Synthetic routes to cyclic and unsymmetric diborane(4) compounds

Article abstract of DOI:10.1039/a909645k

The reaction between 1,2-B₂Cl₂(NMe₂)₂ and disodium catecholate Na₂[1,2-O₂C₆H₄] affords the cyclic diborane(4) compound 1,2- B₂(NMe₂)₂(cat) (cat = 1,2-O₂C₆H₄), whereas a similar reaction using dilithium thiocatecholate Li₂[1,2-S₂C₆H₄] affords the 1,1-isomer 1,1- B₂(NMe₂)₂(thiocat) (thiocat = 1,2-S₂C₆H₄). Both compounds can be used to prepare unsymmetric diborane(4) species.

Full text of DOI:10.1039/a909645k

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Synthetic routes to cyclic and unsymmetric diborane(4) compounds
M. J. Gerald Lesley,* Nicholas C. Norman,* A. Guy Orpen and Jonathan Starbuck
T he University of Bristol, School of Chemistry, CantockÏs Close, Bristol, UK BS8 1T S.
E-mail: N.C.Norman=bristol.ac.uk
L e t t e r
Received (in Montpellier, France) 3rd December 1999, Accepted 12th January 2000
The reaction between 1,2-B Cl (NMe ) and disodium cate-
2 2
cholate Na [1,2-O C H ] a†ords the cyclic diborane(4) com-
2 6
narity of all atoms associated with the BN unit, although this
results in a marked twisting of the molecule in order to avoid
unfavourable steric interactions between the C(5) methyl
groups [torsion angle N(1)ÈB(1)ÈB(1A)ÈN(1A) 31.2¡; angle
between the mean planes O(1), B(1), N(1) and O(1A), B(1A),
N(1A) 27.0¡]. Separate methyl group resonances were also
observed in the 1H NMR spectrum of 6, consistent with hin-
dered rotation about the BÈN bond. We note a related reac-
tion between 4 and K [1,2-(CH ) C H ] that also a†ords a
2
2
2
4
pound 1,2-B (NMe ) (cat) (cat = 1,2-O C H ), whereas a
2
2 2
2 6 4
similar reaction using dilithium thiocatecholate Li [1,2-
2
S C H ] a†ords the 1,1-isomer 1,1-B (NMe ) (thiocat)
2 6
4
2
2 2
(thiocat = 1,2-S C H ). Both compounds can be used to
2 6
4
prepare unsymmetric diborane(4) species.
Diborane(4) compounds have recently been the focus of
renewed interest in part because they are key reagents in tran-
sition metal catalysed diboration reactions1 for which the
most widely used are the diolate species B (pin) (1)
2
2 2 6 4
1,2-isomer, viz. 1,2-B (NMe ) [1,2-(CH ) C H ].10
2 2 2 2 6
2
4
2
2
(pin \ pinacolate) and B (cat) (2) (cat \ 1,2-O C H ).2 In
2
2
2 6 4
addition to the boron(II) halides,3 other well characterised and
synthetically useful compounds include the amido species
B (NMe ) 4 (3) and 1,2-B Cl (NMe ) 5 (4) and the thiocate-
2
2 4
2
2
2 2
cholate derivative B (1,2-S C H ) (5).2b From a structural
The reaction between 6 and catechol in Et O, followed by
2
2 6 4 2
2
perspective, it is noteworthy that compounds 1, 2 and 5, and
almost all related species that have been structurally charac-
terised, exist as 1,1-isomers,6 as shown in the diagram for 2,
rather than as an alternative 1,2-isomer (e.g., 2a), although 1,2-
isomers are known, as in the case of the binol derivative
addition of 2 equiv. of HCl, was carried out with the expecta-
tion, by analogy with the reaction between 3 and catechol,2b,°
of forming B (cat) as the 1,2-isomer 2a. However, although
2
2
the reaction proceeded cleanly, the compound isolated was
found to be the 1,1-isomer 2 as conÐrmed by spectroscopic2b
and X-ray crystallographic methods.6 The fact that 2 rather
than 2a was formed indicates that the 1,1-isomer is probably
thermodynamically favoured and results from a catecholate
group rearrangement process in solution Msimilar isomeric
rearrangements have been suggested by Noth and co-workers
B (O C
H
) (O C
H
\ binaphthalenolate or binolate).7
2
2 20 12
2 20 12
In fact, this topic of 1,1- vs. 1,2-isomers has a long history and
was Ðrst addressed by Shore and co-workers8 for a range of
O, S and N substituted species and later by Noth et al.,9
although in most cases deÐnitive structural data were lacking.
