Platinum(0)-catalyzed diboration of alkynylboronates and alkynylphosphonates with bis(pinacolato)diborane(4) (see abstract)

Article abstract of DOI:10.1021/om020334s

The platinum(0)-catalyzed diboration addition reaction of bis(pinacolato)diborane(4) [(Me₄C₂O₂)BB(O₂C₂Me₄ ), 1] with various 1-alkynylphosphonates and 1-alkynylboronates 3a-d gave the desired novel cis-1,2-diboronated vinylphosphonate and trisboronated alkene products 5a, 5b and 5c, 5d respectively, in high yields. No detectable amount of the desired trisboronated alkene products 5c and 5d were isolated when the alkynylboronate was immediately added to a toluene solution of the catalyst and bis(pinacolato)diborane(4) followed by stirring overnight at 80 °C. Under these conditions, cis-1,2-diboronated alkenes 6c and 6d were obtained in 100% conversion yields. Only after changing the reaction conditions were 5c and 5d obtained in high yields. The structure and configuration of the new compounds have been fully characterized by ¹H, ¹³C, ³¹P, and ¹¹B NMR, GCMS, elemental analysis, and single-crystal X-ray structure determination. The structure of 5d was found to be fully isomorphous to that of 5c, with the C₆H₅ ring located in place (and similarly disordered about the 2-fold symmetry axis) of the C₄H₉ residue; therefore the results of the diffraction analysis, crystal data, and details of the structure determination of 5d are not included.

Full text of DOI:10.1021/om020334s

Organometallics 2002, 21, 4533-4539
4533
P la tin u m (0)-Ca ta lyzed Dibor a tion of Alk yn ylbor on a tes
a n d Alk yn ylp h osp h on a tes w ith
Bis(p in a cola to)d ibor a n e(4): Molecu la r Str u ctu r es of
[((Me4C2O2)B)(C6H5)CdC(P (O)(OC2H5)2)(B(O2C2Me4))] a n d
[((Me4C2O2)B)(C4H9)CdC(B(O2C2Me4))2]
Hijazi Abu Ali,† Abed El Aziz Al Quntar,† Israel Goldberg,‡ and Morris Srebnik*,†
Department of Medicinal Chemistry and Natural Products, School of Pharmacy, Hebrew
University in J erusalem, J erusalem, Israel, and School of Chemistry, Tel-Aviv University,
Ramat-Aviv 69987, Tel-Aviv, Israel
Received April 26, 2002
The platinum(0)-catalyzed diboration addition reaction of bis(pinacolato)diborane(4)
[(Me4C2O2)BB(O2C2Me4), 1] with various 1-alkynylphosphonates and 1-alkynylboronates
3a -d gave the desired novel cis-1,2-diboronated vinylphosphonate and trisboronated alkene
products 5a , 5b and 5c, 5d respectively, in high yields. No detectable amount of the desired
trisboronated alkene products 5c and 5d were isolated when the alkynylboronate was
immediately added to a toluene solution of the catalyst and bis(pinacolato)diborane(4)
followed by stirring overnight at 80 °C. Under these conditions, cis-1,2-diboronated alkenes
6c and 6d were obtained in 100% conversion yields. Only after changing the reaction
conditions were 5c and 5d obtained in high yields. The structure and configuration of the
new compounds have been fully characterized by 1H, 13C, 31P, and 11B NMR, GCMS, elemental
analysis, and single-crystal X-ray structure determination. The structure of 5d was found
to be fully isomorphous to that of 5c, with the C6H5 ring located in place (and similarly
disordered about the 2-fold symmetry axis) of the C4H9 residue; therefore the results of the
diffraction analysis, crystal data, and details of the structure determination of 5d are not
included.
In tr od u ction
The development of new strategies in organic syn-
theses with a minimum of chemical steps is becoming
more and more important for the efficient assembly of
complex molecular structures.7
Derivatives of diborane(4) are an important class of
compounds in boron chemistry. Diborane(4) itself, B2H4,
is stable only when complexed by Lewis base ligands
such as amines or phosphines. Although the tetraha-
lides, B2X4 (X ) F, Cl, Br, I), have a reasonably well-
established chemistry, they suffer from low thermal
stability (with the exception of B2F4) and preparative
difficulties. Tetraorganodiborane compounds, B2R4, are
stable only when substituted with sterically demanding
R groups such as t-Bu, CH2-t-Bu, and mesityl. The most
stable derivatives are those in which good π-donor
groups are present such as amido (NR2) or alkoxy
(OR).1,2 More recently, as part of the interest in the
oxidative addition chemistry of the B-B bond and
metal-catalyzed diborations of alkenes3 and alkynes,4
syntheses of stable, crystalline bispinacolato and bis-
(catecholato)diborane(4) derivatives have been re-
ported.5,6
So the combination of multiple reactions in a single
operation represents a particularly efficient approach.
