The platinum catalysed diboration of alkynes using 1,2- B2Cl2(NMe2)2: Formation of 1-azonia-2-borata-5-borole derivatives

Article abstract of DOI:10.1039/a907255a

The platinum catalysed diboration of alkynes using the diborane(4) compound 1,2-B₂Cl₂(NMe₂)₂ affords high yields of cyclic 1-azonia-2- borata-5-borole compounds, which arise from redistribution of B-Cl and B- NMe₂ bonds.

Full text of DOI:10.1039/a907255a

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The platinum catalysed diboration of alkynes using 1,2-
B Cl (NMe ) : formation of 1-azonia-2-borata-5-borole derivatives
2 2 2 2
Kirsty M. Anderson, M. J. Gerald Lesley,* Nicholas C. Norman,* A. Guy Orpen and Jonathan
Starbuck
L e t t e r
T he University of Bristol, School of Chemistry, CantockÏs Close, Bristol, UK BS8 1T S
Received (in Montpellier, France) 6th September 1999, Accepted 22nd September 1999
The platinum catalysed diboration of alkynes using the
diborane(4) compound 1,2-B Cl (NMe ) a†ords high yields of
being attributable to the coordination numbers of the two
boron atoms [in the diagrams, the B(1)ÈN(1) bond is rep-
resented as a coordinate bond]. In all of 3a–d, the solution
NMR data are consistent with the retention of this structure
in solution. In particular, the disparate 11B chemical shifts are
characteristic of three- and four-coordinate boron environ-
ments, and the observed inequivalence of the exocyclic amido
methyl groups (at room temperature) is consistent with hin-
dered rotation about the exocyclic BÈN bond.
2
2
2 2
cyclic 1-azonia-2-borata-5-borole compounds, which arise from
redistribution of B–Cl and B–NMe bonds.
2
Transition metal boryl compounds are known to be active
catalysts for the diboration of unsaturated organic substrates1
although in most cases the diborane(4) compounds employed
have been either B (cat) (cat \ 1,2-O C H )1,2 (and alkylated
2
2
2 6 4
The mechanism by which 3a–d are formed remains unclear
although an initial diboration product A, which subsequently
rearranges to 3, is likely in view of the known structure of
derivatives thereof) or B (pin) (pin \ 1,2-O C Me ).1,3
2
2
2 2
4
Herein we describe some preliminary results from a study of
platinum catalysed alkyne diboration4 using 1,2-
B Cl (NMe ) (1)5 as the diboration reagent, the chemistry of
which is signiÐcantly di†erent. Prior reactivity studies involv-
ing transition metal complexes and 1 have yielded products
derived from both BÈB and BÈCl activation, an example
of the former being our own report of the reaction be-
the
platinum
boryl
catalyst
precursor
cis-
2
2
2 2
[Pt(PPh ) MBCl(NMe )N ]6 in which each boron retains the
3 2
2 2
one chlorine and one amido group present in the starting
diborane(4) compound 1. In relation to this study, particularly
with regard to any mechanistic speculation, we note that
Berndt et al. have described the uncatalysed reaction between
1 and the silyl alkynes Me SiC3 CSiMe and PhC3 CSiMe at
tween
1
and
[Pt(PPh ) (g-C H )]
a†ording
cis-
3 2
2
4
[Pt(PPh ) MBCl(NMe )N ]6 and the latter exempliÐed by the
3
3
3
3 2
2 2
150 ¡C. Spectroscopic data on the products formed are consis-
tent with species containing unrearranged boryl groups but in
work of Braunschweig involving the formation of diboran(4)yl
derivatives.7
The diboration of internal alkynes (2a–d) employing 1 in the
presence of 5 mol% [Pt(PPh ) (g-C H )] proceeded over 24 h
3 2
2 4
in toluene at 95 ¡C to give compounds formulated as cyclic
1-azonia-2-borata-5-borole derivatives (3a-d) in excellent yield
and purity (Scheme 1).¤ Analytical and spectroscopic data¤
were in accord with the proposed structures, which, for 3a, 3c
and 3d, were conÐrmed by X-ray crystallography.” A view of
3c is shown in Fig. 1. Key features of the 1-azonia-2-borata-5-
borole structure are the presence of three- and four-coordinate
boron centres [B(2) and B(1), respectively] and three- and
four-coordinate nitrogen centres [N(2) and N(1), respectively].
