within well-defined
ranges:
l
14.4–15.4, (R3SiO)3B;
l
17.2–19.0,

M.A. Beckett et al. / Journal of Organometallic Chemistry 588 (1999) 107–112
109
(MeO)3B3O3 have AN values of 23 and 79, respectively.
The most highly acidic of those derivatives measured
was 23 {(PhMe2SiO)3B3O3, AN=81} whose Lewis
acidity approaches that of BF3 and is greater than that
of (MeO)3B3O3. As was found for the organic esters [2],
increasing the steric bulk of the R3M moiety signifi-
cantly reduces the AN value for a particular com-
pound, and there also appears to be little differentiation
between corresponding R3Si and R3Sn derivatives, e.g.
orthoborates: 1\2\3ꢀ(5), metaborates: 18\7\
15\16ꢀ(8)\17 (R3Sn derivatives are in par-
entheses).
These AN values also indicate that there is an elec-
tron-withdrawing electronic effect, associated with in-
corporating Si or Sn atoms into the ester linkage, which
results in increased Lewis acidity at B. The AN data for
the orthoborate esters clearly demonstrate that the Si
and Sn derivatives should be stronger Lewis acids than
the isomeric C analogues, i.e. 1, 2, 5, 10, and 11 would
all be expected from steric considerations to be less
acidic than (H3CO)3B (AN=23 [2]) and yet these com-
pounds have AN values which are higher. Similar elec-
tron-withdrawing effects associated with SiR3 groups
2.2. Acceptor number (AN) 6alues
Gutmann and co-workers have described a quantita-
tive parameter (Acceptor number, AN), derived from
the 31P-NMR shifts produced in Et3PO by electrophilic
solvent interactions that lead to deshielding of the P
atoms by inductive effects involving electron donation
from O to the solvent, as a measure of the Lewis acidity
of the solvent [14]. For comparative purposes this AN
scale has arbitrary fixed points for hexane (0) and SbCl5
(100). In relation to the Lewis acidity of trigonal boron
compounds, the AN value is a measure of how well the
O donor atom of the Et3PO competes with the het-
eroatoms bound to boron for its acceptor orbital; thus
BF3 has an AN value of 89 whereas B(NMe2)3 has an
AN value of 9 [2]. The liquid triorganosilyl and tri-
organostannyl orthoborate esters and arylboronic acid
esters are poor Lewis acids (AN values 23–36, and
23–31, respectively) whilst the liquid esters of meta-
boric acid are considerably stronger Lewis acids (AN=
44–81). These AN values for the esters with BꢀOꢀSi or
BꢀOꢀSn linkages are in the same general ranges as
those observed [2] for organic esters, e.g. (MeO)3B and
Table 1
Elemental analysis a of new triorganosilyl and triorganylstannyl borate esters
Compound
Yield/%
M.p. (b.p./mmHg)/°C