Among different strategies,8 geminated organobisme-
tallic derivatives (1,1-bisanions) are becoming more and
more useful. During the past decades considerable
efforts have been made to find new routes for the
preparation of geminated sp2 organobismetallic deriva-
tives and for their selective reactions with several
electrophiles.9 The addition of tetrakis(alkoxy)diborane-
(4) derivatives to unsaturated hydrocarbons is an at-
tractive and straightforward method to introduce two
boryl units or more into organic molecules.4a,10,11 In
(4) (a) Ishiyama, T.; Matsuda, N.; Murata, M.; Ozawa, F.; Suzuki,
A.; Miyaura, N. Organometallics 1996, 15, 713. (b) Iverson, C. N.;
Smith, M. R. J . Am. Chem. Soc. 1995, 117, 4403. (c) Cui, Q.; Musaev,
D. G. M.; Morokuma, K. Organometallics 1998, 17, 742. (d) Cui, Q.;
Musaev, D. G. M.; Morokuma, K. Organometallics 1998, 17, 1383. (e)
Siebert, W.; Pritzkow, H.; Maderna, A. Angew. Chem. Int. Ed. Engl.
1996, 35, 1501. (f) Clegg, W.; Scott, A. J .; Lesley, G.; Marder, T. B.;
Norman, N. C. Acta Crystallogr. 1996, C52, 1989 and 1991.
(5) No¨th, H. Z. Naturforsch. 1984, 39b, 1463.
† Hebrew University in J erusalem.
‡ Tel-Aviv University.
(1) Abu Ali, H.; Goldberg, I.; Srebnik, M. Eur. J . Inorg. Chem. 2002,
73.
(2) Brotherton, R. J .; McCloskey, J . L.; Petterson, L. L.; Steinberg,
H. J . Am. Chem. Soc. 1960, 82, 6242.
(3) (a) Baker, R. T.; Nguyen, P.; Marder, T. B.; Westcott, S. A. Angew.
Chem., Int. Ed. Engl. 1995, 34, 1336. (b) Ishiyama, T.; Yamamoto, M.;
Miyaura, N. Chem. Commun. 1997, 689. (c) Marder, T. B.; Norman,
N. C.; Rice, C. R. Tetrahedron Lett. 1998, 39, 155. (d) Iverson, C. N.;
Smith, M. R. Organometallics 1997, 16, 2575.
(6) Lawlor, F. J .; Norman, N. C.; Pickett, N. L.; Robins, E. G. Inorg.
Chem. 1998, 37, 1777.
(7) Corey, E. J .; Cheng, X. The Logic of Chemical Synthesis; Wiley-
Interscience: New York, 1989.
(8) Frontiers in Organic Synthesis thematic issue. Chem. Rev. 1996,
96, 1.
(9) Mark, I. Chem. Rev. 2000, 100, 2887.
10.1021/om020334s CCC: $22.00 © 2002 American Chemical Society
Publication on Web 09/18/2002

4534 Organometallics, Vol. 21, No. 21, 2002
Sch em e 1. Syn th esis of 3a -d a n d 5a -d
Abu Ali et al.
1993, Ishiyama et al. reported the first synthesis of
isomerically pure cis-1,2-bis(boryl)alkenes via the plati-
num(0)-catalyzed addition of tetrakis(alkoxy)diborane-
(4) compounds to alkynes.12a,b Organoboron compounds
are among the most useful reagents in organic synthesis
since the carbon-boron bond can be cleaved in different
ways.11 The combination of the chemistry of 1,1-bisme-
tallic and 1,2-bismetallic compounds should open a way
to many interesting synthetic transformations. One
particularly useful common example is the Suzuki-
Miyaura palladium-catalyzed cross-coupling reaction
between aryl boronates or vinyl boronates for carbon-
carbon bond formation.12c This encouraged us to search
for new approaches for the synthesis of novel trisbor-
onated double-bond compounds and bisboronated vinyl
phosphonates through diboration of 1-alkynylboronates
and 1-alkynylphosphonates as described in Scheme 1.
The first 1-alkynylphosphonate was described in
1957,12d and the syntheses forming the basis of modern
1-alkynylphosphonates chemistry were elaborated in
the 1960s. On the other hand, the chemistry of 1-alkynyl-
boronates has been relatively little explored, apparently
due to the lack of suitable synthetic methodology for the
synthesis of these compounds.13 The breakthrough came
in 1983, when H. C. Brown reported a general synthesis
of 1-alkynyldiisopropoxyboranes in high yield.14 Later
on, this strategy was used for the preparation of large-
scale 1-alkynylboronic ester compounds.15 While it is
known that diboron tetrahalides B2X4 add across un-
saturated substrates in the absence of a catalyst,16 the
tetrakis(alkoxy)diborane(4) compounds fail to add to
alkenes or alkynes under conventional reaction condi-
tions.17,18 Thus, a metal catalyst is required to cleave
the B-B bond, generally via oxidative addition, to form
the intermediate metal bis(boryl) complex 2, as de-
scribed in detail in the proposed mechanism in Figure
1. Platinum complexes lead to different selectivities19a
or enable processes that are not possible with other
transition metal homogeneous catalysts. It is worth
highlighting that platinum complexes have two key
advantages as tools to investigate reaction mecha-
nisms: 195Pt is an NMR active isotope, and consequently
NMR spectroscopy is a powerful and useful tool for
characterization. Second, many species that are tran-
sient intermediates for other metals were isolated and
fully characterized for similar platinum complexes.4a
Resu lts a n d Discu ssion
The diboration reaction of bis(pinacolato)diborane(4),
1, with diethyl 1-hexynylphosphonate, 3a , diethyl phen-
ylethynyl phosphonate, 3b, 1-hexynylpinacolatoborane,
3c, and phenylethynylpinacolatoborane, 3d , in the pre-
sence of a catalytic amount of Pt(PPh3)4 (3 mol %) in
toluene at 80 °C overnight gave the desired novel cis-
(15) Brown, H. C.; Bhat, N. G.; Srebnik, M. Tetrahedron Lett. 1988,
29, 2631.