The two three-coordinate centres are associated with the
shortest BÈN bond [B(2)ÈN(2) 1.381(2) Ó], consistent with a
BÈN double bond resulting from dative N ] B p-bonding.
The bonds B(1)ÈN(1) [1.622(2) Ó] and B(2)ÈN(1) [1.563(2) Ó]
are both consistent with BÈN single bonds, the di†erence
Fig. 1 A view of the molecular structure of 3c with key atoms
labelled. Atoms are drawn as spheres of arbitrary radius and hydro-
gen atoms omitted for clarity. Selected bond lengths (Ó) and angles
(deg) include B(1)ÈN(1) 1.622(2), B(2)ÈN(1) 1.563(2), B(2)ÈN(2) 1.381(2),
B(1)ÈCl(1) 1.894(2), B(1)ÈCl(2) 1.853(2), B(1)ÈN(1)ÈB(2) 98.76(11),
C(12)ÈB(1)ÈN(1) 102.32(12), C(13)ÈB(2)ÈN(1) 106.61(13), B(1)ÈC(12)È
C(13) 109.11(13), B(2)ÈC(13)ÈC(12) 109.60(13).
Scheme 1
New J. Chem., 1999, 23, 1053È1055
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 1999
1053

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which silyl/boryl rearrangement has occurred, resulting in 1,1-
isomers, such as (Me Si)(R)C2C[BCl(NMe )] (R \ Ph,
3
2 2
Me Si).8 More recently, Siebert et al. have presented spectro-
3
scopic data that reveal that alkenes derived from cis-1,2-
addition of B Cl
to alkynes, for example cis-(R)-
2
4
(BCl )C2C(BCl )(R@) (B), react with the silyl amide
2
2
Me SiNMe to a†ord compounds analogous to either 3
3
2
(R \ R@ \ Et, Me; R \ But, R@ \ H) or A (R \ R@ \ But).9
Reactions between B and Pri NH also a†orded species of type
2
A (R \ R@ \ H, Me, Et; R \ H, R@ \ But). MThis report9
described the structure of the bromo derivative cis-(But)-
[BBr(NMe )]C2C[BBr(NMe )](But), a structure analogous to
2
2
A, although this compound was not prepared by a route rele-
vant to this study.N The results of Siebert et al. indicate that
structures 3 and A are probably close in energy (with a subtle
dependence on the nature of the R group) and our preliminary
ab initio calculations10 carried out on 3b and its isomer of
type A conÐrm that the two isomers are very close in energy.°
Fig. 2 A view of the molecular structure of 4 with key atoms
labelled. Atoms are drawn as spheres of arbitrary radius and hydro-
gen atoms omitted for clarity with the exception of H(98). Selected
bond lengths (Ó) and angles (deg) include B(1)ÈN(1) 1.607(6), B(2)È
N(1) 1.604(5), B(2)ÈN(2) 1.616(6), B(1)ÈCl(1) 1.862(5), B(1)ÈCl(2)
1.904(5), B(2)ÈCl(3) 1.867(5), B(1)ÈN(1)ÈB(2) 101.3(3), C(12)ÈB(1)ÈN(1)
102.8(3), C(13)ÈB(2)ÈN(1) 101.6(3), B(1)ÈC(12)ÈC(13) 111.3(3), B(2)È
C(13)ÈC(12) 109.7(3).
