Novel C1-bridged bisboronate derivatives by insertion of diazoalkanes into bis(pinacolato)diborane(4)

Article abstract of DOI:10.1021/om011012b

The insertion reaction of bis(pinacolato)diborane(4) [(Me₄C₂O₂)BB(O₂C₂Me₄ ), 1] with various diazoalkanes provided novel representatives of a new class of substituted C1-bridged bis(pinacolato)diborane(4) derivatives 5a, 5b, 5c, and 5d in a range of 75-78% isolated yields. The reaction was efficiently catalyzed by Pt(PPh₃)₄ in toluene at 110 °C. Single-crystal X-ray diffraction, GCMS, and NMR multinuclear spectroscopies fully confirmed the structure and configuration of the new compounds. Crystals of 5a and 5c are centrosymmetric, while those of 5d are chiral. 5b crystallized with a somewhat unusual Fdd2 space symmetry. While its crystals are noncentrosymmetric, they contain mirror-related species. The degree of puckering of the five-membered C₂O₂B rings, as well as the dihedral angles between the mean planes of these rings, vary from one compound to another and to some extent from one ring to another within the same compound.

Full text of DOI:10.1021/om011012b

1870
Organometallics 2002, 21, 1870-1876
Novel C1-Br id ged Bisbor on a te Der iva tives by In ser tion
of Dia zoa lk a n es in to Bis(p in a cola to)d ibor a n e(4)
Hijazi Abu Ali,† Israel Goldberg,‡ Dieter Kaufmann,§ Christian Burmeister,§ and
Morris Srebnik*,†
Department of Medicinal Chemistry and Natural Products, School of Pharmacy,
Hebrew University in J erusalem, J erusalem, Israel, School of Chemistry, Tel-Aviv University,
Ramat-Aviv 69987, Tel-Aviv, Israel, and Institut fu¨r Organische Chemie, Leibnizstrsse 6,
38678 Clausthal-Zellerfeld, Germany
Received November 26, 2001
The insertion reaction of bis(pinacolato)diborane(4) [(Me4C2O2)BB(O2C2Me4), 1] with various
diazoalkanes provided novel representatives of a new class of substituted C1-bridged bis-
(pinacolato)diborane(4) derivatives 5a , 5b, 5c, and 5d in a range of 75-78% isolated yields.
The reaction was efficiently catalyzed by Pt(PPh3)4 in toluene at 110 °C. Single-crystal X-ray
diffraction, GCMS, and NMR multinuclear spectroscopies fully confirmed the structure and
configuration of the new compounds. Crystals of 5a and 5c are centrosymmetric, while those
of 5d are chiral. 5b crystallized with a somewhat unusual Fdd2 space symmetry. While its
crystals are noncentrosymmetric, they contain mirror-related species. The degree of puckering
of the five-membered C2O2B rings, as well as the dihedral angles between the mean planes
of these rings, vary from one compound to another and to some extent from one ring to
another within the same compound.
In tr od u ction
In addition to the above-mentioned uses, 1,1-dibora
reagents can also undergo interesting chemical trans-
formations. Thus, 1,1-diboraalkanes react with 2 molar
equiv of methyl- or butyllithium to furnish 1-borato-1-
lithio alkanes, which can be reacted with carbon dioxide
to yield alkylmalonic acids.11 The use of 1 equiv of
n-BuLi produces 1,1-lithioboranes, which react with
aldehydes and ketones to form alkenes.12 Sequential
reaction of the 1,1-lithioboranes is also possible.13
Oxidation of 1,1-diboraalkanes with H2O2 in acidic
medium proceeds very smoothly, providing aldehydes
in good yield.14 Heating 1,1-bis(dialkylbora)-3-chloro-
propanes with sodium tetraethylborate induces cycliza-
tion, forming cyclopropyldialkylboranes.15 Treatment of
the dihydroborated product of 3-butyn-1-ol with meth-
yllithium leads to the formation of a cyclobutylborane.16
1,1-Bis(dimethoxybora)alkanes react with mercuric chlo-
ride to form, 1,1-bis(chloromercurio)alkanes.17 C1-
bridged 1,1-dibora reagents are usually obtained by
Bidentate Lewis acids are gaining increased utiliza-
tion in synthetic transformations and as complexation
reagents since it is recognized that they tend to increase
reaction selectivity and substrate activation by acting
in concert.1 They have been shown to be effective in the
activation of organic reactions,2 catalysis,3 and molec-
ular recognition4 and for enhanced activation of basic
substrates and the selective binding of anions. These
effects have been demonstrated with aluminum,5 mer-
cury,6 titanium,7 tin,8 boron,9 and indium.10 Of particu-
lar interest to us are C1-bridged 1,1-dibora reagents.
This paper is dedicated to Professor Herbert C. Brown, a pioneer
in organoboranes and their uses in synthesis, on the occasion of his
90th birthday.
† Hebrew University in J erusalem.
‡ Tel-Aviv University.
§ Institut fu¨r Organische Chemie.
(1) Piers, W. E.; Irvine, G. J .; Williams, V. C. Eur. J . Inorg. Chem.
2000, 2131.
(2) Maruoka, K. Catal. Today 2001, 66, 33.
(3) J ia, L.; Yang, X.; Stern, C.; Marks, T. J . Organometallics 1994,
13, 3755.
(8) Newcomb, M.; Homer, J . H.; Blanda, M. T. J . Am. Chem. Soc.
1987, 109, 7878. (b) J urkschat, K.; Kuivila, H. G.; Liu, S.; Zubieta, J .
A. Organometallics 1989, 8, 2755. (c) Altmann, R.; J urkschat, K.;
Schu¨rmann, M. Organometallics 1998, 17, 5858, and references cited.