(16) (a) Massey, A. G. Adv. Inorg. Chem. Radiochem. 1983, 26, 1.
(b) Morrison, J . A. Chem. Rev. 1991, 91, 35. (c) Ahmed, I.; Castillo, J .;
Saulys, D. A.; Morrison, J . A. Inorg. Chem. 1992, 31, 706. (c) Ceron,
B.; Finch, A.; Frey, J .; Kerrigan, J .; Parsons, T.; Urry, G.; Schlesinger,
H. I. J . Am. Chem. Soc. 1959, 81, 6368.
(10) Bluhm, M.; Maderna, A.; Pritzkow, H.; Bethke, S.; Gleiter, R.;
Siebert, W. Eur. J . Inorg. Chem. 1999, 1693.
(11) (a) Onak, T. Organoborane Chemistry; Academic Press: New
York, 1975; p 38. (b) Colyle, T. D.; Ritter, J . J . Advances in Organo-
metallic Chemistry; Academic Press: New York, 1972: Vol. 10, p 237.
(c) Muetterties, E. L. The Chemistry of Boron and its Compounds;
Wiley: New York, 1967; p 398.
(12) (a) Ishiyama, T.; Matsuda, N.; Miyaura, N.; Suzuki, A. J . Am.
Chem. Soc. 1993, 115, 11018. (b) Thomas, R. LI; Souza, F. E. S.;
Marder, T. B. J . Chem. Soc. Dalton Trans. 2001, 1650. (c) Miyaura,
N.; Suzuki, A. Chem. Rev. 1995, 95, 2457. (d) J acobson, H. I.; Griffin,
M. J .; Preis, S.; J ensen, E. V. J . Am. Chem. Soc. 1957, 79, 2608.
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(b) Woods, W. G.; Strong, P. L. J . Organomet. Chem. 1967, 7, 371.
(14) Brown, H. C.; Cole, T. E. Organometallics 1983, 2, 1316.
(17) Lesley, G.; Marder, T. B.; Norman, N. C.; Rice, C. R. Main
Group Chem. News 1997, 5, 4.
(18) Clegg, W.; Dai, C.; Lawlor, F. J .; Lesley, G.; Marder, T. B.;
Nguyem, P.; Norman, N. C.; Pickett, N. L.; Rice, C. R.; Robins, E. G.;
Scott, A. G.; Taylor, N. J . In Advances in Boron Chemistry; Siebert,
W., Ed.; Royal Society of Chemistry: Cambridge, 1997; p 389.
(19) (a) Clarke, M. L. Polyhedron 2001, 20, 151. (b) The authors wish
to acknowledge a reviewer’s valuable explanation on the mechanism
involved into the preequilibrium step (see text).

Platinum(0)-Catalyzed Diboration
Organometallics, Vol. 21, No. 21, 2002 4535
F igu r e 1. Suggested catalytic cycle for the di- and triboration reactions.
1,2-diboronated vinylphosphonate and trisboronated
alkene addition products 5a , 5b, 5c, and 5d , respec-
tively, in high yields (Scheme 2).
bration procedure. Preequilibration of Pt(PPh3)4
+
excess B-B would lead to (PPh3)2Pt(B)2. If adventitious
water were present, (PPh3)2Pt(B)2 would react with H2O
to give (PPh3)2PtH2 + B-O-B. (PPh3)2PtH2 would lose
H2 and with excess B-B give more (PPh3)2Pt(B)2, which,
using PPh3 and B-B, is in equilibrium with (PPh3)3Pt
or (PPh3)4Pt + B-B, by the way, as well as with the
(PPh3)Pt(B)2 species. So, in effect, Pt can catalyze the
hydrolysis of the B-B bond. There is no other source of
H, and the preequilibration step just serves to dry the
system before the 1-alkynylboronate is added.19b
In general no detectable amount of addition product
was observed without catalyst, but the reaction was
efficiently catalyzed by Pt(PPh3)4. It is worth highlight-
ing a very important finding that no detectable amount
of the desired trisboronated alkene products 5c and 5d
were indicated when the alkynylboronate was added
directly to the reaction mixture of the catalyst and bis-
(pinacolato)diborane(4) in dry toluene and stirred over-
night at 80 °C, but the cis-1,2-diboronated alkenes 6c
and 6d were obtained in 100% conversion yields. This
may be due to the cleavage of the labile C-B bond, first
leading to nonboronated alkyne, the latter eventually
being diboronated to give cis-1,2-diboronated alkenes 6c
and 6d (Scheme 3).
The apparent hydrodeboronation followed by debor-
onation when the B-B and Pt compounds are not
preequilibrated may be due to either water or other such
protic impurities since it is known that 1-alkynylbor-
onates are very susceptible to cleavage by water.