In contrast to the reactions of internal alkynes, terminal
alkynes reacted more slowly. Thus, the platinum catalysed
reaction between 1 and 1-octyne (2e) gave 3e but was com-
plete only after three days. Reactions using phenylacetylene
(2f) were incomplete even after several days although the reac-
tion between phenylacetylene and 1 in the presence of a stoi-
chiometric amount of [Pt(PPh ) (g-C H )] did a†ord the
desired product (3f) after 3 h (Scheme 1).Ò Competing activa-
tion of the CH bonds in terminal alkynes has been noted in
catalytic reactions involving alkoxy substituted diborane(4)
compounds with phenylacetylene.4e
crystallography** (Fig. 2). In most respects, the structure of 4
is similar to that of 3c with the exception of the environment
around B(2), which now carries a chlorine and a datively
3 2
2 4
bound NHMe amine ligand; pertinent metric data are given
2
in the caption to Fig. 2, which reveal that all BÈN bond
The availability of crystalline 3a-d prepared in high yields
under mild conditions has enabled an initial study of their
reactivity to be carried out, preliminary details of which are
reported here for 3c with HCl, alcohols, diols and dilithiofer-
rocene (Scheme 2). A previous study by Siebert et al., wherein
cis-(R)[BCl(NPri )]C2C[BCl(NPri )](R) (R \ H, Et) was
lengths are the same within experimental error, as expected
for bonds between four-coordinate boron and nitrogen
centres. Attempts to prepare an amine adduct of
a
bis(dichloroboryl)ethene derivative (cf., B above) through
addition of a second equivalent of HCl across the B(2)ÈN(1)
bond were not successful. Thus, the reaction between 3c and 4
equiv. HCl also a†orded 4 whilst with the use of 10 equiv. of
HCl, signiÐcant decomposition was evident by NMR.
2
2
reacted with Me SnLi a†ording cis-(R)[B(SnMe )(NPri )]-
3
3
2
C2C[B(SnMe )(NPri )](R), is noted.11 Thus, 3c reacts with
3
2
two equiv. of HCl in Et O to give 4 in high yield,A resulting
The addition of one equivalent of water in ethanol to a
solution of 3c in toluene a†orded a compound formulated as
the oxaborole species 5a on the basis of the 11B NMR chemi-
cal shift and the 1H NMR spectrum integration (two equiva-
lents of [NH Me ]Cl were also produced).¤¤ In the absence
2
from the addition of HCl across the exocyclic BÈN bond. 11B
NMR data were consistent with the presence of two distinct
four-coordinate boron centres whilst the 1H NMR spectrum
revealed the presence of a coordinated NHMe amine ligand;
2
2
2
the asymmetric boron centre [B(2)] results in inequivalent
of ethanol, reaction with 4 equiv. of water in toluene yielded
(diastereotopic) ring NMe and coordinated NHMe methyl
5b plus two equivalents of [NH Me ]Cl.¤¤ The formulation
2
2
2
2
environments. The structure of 4 was conÐrmed by X-ray
of 5b as an oxaborole species rather than a bis(boronic acid)12
Mi.e., (R)[B(OH) ]C2C[B(OH) ](R)N was conÐrmed by obtain-
2
2
ing a 1H NMR spectrum in CD OD wherein an OH signal
3
integrating to six protons was observed, corresponding to four
from exchange with the [NH Me ]` cations and two from
2
2
BÈOH groups.”” Even in the presence of a large excess of
water, no evidence was obtained for the formation of a
bis(boronic acid) with only stilbene and boron containing
compounds of unknown composition being formed. Reaction
of 3c with 2.2 equiv. catechol a†orded the known4e
bis(catecholatoboryl) derivative 6 quantitatively according to
in situ 1H and 11B NMR spectroscopy.
Finally, the reaction between 3c and 1,1@-dilithioferrocene
a†orded a species formulated as the [4]ferrocenophane com-
pound 7.°° The boron chemical shift of 7 (d 38.6) is consistent
with that found for three-coordinate boron centres in related
boron containing ferrocenophane derivatives, which have
attracted some attention as precursors to polyferrocene poly-
mers,13h15 but the fact that the symmetrical species 7 is
Scheme 2 R \ 4-MeC H (i) HCl; (ii) a, H O/EtOH, b, H O; (iii)
6
4
2
2
1,2-(HO) C H ; (iv) [Fe(C H Li) ]
2
6
4
5
4
2
1054
New J. Chem., 1999, 23, 1053È1055

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** Crystal data for 4: C
H
B Cl N , M \ 423.41, monoclinic,
formed here indicates that 3c is a synthetic equivalent of the
type A structure.
