Unexpected formation of new fluoroboranes from the reaction of NMe4B3H8 with BF3 and MeC≡CH: exo-2-FB4H9 and trans-MeCH≡CHBF2

Article abstract of DOI:10.1039/b206102c

The new fluoroboranes exo-2-FB₄H₉ 1 and trans-MeCH≡CHBF₂ 2 have been obtained unexpectedly and in good yield from the reaction of tetramethylammonium octahydrotriborate (NMe₄B₃H₈) with boro

Full text of DOI:10.1039/b206102c

Unexpected formation of new fluoroboranes from the reaction of
NMe4B3H8 with BF3 and MeC·CH: exo-2-FB4H9 and
trans-MeCHNCHBF2†
Mark A. Fox,a Robert Greatrex*b and Daniel L. Ormsby‡b
a Department of Chemistry, University of Durham, South Road, Durham, UK DH1 3LE
b Department of Chemistry, University of Leeds, Leeds, UK LS2 9JT. E-mail: r.greatrex@chem.leeds.ac.uk
Received (in Cambridge, UK) 24th June 2002, Accepted 22nd July 2002
First published as an Advance Article on the web 13th August 2002
The new fluoroboranes exo-2-FB4H9
1
and trans-
couples to the terminal fluorine substituent. The proton-
decoupled 11B spectrum comprised three peaks with intensities
in the ratio 1+1+2 and shifts of 10.5, 216.5 and 241.0 ppm,
assigned to boron atoms B2, B4 and B1,3 respectively.
However, as in other 2-substituted tetraboranes, such as
MeCHNCHBF2 2 have been obtained unexpectedly and in
good yield from the reaction of tetramethylammonium
octahydrotriborate (NMe4B3H8) with boron trifluoride and
propyne (MeC·CH).
2-BrB4H9 and 2-MeB4H9,8 the precise location (exo or endo)
7
Fluoropolyborane compounds such as fluorinated borane ani-
ons1 and polyboron fluorides2 are the subject of significant
current interest. However, only one neutral polyborane contain-
ing a fluorine atom attached to a cluster boron atom has been
reported, namely the pentaborane(9) derivative 2-FB5H8.3
During the course of our investigations4 into the role played by
the reactive intermediate {B3H7} in the synthesis and mecha-
nisms of formation of small carbaboranes we have now
discovered, quite unexpectedly, a one-pot reaction that gen-
erates two new fluoroboranes, in good yield.
To avoid the difficulties involved in preparing and handling
B3H7CO5 we decided to continue this work by exploring the
reaction of propyne with {B3H7} produced in situ by hydride-
ion abstraction from [B3H8]2. In the absence of alkyne the
reaction of NMe4B3H8 and BF3 is known to generate pure
B4H10 in high yield.6 Surprisingly, in the presence of MeC·CH
at 220 °C for 3 h we now find that the main volatile products
are 2-FB4H9 1 and MeCHNCHBF2 2.§ Tetraborane(10) is not
observed. The new fluoroborane, arachno-2-FB4H9 1, is
unstable above 220 °C, decomposing rapidly to B4H10 as the
only volatile compound at room temperature, whereas 2 is
stable at ambient temperature under nitrogen.
of the fluorine substituent in 1 could not be established from the
NMR spectroscopic data alone.
Optimised geometries of 2-FB4H9 1 at the correlated MP2/
6-31G* level of theory showed the exo isomer 1a to be more
stable than the endo isomer 1b by 2.2 kcal mol21 (Fig. 2).∑
Moreover, as indicated in Table 1, comparison of calculated
(GIAO-B3LYP/6-311G*) chemical shifts generated from these
optimised geometries with the shifts observed experimentally
confirms that the fluorotetraborane is indeed the exo-isomer 1a.
Ab initio calculations (see ESI†) on the related boranes
2-MeB4H9 and 2-BrB4H9 indicate that the exo configuration is
the more stable in each case.
trans-MeCHNCHBF2 2, a derivative of CH2NCHBF2,9 was
characterised by multinuclear (1H, 11B, 19F and 13C) NMR
spectroscopy and mass spectrometry. The trans-configuration
in 2 was indicated by the coupling constant of 18 Hz for
3JHCNCH, which falls within the range expected for trans-
RCHNCHRA isomers.10 Further evidence in favour of the trans
Boron–hydrogen couplings consistent with those expected
for a tetraborane molecule with a substituent at the wingtip
boron B2 were observed for boron atoms B1, B3 and B4 in the
line-narrowed 11B spectrum for 2-FB4H9 1 (Fig. 1).¶ Couplings
from bridging hydrogen atoms were obscured for B2 which
Fig. 2 Optimised geometries of 2-FB4H9 1 and trans-MeCHNCHBF2 2.
Table 1 Observed (1) and calculated (1a and 1b) 11B, 1H and 19F NMR
chemical shifts for 2-FB4H9
1a
1
1b
B2
B4
B1,3
H2
8.1
10.5
216.8
242.2
18.5
219.5
211.0
247.7
242.6
Fig. 1 Boron NMR spectra for 2-FB4H9 1; (a) proton-decoupled and (b)
4.07
3.64
4.90
line-narrowed.
endo-H4
exo-H4
exo-H1,3
m-H1/2,2/3
m-H1/4,3/4
F
2.35
2.32
1.75
1.92
1.92
1.40
3.81
2.73
1.75
† Electronic supplementary information (ESI) available: boron NMR shifts
and relative energies for 2-MeB4H9 and 2-BrB4H9 and .xyz files. See http:
//www.rsc.org/suppdata/cc/b2/b206102c/
‡ Current address: Accelrys Ltd., 334 Cambridge Science Park, Cambridge,
UK CB4, 0WN.
0.79
0.95
1.07
21.76
2142
21.32
2152
21.14
2170
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This journal is © The Royal Society of Chemistry 2002