(9) (a) Shriver, D. F.; Biallas, M. J . J . Am. Chem. Soc. 1967, 89,
1078. (b) Katz, H. E. J . Org. Chem. 1985, 50, 5027. (c) Katz, H. E. J .
Am. Chem. Soc. 1986, 108, 7640.
(10) Tschinkl, M.; Schier, A.; Riede, J .; Gabba¨ı, F. P. Inorg. Chem.
1997, 36, 5706.
(11) Cainelli, G.; Dal Bello, G.; Zubiani, G. Tetrahedron Lett. 1965,
3429.
(4) (a) Wuest, Acc. Chem. Res. 1998, 32, 81. (b) Gabba¨ı, F. P.; Schier,
A.; Riede, J .; Hynes, M. J . Chem. Commun. 1998, 897. (b) Williams,
V. C.; Piers, W. E.; Clegg, W.; Elsegood, M. R. J .; Collins, S.; Marder,
T. B. J . Am. Chem. Soc. 1999, 121, 3244. (c) Uhl, W.; Hannemann, F.;
Saak, W.; Wartchow, R. Eur. J . Inorg. Chem. 1998, 921.
(5) (a) Sharma, V.; Simard, M.; Wuest, J . D.; J . Am. Chem. Soc.
1992, 114, 7931. (b) Kaul, F. A. R.; Tschinkl, M.; Gabba¨ı, F. P. J .
Organomet. Chem. 1997, 539, 187.
(6) (a) Wuest, J . D.; Zacharie, B. Organometallics 1985, 4, 410. (b)
Schmidbaur, H.; O¨ ller, H.-J .; Wilkinson, D. L.; Huber, B. Chem. Ber.
1989, 122, 31. (c) Tikhonova, I. A.; Dolgushin, F. M.; Yanovsky. A. I.;
Struchkov, Yu.; Gavrilova, A. N.; Saitkulova, L. N.; Shubina, E. S.;
Epstein, L. M.; Furin, G. G.; Shur, V. B. J . Organomet. Chem. 1996,
508, 271. (d) Beauchamp, A. L.; Olivier, M. J .; Wuest, J . D.; Zacharie,
B. Organometallics 1987, 6, 153. (e) Vaugeois, J .; Simard, M.; Wuest,
J . D. Organometallics 1998, 17, 1208.
(12) Cainelli, G.; Dal Bello, G.; Zubiani, G. Tetrahedron Lett. 1966,
4135.
(13) Zweifel, G.; Arzoumanian, H. Tetrahedron Lett. 1966, 2535.
(14) (a) Mikhailov, B. M.; Aronovich, P. M.; Kiselev, V. G. Izv. Akad.
Nauk. SSSR, Ser. Khim. 1968, 146. (b) Aronovich, P. M.; Bogdanov,
V. S.; Mikhailov, B. M. Izv. Akad. Nauk. SSSR, Ser. Khim. 1969, 362.
(c) Matteson, D. S.; Shdo, J . G. J . Org. Chem. 1964, 29, 2742.
(15) Binger, P.; Ko¨ster, R. Angew. Chem. 1962, 74, 652.
(16) Brown, H. C.; Rhodes, S. P. J . Am. Chem. Soc. 1969, 91, 4306.
(17) Larock, R. C. J . Organomet. Chem. 1973, 61, 27.
(7) Asao, N.; Kii, S.; Hanawa, H.; Maruoka, K. Tetrahedron Lett.
1998, 39, 3729.
10.1021/om011012b CCC: $22.00 © 2002 American Chemical Society
Publication on Web 03/28/2002

Novel C1-Bridged Bisboronate Derivatives
Organometallics, Vol. 21, No. 9, 2002 1871
hydroboration of terminal alkynes with a hydroborating
reagent. The hydroborating reagent of choice is 9-BBN.18
Use of diborane leads to mixtures of products.19a
(C2H4O2B)2C(Et)2 was obtained from lithiodeboronation
of [(MeO)2B]4C, alkylation with ethyl iodide, and trans-
estrification with ethylene glycol.19b Alkylation of the
lithiated 1,1-diboryl esters is another method that was
used by Matteson and Moody, to get such boronic
esters.19c Dihydroboration of internal alkynes also leads
to mixtures and is not practical.19d,e Thermal isomer-
ization of the products obtained from hydroboration of
dienes with diborane leads to a 1,1-bis(bora)alkanes.20
Preperation of pinacol methylenediboronate was previ-
ously reported by Castle and Matteson.21a For higher
homologues, the method of hydroboration of alkynes
with HBCl2 used by Matteson and Soundararjan may
be the most efficient route to many compounds of this
class.21b,c We have recently reported a new reaction
involving bis(pinacolato)diborane(4), 1 (eq 1). Treatment
of 1 with an etheral solution of diazomethane in the
presence of Pt(PPh3)4 resulted in the high-yield synthe-
sis of 6 (R1 ) R2 ) H).21d
tives 5a , 5b, 5c, and 5d with evolution of N2 gas
(Scheme 1).
No detectable amount of insertion product was ob-
served without catalyst, but the reaction was efficiently
catalyzed by Pt(PPh3)4. The desired products 5a , 5b, 5c,
and 5d were produced in an isolated yields of 78%, 75%,
76%, and 75%, respectively. The reaction initially
requires a Pt(0) complex, which we believe generates
the active Pt(II) catalytic species. The reaction is ap-
parently similar to other reactions catalyzed by transi-
tion metals involving oxidative addition of B-H,23 Si-
Si,24 Sn-Sn,25 and Si-Sn26a compounds to low-valent
transiton-metal complexes, the key step in the catalytic
diboration,26b,c hydroboration,27 and addition of disi-
lanes,28 distannanes,29 or silylstannanes30 to alkenes
and alkynes.