However, we have repeated this reaction on numerous
occasions under anhydrous conditions and the results
were always duplicated. Preequilibrium is necessary,
otherwise the products of diboration of the alkyne are
obtained. The same solvent was used in the preequili-
The desired products 5a , 5b, 5c, and 5d were pro-
duced in 100% conversion yields and an isolated yields
of 82%, 83%, 86%, and 87%, respectively, only after we
changed the reaction conditions as follows: the catalyst
and bis(pinacolato)diborane(4) in toluene were allowed
to react for 3 h at 80 °C and then the 1-alkynylboronate
dissolved in toluene was added, and the whole mixture
was stirred overnight at 80 °C (Scheme 2). The reaction
initially requires a Pt(0) complex, which we believe
generates the active Pt(II) catalytic species. The reaction
is apparently similar to other reactions catalyzed by
transition metals involving oxidative addition of B-H,20
Si-Si,21 Sn-Sn,22 and Si-Sn23 compounds to low-valent
transiton metal complexes, the key step in the catalytic
diboration,24 hydroboration,25 and addition of disi-

4536 Organometallics, Vol. 21, No. 21, 2002
Abu Ali et al.
Sch em e 2. Syn th esis of Novel Tr isbor on a ted Dou ble-Bon d Com p ou n d s a n d Bisbor on a ted vin yl
P h osp h on a tes
Sch em e 3. Dibor a tion of 1-Alk yn ylbor on a te If All th e Rea cta n ts Wer e Im m ed ia tely Ad d ed Togeth er
lanes,26 distannanes,27 or silylstannanes28 to alkenes28e
and alkynes.12b
that the addition of PPh3 or the presence of a more
strongly bound bidentate phosphine ligand inhibits the
reaction;24 (c) the probably rapid reductive elimination
of the diboronated alkene product to give 5 and complete
the reaction; and (d) the role of the phosphine in the
later stages of the proposed catalytic cycle is not clear.
Either the phosphine can add to the metal center in the
final reductive elimination step or the mono(phosphine)
Extensive work have been done on the catalytic
diboration addition to alkynes and alkenes.3,4,12a,24,29
Figure 1 illustrates a more detailed description of the
suggested catalytic cycle in the present reaction and
similar catalytic diboration of alkynes.4c,19a The mech-
anism may involve (a) rapid B-B bond activation by
oxidative addition of 1 to the platinum(0) complex to
form a bis(boryl) platinum(II) intermediate 2; (b) the
dissociation of a phosphine ligand and association of the
alkyne to give the intermediate 4, as it has been shown
(25) (a) Gridnev, I. D.; Miyaura, N.; Suzuki, A. Organometallics
1993, 12, 589. (b) Burgess, K.; Donk, W. A.; Westcott, S. A.; Marder,
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Platinum(0)-Catalyzed Diboration
Organometallics, Vol. 21, No. 21, 2002 4537
species may add another 1 equiv of diborane and
continue the catalytic cycle.
The structure of bis(pinacolato)diborane(4) plati-
num(II) intermediate 2 was confirmed by Ishiyama et
al.,4a who obtained a single crystal of 2 suitable for X-ray
analysis in an 82% yield by treatment of Pt(PPh3)4 with
20 equiv of 1 in hexane at 80 °C for 2 h, followed by
recrystallization from hexane/toluene. Lesley et al.24
also determined various molecular structures of bis-
(boryl)diborane(4) platinum(II) intermediates, and they
1
also show by means of H and 31P NMR studies that a
mono(phosphine) species has not been detected, al-
though they do indicate the presence of bis(phosphine)
complex 2 through the course of the reaction. This may
indicate that 2 is the catalyst resting state and that
alkyne addition or insertion into the B-Pt bond may
be rate determining (vide infra).
F igu r e 2. Perspective view of the molecular structure of
5b. For clarity, only the major conformation is shown.
Ellipsoids represent thermal displacement parameters at
the 40% probability level. Their relatively large amplitudes
reflect on the structural disorder even at low temperature.
Selected bond distances (Å) and angles and torsion angles
(deg): B(1)-O(2) ) 1.387(7), B(1)-O(3) ) 1.397(7), B(1)-
C(19) ) 1.583(6), B(10)-O(11) 1.350(6), B(1)-O(12) )
1.344(6), B(10)-C(20) ) 1.578(6), C(19)-C(20) ) 1.351(5),
C(20)-P(27) ) 1.796(4), P(27)-O(28) ) 1.468(3), P(27)-
O(29) ) 1.574(4), P(27)-O(32) ) 1.584(3); C(19)-C(20)-
B(10) ) 124.1(4), C(19)-C(20)-P(27) ) 123.4(4), B(10)-
C(20)-P(27) ) 112.5(3), O(11)-B(10)-C(20) ) 124.1(4),
O(2)-B(1)-C(19) ) 123.1(4); B(1)-C(19)-C(20)-B(10) )
3.0(6), B(1)-C(19)-C(20)-P(27) ) 178.5(3).