condensable gas (probably H2) was removed by evacuation. The volatile
products were then subjected to cold-column fractionation,12 with mass
spectrometric monitoring. The products were collected in order of
decreasing volatility (highest mass cut-off and % yield based on the total
volatile product fraction in parentheses) B2H6 (m/z 28, 5%), trans-
MeCHNCHBF2 2 (m/z 90, 40%), 2-FB4H9 1 (m/z 38, 30%) and B5H9 (m/z
64, 10%). The parent mass cut-off for the fluoroborane 1 is not observed and
the mass cut-off of 38 is likely to be F2. The yield of the stable compound
trans-MeCHNCHBF2 2 was ca 3 mmol.
configuration was provided by the excellent agreement between
the experimental NMR chemical shifts with those calculated
from the ab-initio optimised geometry 2a in which the fluorine
atoms lie in the same plane as the double bond (Fig. 2). The
barrier to rotation of the BF2 group about the B–C bond vector
via 2b was computed to be 6.8 kcal mol21, very similar to the
value of 4.56 kcal mol21 determined recently by variable
temperature 19F NMR spectroscopy for the related vinyldi-
fluoroborane.11 Examination of the orbitals computed for the
optimised geometries 2a and 2b revealed a small interaction in
the HOMO for 2a between the CNC p-orbital and the vacant
boron p-orbital which is absent in the HOMO for 2b. This
interaction in 2a, which may be described as hyperconjugation,
is largely responsible for the rotational barrier.
¶ NMR data for 1 (in CDCl3 at 233 K): dB 10.5 (dd, JBHt = 174 Hz, JBF
75; B2), 216.8 (tt, JBHt 132, JBHm 38; B4), 242.2 (dt, JBHt 150, JBHm
54; B1, B3). 1H NMR: dH 3.64 (d, 2JHBF 38; B2H), 1.92 (s; B4H), 1.40 (s;
B1H, B3H), 0.95 (s; Hm1/2, Hm2/3), 21.32 (s; Hm3/4, Hm1/4), dF 2152
1
(br). NMR data for 2: dB 24.1 (t, JBF = 66 Hz). H NMR: dH 7.02 (dq,
3JHCNCH 18, 3JHCCH3 6.5; CHCH3), 5.41 (dtq, 3JHCNCH 18, 3JHCBF2
6, 4JHCNCHCH3 1; CHBF2), 1.95 (d, 3JHCCH3 6.5; CH3), dC 159.2 (t,
4JCNCBF 18; CH3CH), 117.7 (s br; CHBF2), 22.4 (s; CH3), dF 289 (br).
The overall stoichiometry, as indicated in the following
oversimplified scheme, indicates that three moles of BF3 are
required for one mole of 1 and one of 2, and on this basis the
yield for 2 is 69%.
∑
Computational: all ab initio computations were carried out with the
Gaussian 94 package.13 The geometries 1a, 1b, 2a and 2b were initially
optimised at the HF/6-31G* level. Frequency calculations were computed
on these optimised geometries at the HF/6-31G* level for imaginary
frequencies. Optimisations of these geometries were then carried out at the
computationally intensive MP2/6-31G* level. NMR shifts were calculated
on these MP2/6-31G* geometries at the GIAO-B3LYP/6-311G* level.
Theoretical 11B chemical shifts listed in the table and in the experimental
section have been referenced to B2H6 (16.6 ppm) and converted to the usual
BF3·OEt2 scale; d(11B) = 102.83 2 s(11B). 13C and 1H chemical shifts
were referenced to TMS; d(13C) = 184.11 2 s(13C), d(1H) = 32.28 2
s(1H). 19F chemical shifts were referenced to HF and converted to the usual
CFCl3 scale; d(19F) = 185 2 s(19F). Calculated NMR data from geometry
2a dB 24.3, dH 7.39 (CHCH3), 5.42 (CHBF2), 1.97 (CH3), dC 169.6