We set out to determine whether a 1,2-isomer of 2 (i.e., 2a)
could be prepared and describe some preliminary results
herein.
for
the
compounds
B (NR ) [N(Me)CH CH N(Me)]
2
2 2
2
2
The reaction between 1,2-B Cl (NMe ) (4) and disodium
2
2
2 2
catecholate (Na [1,2-O C H ]) in toluene a†orded, after
2
2 6 4
work-up, good yields of a colourless crystalline solid charac-
terised as 1,2-B (NMe ) (cat) (6).¤ The appearance of a single
2
2 2
11B NMR chemical shift (d 29.1)¤ was consistent with 6 being
B
a 1,2-isomer [two signals close to the values for 2 (d 29.02b)
B
and 3 (d 34.92b,4) would be expected for the inequivalent
B
borons in a 1,1-isomer] and this was conÐrmed by X-ray crys-
tallography (Fig. 1).” The molecules of 6 lie on a crystallo-
graphic C axis and the BÈB bond is bridged in a 1,2-fashion
2
by the catecholate, with each boron carrying a terminal NMe
2
group. The BÈO [1.411(3) A] distance in 6 di†ers from those
observed in 2 [BÈO 1.382(2), 1.394(2) AŽ 6], probably as a result
of signiÐcantly less p-donation from oxygen to boron in 6, due
Fig. 1 A view of the molecular structure of 6 showing the atom
numbering scheme. Atoms are drawn as spheres of arbitrary radius.
Selected bond lengths (A) and angles (deg) include B(1)ÈB(1A) 1.707(4),
B(1)ÈN(1) 1.391(3), B(1)ÈO(1) 1.411(3), O(1)ÈC(3) 1.373(2), B(1A)ÈB(1)È
N(1) 131.52(13), B(1A)ÈB(1)ÈO(1) 113.73(11), N(1)ÈB(1)ÈO(1)
114.68(18).
to the presence of a strongly p-donating NMe group [BÈN
2
1.391(3) A]. The BÈB bond is also longer than in 2 [BÈB
1.707(4) A, cf. 1.675(3) A in 26], possibly as a result of its
incorporation into a six-membered ring. That N ] B p-
donation is important is further established by the near copla-
DOI: 10.1039/a909645k
New J. Chem., 2000, 24, 115È117
115
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2000

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(R \ Me, Et) based on spectroscopic data9bN. However, this
observation indicated that reactions between 6 and other diols
might a†ord unsymmetrical diborane(4) compounds (an aim
originally envisaged though not realised by Welch and
Notes and references
¤ Equimolar quantities of 4 (3.518 g, 19.5 mmol) and Na [1,2-
2
O C H ] (3.00 g, 19.5 mmol) were stirred in toluene for 3 h. Fil-
2
6 4
tration, reduction of the Ðltrate volume and cooling a†orded colour-
Shore8a). Thus, B (cat)(pin) (7)Ò (probably as a 1,1-isomerÒ)
2
less crystals of 6 (66%). Recrystallisation from MeCN and cooling to
was the major product in the reaction between 6 and pinacol
[26 ¡C a†orded X-ray quality crystals. Traces of 4 (d 35.7) and
although appreciable amounts of 1 and 2 were also formed.
The reaction between 6 and tetrachlorocatechol also a†orded
mixtures of 2, B (1,2-O C Cl ) 13 and B (cat)(1,2-O C Cl )
B
B(NMe )(cat) (d 23.8) were observed by 11B NMR. Spectroscopic
2
B
data for 6: NMR (CD Cl ) 1H d 6.98 (m, 2H, 1,2-O C H ), 6.85 (m,
2
2
2 6 4
2H, 1,2-O C H ), 2.90 (s, 6H, NMe ), 2.83 (s, 6H, NMe ); 13C-M1HN d
2
2 6 4 2
2
2 6 4
2
6
4
2
2
(8) as evidenced by mass spectrometry.Ò Both reactions indi-
cate that intermolecular (and possibly intramolecular) diolate
group rearrangement is occurring (see also ref. 9b), possibly
catalysed by any excess HCl present.