The structure and configuration of the new com-
1
pounds have been fully characterized by H, 13C, 31P,
and 11B NMR, GC/MS, elemental analysis, and single-
crystal X-ray structure determination. The compounds
are all thermally and air stable in their solid state as
well as in solution. An important finding worth men-
tioning is that the vinylphosphonate products 5a and
5b were not stable to silica gel column chromatography
since they decomposed to pinacol and unidentified
products. These two compounds were purified by mul-
tiple pentane extraction and recrystallization, as cited
in detail in the Experimental Section. The solubility of
the compounds in common organic solvents ranges from
quite good (alkanes) to excellent (chloronated solvents,
ethers, and aromatic solvents). The solubility of the
triboronated compounds 5c and 5d in alkanes is lower
than the diboronated similar alkenes, which in turn are
1
lower than the monoboronated derivatives.30 In the H
NMR spectra (CDCl3) of compounds 5a and 5b the
pinacolato methyl resonances are observed as two
singlets at 1.26 ppm (s, 12H) and 1.31 ppm (s, 12H) for
5a and at 1.23 ppm (s, 12H) and 1.36 ppm (s, 12H) for
5b as expected. From this work and similar previous
reports24 the downfield shifts at 1.31 and 1.36 ppm
might be assigned for the methyl pinacolato boryl
groups located trans to the phosphorus moiety, respec-
tively. The pinacolato methyl resonances are observed
as three singlets for the triboronated compounds 5c and
5d and are found at 1.23 (s, 12H), 1.24 (s, 12H), and
1.28 ppm (s, 12H) for 5c and at 1.08 (s, 12H), 1.27 (s,
12H), and 1.30 ppm (s, 12H) for 5d . The downfield shifts
at 1.28 and 1.30 ppm for 5c and 5d , respectively, could
be assigned for the methyl pinacolato boryl groups
located on C(16), where an n-Bu or phenyl group is
attached (see Figure 3). The pronounced upfield shift
at 1.08 ppm for 5d might be assigned to the boryl group
cis to the phenyl group. The assignment of the 13C NMR
spectra (CDCl3) is straightforward except for the C-C
carbon atoms due to the quadrupole effect of the closely
F igu r e 3. Perspective view of the molecular structure of
5c. Ellipsoids represent thermal displacement parameters
at the 40% probability level. The molecules are located in
the crystal on, and are orientationally disordered about, a
2-fold symmetry axis (1 - x, 1 - y, z), which passes
diagonally through B(10), center of C(15)dC(16) and the
alkyl residue. This rotation axis relates atoms shown with
the same labels. The elongated ellipsoids of B(1), O(2), and
O(3) indicate additional unresolved disorder in the molec-
ular plane of this fragment. Selected bond distances (Å)
and angles and torsion angles (deg): B(1)-O(2) ) 1.356-
(7), B(1)-O(3) ) 1.346(6), B(10)-O(11) ) 1.371(3), B(1)-
C(15) ) 1.590(6), B(10)-C(15) ) 1.578(6), C(15)-C(16) )
1.353(7); O(3)-B(1)-C(16) ) 108.8(5), C(16)-C(15)-B(10)
) 150.8(5), O(2)-B(1)-C(15) ) 102.2(5), O(2)-B(1)-C(16)
) 138.4(4); B(1)-C(15)-C(16)-B(10) ) 10.6(6), B(1)-
C(15)-C(16)-B(1) ) 167.6(3).
1
attached boron atoms. In the H and 13C NMR spectra
the coupling constants are consistent with the assign-
ments (see Experimental Section). In the 31P NMR
(29) (a) Ishiyama, T.; Miyaura, N. J . Organomet. Chem. 2000, 392,
611. (b) Marder, T. B.; Norman, N. C. Top. Catal. 1998, 63, 5. (c) Cui,
Q.; Musaev, D. G. M.; Morokuma, K. Organometallics 1997, 16, 1355.
(30) (a) Abu Ali, H.; Goldberg, I.; Srebnik, M. Organometallics 2001,
20, 3962. (b) Abu Ali, H.; Goldberg, I.; Kaufmann, D.; Burmeister, C.;
Srebnik, M. Organometallics 2002, 21, 1870.
spectra (CDCl3) of compounds 5a and 5b the resonances
are observed at 19.65 and 18.47 ppm, showing shifts of
24.77 and 23.59 downfield, respectively, compared to the
1-alkynylphosphonate starting materials. Only broad

4538 Organometallics, Vol. 21, No. 21, 2002
Abu Ali et al.
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e Refin em en t
for 5b a n d 5c
was found to be fully isomorphous to that of 5c, with
the C6H5 ring located in place (and similarly disordered
about the 2-fold symmetry axis) of the C4H9 residue;
therefore the results of the diffraction analysis, crystal
data, and details of the structure determination of 5d
are not included.