C
H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
(nBuMe2SiO)3B
(PhMe2SiO)3B
72
75
83
87
83
81
77
83
80
69
83
69
72
81
67
91
66
66
71
63
85
86
51
89
94
74
80
90
81
85
(170/15)
53.5(53.4)
62.2(62.1)
63.8(64.2)
72.7(72.5)
71.9(71.3)
54.3(54.0)
59.7(59.5)
66.5(66.4)
37.0(37.0)
46.0(45.7)
57.1(57.2)
51.5(51.6)
67.6(67.8)
54.0(54.0)
64.2(65.0)
71.8(71.6)
66.6(66.3)
69.6(69.5)
74.9(74.7)
69.9(70.2)
55.9(56.2)
79.4(79.1)
66.6(66.9)
55.0(55.0)
17.3(17.4)
50.9(51.2)
61.3(61.5)
55.8(56.1)
46.0(46.1)
62.1(61.9)
11.5(11.2)
7.2(7.2)
(151–154/5.0)
(\250/1.5)
37–39
(CyclohexenylEt)Me2SiO]3B
(nOctadecylMe2SiO)3B
(nHex3SiO)3B
10.1(10.2)
12.9(13.1)
13.0(13.0)
10.8(10.6)
11.4(11.2)
11.7(12.0)
8.5(8.5)
9.7(9.6)
9.4(9.1)
10.6(10.3)
12.4(12.2)
6.5(6.2)
6.1(5.5)
5.1(5.0)
11.1(10.9)
11.4(11.5)
5.9(6.1)
5.2(4.9)
9.4(9.1)
5.9(5.7)
11.2(11.0)
4.2(3.9)
(250/1.0)
(260/1.5)
(238–241/0.15)
(250/0.1)
(185–190/0.08)
(170–175/0.1)
Visc. liq b
(\260/0.1)
39–42
(nPr3SiO)3B3O3
(nBu3SiO)3B3O3
(nHex3SiO)3B3O3
(EtMe2SiO)3B3O3
(nBuMe2SiO)3B3O3
[(CyclohexenylEt)Me2SiO]3B3O3
[(Me2HC)(Me)2C]Me2SiO)3B3O3
(nOctadecylMe2SiO)3B3O3
(PhMe2SiO)3B3O3
(Ph2MeSiO)3B3O3
(Ph3SiO)3B3O3
(\250/0.1)
(\250/0.1)
165–8
(nPr3SiO)2BPh
Visc. liq b
Visc. liq b
(\250/0.1)
121–4
(nBu3SiO)2BPh
(Ph2MeSiO)2BPh
(Ph3SiO)2B(C6H4Br-4)
(nPr3SiO)2B(C6H4Br-4)
(Ph3SiO)2B(C6H4Me-2)
(nPr3SiO)2B(C6H4Me-2)
(Ph3SnO)3B3O3
Visc. liq b
88–92
Visc. liq b
118–122
(Me3SnO)3B3O3
98–102
(250/1.0)
115–19
104–111
(250/1.0)
110–14
4.7(4.4)
8.6(8.6)
4.3(4.3)
4.1(3.8)
7.4(7.5)
4.8(4.5)
(nBu3SnO)2BPh
(Ph3SnO)2BPh
(Ph3SnO)2B(C6H4Br-4)
(nBu3SnO)2B(C6H4Br-4)
(Ph3SnO)2B(C6H4Me-2)
a Calculated in parenthesis.
b Did not distill.

110
M.A. Beckett et al. / Journal of Organometallic Chemistry 588 (1999) 107–112
Table 2
perimental data concerning OꢀSi bonds favours
(p–s*)p hyperconjugation as the mechanism of the
electronic interaction in silyl ethers and hydroxysilyl
compounds [20]. The availability of an empty, low
energy CꢀSi s* orbital on Si is a consequence of its low
electronegativity and a similar situation should arise for
Sn which is equally less electronegative. The enhanced
Lewis acidity at B is thus due to OꢀM (p–s*)p bonding
competing with, and reducing O–B (p–p)p bonding. As
might be expected, the electronic effect at B is moder-
ated in the metaborate esters where the M:B ratio is
reduced to 1:1. However, the effect is still observable
and compounds 15, 18, 19, 20, 23 and 24 have AN
values higher or similar to that for (nPrCH2O)3B3O3
(AN=65 [2]) whereas their acidities would be expected
to be lower if the steric factor were dominating.
