Rearrangement of 1,2,4-Triboracyclopentanes into 2-Boryl-1,3-diboracyclobutanes
FULL PAPER
= 107 Hz, BCH], 20.5, 19.5 (both q, Dur-CH3), 13.9 [br. q, 1J(C,H)
= 120 Hz, BCH3], 2.8 (q, SiMe3) ppm. 11B NMR (96 MHz, CDCl3,
25 °C): δ = 104 (2B), 88 (1B) ppm.
constitution. Yield: 1.97 g (35 %) of white 2b, m.p. 128 °C. 1H
NMR (500 MHz, CDCl3, 25 °C): δ = 7.5 (br., BHB, located by 1H/
1H COSY to d at 1.43), 7.5, 7.4 (br.s, 6 H in total, Ph–H and B–
H–B), 7.17, 6.91 (both s, 1 H each, Dur-H), 2.61, 2.32, 2.28, 2.20,
4-Phenyl-1,2-bis(2Ј,3Ј,5Ј,6Ј-tetramethylphenyl)-trans-3,5-bis(trimeth-
ylsilyl)-1,2,4-triboracyclopentane (1b): PhLi solution (cyclohexane/
Et2O, 1.85 , 2.05 mL) was added to a solution of 1d (1.91 g,
3.8 mmol) in Et2O (50 mL) at –78 °C. The mixture was allowed to
reach room temperature over 2 h and stirring was continued for 1 h
at this temperature. The solvents were removed in vacuo and the
residue was digested with pentane (50 mL). The salts were sepa-
rated from the deep yellow solution by filtration with a reversed
frit and washed with pentane (10 mL). The filtrates were reduced
to dryness in vacuo yielding spectroscopically pure 1b. Yield: 2.04 g
(99%) of deep yellow 1b, m.p. 99 °C (dec.). For spectroscopic data
see ref.[6]
3
2.2 (br.), 1.7 (br) (all s, 24 H in total, Dur-Me), 1.43 [d, J(H,H) =
4.6 Hz, 1 H, CHSi, identified by a cross peak to δ13C = 26.8 in a
C,H correlated spectrum], 0.09, –0.12 (both s, 9 H each,
SiMe3) ppm. 1H NMR (500 MHz, CDCl3, –40 °C) 7.7–7.6 (br., 6
H in total, Ph–H and B–H–B), 7.24, 6.99 (both s, 1 H each, Dur-
H), 2.65, 2.36, 2.31, 2.29, 2.28, 2.18, 2.02, 1.67 (all s, 3 H each,
Dur-Me), 1.51 [d, 3J(H,H) = 4.6 Hz, 1 H, CHSi], 0.09, –0.12 (both
s, 9 H each, SiMe3) ppm. 13C NMR (75 MHz, CDCl3, 25 °C): δ =
146.8 (br. s, i-C), 140.8, 139.8, 134.0, 133.5, 132.9, 132.5 (all s, Dur-
o- and m-C), 136.7, 127.3 (both d, Ph-o- or m-C), 135.5, 130.0
(both d, Dur-p-C), 128.3 (d, Ph-p-C), 71.9 (br. s, B2C), 26.8 [d,
1J(C,H) = 113 Hz, B2CH], 21.1, 20.3, 20.2, 20.0, 19.55, 19.50 (all
q, Dur-CH3), 1.6, 0.8 ppm (both q, SiMe3), Ph-i-C and one Dur-i-
C not observed. 13C NMR (125 MHz, CDCl3, –40 °C): δ = 146.5,
139.2 (both br., Dur-i-CЈs), 141.0, 139.7, 133.9, 133.4, 133.0,. 132.9,
132.6, 132.3 (Dur-o- and m-CЈs), 136.6, 127.2 (br., Ph-o- or m-CЈs),
135.5, 129.7 (Dur-p-CЈs), 133.1 (br., Ph-i-C), 128.2 (Ph-p-C), 70.7
(B2C), 26.4 (B2CH), 21.0, 20.6, 20.4, 20.3, 20.0, 19.9, 19.6, 19.5
(Dur-CH3), 1.4, 0.6 (SiMe3) ppm. 11B NMR (160 MHz, CDCl3,
25 °C): δ = 57, 42, –2 ppm (each 1B).
