Lewis and Bronsted multifunctionality: An unusual heterocycle from the reaction of bis(pentafluorophenyl)borinic acid with nitriles

Article abstract of DOI:10.1039/b516262a

The combination of Lewis and Bronsted acidity as well as Lewis basicity in (C₆F₅)₂BOH results in a remarkable reactivity towards organonitriles to give novel heterocyclic compounds containing a BOBOCN six-membered ring. Th

Full text of DOI:10.1039/b516262a

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Lewis and Brønsted multifunctionality: an unusual heterocycle from
the reaction of bis(pentafluorophenyl)borinic acid with nitriles{
George J. P. Britovsek,* Juri Ugolotti, Patricia Hunt and Andrew J. P. White
Received (in Cambridge, UK) 16th November 2005, Accepted 23rd January 2006
First published as an Advance Article on the web 10th February 2006
DOI: 10.1039/b516262a
isolated as a white solid in 92% yield, which was recrystallised from
pentane. Single crystal X-ray analysis revealed the formation of a
rather unusual six-membered heterocyclic compound 2a, contain-
ing two boron and two oxygen atoms, one carbon and one
nitrogen atom, obviously made from two (C6F5)2BOH and one
acetonitrile molecules (see Fig. 1). Similar reactions are observed
with other nitriles such as propionitrile or benzonitrile, yielding the
corresponding ring compounds 2b and 2c (Eq. 2).{ No reaction
was observed when using the less Lewis and Brønsted acidic
diphenylborinic acid (C6H5)2BOH. When the reaction was carried
out in non-dried acetonitrile, the formation of 2a was considerably
faster (, 1 day). We believe that water plays an important role in
the rate of the formation of 2a, as it does in the trimerisation
process shown in Eq. 1.10
The combination of Lewis and Brønsted acidity as well as
Lewis basicity in (C6F5)2BOH results in a remarkable reactivity
towards organonitriles to give novel heterocyclic compounds
containing a BOBOCN six-membered ring.
Bis(pentafluorophenyl)borinic acid, (C6F5)2BOH (1), is an intri-
guing example of multifunctionality in terms of chemical reactivity,
due to the presence of both relatively strong Lewis and Brønsted
acidity as well as a Lewis basic site within the same molecule. It
was first prepared more than 40 years ago,1 but has found renewed
interest recently due to applications as a catalyst in organic
synthesis,2,3 as a co-catalyst in olefin polymerisation4–7 and in
battery electrolytes.8 The presence of both Lewis acidic and basic
sites leads to the formation of a trimer in the solid state which, in
solution, is in equilibrium with a monomeric form (Eq. 1).9 The
position of this equilibrium was found to be strongly solvent
dependent and the rate of inter-conversion affected by the presence
of water.10
ð2Þ
ð1Þ
The only example of a similar compound reported previously is
compound I, prepared by a different route from diphenylborinic
acid and 2-hydroxypyridine.13 The resulting borinic ester forms an
adduct with (C6H5)2BOH to give the heterocyclic compound I.
Also related is compound II, which was prepared from
(F2BOMe)3 and acetic anhydride. This compound can be viewed
as an adduct between the mixed anhydride of difluoroborinic acid
and acetic acid together with the methyl difluoroborinic ester.14
The variety in the substituents in these examples suggests
substantial scope in this unexplored class of compounds.
In the course of our investigations on the Lewis acidic properties
of perfluorinated borinic and boronic esters such as
(C6F5)2BO(C6F5),11 we found that these Lewis acids readily form
adducts with nitriles such as acetonitrile, as indicated by a
characteristic shift in the 11B NMR spectrum, for example from
41.2 to 22.1 ppm in the case of (C6F5)2BO(C6F5).
Tris(pentafluorophenyl)borane B(C6F5)3 displays a similar reacti-
vity with a change in chemical shift from 59 to 210.3 ppm.12 In
contrast, we found that (C6F5)2BOH does not appear to form an
adduct when dissolved in CD3CN, as indicated by the 11B NMR
chemical shift of 38.8 ppm (cf. 42.2 ppm in toluene-d8).9 However,
during the course of a day this signal at 38.8 ppm slowly decreases
and two new sharp peaks of equal intensity appear at 1.2 and
23.4 ppm (in CD3CN). In the 19F NMR spectrum, two new sets
of signals due to the ortho, meta and para fluorine atoms are
formed, while those corresponding to (C6F5)2BOH slowly
decrease. It appears therefore that a new compound is formed
containing two similar but inequivalent (C6F5)2B units. After
removal of the acetonitrile solvent, the new compound was
The X-ray analysis of a crystal of 2a revealed the presence of
two independent molecules (A and B) with different conformations
for the central BOBOCN six-membered ring. In both cases the
ring has a slightly distorted sofa conformation, but whereas in
Department of Chemistry, Imperial College London, Exhibition Road,
London, UK SW7 2AY. E-mail: g.britovsek@imperial.ac.uk;
Fax: +44(0)2075945804; Tel: +44(0)2075945863
{ Electronic supplementary information (ESI) available: X-ray crystal-
lography. See DOI: 10.1039/b516262a
˚
molecule A (Fig. 1) the oxygen atom O(5) lies ca. 0.52 A out of the
plane of the other five atoms (which are coplanar to within ca.
˚
0.06 A), in molecule B (Fig. S1 in the supporting information) it is
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Chem. Commun., 2006, 1295–1297 | 1295