The present insertion reaction shown in (Scheme 1)
may involve (a) B-B bond activation by oxidative
addition of 1 to the platinum(0) complex to form a bis-
(boryl) platinum(II) intermediate 3, (b) the insertion of
a generated carbene from various diazo methanes to the
B-Pt bond to form 4, and finally (c) the reductive
elemination to give 5 (Figure 1).
The structure of bis(boryl) platinum(II) intermediate
3 was confirmed by Ishiyama et al.,31 who obtained a
single crystal of 3 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 (eq 2).
(23) (a) Westcott, S. A.; Marder, T. B.; Baker, R. T.; Calabrese, J .
C. Can. J . Chem. 1993, 71, 930. (b) Westcott, S. A.; Tayler, N. J .;
Marder, T. B.; Baker, R. T.; J ones, N. J .; Calabrese, J . C. J . Chem.
Soc., Chem Commun. 1991, 304. (c) Baker, R. T.; Ovenall, D. W.;
Calabrese, J . C.; Westcott, S. A.; Tayler, N. J .; Williams, I. D.; Marder,
T. B. J . Am. Chem. Soc. 1990, 112, 9399. (d) Knorr, J . R.; Merola, J . S.
Organometallics 1990, 9, 3008. (e) Kono, H.; Ito, K.; Nagia, Y. Chem.
Lett. 1975, 1095. (f) Churchil, M. R.; Hackbarth, J . J .; Davison, A.;
Traficante, D. D.; Wreford, S. S. J . Am. Chem. Soc. 1974, 96, 4041.
(24) (a) Yamashita, H.; Kobayashi, T. A.; Hayashi, T.; Tanaka, M.
Chem. Lett. 1990, 1447. (b) Eaborn, C.; Griffiths, R. W.; Pidock, A. J .
Organomet. Chem. 1982, 225, 331. (c) Glockling, F.; Houston, R. E. J .
Organomet. Chem. 1973, 50, C31-C32. (d) Schmid, G.; Balk, H. J .
Chem. Ber. 1970, 103, 2240.
(25) (a) Weichmann, H. J . Organomet. Chem. 1982, 238, C49-C52.
(b) Akhtar, M.; Clark, H. C. J . Organomet. Chem. 1970, 22, 233.
(26) (a) Murakami, M.; Yoshida, T.; Kawanami, S.; Ito, Y. J . Am.
Chem. Soc. 1995, 117, 6408. (b) Lesley, G.; Nguyen, P.; Taylor, N. J .;
Marder, T. B.; Scott, A. J .; Clegg, W.; Norman, N. C. Organometallics
1996, 15, 5137. (c) Iverson, C. N.; Smith, M. R. Organometallics 1996,
15, 5155.
(27) (a) Gridnev, I. D.; Miyaura, N.; Suzuki, A. Organometallics
1993, 12, 589. (b) Burgess, K.; Donk, W. A.; Westcott, S. A.; Marder,
T. B.; Baker, R. T.; Calabrese, J . C. J . Am. Chem. Soc. 1992, 114, 9350.
(c) Westcott, S. A.; Blom, H. B.; Marder, T. B.; Baker, R. T. J . Am.
Chem. Soc. 1992, 114, 8863. (d) Evans, D. A.; Fu, G. C.; Anderson, B.
A. J . Am. Chem. Soc. 1992, 114, 6679. (e) Satoh, M.; Nomoto, Y.;
Miyaura, N.; Suzuki, A. Tetrahedron Lett. 1989, 30, 3789. (f) Hyashi,
T.; Matsumoto, Y.; Ito, Y. J . Am. Chem. Soc. 1989, 111, 3426. (g)
Wilczynsky, R.; Sneddon, L. G. J . Am. Chem. Soc. 1980, 102, 2857.
(28) (a) Ozawa, F.; Sugawara, M.; Hayashi, T. Organometallics 1994,
13, 3237. (b) Murakami, M.; Suginome, M.; Fujimoto, K.; Nakamura,
H.; Anderson, P. G.; Ito, Y. J . Am. Chem. Soc. 1993, 115, 6487. (c) Ito,
Y.; Suginome, M.; Murakami, M. J . Org. Chem. 1991, 56, 1948. (d)
Watanabe, H.; Kobayashi, M.; Higuchi, K.; Nagia, Y. J . Organomet.
Chem. 1980, 186, 51.
(29) (a) Michell, T. N.; Amamria, A.; Killing, H.; Rutschow, D. J .
Organomet. Chem. 1986, 304, 257. (b) Killing, H.; Michell, T. N.
Organometallics 1984, 3, 1318. (c) Michell, T. N.; Amamria, A.; Killing,
H.; Rutschow, D. J . Organomet. Chem. 1983, 241, C45-C47.
(30) (a) Murakami, M.; Amii, H.; Takizawa, N.; Ito, Y. Organome-
tallics 1993, 12, 4223. (b) Tsuji, Y.; Obora, Y. J . Am. Chem. Soc. 1991,
113, 9368. (c) Michell, T. N.; Wickenkamp, R.; Amamria, A.; Dicke,
R.; Schneider, U. J . Org. Chem. 1987, 52, 4868. (d) Chenard, B. L.;
Van Zyl, C. M. J . Org. Chem. 1986, 51, 3561.
This is a new carbenoid insertion reaction into the
B-B bond.22 The reaction possesses the possibility of
preparing C1-bridged bis-boronates with a quaternary
carbon atom, and it is the subject of our present paper.