20 27 2
3 2
space group P2 /c (no. 14), a \ 20.496(9), b \ 9.775(4), c \ 23.894(13)
1
Ó, b \ 114.37(6)¡, U \ 4360(3) Ó3, Z \ 8, j \ 0.71073 Ó, k \ 0.428
mm~1, D \ 1.290 Mg m~3, F(000) \ 1776, T \ 173(2) K, 7670
calc
Acknowledgements
NCN and AGO thank the EPSRC for research support and
Johnson Matthey Ltd. are thanked for generous supplies of
platinum salts.
unique data, R1 \ 0.0563.
CCDC reference number 440/150. See http://www.rsc.org/
suppdata/nj/1999/1053/ for crystallographic Ðles in .cif format.
¤¤ NMR data for 5a (CD Cl ): 11B-M1HN d 30.2; 1H d 7.13 (d, 4H,
2
2
C H , J \ 7.8 Hz), 7.02 (d, 4H, C H , J \ 7.8 Hz), 4.19 (q, 4H,
6
4
HH
6
4
HH
CH CH ), 2.29 (s, 6H, C H Me), 1.31 (t, 6H, CH CH ). 5b (CD OD):
2
3
6
4
2
3
3
11B-M1HN d 24.5; 1H (including data for the [NH Me ]` cation, see
Notes and references
2
2
text) d 6.91 (s, 8H, C H ), 4.88 (s, 6H, OH), 2.69 (s, 12H, NH Me ),
6
4
2
2
2.24 (s, 6H, C H Me); 13C-M1HN d 139.2 (ipso-C), 136.9 (p-C), 130.2 (o-
¤ In a typical reaction, a solution of 1 (0.502 g, 2.78 mmol) in toluene
(5 cm3) containing [Pt(PPh ) (g-C H )]4e (0.035 g, 5 mol%) was
6
4
C), 129.7 (m-C), 21.3 (C H Me).
6
4
3 2
2 4
”” Evidence for oxaborole species was also seen in the mass spectra of
3b-d. Thus, in the case of 3d, HRMS data were obtained for the
species Cw(4-MeOC H )2C(4-MeOC H )B(NMe )OBz (NMe) ). Calcd
transferred to a YoungÏs tap tube containing 2c (0.478 g, 2.32 mmol),
which was then sealed and heated to 95 ¡C overnight. The toluene was
removed in vacuo and addition of Et O (5 cm3) resulted in the forma-
6 4 6 4 2
H B N O 364.213959, found 364.212954.
2
2
C
tion of a precipitate 3c (0.680 g, 76%) that was isolated by Ðltration
20 26 2
2 3
°° NMR data for 7 (CDCl ): 11B-M1HN d 38.6; 1H d 7.02 (d, 4H,
and washed with hexane (3 ] 5 cm3). Slow di†usion of hexane into
the initial Ðltrate resulted in the formation of a crop of red crystals of
3c, one of which was used for X-ray crystallography. NMR data for 3c
(C D ): 11B-M1HN d 34.4 (BÈNMe ), 9.1 (BCl ); 1H d 7.51 (d, 2H,
3
C H , J \ 8.6 Hz), 6.68 (d, 4H, C H , J \ 8.6 Hz), 4.28 (br s,
6
4
HH
6
4
HH
4H, C H ), 4.19 (br s, 4H, C H ), 3.74 (s, 6H, NMe ), 2.98 (s, 6H,
5
5
5
5
2
NMe ), 2.86 (s, 6H, C H Me).
2
6 4
7
8
2
2
C H , J \ 8.0 Hz), 6.89 (m, 4H, C H ), 6.87 (d, 2H, C H , J
\
6
4
HH
6
4
6
4
HH
8.0 Hz), 2.51 (s, 6H, B NMe ), 2.11 (s, 3H, BNMe ), 2.08 (s, 3H,
1 G. J. Irvine, M. J. G. Lesley, T. B. Marder, N. C. Norman, E. G.
Robins, W. Roper, C. R. Rice, G. R. Whittell and G. J. Wright,
Chem. Rev., 1998, 98, 2685; T. B. Marder and N. C. Norman,
T opics in Catalysis, ed. W. Leitner and D. G. Blackmond, Baltzer
Science Publishers, Amsterdam, 1998, vol. 5, p. 63.