Acceptor number (AN) values and spectroscopic (solution/solid-state
MAS-11B-NMR and IR) data
a
Compound AN
l(11B) b
/ppm
liso
(
11B) c
wmax(BꢀO) d
/cm−1
/ppm
e
e
e
1
2
3
4
5
6
7
8
34
28
14.6
15.4
15.4
15.1
18.1
22.7
17.7
20.2
27.1
14.4
15.0
14.7
15.4
15.0
15.4
19.0
17.2
17.2
18.1
17.2
18.2
17.9
17.7
17.3
19.0
26.6
26.2
26.3
27.4
25.9
27.9
26.3
21.5
19.5
18.2
22.1
19.0
18.2
18.5
1329
1328
1329
1320
1300
1301
1393
1377
1322
1326
1336
1335
1332
1325
1385
1386
1381
1375
1380
1381
1383
1381
1372
1350
1390
1332
1335
1322
1315
1325
1320
1326
1301
1301
1376
1304
1301
1374
1301
23
16.1 g
f
e
25
21.4 g
f
e
72
e
54
f
9
27.9 h
e
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
36
29
e
e
i
23
f
i
e
e
e
e
e
e
e
e
i
2.3. Conclusions
63
56
44
74
71
70
The synthesis and Lewis acidity of a series of tri-
organosiyl and triorganostannyl esters of orthoboric,
metaboric and arylboronic acids have been reported
with AN ranges for the classes of compound shown to
be 23–36, 44–81 and 23–31, respectively. The Lewis
acidity at B in BꢀOꢀMR3 (M/Si, Sn) derivatives was
enhanced relative to isomeric organic esters, and the
most highly acidic of those derivatives measured was
(PhMe2SiO)3B3O3 (AN=81) whose Lewis acidity ap-
proaches that of BF3 and is greater than that of
(MeO)3B3O3 or (nBuO)3B3O3.
59
f
e
e
i
81
74
f
e
e
e
23
i
i
28.7 h
f
i
e
29.2 h
f
e
31
3. Experimental section
f
i
f
i
e
30
3.1. General
21.0 g
f
20.7 g
f
Reactions were carried out under N2. IR spectra were
recorded on a Perkin–Elmer FT-IR 1600 spectrometer
as KBr discs. Multi-element solution NMR were
recorded on a Bruker AC 250 CP/MAS-NMR spec-
e
30
21.0 g
f
a Calculated from l(31P) of Et3PO dissolved in neat (liquid) com-
1
pound from AN=(l(sample)−41.0)×{100/(86.14−41.0)} [2].
b From proton-decoupled solution spectra in CDCl3 at room tem-
perature.
trometer operating at 250 MHz for H, 62.9 MHz for
13C-{1H}, 31 P-{1H) and 80.2 MHz for 11B-{1H).
Chemical shifts (l) are given in ppm with positive
values towards high frequency (downfield) from SiMe4
(1H and 13C), BF3·OEt2 (11B) or 85% H3PO4 (31P).
Elemental analyses (C, H) were obtained on a Carlo
Erba MOD-1106 instrument using helium as carrier
gas. AN values for liquid samples were obtained as
described previously and were referenced against PPh3
(l= −6.0) dissolved in the CDCl3 used as a lock [2].
R3SiOH derivatives [21,22], 2-MeC6H4B(OH)2 [23] and
Me3SnOH [24] were prepared by literature procedures.
Et3PO, H3BO3, ArB(OH)2 (Ar=Ph, 4-BrC6H4),
Me3SnCl, R3SiCl, and (nBu3Sn)2O were obtained com-
mercially and used as supplied. Details of the prepara-
tions of 10, 15, 26 and 35, typical of those shown in
c Solid state MAS-NMR: isotropic shift, liso=lcg−|qs (lcg is the
centre of gravity of the observed signal),
|qs=wCG−wL=−
106(Cq)2(1+0.33p2)/40(wL)2 (wL=96.234 MHz) [18].
d Recorded as thin film between NaCl plates (liquids) or KBr discs
(solids).
e Liquid at room temperature.
f Solid at room temperature.
g Simulated data: |qs=−17.3, Cq=2.53 MHz, p=0.0.
h Simulated data: |qs=−27.2, Cq=3.05 MHz, p=0.5.
i Not recorded.
are well documented [19] and have been observed as
enhanced Brønsted acidity of hydroxy groups on Si,
reduced donor properties of silyl ethers, and stabiliza-
tion of a-silylcarbanions. Current interpretation of ex-

M.A. Beckett et al. / Journal of Organometallic Chemistry 588 (1999) 107–112
111
Eqs. (1)–(4) are given below together with 1H- and
9H), 1.2 (m, 96H). 13C-NMR: l=0.0, 14.1, 17.5, 17.8,
22.7, 23.2, 29.4, 29.7, 31.9, 33.5. (nHex3SiO)3B (14):
1H-NMR: l=0.4 (t, 18H), 0.7 (t, 27H), 1.1 (m, 72H).