4-Chloro-1,2-bis(2Ј,3Ј,5Ј,6Ј-tetramethylphenyl)-trans-3,5-bis(trimeth-
ylsilyl)-1,2,4-triboracyclopentane (1d): B(OMe)3 (9.15 g, 88.0 mmol,
10 mL) was added slowly by syringe to a suspension of dilithium-
2,3-bis(2Ј,3Ј,5Ј,6Ј-tetramethylphenyl)-1,4-bis(trimethylsilyl)-2,3-di-
boratabuta-1,3-diene[6] (13.4 g, 21.5 mmol) in pentane (200 mL)
at –78 °C. The mixture was allowed to reach room temperature
during which time it gradually became yellow. Stirring was contin-
ued overnight and the solvents were then removed in vacuo. The
viscous residue was thoroughly dried in vacuo for 2 h and was then
digested with pentane (200 mL) and cooled again to –78 °C. After
addition of BCl3 (30 g, 256 mmol) with a syringe, the mixture was
warmed to room temperature over 2 h and stirring was continued
for 30 min at this temperature. All volatile materials were then re-
moved in vacuo and the solid residue digested with pentane
(200 mL). The salts were filtered through a reversed frit and washed
with pentane (2×20 mL). The combined filtrates were reduced to
dryness in vacuo and thoroughly dried thereafter. The deep yellow
residue was of almost spectroscopic purity. Crystallisation from
pentane (50 mL) at –30 °C yielded 1d as yellow crystals. Yield:
8.17 g (75%) of bright-yellow 1d, m.p. 134 °C (dec.). For spectro-
scopic data see ref.[6]
1,2,4-Tris(2Ј,3Ј,5Ј,6Ј-tetramethylphenyl)-3,5-bis(trimethylsilyl)-
µ(1,2)-hydro-3-dehydro-1,2,4-triboracyclopentane (2c): A mixture of
1d (5.5 g, 10.85 mmol) and lithium powder (0.25 g) in diethyl ether
(70 mL) was stirred at room temperature and monitored frequently
by 11B NMR spectroscopy until the signals of 1d were replaced by
those of 3 at δ = 24 and 21 ppm (about 1 h). After cooling to
–78 °C, duryllithium (1.53 g, 10.85 mmol) was added with stirring.
Formation of the dianion of 1c during warming to room tempera-
ture was indicated by the presence of a 11B NMR signal at δ =
–16 ppm. After disappearance of the signals of 3 and cooling to
–78 °C, 2,3-dibromo-2,3-dimethylbutane (2.6 g, 10.85 mmol) was
added and the mixture allowed to warm to room temperature with
stirring for an additional 1 h. 11B NMR signals at δ = 97 and
80 ppm unambiguously indicated the formation of 1c. All volatile
materials were removed in vacuo and the residue was dissolved in
pentane (50 mL). The salts were separated by filtration with a re-
versed frit and washed further with pentane (10 mL). The filtrate
was reduced to dryness in vacuo yielding yellow 2c which could be
crystallised at –30 °C from pentane. Yield: 6.2 g (84%) of yellow
2c, m.p. 134 °C. 1H NMR (500 MHz, CD2Cl2, 25 °C): δ = 7.87 (br.,
1 H, BHB), 7.02, 6.98 (both s, 1H and 2 H, respectively, p-H), 2.30,
2.26, 2.20, 2.10, 1.95 (all s, 36 H in total, Dur-Me), 1.17 (d, 1J =
1-Methyl-2,4-bis(2Ј,3Ј,5Ј,6Ј-tetramethylphenyl)-3,5-bis(trimethylsi-
lyl)-µ(1,2)-hydro-3-dehydro-1,2,4-triboracyclopentane (2a): A solu-
tion of freshly prepared 1a (3.00 g, 6.17 mmol) in CDCl3 (20 mL)
was stored for 4 d at room temperature and the solution was moni-
tored frequently by 1H NMR spectroscopy. After completion of
the reaction all volatile materials were removed in vacuo. The white
residue consisted of essentially pure 2a which could be crystallised
at –30° from toluene. Yield: 2.97 g (99%) of white 2a, m.p. 113 °C.