Resu lts a n d Discu ssion
Reaction of bis(pinacolato)diborane(4) with diphenyl
diazomethane, 1-diazo-1,2,3,4-tetrahydronaphthalene,
methyl phenyl diazomethane, and 2-methoxyphenyl
methyl diazomethane in the presence of a catalytic
amount of Pt(PPh3)4 (3 mol %) in toluene at 110 °C
overnight gave the desired insertion products of sub-
stituted C1-bridged bis(pinacolato)diborane(4) deriva-
(18) Brown, H. C.; Scouten, C. G.; Liotta, R. J . Am. Chem. Soc. 1979,
101, 96.
(19) (a) Brown, H. C.; Zweifel, G. J . Am. Chem. Soc. 1961, 83, 3834.
(b) Matteson, D. S.; Thomas, J . R. J . Organomet. Chem. 1970, 24, 263.
(c) Matteson, D. S.; Moody, R. J . Organometallics 1982, 1, 20. (d) Pasto,
D. J . J . Am. Chem. Soc. 1964, 86, 3039. (e) Zweifel, G.; Arzoumanian,
H. J . Am. Chem. Soc. 1967, 89, 291.
(20) Negishi, B.; Burke, P. L.; Brown, H. C. J . Am. Chem. Soc. 1972,
94, 7431. (b) Brown, H. C.; Negishi, E.; Gupta, S. K. J . Am. Chem.
Soc. 1970, 92, 2460.
(21) (a) Castle, R. B.; Matteson, D. S. J . Organomet. Chem. 1969,
20, 19. (b) Soundararajan, R.; Matteson, D, S. J . Org. Chem. 1990, 55,
2274. (c) Matteson, D. S.; Soundararajan, R. Organometallics 1995,
14, 4157. (d) Abu Ali, H.; Goldberg, I.; Srebnik, M. Organometallics
2001, 20, 3962.
(22) For carbenoid insertion into B-C bonds: Vasella, A.; Wenger,
W.; Rajamannar, T. Chem. Commun. 1999, 2215.
(31) Ishiyama, T.; Matsuda, N.; Murata, M.; Ozawa, F.; Suzuki, A.;
Miyaura, N. Organometallics 1996, 15, 713.

1872 Organometallics, Vol. 21, No. 9, 2002
Abu Ali et al.
Sch em e 1. In ser tion Rea ction of Dia zoa lk a n es w ith Bis(p in a cola to)d ibor a n e(4)
5c (centrosymmetric), as well as of 5b (noncentrosym-
metric), are achiral; those of 5d are chiral. This varia-
tion may be due to the synthesis or to the asymmetric
substituents used in 5b and 5d . In solution, the
compounds are stable, and their solubility in common
organic solvents ranges from quite good (alkanes) to
excellent (in aromatic solvents, chlorinated solvents, and
ethers). This reflects the monomeric structure of the
compounds. In the 1H NMR spectra (CDCl3) of com-
pounds 5c and 5d the methyl protons appear upfield
compared to their parent keto and diazo compounds.
The methyl resonances are found at 1.45 ppm for 5c
and 1.35 ppm for 5d . In 5d the methoxy protons also
appear upfield at 3.78 ppm. The resonances of the
aromatic protons for 5a , 5b, 5c, and 5d are shifted
upfield and observed at 7.27 ppm (m, 10H, Ph) for 5a ,
at 6.97 ppm (m, 4H, Ph) for 5b, at 7.28 ppm (m, 5H,
3
Ph) for 5c, and at 6.78 ppm (dd, 1H, CH, J H-H ) 7.8,
3
4J H-H ) 0.9 Hz), 6.85 ppm (dt, 1H, CH, J H-H ) 7.5,
F igu r e 1. Suggested catalytic cycle for the insertion
reaction.
3
4J H-H ) 1.2 Hz), 7.10 ppm (dt, 1H, CH, J H-H ) 8.01,
3
4J H-H ) 2.1 Hz), 7.24 ppm (dd, 1H, CH, J H-H ) 7.5,
4J H-H ) 1.5 Hz) for 5d . The pinacolato methyl reso-
nances for 5c and 5d are observed as a pair of singlets
at 1.22 ppm (s, 12H) and 1.23 ppm (s, 12H) for 5c and
at 1.26 ppm (s, 12H) and 1.27 ppm (s, 12H) for 5d . This
could be explained as follows: If we assume fast rotation
about the B-C bonds on the NMR time scale, the
methyl groups a and b in the figure below are diaste-
reotopic when R1 * R2, and two resonances are expected.
1
The new compounds have been characterized by H,
13C, and 11B NMR, GC/MS, elemental analysis, and
single-crystal X-ray structure determination. The com-
pounds are all monomers in the solid state. In the
crystalline form they are air-stable. Crystals of 5a and

Novel C1-Bridged Bisboronate Derivatives
Organometallics, Vol. 21, No. 9, 2002 1873
Ta ble 1. Selected P a r a m eter s of Molecu la r Geom etr y
5a a
5b
5c
5d
average
1.371
1.365(2)
1.372(2)
1.369(4)
1.376(4)
1.371(4)
1.373(4)
1.586(4)
1.574(5)
112.8(3)
112.8(3)
120.2(3)
126.9(3)
123.5(3)
123.5(3)
1.368(2)
1.370(2)
1.368(2)
1.373(2)
1.587(2)
1.582(2)
113.4(1)
113.3(1)
123.5(3)
123.7(1)
123.7(1)
121.9(1)
1.371(2)
1.374(2)
1.369(2)
1.373(2)
1.589(2)
1.578(2)
113.0(1)
112.8(2)
123.3(1)
122.8(1)
124.2(1)
123.0(2)
B-O bond lengths, Å
B-C(sp3) bond lengths, Å
O-B-O bond angles, deg
1.587(2)
113.4(1)
1.583
113.1
123.4
124.1(1)
122.3(1)
O-B-C bond angles, deg
puckering of C2O2B rings39
q2, Å
0.269(1)
90.1(2)
46.3(1)
0.671(3)
0.748(3)
-152.6(2)
-157.8(2)
119.5(1)
0.284(1)
0.307(1)
93.1(2)
-91.1(2)
83.5(1)
0.219(2)
0.255(2)
-92.6(4)
91.4(4)
φ2, deg
dihedral angles between
the two C2O2B rings, deg
79.3(1)
a
Molecules are located on 2-fold axes of crystallographic rotational symmetry.