13C-NMR: l=14.1, 14.8, 22.7, 23.2, 31.7, 33.6.
13C-NMR data (CDCl3, RT) of compounds 10–39.
3.2. Preparation of (nBuMe2SiO)3B (10)
1
(nPr3SiO)3B3O3 (15): H-NMR: l=0.55 (m, 18H), 0.80
nBuMe2SiOH (3.73 g, 0.03 mol) was added to H3BO3
(0.58 g, 0.01 mol) in MePh (100 ml) and heated in a
Dean–Stark apparatus for 20 h until H2O (0.5 ml, 0.03
mol) had separated. The PhMe was removed from the
resulting solution by rotary evaporation leaving the
product as an analytically pure clear colourless liquid
(3.24 g, 72%) which distilled at 170°C/15 mmHg.
(t, 27H), 1.3 (m, 18H). 13C-NMR: l=16.6, 17.5, 18.3.
1
(nBu3SiO)3B3O3 (16): H-NMR: l=0.7 ( t, 27H), 0.8
(m, 18H), 1.2 (m, 36H). 13C-NMR: l=13.7, 15.4, 25.4,
26.7. (nHexyl3SiO)3B3O3 (17): 1H-NMR: l=0.7 (t,
18H), 0.9 (t, 27H), 1.3 (m, 72H). 13C-NMR: l=14.1,
1
14.5, 22.6, 22.9, 31.6, 33.2. (EtMe2SiO)3B3O3 (18): H-
NMR: l=0.2 (s, 18H), 0.65 (q, 6H), 1.0 (t, 9H).
13C-NMR: l=0.1, 6.4, 8.9. (nBuMe2SiO)3B3O3 (19):
1H-NMR: l=0.25 (s, 18H), 0.7 (t, 6H), 0.95 (t, 9H),
1.35 (q, 12H). 13C-NMR: l=0.0, 13.7, 17.1, 25.4, 26.4.
[(2-{3-cyclohexenyl}Et)Me2SiO]3B3O3 (20): 1H-NMR:
l=0.2 (s, 18H), 0.7 (m, 6H), 1.15–2.15 (m, 27H), 5.65
(q, 6H). 13C-NMR: l=0.0, 14.2, 25.3, 28.1, 29.6, 31.6,
36.3, 126.6, 126.7. [(Me2HC)(Me)2C]Me2SiO)3B3O3
3.3. Preparation of (nPr3SiO)3B3O3 (15)
nPr3SiOH (4.50 g, 0.026 mol) was added to H3BO3
(1.59 g, 0.026 mol) in MePh (100 ml) and heated in a
Dean–Stark apparatus for 20 h until H2O (0.94 ml,
0.052 mol) had separated. The PhMe was removed
from the resulting solution by rotary evaporation leav-
ing the crude product (5.04 g, 83%). The crude product
was dissolved in the minimum amount of Et2O and
filtered to removed unreacted H3BO3 (0.12 g, 0.002
mol). The Et2O was then removed under vacuum to
yield a clear colourless liquid (4.92 g, 81%) which
distilled at 260°C/1.5 mmHg.
1
(21): H-NMR: l=0.2 (s, 18H), 0.9 (s, 36H), 1.7 (sept,
3H). 13C-NMR: l=0.0, 18.5, 20.0, 24.8, 34.1.