5b
5c
formula
fw
habit
C
24H39B2O7P
C24H45B3O6
462.03
prisms
colorless
110(2)
Mo KR
492.14
prisms
colorless
110(2)
color
temp, K
radiation
cryst size, mm
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Mo KR
Exp er im en ta l Section
0.35 × 0.20 × 0.10 0.15 × 0.15 × 0.10
orthorhombic
P212121
10.2840(2)
11.9170(2)
21.7700(5)
90.00
orthorhombic
Aba2
12.4210(5)
12.6030(7)
17.9240(6)
90.00
90.00
90.00
2806(4)
4
1.094
Gen er a l Com m en ts. All reactions were carried out under
a
nitrogen atmosphere using vacuum line and glovebox
techniques. Solvents were purified by distillation from ap-
propriate drying agents under a nitrogen atmosphere. Starting
materials were purchased from commercial suppliers and used
without further purification. 1H and 13C NMR spectra were
recorded in CDCl3 solution with a Varian Unity spectrometer
(300 or 75 MHz) using Me4Si as an internal standard. 11B NMR
spectra were also recorded in CDCl3 solution with a Varian
Unity spectrometer (96 MHz) using BF3‚OEt2 as an external
standard. 31P NMR spectra were recorded in CDCl3 solution
with a Varian Unity spectrometer (121 MHz) using 85% H3-
PO4 as an external standard. GC/MS analyses were performed
on an HP GC/MS instrument (Model GCD PLUS), with an EI
detector and 30 m methyl silicone column.
90.00
90.00
2668.0(8)
4
1.225
Z
d
calcd, g cm-3
F(000)
1056
0.143
3.90-55.8
3572
1008
0.074
7.26-55.74
3107
µ, mm-1
2θ range, deg
no. of unique reflns
restraints
0
1
hkl limits
0,13/0,15/0,28
-16,16/-16,16/
-22,22
184
Dibor on Rea gen t. Bis(pinacolato)diborane(4) was pre-
pared from tetrakis(pyrrolidino)diborane(4) by Wurtz coupling
of bis(pyrrolidino)bromoborane and then obtained by solvolysis
of tetrakis(pyrrolidino)diborane(4) with 2 equiv of pinacol in
benzene solution at room temperature.1
no. of variables
404
no. of reflns with [I>2σ(I)] 2653
2129
final R indicesa [I>2σ((I)]
R1
0.058
0.141
e0.478
0.998
0.062
0.111
e0.003
1.081
wR2
|∆F|, e Å-3
1-Alk yn ylbor on a tes. Borylation reaction of pinacol with
triisopropylborate gave 2-isopropyloxy-4,4,5,5-tetramethyl-
1,3,2-dioxaborolane.31 This borate ester reacted with 1-alkynyl-
lithium to produce, after treatment with anhydrous hydrogen
chloride, the corresponding 1-alkynylpinacolatoborane.
Gen er a l P r oced u r e for th e P r ep a r a tion of 1-Alk yn yl-
bor on a tes. n-BuLi (31.3 mL, 1.6 M solution in hexane, 50
mmol) was added slowly to a stirred solution of 1-alkyne (50.1
mmol) dissolved in diethyl ether (50 mL) at -78 °C. Using a
douple-ended needle the whole mixture was then slowly added
to 2-isopropyloxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (9.3
g, 50 mmol) in diethyl ether (50 mL) at -78 °C. The reaction
was maintained at the same temperature for 2 h, and then
the mixture was stirred overnight at room temperature.
Anhydrous hydrogen chloride (20 mL, 3 M solution in ether,
60 mmol) was then added at -50 °C to the reaction mixture,
and the solution was allowed to warm to room temperature.
The mixture was filtered under a nitrogen atmosphere to
remove the undesired solids. The volatiles were removed from
the clear solution under vacuum, and the residue was distilled
under vacuum to give the product as a pure oily liquid.
1-Alkyn ylph osph on ates. 1-Alkynylphosphonates were pre-
pared from the corresponding alkynes and diethylchlorophos-
phonate according to literature procedure as follows: Alkynes
were metalated with n-BuLi in THF solution at low temper-
ature, and the resultant lithium acetylides were treated with
diethylchlorophosphonate at the same temperature. This
procedure minimizes side reactions and provides high and
reproducible yields of diethyl 1-alkynyl phosphonates.32
Gen er a l P r oced u r e for Dibor a tion of Alk yn ylbor -
on a tes a n d Alk yn ylp h osp h on a tes w ith Bis(p in a cola to)-
d ibor a n e(4). A dry nitrogen flushed 50 mL flask equipped
with a magnetic stirring bar was charged with Pt(PPh3)4 (74.7
mg, 0.06 mmol) and bis(pinacolato)diborane(4) (508 mg, 2
mmol); 15 mL of toluene was then added to the mixture. After
the mixture was stirred for 3 h at 80 °C the alkyne (1.9 mmol)
dissolved in 10 mL of toluene was then added. The mixture
GOF
R1 ) ∑||Fo| - |Fc||/∑|Fo|. wR2 ) {∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]}1/2
.
a
singlets for 5a , 5b, 5c, and 5d were observed in the 11B
NMR spectra due to the broad nature of the 11B NMR
resonances and are comparable with those found in the
literature for similar boronic acid ester derivatives.24,30
Mass spectra for 5a and 5b show molecular ion (M -
2C2H5)+ peaks, respectively, at m/e values of 414 and
434 mass units, while 5c shows a molecular ion (M -
CH3)+ peak at an m/e value of 447 mass units and 5d
shows a molecular ion (M - 4CH3)+ peak at an m/e
value of 422 mass units. The most abundant ion was
different from one compound to another, but a promi-
nent fragment at (M - 100)+ was pronounced.