Since the differences in chemical shifts are only 0.01
ppm, perhaps the resonances are indistinguishable in
5b. In compound 5a , where R1 ) R2 ) Ph, a single
resonance is expected. The signals of the aliphatic
protons in the tetralone moiety in 5b are also shifted
3
up and observed at 1.73 ppm (t, 2H, CH2, J H-H ) 6
3
Hz), 1.93 ppm (m, 2H, CH2 , J H-H ) 6.2 Hz), and 2.74
ppm (t, 2H, CH2
,
3J H-H ) 6.2 Hz). The pronounced
upfield shifts in the H and 13C NMR spectra of 5a , 5b,
5c, and 5d are due to the close proximity of the
substituents to two boron atoms instead of oxygen and
nitrogen atoms in the parent keto and diazo compounds.
The assignment of the 13C NMR spectra (CDCl3) is also
straightforward except for the central carbon atom,
which cannot be detected due to the quadrupole effect
of the closely attached boron atoms. Chemical shifts and
coupling constants (see Experimental Section) in the 1H,
13C, and 11B NMR spectra are comparable with those
found in the literature for boronic acid esters.32-34 Mass
spectra for 5a , 5b, and 5c show monomeric molecular
ion (M)+ peaks, respectively, at m/e values of 420, 384,
and 358 mass units, while 5d shows a molecular ion
(M - CH3)+ peak at an m/e value of 373 mass units. In
all cases the most abundant ion was at an m/e value of
83 mass units and a prominent fragment at (M - 100)+
mass units.
Cr ysta llogr a p h y. The molecular structures of 5a ,
5b, 5c, and 5d were determined by single-crystal X-ray
diffraction. The low-temperature diffraction experi-
ments of the four different derivatives (which contain
seven crystallographically independent units of the
boronic ester moiety) at similar conditions allowed
precise determination of the covalent parameters around
the boron atom. The results of the diffraction analysis,
crystal data, and details of the structure determination
are shown in Figures 2, 3, 4, and 5, and the data are
summarized in Tables 1 and 2. The molecular structures
of 5a , 5b, 5c, and 5d consist of a slightly distorted
tetrahedral geometry for the central bridged carbon
1
F igu r e 2. View of the molecular structure of 5a , showing
the atom-numbering scheme. Ellipsoids represent thermal
displacement parameters at the 50% probability level. The
molecules are located in the crystal on 2-fold symmetry
axes. Selected bond distances (Å) and angles (deg): B(1)-
C(10) ) 1.5873(15), C(10)-C(11) ) 1.5397(14), B(1)-O(2)
) 1.3715(15), B(1)-O(3) ) 1.3649(15), O(3)-B(1)-O(2) )
113.44(10), B(1)-O(2)-C(5) ) 107.1(9), B(1)-C(10)-B(1)
) 113.99(13), B(1)-C(10)-C(11) ) 112.76(13), C(11)-
C(10)-C(11) ) 114.15(13).
atom, for which the significant bond angles and bond
lengths are summarized in Table 1. The average
B-C(sp3) bond lengths of 1.583 Å and the average B-O
bond lengths of 1.371 Å are closely similar to the
previously reported B-C bond lengths (1.575(5)-
1.589(5) Å) and B-O bond lengths (1.364(9)-1.370(7)
Å) in boronic acid esters.32-35 The coordination at B(1)
or B(2) is distorted trigonal planar with O-B-O aver-
age bond angles of 113.1° and O-B-C average bond
angles of 123.4° and is in agreement with previously
reported boronic esters.35-37 The dioxaborolane rings in
5a , 5b, 5c, and 5d are slightly distorted to release the
steric repulsion between the vicinal methyl groups in
contrast to the almost planar structure of bis(pinaco-
lato)diborane(4) and bis(catecolato)diborane(4).36,37 The
(32) Hyslop, A. M.; Kellett, M. A.; Iovine, P. M.; Therien, M. J . J .
Am. Chem. Soc. 1998, 120, 12676.
(33) Grigsby, W. J .; Power, P. P. Chem. Commun. 1996, 2235.
(34) Zheng, B.; Deloux, L.; Skrzypczak-J ankun, E.; Cheesman, B.
V.; Pereira, S.; Srebnik, M.; Sabat, M. J . J . Mol. Struct. 1996, 374,
291.
(35) Brauer, D. J .; Machnitzki, P.; Nickel, T.; Stelzer, O. Eur. J .
Inorg. Chem. 2000, 65.
(36) Clegg, W.; Elsegood, M. R. J ., Lawlor, F. J .; Norman, N. C.;
Picket, N. L.; Robins, E. G.; Scott, A. J . Inorg. Chem. 1998, 37, 5289.
(37) No¨th, H. Z. Naturforsch. 1984, 39b, 1463.

1874 Organometallics, Vol. 21, No. 9, 2002
Ta ble 2. Cr ysta l Da ta a n d Str u ctu r e Refin em en t for 5a , 5b, 5c, a n d 5d
Abu Ali et al.