(nOctadecylMe2SiO)3B3O3 (22): 1H-NMR: l=0.2 (s,
18H), 0.65 (q, 6H), 0.9 (t, 9H), 1.25 (m, 96H). 13C-
NMR: l=0.0, 14.1, 17.2, 17.5, 22.7, 23.0, 29.4, 29.7,
31.9, 33.4. (PhMe2SiO)3B3O3 (23): 1H-NMR: l=0.5 (s,
18H), 7.4 (d, 9H), 7.6 (m, 6H). 13C-NMR: l=0.0,
1
128.1, 130.0, 133.4, 139.0. (Ph2MeSiO)3B3O3 (24): H-
3.4. Preparation of (nPr3SiO)2BPh (26)
NMR: l=0.9 (s, 9H), 7.6 (m, 18H), 7.85 (m, 12H).
13C-NMR: l=0.0, 127.8, 128.0, 130.1, 134.2.
nPr3SiOH (3.04 g, 0.017 mol) was added to PhB(OH)2
(1.06 g, 0.00872 mol) in MePh (100 ml) and heated in a
Dean–Stark apparatus for 20 h until H2O (0.31 ml,
0.017 mol) had separated. The PhMe was removed
from the resulting solution by rotary evaporation leav-
ing the product as an analytically pure clear colourless
liquid (2.7 g, 66%).
(Ph3SiO)3B3O3 (25): H-NMR: l=7.1–7.6 (m, 45H).
1
13C-NMR: l=127.6, 129.7, 135.3. (nPr3SiO)2BPh (26):
1H-NMR: l=0.85 (m, 12H), 1.0 (t, 18H), 1.55 (m,
12H), 7.5 (d, 3H), 7.9 (t, 2H). 13C-NMR: l=16.8, 18.1,
18.3, 125.1, 127.4, 129.0, 134.9. (nBu3SiO)2BPh (27):
1H-NMR: l=0.55 (m, 12H), 0.75 (t, 18H), 1.25 (m,
24H), 7.25 (d, 3H), 7.5 (t, 2H). 13C-NMR: l=13.7,
15.0, 25.6, 26.6, 125.2, 127.2, 130.2, 134.9.
3.5. Preparation of (nBu3SnO)2BPh (35)
(Ph2MeSiO)2BPh (28): H-NMR: l=0.7 (s, 6H), 7.3–
1
7.55 (m, 20H), 7.65 (m, 5H). 13C-NMR: l=0.8, 125.3,
(nBu3Sn)2O (3.0 g, 0.0051 mol) was added to Ph-
B(OH)2 (0.63 g, 0.0051 mol) in MePh (100 ml) and
heated in a Dean–Stark apparatus for 20 h until H2O
(0.1 ml, 0.0051 mol) had separated. The PhMe was
removed from the resulting solution by rotary evapora-
tion leaving the product as an analytically pure clear
colourless liquid (2.61 g, 74%) which distilled at 250°C/
1 mmHg.
127.7, 127.9, 129.6, 130.3, 135.3, 134.2, 137.0.