Cr ysta llogr a p h y. The molecular structures of 5b
and 5c were determined by single-crystal X-ray diffrac-
tion. The results of the diffraction analysis, crystal data,
and details of the structure determination are shown
in Figures 2 and 3, and the data are summarized in
Table 1. The two pinacol rings and one of the O-ethyl
residues in 5b reveal severe conformational disorder in
the loosely packed crystal even at the low temperature.
They were represented in the crystallographic refine-
ment by a two-site model with correspondingly con-
strained occupancies. In the crystal of 5c, the molecules
are positioned on, and disordered about, symmetry axes
of 2-fold rotation (parallel to c). The symmetry axis
crosses the central CdC bond, one of the pinacol rings,
and the aliphatic residue aligned trans to it. The
rotational disorder also persists when the crystal struc-
ture was assigned lower monoclinic or triclinic sym-
metry, indicating that it is a genuine phenomenon (due
to an approximate square shape of the molecule) not
artificially imposed by the choice of the higher orthor-
hombic mm2-type space symmetry. The structure of 5d
(31) Andersen, M. W.; Heldebrandt, B. Ko¨ster, G.; Hoffmann, R. W.
Chem. Ber. 1989, 122, 1777.
(32) Iorga, B.; Eymery, F.; Carmichael, D.; Saviganc, P. Eur. J . Org.
Chem. 2000, 3103.

Platinum(0)-Catalyzed Diboration
Organometallics, Vol. 21, No. 21, 2002 4539
was stirred overnight at 80 °C, and the toluene was removed
under vacuum. The residue was extracted with pentane, which
was then washed twice with water and dried over anhydrous
sodium sulfate. The solvent was then removed under vacuum,
and the residue was dissolved in hot pentane and filtered to
remove any undissolved impurites. The vinylboronates were
purified by silica gel column chromatography with 10% ether/
petroleum ether as eluent to give the product as colorless
crystals. Vinylphosphonate diborane derivatives were purified
by pentane extraction and recrystallization since they are not
stable to silica gel column chromatography. The solid products
were crystallized from hot pentane.
129 (1.0), 101 (7.9), 84 (100), 69 (16.6), 57 (8.3), 43 (9.3), 28
(29.4). Anal. Calcd for C24H45B3O6: C, 62.39; H, 9.82. Found:
C, 61.95; H, 9.72.
[((Me4C2O2)B)(C6H5)CdC(B(O2C2Me4))2], 5d : 87% (0.80
g) yield, mp 251 °C; 1H NMR (CDCl3) δ 1.08 (s, 12H, CH3),
1.27(s, 12H, CH3), 1.30 (s, 12H, CH3), 7.22 (m, 5H, Ph); 13C-
{1H} NMR (CDCl3) δ 24.45 (CCH3), 24.79 (CCH3), 24.89
(CCH3), 83.13 (CCH3), 83.40 (CCH3), 83.82 (CCH3), 126.64
(para CH), 127.60 (ortho or meta CH), 127.68 (ortho or meta
CH), 145.20 (ipso) (C-CB cannot be detected); 11B NMR
(CDCl3) δ 30.58. MS (EI) m/z (%): 422 (M+-60, 0.1), 382 (0.7),
325 (0.2), 299 (3.6), 282 (0.2), 254 (0.2), 239 (1.1), 229 (2.2),
154 (0.5), 143 (0.8), 129 (3.0), 127 (0.3), 101 (4.7), 84 (100), 77
(0.4), 69 (11.8), 55 (7.5), 42 (1.1). Anal. Calcd for C26H41B3O6:
C, 64.78; H, 8.57. Found: C, 64.14; H, 8.49.
X-r a y Cr ysta llogr a p h ic Stu d y. Colorless single crystals
of 5b and 5c suitable for X-ray diffraction analysis were
obtained from a saturated pentane solution at ca. -20 °C. The
X-ray diffraction measurements were carried out at ca. 110 K
on a Nonius KappaCCD diffractometer, using Mo KR (λ )
0.7107 Å) radiation and 1.0° φ scans. The intensity data were
integrated and scaled by the programs DENZO-SMN and
Scalepack.33 The analyzed crystals were coated with a layer
of hydrocarbon oil (in order to avoid deterioration) and
mounted on a glass fiber. The structures were solved by direct
methods (SIR-92 and SIR-97)34 and refined by full-matrix
least-squares based on F2 for all reflections (SHELXL-97).35
All non-hydrogen atoms were refined with anisotropic dis-
placement parameters. The hydrogen atoms were located in
calculated positions to correspond to standard bond lengths
and angles and were included in the refinement with isotropic
displacement parameters using a riding model. The two crystal
structures suffer from a considerable rotational and/or con-
formational disorder, which affected the precision of the
crystallographic determination. The crystal and experimental
data are summarized in Table 1.