2
2
2
BNMe ), 2.03 (s, 3H, C H Me), 2.02 (s, 3H, C H Me); 13C-M1HN d
2
6
4
6 4
138.9, 137.3 (ipso-C), 136.2, 135.5 (p-C), 129.6, 129.4 (o-C), 129.2, 128.5
(m-C), 44.4 (B NMe ), 42.2, 38.9 (BNMe ), 21.2, 21.1 (C H Me). 11B,
2
2
2
6 4
1H and 13C spectra were referenced to BF É Et O, Me Si and Me Si,
3 2 4 4
respectively. HRMS (EI) for C
H B Cl N : Calcd 384.173 181,
2 F. J. Lawlor, N. C. Norman, N. L. Pickett, E. G. Robins, P.
Nguyen, G. Lesley, T. B. Marder, J. A. Ashmore and J. C. Green,
Inorg.Chem., 1998, 37, 5282.
20 26 2
2 2
found 384.171 333. Compounds 3a, 3b and 3d were prepared similarly.
Selected data for 3a: 11B-M1HN NMR (C D ) d 33.8 (BÈNMe ), 9.5
7
8
2
(BCl ); 3b: 11B-M1HN NMR (C D ) d 34.2 (BÈNMe ), 9.0 (BCl ),
3 H. Noth, Z. Naturforsch., T eil B, 1984, 39, 1463.
2
7
8
2 2
Calcd 356.141 880, found
HRMS (EI) for
C
H
B Cl N
:
4 (a) T. Ishiyama, N. Matsuda, N. Miyaura and A. Suzuki, J. Am.
Chem. Soc., 1993, 115, 11018; (b) F. Ozaura, A. Suzuki and N.
Miyaura, Organometallics, 1996, 15, 713; (c) C. N. Iverson and M.
R. Smith, J. Am. Chem. Soc., 1995, 117, 4403; (d) C. N. Iverson and
M. R. Smith, Organometallics, 1996, 15, 5155; (e) M. J. G. Lesley,
P. Nguyen, N. J. Taylor, T. B. Marder, A. J. Scott, W. Clegg and
N. C. Norman, Organometallics, 1996, 15, 5137.
18 22 2
2
2
356.140 419; 3d: 11B-M1HN NMR (C D ) d 34.7 (BÈNMe ), 8.7 (BCl ),
7 8 2 2
HRMS (EI) for
C H B Cl N O : Calcd 416.163 010, found
20 26 2
2 2 2
416.162 476. The 11B resonances for the BÈNMe borons for all 3 are
2
signiÐcantly broader than those for the BCl borons consistent with
2
their respective coordination geometries. Satisfactory 1H and 13C
NMR data were also obtained for 3a, 3b and 3d. All alkynes were
either procured commercially or prepared by literature methods.
5 H. Noth and W. Meister, Z. Naturforsch., T eil B, 1962, 17, 714.
6 D. Curtis, M. J. G. Lesley, N. C. Norman, A. G. Orpen and J.
Starbuck, J. Chem. Soc., Dalton T rans., 1999, 1687.
7 H. Braunschweig, Angew. Chem., Int. Ed., 1998, 37, 1787.
8 H. Klusik, C. Pues and A. Berndt, Z. Naturforsch., T eil B, 1984,
39, 1042.
9 W. Siebert, M. Hildenbrand, P. Hornbach, G. Karger and H.
Pritzkow, Z. Naturforsch., T eil B, 1989, 44, 1179.
10 M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G.
Johnson, M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Petersson,
J. A. Montgomery, K. Raghavachari, M. A. Al-Laham, V. G.
Zakrzeqski, J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Stefa-
nov, A. Nanayakkara, M. Challacombe, C. Y. Peng, P. Y. Ayala,
W. Chen, M. W. Wong, J. L. Andres, E. S. Replogle, R. Gomperts,
R. L. Martin, D. J. Fox, J. S. Binkley, D. J. Defrees, J. Baker, J. P.
Stewart, M. Head-Gordon, C. Gonzalez and J. A. Pople, Gaussian
94, revision D.4, Gaussian Inc., Pittsburgh, PA, 1995.