(CH3CH), 124.0 (CHBF2), 25.3 (s; CH3), dF 274.
[B3H8]2 + BF3 ? {B3H7} + [HBF3]2
2{B3H7} + BF3 + MeC·CH ? B4H9F (1) + MeCHNCHBF2 (2)
+ 1/n(B2H4)n
2[B3H8]2 + 3BF3 + MeC·CH ? B4H9F (1) + 2[HBF3]2
+
MeCHNCHBF2 (2) + 1/n(B2H4)n
In considering the mechanism of this reaction, several addi-
tional pieces of experimental evidence are highly informative:
(i) no reaction occurs at 0 °C between B4H10 and BF3 in the
presence of propyne, (ii) no reaction takes place when 6 mmol
each of B4H10 and MeC·CH are condensed into a flask
containing the solid ‘[HBF3]2’, and stirred for 4 h at 0 °C, (iii)
no reaction is observed between BF3 and propyne in the absence
of [B3H8]2.
Cartesian coordinates and relative energies of optimised geometries of
2-FB4H9 (1a, 1b), trans-MeCHNCHBF2 (2a, 2b), 2-MeB4H9 and 2-BrB4H9
and calculated NMR shifts generated from optimised geometries of
2-MeB4H9 and 2-BrB4H9 are available as ESI†.
From these observations it is clear that the fluorotetraborane
does not result from the rapid reaction with either BF3 or
[HBF3]2 of any B4H10 that might have been formed in situ. It
therefore seems reasonable to conclude that the first step does
indeed involve the abstraction of a hydride ion from [B3H8]2 to
generate {B3H7}, but that this reactive intermediate (which in
the absence of propyne would react with itself to give B4H10) is
then trapped by the alkyne in a hydroboration process. The
resulting intermediate (presumably MeCHNCHB3H6), could
then react with BF3 in a double displacement reaction to yield
the fluoroborane 2 and {B3H6F}. Subsequent reaction of the
latter with {B3H7} would account for the formation of the
fluorotetraborane 1. This interpretation of the evidence implies
that, under the conditions of the experiment, {B3H7} reacts
more readily with both the alkyne and {B3H6F} than with
itself.
In summary, the syntheses of exo-2-FB4H9 1 and trans-
MeCHNCHBF2 2 have been obtained unexpectedly, in good
yields, from a one-pot reaction involving commercially availa-
ble starting materials. The known compounds, 2-bromote-
traborane6,7 and vinyldifluoroborane,9 related respectively to 1
and 2, have been reported as products from difficult, multi-step
syntheses, the latter involving divinylmercury as a reagent. The
present study suggests that these compounds might be obtained
much more conveniently from the appropriate boron trihalide
and alkyne. There is also scope for extending this work to the
exploration of reactions involving other borane anions and
unsaturated hydrocarbons.
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We thank the EPSRC for funding, for the award of an EPSRC
Advanced Research Fellowship to M. A. F., and a quota award
to D. L. O.
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Notes and references
§ Syntheses, separation and mass spectra of B4H9F 1 and MeCHNCHBF2 2:
the gases BF3 (13 mmol) and MeC·CH (13 mmol) were condensed at 2196
°C into a 1 l flask containing 3.03 g (13 mmol) of NMe4B3H8 and a magnetic
stirrer bar. The flask was warmed to 220 °C (carbon tetrachloride–liquid
nitrogen slush) for 3 h followed by cooling to 2196 °C and the non-
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