145.0 (CÈO), 121.5 (cat CH), 119.3 (cat CH), 40.9 (NMe ), 35.4
2
(NMe ); 11B-M1HN d 29.1 (br s). C
H
B N O requires C, 55.15; H,
2
10 16 2 2 2
7.40; N, 12.85. Found C, 53.40; H, 6.35; N, 12.65.
” Crystal data for 6: C
10 16 2
group C2/c, a \ 11.931(7), b \ 11.105(11), c \ 8.929(5) A,
H B N O , M \ 217.86, monoclinic, space
2
2
Attempts to prepare B (NMe ) (pin) (9), the pinacolate ana-
b \ 90.96(4)¡, U \ 1182.8(15) AŽ 3, Z \ 4, j \ 0.710 73 A, k \ 0.082
2
2 2
logue of 6, by the reaction of 4 with disodium pinacolate
mm~1, T \ 173(2) K, D \ 1.223 Mg m~3, F(000) \ 464, 930
calc
Na [O C Me ] (the dilithium salt reacts similarly) a†orded a
unique data, R \ 0.0402. CCDC reference number 440/162. See
1
2
2 2
4
http://www.rsc.org/suppdata/nj/a9/a909645k/ for crystallographic
mixture of 1 and 3 although 11B NMR data were consistent
Ðles in .cif format.
with the presence of small amounts of 1,1-B (NMe ) (pin) (d
2
2 2
B
° The reaction between 4 and 2 equiv. of either catechol or pinacol
provides an alternative synthetic route to 2 and 1, respectively,
although it o†ers no particular advantage over published procedures
starting from 3.2,11 The complex [B Cl (NHMe ) ]2b,12 may also be
38.8, 29.7; cf. 3 d 34.9, 1 d 28.5); the possibility that a 1,2-
B
B
isomer is present in solution cannot be ruled out. The reaction
between 4 and the dilithium salt of dithiocatechol in tolueneÈ
THF, however, proceeded much more cleanly, a†ording high
yields of 1,1-B (NMe ) (thiocat) (10) (thiocat \ 1,2-S C H )
2
4
2 2
employed as the boron source.
Ò Compound 6 was generated in situ in THF, which was followed by
2
2 2
2 6 4
addition of 1 equiv. of pinacol and a slight excess of HCl in Et O ([2
identiÐed unambiguously as the 1,1-isomer by the disparate
2
equiv.) at [78 ¡C. Filtration, removal of all volatiles from the Ðltrate
11B NMR chemical shifts (d 61.6, 31.8, cf. 5 d 57.9,2b 3 d
and extraction in hexane a†orded 1 and 7; subsequent extraction in
benzene a†orded 2. Spectroscopic data for 7: NMR (C D ) 1H d 7.03
B
B
B
34.9).p The reason why 6 is formed as a 1,2-isomer whereas 10
is observed as a 1,1-isomer is unclear although the fact that 6
is not very stable in solution,¤ together with the diolate
rearrangements referred to above, indicate that the 1,2-isomer-
ic forms might be kinetic products whereas the 1,1-isomers are
thermodynamically more stable. To shed light on this matter
ab initio electronic structure calculations on the 1,1- and 1,2-
isomers of 2, 5, 6 and 10 were carried out. Geometries were
optimised at the HF/6-31G level of theory with 2 and 5
having D symmetry and 6 and 10 having C symmetry.14
6
6
(m, 2H, 1,2-O C H ), 6.78 (m, 2H, 1,2-O C H ), 1.04 (s, 12H, pin);
2
6
4
2 6 4
11B-M1HN d 29.3 (br s) [although only one 11B NMR signal was seen
for 7, the chemical shifts of 1 (28.5) and 2 (29.0) are very similar,
making it difficult to distinguish between possible 1,1- and 1,2-
isomers]. HRMS calculated for C
246.122 677. Compound 8 was prepared by generating 6 in toluene
and adding tetrachlorocatechol. All volatiles were then removed and
H B O 246.123 470. Found
12 16 2
4
the white solid dissolved in Et O to which HCl was added. Extraction
2
in toluene a†orded mostly 2, leaving a mixture of 8 and B (1,2-
2
O C Cl ) as a white solid. Spectroscopic data for 8: LRMS m/z 376
2
6 4 2
2h
2
(M`).