5a a
5b
5c
5d b
formula
fw
C25H34B2O4
420.14
C22H34B2O4
384.11
C20H32B2O4
358.08
C21H34B2O5
388.10
habit
color
prisms
colorless
110(2)
Mo KR
0.25 × 0.20 × 0.10
monoclinic
C2/c
12.4680(2)
14.3490(3)
14.2030(3)
90.00
109.57
90.00
2394.16(8)
4
1.166
prisms
colorless
110(2)
prisms
colorless
110(2)
plates
colorless
110(2)
Mo KR
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
Mo KR
0.25 × 0.20 × 0.20
monoclinic
P21/c
13.0510(2)
14.5220(2)
12.4110(2)
90.00
117.25
90.00
2091.25(5)
4
1.137
0.35 × 0.25 × 0.20
orthorhombic
Fdd2
0.45 × 0.25 × 0.15
monoclinic
P21
9.1980(2)
9.4150(2)
13.5900(4)
90.00
106.38
90.00
1129.12(5)
53.9430(3)
10.3370(3)
15.7610(6)
90.00
90.00
90.00
8788.50(4)
16
1.161
Z
d
2
calcd, g cm-3
1.142
F(000)
904
0.076
0-56
3854
3328
0.076
0-50
1920
776
0.075
0-56
4834
420
0.078
0-56
4537
µ, mm-1
2θ range, deg
no. of unique reflns
no. of restraints
hkl limits
0
1
0
1
0,16/0,18/-18,17
145
2273
-51,51/0,12/0,18
266
1698
-17,15/-17,0/0,16
244
3964
-10,10/-11,11/-18,18
263
3943
no. of variables
no. of reflns with [I > 2σ(I)]
c
final R indices [I > 2σ(I)]
R1
0.042
0.03
0.041
0.038
wR2
0.114
0.349 and -0.261
1.02
0.098
0.556 and -0.197
1.06
0.104
0.315 and -0.231
1.00
0.104
0.185 and -0.185
0.98
max and min peak, e Å-3
GOF
a
b
Molecules are located on 2-fold axes of crystallographic rotational symmetry. Chiral structure. c R1 ) ∑||Fo - |Fc||/Fo|. wR2 )
{∑[w(Fo - Fc2)2]/∑[w(Fo2)2]}1/2
.
2
F igu r e 4. View of the molecular structure of 5c, showing
the atom-numbering scheme. Ellipsoids represent thermal
displacement parameters at the 50% probability level.
Selected bond distances (Å) and angles (deg): B(1)-C(19)
) 1.5868(17), C(19)-C(21) ) 1.5352(16), C(19)-C(20) )
1.5483(15), B(2)-O(4) ) 1.3727 (16), B(1)-O(6) ) 1.3675-
(15), O(4)-B(2)-O(3) ) 113.25(10), B(2)-O(3)-C(9) )
106.65(9), B(1)-C(19)-B(2) ) 109.23(9), B(1)-C(19)-C(20)
) 108.83(9), C(20)-C(19)-C(21) ) 113.09(10).
F igu r e 3. View of the molecular structure of 5b, showing
the atom-numbering scheme. Ellipsoids represent thermal
displacement parameters at the 50% probability level.
Selected bond distances (Å) and angles (deg): B(1)-C(19)
) 1.586(4), C(19)-C(20) ) 1.530(4), B(2)-O(4) ) 1.371 (4),
B(1)-O(6) ) 1.369(4), O(4)-B(2)-O(3) ) 112.8(3), B(2)-
O(3)-C(9) ) 106.4(2), B(1)-C(19)-B(2) ) 101.3(2), B(1)-
C(19)-C(28) ) 108.4(2), C(20)-C(19)-C(28) ) 111.7(2).
stituents on the central carbon. The degree of ring
puckering was quantitavely defined by Cremer and
Pople38 and represented by the ring-puckering coordi-
nates qm (Å) and φm (deg), where qm is a puckering
amplitude (qm g 0), φm is a phase angle describing
various kinds of puckering (0 e φm < 2π), and m ) 2, 3,
..., (N - 1)/2, where m is the number of coordinates and
N is the number of atoms in the ring. Thus, for 5a -d
m ) 2. The largest puckering of the C2O2B rings was
observed in the mostly constrained tetralone derivative
observed bond angles in 5a , O(3)-B(1)-O(2) 113.44°,
B(1)-O(2)-C(5) 107.1°, B(1)-O(3)-C(5) 107.2°, and
O(3)-C(4)-C(5) 102.1°, are also similar to those ob-
served for 5b, 5c, and 5d . The degree of puckering of
the five-membered C2O2B rings, as well as the dihedral
angles between the mean planes of these rings, vary
from one compound to another and to some extent from
one ring to another within the same compound, Table
1. This can be attributed mainly due to the different
intramolecular steric constraints imposed by the sub-
(38) Cremer, D.; Pople, J . A. J . Am. Chem. Soc. 1975, 97, 1354.

Novel C1-Bridged Bisboronate Derivatives
Organometallics, Vol. 21, No. 9, 2002 1875
of tetrakis(pyrrolidino)diborane(4) with 2 equiv of pinacol in
benzene solution at room temperature.40
Dia zoa lk a n es were prepared from the corresponding ke-
tones and hydrazones according to literature procedures.41-44
Hyd r a zon es were obtained by refluxing the ketones (2 g)
and hydrazine hydrate (2 g) in 2 mL of n-butanol for 2 h. The
hydrazone that usually separated out on cooling was filtered
off and washed with hexane and can be used directly after
drying for the preparation of the diazoalkane.