1H NMR (500 MHz, CDCl3, –40 °C): δ = 7.21, 6.99 (both s, 1 H
each, Dur-H), 6.72 (br. s, 1 H, B–H–B), 2.49, 2.43, 2.37, 2.31, 2.28,
2.19 (all s, 24 H in total, Dur-CH3), 1.10 (s, 1 H, CHSi), 0.66 (s, 3
H, BCH3), 0.01, –0.26 (both s, 9 H each, SiMe3) ppm. 13C NMR
(125 MHz, CDCl3, –40 °C): δ = 146.7, 133.9 (each br. s, i-C), 140.0,
138.4, 134.0, 133.1, 133.05, 132.9, 132.8, 131.8 (each s, o- and m-
C), 134.9, 129.6 (both d, p-C), 74.6 (s, B2C), 27.6 [d, 1J(C,H) =
116 Hz, B2CH], 20.6, 20.5, 20.3, 20.1, 19.6, 19.3 (all q, Dur-CH3),
4.9 [br. q, 1J(C,H) = 117 Hz, BCH3], 0.9, 0.2 (both q, SiMe3) ppm.
11B NMR (96 MHz, CDCl3, 25 °C): δ = 56, 48, 0 (each 1B) ppm.
6.2 Hz, 1 H, CHSi), 0.06, –0.15 (both s, 9 H each, SiMe3) ppm. 13
C
NMR (125 MHz, CD2Cl2, 25 °C): δ = 143.4 (br. s, i-C), 136.9,
133.9, 133.6, 133.4 (all s, Dur-o- and m-C), 132.6, 131.0 (all d, 1C
and 2C, p-C), 102.3 (br. s, B2C), 25.0 (br. B2CH), 21.1, 19.9, 19.7,
19.6 (all q, Dur-CH3), 1.6, 0.9 (both q, SiMe3) ppm. 13C NMR
(125 MHz, CD2Cl2, –90 °C): δ = 149.0, 146.0, 142.3, 137.5, 136.0,
135.6, 135.0, 134.5, 133.3,. 132.5, 132.1, 131.6 (o- m- and i-CЈs),
131.6, 130.1, 129.1 (p-C), 89.1 (B2C), 21.6 (B2CH), 22.2, 21.6, 21,4,
20.3, 20.2, 20.1, 19.1, 18.7, 18.1 (Dur-CH3), –0.01, –0.21
(SiMe3) ppm. 11B NMR (96 MHz, CDCl3, 25 °C): δ = ca. 47 (2B),
ca. 30 ppm (1B).
1-Phenyl-2,4-bis(2Ј,3Ј,5Ј,6Ј-tetramethylphenyl)-3,5-bis(trimethylsi-
lyl)-µ(1,2)-hydro-3-dehydro-1,2,4-triboracyclopentane (2b): A solu-
tion of 1b (5.67 g, 10.3 mmol) in CDCl3 (25 mL) was stored for 9 d X-ray Crystal Structure Analyses of 1a, 2a, 2b and 2c·pentane: Sin-
at room temperature and was monitored frequently by 1H NMR
spectroscopy. All volatile materials were then removed in vacuo and
the residue was dissolved in a 1:1-mixture of Et2O and pentane
(30 mL). After four days at –30 °C 2b precipitated as a white solid
which was contaminated with approx. 5% of a product of unknown
gle crystals were grown from toluene/Et2O at 0 °C for 2a and pen-
tane at –30 °C for 2b and 2c. Those of 1a were obtained from di-
ethyl ether at –30 °C. The crystals were investigated on imaging
plate systems (IPDS Stoe) using graphite monochromated Mo-Kα
radiation at 193 K. The space groups were determined from the
Eur. J. Inorg. Chem. 2005, 4078–4085
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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