Single point B3LYP/6-31G*//HF/6-31G calculations were
carried out to obtain reliable energies. These results are in
good agreement with the experimental observations: the 1,1-
isomers of 2, 5 and 10 are the more stable (by 3.0, 19.3 and 6.0
kcal mol~1, respectively) whereas the 1,2-isomer of 6 is the
more stable by 7.7 kcal mol~1.**
p A solution of 4 (0.470 g, 2.60 mmol) in toluene (20 cm3) was added
via syringe to a solution of Li [1,2-S C H ] É THF in THF (0.576 g,
2
2 6 4
2.25 mmol) cooled to 0 ¡C. After stirring overnight and warming to
room temperature, a white precipitate was removed by Ðltration and
all volatiles removed from the Ðltrate. Extraction into hexane, further
Ðltration and vacuum pumping a†orded an oily solid comprising 10
(90%). Spectroscopic data for 10: NMR (CDCl ) 1H d 7.85 (m, 2H,
Treatment of 10 in THF with either pinacol and HCl or
3
1,2-S C H ), 7.29 (m, 2H, 1,2-S C H ), 2.72 (s, 12H, NMe ); 11B-M1HN
catechol and HCl a†orded B (thiocat)(pin) (11) or
2
6
4
2
6
4
2
d 61.6, 31.8 (br s).
2
B (thiocat)(cat) (12), respectively (presumably as 1,1-isomers),
2
** To calibrate and validate this computational approach, further cal-
culations were carried out on models of 2, 5, 6 and 10. The models
di†er from the structures discussed in the text in that the 1,2-
disubstituted benzene rings are replaced with cis-1,2-disubstituted
ethene units and the dimethylamino fragments are replaced by amino
groups. For the model systems, B3LYP/6-31G*//HF/6-31G energies
were calculated as above, yielding similar results as for the full com-
pounds; full geometry optimisation at the B3LYP/6-31G* level was
also performed. The geometries were very similar to the HF/6-31G
values and the relative energies were essentially identical. The e†ects
of larger basis sets and other methods for treating electron correlation
were explored by performing full B3LYP/6-311]G* optimisations as
well as single point MP2/6-31G*//HF/6-31G calculations. All of these
calculations were in very close agreement with the B3LYP/6-31G*//
HF/6-31G results.
identiÐed by mass spectrometry, albeit as mixtures with the
respective symmetrical species 1 and 5 or 2 and 5.¤¤
¤¤ Spectroscopic data for 11: NMR (C D ) 1H d 7.53 (m, 2H,
6
6
1,2S C H ), 6.93 (m, 2H, 1,2-S C H ), 1.10 (s, 12H, pin); 11B-M1HN d
In conclusion, these preliminary results reveal that although
cyclic 1,2-species such as 6 can be formed, facile rearrange-
ment of diolate or dithiolate groups in solution allows access
to favoured 1,1-isomers with an unsupported BÈB bond. In
most cases, the formation of mixtures of unsymmetrical and
symmetrical products is synthetically undesirable although 6
and 10 can be prepared and isolated in high yields.
2
6
4
2 6 4
55.5, 29.7 (br s). HRMS calculated for C
12 16 2 2 2
Found 278.077 507. Spectroscopic data for 12: NMR (C D ) 1H d
H B O S 278.077 784.
6
6
7.51 (m, 2H, 1,2-S C H ), 7.08, (m, 2H, 1,2-O C H ), 6.90 (m, 2H,
2
6
4
2 6 4
1,2S C H ), 6.82, (m, 2H, 1,2-O C H ); 11B-M1HN d 53.2, 28.8 (br s).
2
6
4
2 6 4
HRMS calculated for C H B O S 270.015 184. Found 270.014 923.
12 8 2 2 2
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Acknowledgements
NCN and AGO thank the EPSRC for research support.
116
New J. Chem., 2000, 24, 115È117

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