11 P. Frankhauser, H. Pritzkow and W. Siebert, Z. Naturforsch., T eil
B, 1994, 49, 250.
12 N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457.
13 M. J. G. Lesley, U. Mock, N. C. Norman, A. G. Orpen, C. R. Rice
and J. Starbuck, J. Organomet. Chem., 1999, 582, 116.
14 M. Herberhold, U. DorÑer and B. Wrackmeyer, J. Organomet.
Chem., 1997, 530, 117.
15 H. Braunschweig, R. Dirk, M. Muller, P. Nguyen, R. Resendes, D.
P. Gates and I. Manners, Angew. Chem., Int. Ed. Engl., 1997, 36,
2338.
” Crystal data for 3a: C
H B Cl N , M \ 296.83, monoclinic,
13 20 2
2 2
space group P2 /c (no. 14), a \ 16.121(3), b \ 7.1894(6),
1
c \ 13.5772(17) Ó, b \ 95.567(15)¡, U \ 1566.2(4) Ó3, Z \ 4,
j \ 0.71073 Ó, k \ 0.40 mm~1, T \ 173(2) K. Crystal data for 3c:
C
H B Cl N , M \ 386.95, monoclinic, space group P2/c (no. 13),
20 26 2
2 2
a \ 12.410(2), b \ 12.8779(14), c \ 13.0655(16) Ó, b \ 90.428(14)¡,
U \ 2088.0(5) Ó3, Z \ 4, j \ 0.71073 Ó, k \ 0.317 mm~1, D
calc
1.231 Mg m~3, F(000) \ 816, T \ 173(2) K, 4779 unique data,
\
R1 \ 0.0372. Crystal data for 3d: C
H B Cl N O , M \ 418.95,
20 26 2
2 2 2
monoclinic, space group P2 /c (no. 14), a \ 10.870(3), b \ 12.404(2),
1
c \ 16.086(3) Ó, b \ 92.185(9)¡, U \ 2167.3(7) Ó3, Z \ 4, j \ 0.71073
Ó, k \ 0.32 mm~1, T \ 173(2) K.
° Gas phase energies of 3b and its type A isomer were evaluated by
full geometry optimisation at the SCF 6-31G level, followed by single
point energy calculations with 6-31G** basis sets. At this level the
acyclic A isomer is 6.9 kcal mol~1 more stable. We note however, that
these calculations do not model any intermolecular interactions that
might inÑuence the preferred geometry in condensed phases.
Ò 11B NMR data (C D ) for 3e: 11B-M1HN d 37.9 (BÈNMe ), 11.1
7
8
2
(BCl ); 3f: 36.2 (BÈNMe ), 9.1 (BCl ). Satisfactory 1H NMR data
2
2
2
were also obtained.
p NMR data for 4 (CD Cl ): 11B-M1HN d 8.7 (sh), 7.3 (sh); 1H d 7.03
2
2
(d, 2H, C H , J \ 8.1 Hz), 6.96 (d, 2H, C H , J \ 8.1 Hz), 6.92
HH HH
(s, 4H, C H ), 6.59 (br s, 1H, NHMe ), 2.84 (s, 3H, NMe ), 2.74 (d, 3H,
6
4
6 4
6
4
2
2
NHMe , 3J \ 5.5 Hz), 2.72 (s, 3H, NMe ), 2.37 (d, 3H, NHMe ,
HH
2
2
2
3J \ 5.5 Hz), 2.25 (s, 3H, C H Me), 2.24 (s, 3H, C H Me); 13C-
HH
6
4
6 4
M1HN d 157.8, 147.5 (BC2CB), 139.3, 138.4 (ipso-C), 135.5 (p-C), 130.3,
128.9 (o-C), 128.6, 128.5 (m-C), 49.4, 42.8, 40.1, 40.3 (NMe and
2
NHMe ), 21.4, 21.3 (C H Me).
L etter 9/07255A
2
6 4
New J. Chem., 1999, 23, 1053È1055
1055