Gen er a l P r oced u r e for th e P r ep a r a tion of Dia zoa l-
k a n es. Pieces of sodium (1 g, 44 mmol) were slowly added to
40 mL of dry ether with stirring under a nitrogen atmosphere.
Stirring was continued, and the contents were heated under
reflux for 2 h. A solution of the hydrazone (2 g) in dry ether
(75 mL) was added dropwise. The reaction mixture was stirred
and heated under reflux overnight. Oxygen gas was passed
through for 15 min, and the mixture was allowed to cool.
Unreacted pieces of sodium were removed by filtration. The
ethereal solution was treated with 10% aq. ammonium chlo-
ride, and the organic layer was washed twice with water and
dried over anhydrous sodium sulfate. The solvent was removed
on a rotary evaporator, and the residue was dissolved in
n-hexane (25 mL) and filtered to remove any unreacted
hydrazones. The solid products were crystallized from n-
hexane.
Gen er a l P r oced u r e for th e P r ep a r a tion of (p in a co-
lato)2BC(R1R2)B by Diazoalkan e In ser tion . 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. The diazoalkane (2.1 mmol)
dissolved in 10 mL of toluene was then added. After the
mixture was stirred overnight at 110 °C, the toluene was
removed under vacuum. The residue was extracted with
pentane, which was then removed under vacuum, and the
product was purified by silica gel column chromatography with
8% ether/petroleum ether as eluent to give the product as
colorless crystals.
F igu r e 5. View of the molecular structure of 5d , showing
the atom-numbering scheme. Ellipsoids represent thermal
displacement parameters at the 50% probability level.
Selected bond distances (Å) and angles (deg): B(2)-C(19)
) 1.578(2), C(19)-C(21) ) 1.531(2), C(19)-C(20) ) 1.548-
(2), B(1)-O(6) ) 1.371 (2), B(2)-O(3) ) 1.369(2), O(5)-
B(1)-O(6) ) 112.98(14), B(1)-O(6)-C(7) ) 107.89(12),
B(1)-C(19)-B(2) ) 103.81(13), B(2)-C(19)-C(20) ) 111.07-
(13), C(20)-C(19)-C(21) ) 108.10(13).
5b (φ2 ) -152.6°, -157.8° and q2 ) 0.671, 0.748 Å). For
compounds 5a , 5c, and 5d the φ2 values (90.1°, 93.1°,
-92.6°), respectively, are close to the value (90°) ap-
propriate to one of the twist forms. This twist form
seems to be the most stable conformation for oxalane
and many compounds containing five-membered rings.39
The q2 values for 5a , 5c, and 5d are also close to those
reported in the literature for many such rings which
lie in a narrow range39 (near 0.35 Å).
Compound 5b crystallized in the somewhat unusual
Fdd2 space group symmetry (common to only about
0.4% of organic crystal structures). While these crystals
are noncentrosymmetric, they contain equal amounts
of mirror-related species. On the other hand, compound
5d formed chiral crystals in the space group P21
(common to about 6% of organic crystal structures)
rather than in P21/ c (of about 36% frequency) as its
analogous 5c derivative. This could be attributed to the
additional asymmetry introduced into the molecular
framework by the methoxy substituent.
[(Me4C2O2)BC(C6H5)2B(O2C2Me4)], 5a : 78% (0.66 g) yield,
1
mp 125 °C. H NMR (CDCl3): δ 1.24 (s, 24H, CH3), 7.27 (m,
10H, Ph). 13C{1H} NMR (CDCl3): δ 24.50 (CH3), 83.56 (CCH3),
124.86 (para CH), 127.81 (ortho or meta CH), 130.45 (ortho or
meta CH), 143.63 (ipso), (CB cannot be detected). 11B NMR
(CDCl3): δ 31.82. Anal. Calcd for C25H34O4B2: C, 71.49; H,
8.16. Found: C, 71.06; H, 8.09. MS (EI) m/z (%): 420 (M+, 4.5),
405 (0.8), 363 (0.7), 320 (9.4), 263 (2.8), 237 (41.5), 219 (12.9),
193 (24.5), 165 (21.4), 129 (4.5), 101 (21.0), 83 (100), 77 (1.3),
69 (18.7), 55 (23.0).
[(Me4C2O2)BC(C10H7)B(O2C2Me4)], 5b: 75% (0.58 g) yield,
mp 91 °C. 1H NMR (CDCl3): δ 1.21 (s, 24H, CH3), 1.73 (t, 2H,
3
3
CH2, J H-H ) 6 Hz), 1.93 (m, 2H, CH2 , J H-H ) 6.2 Hz), 2.74
Exp er im en ta l Section
(t, 2H, CH2 , J H-H ) 6.2 Hz), 6.97 (m, 4H, Ph). 13C{1H} NMR
3
(CDCl3): δ 23.03 (CH2), 27.77 (CH2), 24.50 (CH3), 24.66 (CH3),
33.26 (CH2), 83.19 (CCH3), 123.77 (CH), 124.86 (CH), 128.96
(CH), 130.45 (CH), 135.71 (C), 140.51 (C), (CB cannot be
detected). 11B NMR (CDCl3): δ 31.88. Anal. Calcd for
Gen er a l Com m en ts. All reactions were carried out under
nitrogen atmosphere using vacuum line and glovebox
a
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. GC/MS analyses were performed on an HP GCMS
instrument (Model GCD PLUS), with an EI detector and a 30
m methyl silicone column.