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Fig. 2 Computed atomic charges in 2a9.
In conclusion, we have shown that the reaction between
(C6F5)2BOH and organonitriles leads to the formation of
heterocyclic compounds with a BOBOCN ring, the atoms being
held together by a combination of covalent and dative bonds. This
discovery is an exciting manifestation of the multi-functionality of
(C6F5)2BOH, i.e. Lewis and Brønsted acidity as well as Lewis
basicity, which allows a multitude of reactions, such as protona-
tions, Lewis acid–base interactions, H-bonding and the formation
of covalent bonds. We are currently investigating the mechanism
of formation of these heterocycles and their reactivity.
Fig. 1 The molecular structure of one (A) of the two independent
molecules present in the crystals of 2a.
We are grateful to the EPSRC for support (GR/R92042/01).
˚
the boron atom B(19) that lies ca. 0.64 A out of the plane of the
˚
other five atoms (which are again coplanar to within ca. 0.06 A).
Notes and references
There are no statistically significant differences in the equivalent
bond lengths within the central six-membered rings of the two
independent molecules, and the pattern of bonding is similar to
that seen in the related compound I (Table 1).13 The bonds in I are
{ Selected analytical data. 2a: Mp: 130–133 uC. 1H-NMR (CDCl3):
7.90 (m, OH), 7.56 (br. s, NH), 2.53 (s, CH3). 11B-NMR (CDCl3):
2.3 (OBO), 21.2 (NBO). 19F-NMR (CDCl3): 2137.3 (d, 4F, 3JFF = 12 Hz,
o), 2137.5 (d, 4F, o), 2153.1 (t, 2F, 3JFF = 20 Hz, p), 2153.4 (t, 2F, 3JFF
=
˚
all longer than in 2a (on average, by ca. 0.03 A), presumably a
20 Hz, p), 2161.0 (td, 4F, 3JFF = 12 Hz, 3JFF = 21 Hz, m), 2161.9 (td, 4F,
3JFF = 12 Hz, 3JFF = 21 Hz, m). MS-EI (m/z): 765 [M]+, 598 [M 2 (C6F5)]+,
430 [M 2 (C6F5)2H]+. Anal.: Found (Calc.): C, 40.79 (40.83); H, 0.64
(0.66); N, 1.89 (1.83). 2b: 11B NMR (CDCl3): 1.6, 23.2. 19F NMR
(CDCl3): 2134.3 (4F, o), 2134.9 (4F, o), 2157.4 (2F, p), 2157.6 (2F, p),
2165.6 (8F, m). 2c: 11B NMR (CDCl3): 0.7, 21.8. 19F NMR (CDCl3):
2133.6 (4F, o), 2134.0 (4F, o), 2155.8 (2F, p), 2160.0 (2F, p), 2163.9 (8F,
consequence of the change from phenyl to pentafluorophenyl
groups and the increased Lewis acidity of the boron centres in 2a.
The amide C–O and C–N distances seen here in 2a are the same as
˚
each other (at ca. 1.30 A) in contrast to the ca. 0.1 A difference
˚
˚
˚
[C–O ca. 1.23 A, C–N ca. 1.33 A] seen in simple acyclic amides,
indicating a significant contribution from the canonical form as
shown in Eq. 2.
¯
m). Crystal data for 2a: C26H5B2F20NO2, M = 764.93, triclinic, P1 (no. 2), a
˚
= 10.7737(6), b = 14.0513(7), c = 18.8781(10) A, a = 97.147(4), b =
3
˚
96.075(4), c = 104.631(5)u, V = 2715.5(2) A , Z = 4 (2 independent
molecules), Dc = 1.871 g cm23, m(Cu-Ka) = 1.939 mm21, T = 173 K,
colourless blocks, Oxford Diffraction Xcalibur PX Ultra diffractometer;
10017 independent measured reflections, F2 refinement, R1 = 0.046, wR2 =
0.127, 8953 independent observed absorption-corrected reflections [|Fo| >
4s(|Fo|), 2hmax = 142u], 938 parameters. CCDC 288347. For crystal-
lographic data in CIF or other electronic format see DOI: 10.1039/
b516262a
In order to obtain further insight into the type of bonding in this
unusual heterocycle, the formation of 2a was modelled with DFT
calculations.§ As can be seen in Table 1, the bond lengths of the
optimised structure 2a9 are in very good agreement with
the experimental data. Irrespective of the starting geometry for
the calculations, A or B, the optimised structure 2a9 has geometry
B, which indicates that this geometry is probably thermodynami-
cally favoured whereas geometry A arises from crystal packing
§ All calculations were performed at the B3LYP level of theory20–22 and
6-31G(d) basis set23,24 using the Gaussian03 suite of programs.25 Structures
have been fully optimised under no symmetry constraints and confirmed as
minima by frequency analysis.
…
effects in the solid state (F p interactions). Natural bond orbital
analysis (NBO)15–19 reveals a CLN double bond, a covalent
polarised C–O bond and highly ionic B–O and B–N bonds.
The computed atomic charges, as shown in Fig. 2, are consistent
with a highly ionic mode of bonding and confirm the proposed
bonding description with significant positive charge on the carbon
and boron centres and negative charge on the oxygen and nitrogen
centres.
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2a
I
2a9
CLN
NAB
B–O
OAB
B–O
O–C
1.296(3)
1.539(3)
1.523(2)
1.529(2)
1.503(2)
1.295(2)
1.342(7)
1.612(8)
1.553(7)
1.558(8)
1.532(7)
1.325(6)
1.308
1.562
1.527
1.542
1.499
1.293
1296 | Chem. Commun., 2006, 1295–1297
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