[((M e 4 C 2 O 2 )B )(C 4 H 9 )C dC (P (O )(O C 2 H 5 )2 )(B (O 2 C 2 -
Me4))], 5a : 82% (0.73 g) yield; 1H NMR (CDCl3) δ 0.87 (t, 3H,
3
3
CH3, J H-H ) 6.9 Hz), 1.27 (t, 6H, CH3, J H-H ) 7.1 Hz), 1.26
(s, 12H, CH3), 1.31 (s, 12H, CH3), 1.36 (m, 4H, CH2), 2.66 (t,
2H, CH2, J H-H ) 6.0 Hz), 4.06 (m, 4H, OCH2); 13C{1H} NMR
3
3
(CDCl3) δ 13.98 (CH3), 16.36 (d, CH3, J P-C ) 6.2 Hz), 23.05
4
(CH2), 24.71 (CCH3), 25.01 (CCH3), 31.65 (d, CH2, J P-C ) 2.9
3
3
Hz), 35.48 (d, CH2, J P-C ) 16.6 Hz), 61.22 (d, OCH2, J P-C
)
5.9 Hz), 83.96 (CCH3), 84.25 (CCH3) (C-CB cannot be de-
tected); 11B NMR (CDCl3) δ 30.22; 31P NMR (CDCl3) δ 19.65;
MS (EI) m/z (%) 414 (M+ - 58, 100), 399 (6.9), 384 (23.1), 372
(13.1), 330 (11.1), 315 (7.0), 264 (13.1), 245 (12.3), 233 (27.1),
218 (9.0), 204 (47.1), 199 (17.7), 187 (31.2), 179 (11.4), 157
(15.9), 151 (13.3), 127 (12.6), 101 (23.8), 83 (89.6), 69 (4.0), 58
(14.1), 57 (32.1), 45 (13.1), 29 (29.9). Anal. Calcd for
C
22H43B2O7P: C, 55.96; H, 9.18; P, 6.56. Found: C, 55.84; H,
8.99; P, 6.46.
[((M e 4 C 2 O 2 )B )(C 6 H 5 )C dC (P (O )(O C 2 H 5 )2 )(B (O 2 C 2 -
Me4))], 5b: 83% (0.78 g) yield, mp 128 °C; 1H NMR (CDCl3) δ
3
1.05 (t, 6H, CH3, J H-H ) 7.2 Hz), 1.23 (s, 12H, CH3), 1.36 (s,
12H, CH3), 3.74 (m, 4H, OCH2), 7.28 (m, 5H, Ph); 13C{1H} NMR
3
(CDCl3) δ 16.36 (d, CH3, J P-C ) 6.5 Hz), 24.68 (CCH3), 24.94
3
(CCH3), 61.14 (d, OCH2, J P-C ) 6.2 Hz), 84.40 (CCH3), 84.50
(CCH3), 127.10 (para CH), 127.35 (ortho or meta CH), 127.47
3
(ortho or meta CH), 141.65 (ipso, J P-C ) 16.5 Hz), (C-CB
Ack n ow led gm en t. The authors thank the Israeli
Science Foundation for their support of this work.
cannot be detected); 11B NMR (CDCl3) δ 29.76; 31P NMR
(CDCl3) δ 18.47; MS (EI) m/z (%) 434 (M+ - 58, 2.5), 415 (3.7),
292 (5.6), 282 (14.7), 263 (12.2), 254 (4.8), 237 (30.6), 209 (12.9),
193 (14.6), 157 (14.1), 143 (15.4), 129 (100), 127 (10.1), 101
(47.9), 91 (16.8), 84 (32.6), 83 (64.7), 77 (10.8), 69 (27.4), 58
(15.1), 29 (26.4). Anal. Calcd for C24H39B2O7P: C, 58.57; H,
7.99; P, 6.29. Found: C, 57.98; H, 7.95; P, 6.23.
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
positional and thermal displacement parameters, bond dis-
tances, bond angles, and torsion angles, together with details
of data collection and structure solution and refinement as a
CIF file for compounds 5b and 5c. This material is available
free of charge via the Internet at http://pubs.acs.org.
[((Me4C2O2)B)(C4H9)CdC(B(O2C2Me4))2], 5c: 86% (0.75 g)
yield, mp 230 °C; 1H NMR (CDCl3) δ 0.86 (t, 3H, CH3, 3J H-H
)
OM020334S
7.2 Hz), 1.23 (s, 12H, CH3), 1.24 (s, 12H, CH3), 1.28 (s, 12H,
3
CH3), 1.35 (m, 4H, CH2), 2.36 (t, 2H, CH2, J H-H ) 7.6 Hz);
(33) Otwinowski, Z.; Minor, W. Methods Enzymol. 1996, 276, 307-
326. SCALEPACK; Nonius B.V.: The Netherlands, 1998.
(34) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.;
Giacovazzo, C.; Guagliardi, A.; Polidori, G. J . Appl. Crystallogr. 1994,
27, 435.
(35) Sheldrick, G. M. SHELXL-97. Program for the Refinement of
Crystal Structures from Diffraction Data; University of Goettingen:
Germany, 1997.
13C{1H} NMR (CDCl3) δ 14.06 (CH3), 22.82 (CH2), 24.73
(CCH3), 24.76 (CCH3), 24.90 (CCH3), 32.74 (CH2), 37.44 (CH2),
82.82 (CCH3), 83.08 (CCH3), 83.64 (CCH3) (CdCB cannot be
detected); 11B NMR (CDCl3) δ 30.75. MS (EI) m/z (%) 447 (M+
- 15, 0.4), 404 (1.2), 362 (1.4), 279 (7.0), 278 (4.1), 262 (0.8),
254 (0.4), 221 (9.9), 209 (0.8), 178 (1.6), 162 (0.6), 154 (0.6),