C
22H34O4B2: C, 68.81; H, 8.91. Found: C, 68.23; H, 8.82. MS
(EI) m/z (%): 384 (M+, 2.1), 368 (0.6), 327 (1.8), 299 (1.0), 284
(2.0), 227 (2.5), 201 (36.3), 171 (7.62), 157 (19.7), 129 (47.9),
101 (27.4), 83 (100), 69 (19.5), 55 (18.8), 41 (18.0).
[(Me4C2O2)BC(C6H5)(CH3)B(O2C2Me4)], 5c: 76% (0.54 g)
1
yield, mp 71 °C. H NMR (CDCl3): δ 1.22 (s, 12H, CH3), 1.23
(s, 12H, CH3), 1.45 (s, 3H, CH3), 7.28 (m, 5H, Ph). 13C{1H}
NMR (CDCl3): δ 18.50 (CH3), 24.60 (CH3), 83.28 (CCH3),
(40) Abu Ali, H.; Goldberg, I.; Srebnik, M. Eur. J . Inorg. Chem. 2002,
73.
(41) Miller, J . B. J . Org. Chem. 1959, 24, 560.
(42) Giri, B. P.; Prasad, G.; Mehrotra, K. N. Can. J . Chem. 1979,
57, 1157.
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
(43) Scho¨nberg, A.; Kader, A. E.; Sammour, A. E. M. A. J . Am. Chem.
Soc. 1957, 79, 6020.
(44) J onczyk, A.; Wlostowska, J . Synth. Commun. 1978, 8 (8), 569.
(39) Almenningen, A.; Seip, H. M.; Willadsen, T. Acta Chem. Scand.
1969, 23, 2748.

1876 Organometallics, Vol. 21, No. 9, 2002
Abu Ali et al.
124.31 (para CH), 127.80 (ortho or meta CH), 128.17 (ortho or
meta CH), 145.31 (ipso), (CB cannot be detected). 11B NMR
(CDCl3): δ 32.90. Anal. Calcd for C20H32O4B2: C, 67.10; H,
9.01. Found: C, 67.18; H, 8.96. MS (EI) m/z (%): 358 (M+, 3.0),
343 (1.1), 301 (1.6), 285 (0.1), 227 (1.2), 201 (1.4), 175 (11.7),
145 (2.4), 131 (10.5), 129 (2.9), 101 (12.1), 83 (100), 77 (1.5),
69 (11.4), 55 (12.4), 41 (11.7).
[(Me4C2O2)BC(CH3OC6H4)(CH3)B(O2C2Me4)], 5d : 75%
(0.58 g) yield, mp 88 °C. 1H NMR (CDCl3): δ 1.26 (s, 12H, CH3),
1.27(s, 12H, CH3), 1.35 (s, 3H, CH3), 3.78 (s, 3H, OCH3), 6.78
K, using graphite-monochromated Mo KR (λ ) 0.71070 Å)
radiation and 1° æ scans. The intensity data were integrated
and scaled by the programs DENZO-SMN and Scalepack.45
The structures were solved by direct methods (SIR-92)46 and
refined by full-matrix least-squares on F2 (SHELXL-97). All
non-hydrogen atoms were refined anisotropically; the hydro-
gens were located in idealized positions (the methyls being
treated as rigid groups) and allowed to ride with thermal
parameters 1.2 times those of their parent carbon.
3
4
(dd, 1H, CH, J H-H ) 7.8, J H-H ) 0.9 Hz), 6.85 (dt, 1H, CH,
Ack n ow led gm en t. The authors thank the Israeli
Science Foundation for their support for this work.
3J H-H ) 7.5, J H-H ) 1.2 Hz), 7.10 (dt, 1H, CH, J H-H ) 8.01,
4
3
4J H-H ) 2.1 Hz), 7.24 (dd, 1H, CH, J H-H ) 7.5, J H-H ) 1.5
Hz). 13C{1H} NMR (CDCl3): δ 19.06 (CH3), 24.74 (CH3), 54.99
(OCH3), 82.97 (CCH3), 110.26 (ortho or meta CH), 120.64 (ortho
or meta CH), 126.10 (para CH), 129.71 (ortho or meta CH),
135.51 (ipso), 157 (COCH3), (CB cannot be detected). 11B NMR
(CDCl3): δ 34.14. Anal. Calcd for C21H34O5B2: C, 65.01; H,
3
4
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, structure solution, and refinement as a CIF
file for compounds 5a -d . This material is available free of
charge via the Internet at http://pubs.acs.org.
8.83. Found: C, 65.11; H, 8.78. MS (EI) m/z (%): 373 (M+
-
15, 2.0), 330 (12.8), 315 (9.5), 288 (51.0), 273 (22.3), 247 (6.9),
215 (24.2), 188 (14.1), 173 (17.7), 161 (19.4), 146 (26.2), 129
(27.0), 101 (21.5), 83 (100), 77 (6.83), 69 (8.5), 55 (14.4), 41
(11.2).
X-r a y Cr ysta llogr a p h ic Stu d y. Colorless single crystals
of 5a , 5b, 5c, and 5d suitable for X-ray diffraction analysis
were obtained from a saturated pentane solution at ca. -20
°C. The crystal data and structure refinement parameters are
summarized in Table 2. All diffraction measurements were
carried out on a Nonius KappaCCD diffractometer at ca. 110
OM011012B
(45) Otwinowski, Z.; Minor, W. Methods Enzymol. 1996, 276, 307-
326. SCALEPACK; Nonius B.V., The Netherlands, 1998.
(46) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.;
Giacovazzo, C.; Guagliardi, A.; Polidori, G. J . Appl. Crystallogr. 1994,
27, 435.
(47) Sheldrick, G. M. SHELXL-97, Program for the Refinement of
Crystal Structures from Diffraction Data; University of Goettingen,
Germany, 1997.