(Ph3SiO)2B(C6H4Br-4) (29): H-NMR: l=7.2–7.8 (m,
1
34H). 13C-NMR: l=128.0, 128.3, 130.0, 130.2, 135.0,
135.2, 137.5. (nPr3SiO)2B(C6H4Br-4) (30): 1H-NMR:
l=0.75 (t, 12H), 1.05 (t, 18H), 1.4 (m, 12H), 7.55 (d,
2H), 7.65 (d, 2H). 13C-NMR: d=16.8, 18.0, 18.3,
125.3, 130.7, 136.5. (Ph3SiO)2B(C6H4Me-2) (31): 1H-
NMR: l=2.55 (s, 3H), 7.25 (m, 4H), 7.5 (m, 18H),
7.65 (t, 12H). 13C-NMR: l=22.5, 124.1, 127.6, 129.8,
130.1, 132.5, 135.0, 135.1, 135.3. (nPr3SiO)2B(C6H4Me-
1
(nBuMe2SiO)3B (10): H: l=0.05 (s, 18H), 0.55 (t,
6H), 0.85 (t, 9H), 1.3 (m, 12H). 13C: l=0.3, 13.7, 17.2,
1
1
24.4, 26.5. (PhMe2SiO)3B (11): H: l=0.25 (s, 18H),
2) (32): H-NMR: l=0.5 (t, 12H), 0.8 (t, 18H), 1.3 (m,
7.3 (d, 9H), 7.5 (m, 6H). 13C: l=1.3, 128.1, 129.7,
133.4, 140.1. (2-{3-cyclohexenyl}Et)Me2SiO]3B (12):
1H-NMR: l=0.0 (t, 18H), 0.6 (m, 6H), 1.0–2.0 (m,
27H), 5.5 (q, 6H). 13C-NMR: l=0.7, 15.3, 25.2, 28.6,
29.7, 31.7, 36.9, 126.7, 127.0. (nOctadecylMe2SiO)3B
12H), 2.2 (s, 3H), 6.9–7.2 (m, 4H). 13C-NMR: l=16.7,
17.8, 18.3, 28.4, 124.3, 128.5, 129.2, 132.2, 140.6.
1
(Ph3SnO)3B3O3 (33): H-NMR: l=7.2–7.45 (m, 45H).
13C-NMR:
l=128.3,
129.3,
136.2,
139.6.
(Me3SnO)3B3O3 (34): 1H-NMR: l=0.4 (27H). 13C-
1
1
(13): H-NMR: l=0.0 (s, 18H), 0.5 (t, 6H),) 0.8 (t,
NMR: l= −2.7. (nBu3SnO)2BPh (35): H-NMR: l=

112
M.A. Beckett et al. / Journal of Organometallic Chemistry 588 (1999) 107–112
[5] E. Wiberg, U. Kruerke, Z. Naturforsch. B 8 (1953) 609.
[6] (a) B.N. Dolgov, Yu.I. Khudobin, N.P. Kharitonov, Doklady
Akad. Nauk. SSSR 122 (1958) 607. (b) B.N. Dolgov, Yu.I.
Khudobin, N.P. Kharitonov, Chem. Abs. 53 (1959) 4110.
[7] (a) M.G. Voronkov, N.F. Orlov, Latvijas PSR Zinatnu Akad.
Vestis, Kim. Ser. 1 (1961) 93. (b) M.G. Voronkov, N.F. Orlov,
Chem Abs. 58 (1963) 2466.
[8] E.W. Abel, A. Singh, J. Chem. Soc. (1959) 690.
[9] D. Murphy, J.P. Sheehan, T.R. Spalding, G. Ferguson, A.J.
Lough, J.F. Gallagher, J. Mater. Chem. 3 (1993) 1275.
[10] S.K. Mehrotra, G. Srivastava, R.C. Mehrotra, J. Organomet.
Chem. 65 (1974) 361.
[11] (a) R.H. Krieble, US Patent 2440101 (1948). (b) R.H. Krieble,
Chem. Abs. 42 (1948) 6376.
[12] (a) M.G. Voronkov, V.N. Zgonnik, Zh. Obshch. Khim. 27
(1957) 1476. (b) M.G. Voronkov, V.N. Zgonnik, Chem. Abs. 52
(1958) 3673.
1.0 (t, 18H), 1.2 (t, 12H), 1.45 (m, 12H), 1.65 (m, 12H),
7.3 (d, 3H), 7.5 (t, 2H). 13C-NMR: l=9.6, 13.7, 27.4,
29.2, 128.0, 131.6, 136.5, 141.9. (Ph3SnO)2BPh (36):
1H-NMR: l=7.2–7.8 (m, 35H). 13C-NMR: l=128.3,
128.8, 129.1, 129.2, 136.8, 137.3, 139.6, 156.5.
1
(Ph3SnO)2B(C6H4Br-4) (37): H-NMR: l=7.2–7.7 (m,
34H). 13C-NMR: l=127.1, 127.6, 127.8, 128.3, 128.4,
128.6, 129.0, 138.3. (nBu3SnO)2BC6H4Br-4 (38): 1H-
NMR: l=0.65 (t, 18H), 0.95 (t, 12H), 1.2 (m, 12H),
1.4 (m, 12H), 7.15 (d, 2H), 7.25 (d, 2H). 13C-NMR:
l=9.6, 13.7, 27.2, 29.0, 122.8, 131.0, 137.0, 140.5.
1
(Ph3SnO)2BC6H4Me-2 (39): H-NMR: l=2.4 (s, 3H),
7.3 (t, 12H), 7.4 (d, 18H), 7.6 (m, 4H). 13C-NMR:
l=21.1, 128.3, 128.6, 129.0, 129.3, 136.8, 137.2, 139.6.
[13] S.K. Mehrotra, G. Srivastava, R.C. Mehrotra, Ind. J. Chem.
Sect. A 14 (1974) 137.
[14] U. Mayer, V. Gutmann, W. Gerger, Monatshefte Chem. 106
(1975) 1235.
Acknowledgements
[15] H. Noth, B. Wrackmeyer, Nuclear magnetic resonance spec-
troscopy of boron compounds, Tables XI, XII and XVII, p. 136,
139, and 154, in: P. Diehl, E. Fluck,, R. Kosfeld (Series Eds.),
NMR Basic Principles and Progress, vol. 14, Springer-Verlag,
Berlin, 1978.
We thank Dr D. Apperley at the EPSRC Solid
State-NMR Service Centre (Durham University) for
spectra and the EPSRC and Pilkington’s plc for a
CASE studentship.
[16] F.A. Davis, I.J. Turchi, D.N. Greeley, J. Org. Chem. 36 (1971)
1300.
[17] M.A. Beckett, G.C. Strickland, K.S. Varma, D.E. Hibbs, M.B.
Hursthouse, K.M.A. Malik, J. Organomet. Chem. 535 (1997) 33.
[18] G. Engelhardt, D. Michel, High Resolution Solid-State NMR of
Silicates and Zeolites, Wiley, Chichester, 1987.
References
[1] W.W. Porterfield, Inorganic Chemistry: A Unified Approach,
Ch.6, 2nd ed., Academic Press, London, 1993, p. 313.
[2] M.A. Beckett, G.C. Strickland, J.R. Holland, K.S. Varma, Poly-
mer 37 (1996) 4629.
[3] S.K. Mehrotra, G. Srivastava, R.C. Mehrotra, J. Organomet.
Chem. 73 (1974) 277.
[4] (a) N.F. Orlov, B.N. Dolgov, M.G. Voronkov, Khim i Prakt.
Primenenie Kremneorg. Soedinenii, Trundy Konf., Leningrad 1
(1958) 161. (b) N.F. Orlov, B.N. Dolgov, M.G. Voronkov,
Chem Abs. 54 (1960) 4360.
[19] E. Colvin, Silicon in Organic Synthesis, Ch. 2, Butterworths,
London, 1981, p. 4.
[20] J.A. Tossell, in: R. Corriu, P. Jutzi (Eds.), Tailor-Made Silicon–
Oxygen Compounds, Ch. 3, Vieweg, Go¨ttingen, Germany, 1996,
p. 31.
[21] T. Takiguchi, J. Am. Chem. Soc. 81 (1959) 2359.
[22] J.A. Cella, J.C. Carpenter, J. Organomet. Chem. 480 (1994) 23.
[23] F.R. Bean, J.R. Johnson, J. Am. Chem. Soc. 54 (1932) 4415.
[24] (a) J.G.A. Luitjen, Rec. Trav. Chim. 82 (1963) 1179. (b) J.G.A.
Luitjen, Chem. Abs. 60 (1964) 5533.
.