H.-J. Frohn, N. Y. Adonin, V. V. Bardin, V. F. Starichenko
under reduced pressure 3,4,5-trifluorophenyl(dihydroxy)borane
(2.67 g), slightly contaminated by the corresponding anhydride,
was obtained.
(Method B) Compound 4a was prepared from acid 4 (20 mg,
0.10 mmol) and SOCl2 (100 mg. 0.84 mmol) in ether in quantitat-
ive yield.
MS (70 eV): calc. for C18H3B3F12O3: 528.01699 (Mϩ); found Mϩ 528.01641
m/z.
2,6-Difluorophenyl(dihydroxy)borane 2,6-C6H3F2B(OH)2 (7)
IR ν¯ (cmϪ1): (film on Si) 1643 w, 1614 vw, 1482 vs, 1377 m, 1342 s, 1302 w,
1269 w, 1244 w, 1231 w, 1172 m, 1138 w, 1119 w, 1097 w, 1019 w, b, 926 m,
890 w, 867 w, 857 m, 792 w, b, 757 w, 737 w, 710 w, 690 w, 677 w, 619 w,
506 vw, 489 vw.
n-BuLi (2.5 M in hexane; 8 ml, 20 mmol) was slowly added to a
solution of 2,6-difluorobromobenzene (3.86 g, 20 mmol) in ether
(30 ml) at Ϫ60 °C. After stirring for 1 h at Ϫ60 to Ϫ70 °C the solu-
tion was added to the pre-cooled (Ϫ70 °C) and stirred solution
of trimethoxyborane (3.11 g, 30 mmol) in ether (20 ml) within 30
minutes. The reaction mixture was stirred for 1 h at Ϫ70 to Ϫ60 °C,
warmed up to 20 °C within 30 Ϫ 40 min. and finally hydrolyzed
with 5 % HCl (20 ml). The organic layer was separated and the
aqueous layer was extracted with ether. The combined extracts were
dried over MgSO4 and the solvent was distilled off under reduced
pressure to yield 2.46 g (77 %) of acid 7. Recrystallization from hot
toluene gave 2,6-difluorophenyl(dihydroxy)borane (white shining
plates, 0.83 g, 26 %).
C6H5BF2O2 (157.91): calculated C 45.64, H 3.19, F 24.06; found C
45.0, H 3.32, F 23.4 %.
IR ν¯ (cmϪ1): (KBr pellet) 3115 w, 3073 w (C-H), 2238 w, 1943 w, 1859 w,
1770 w, 1626 s, 1588 w, 1567 m, 1463 vs, 1418 m, 1397 s, 1372 w, 1346 s,
1256 m, 1218 m, 1152 m, 1121 m, 1065 m, 1026 m, 986 s, 881 w, 793 s, 737 w,
657 w, 628 m, 565 w, 540 m, 515 w.
Tris(3,4,5-trifluorophenyl)boroxin (3,4,5-C6H2F3BO)3 (6a)
(Method C) Compound 6a was obtained by azeotropic treatment
of acid 6 in toluene followed by crystallisation from boiling toluene
in 56 % yield. Solid 6a was stored over P4O10.
C18H6B3F9O3 (473.67): calculated C 45.64, H 1.28, F 36.10; found
C 46.2, H 1.29, F 36.6 %.
MS (70 eV): calc. for C18H6B3F9O3: 474.04525 (Mϩ); found Mϩ 474.04487
m/z.
IR ν¯ (cmϪ1): (KBr pellet) 3076 w (C-H), 1769 w, 1616 m, 1589 m, 1529 m,
1424 vs, 1360 vs, 1338 vs, 1293 m, 1217 s, 1038 s, 926 w, 879 m, 772 m, 747 m,
737 m, 713 w, 686 m, 622 m, 517 w. (film on Si) 3075 vw (C-H), 1767 vw,
1614 m, 1587 w, 1528 m, 1423 s, 1359 s, 1337 s, 1294 m, 1217 m, 1038 s,
926 w, 879 w, 771 m, 747 w, 737 w, 713 vw, 686 vw, 622 s, 516 w, 490 w.
Tris(2,4-difluorophenyl)boroxin (C6H3F2BO)3 (8a)
(Method C) Compound 8a was obtained by azeotropic treatment
of acid 8 in toluene followed by crystallisation from boiling toluene
in 60 % yield. Solid 8a was stored over P4O10.
C18H9B3F6O3 (419.70): calculated C 51.51, H 2.16, F 27.16; found
C 50.6, H 2.33, F 26.8 %.
IR ν¯ (cmϪ1): (film on Si) 1610 m, 1588 s, 1499 w, 1432 s, 1368 s, 1352 sh,
1308 m, 1259 s, 1226 m, 1137 m, 1109 m, 1071 w, sh, 964 m, 852 m, 817 w,
740 m, 675 m, 554 w, 523 m, 462 w.
2,4-Difluorophenyl(dihydroxy)borane 2,4-C6H3F2B(OH)2 (8)
2,4-Difluorophenyllithium was prepared from 1-bromo-2,4-
difluorobenzene (3.86 g, 20 mmol) and n-BuLi (2.5 M in hexane;
8.8 ml, 22 mmol) in ether (40 ml) at Ϫ70 °C. The cold reagent was
gradually added to the cold solution of trimethoxyborane (3.11 g,
30 mmol) in ether (40 ml). The reaction mixture was stirred at
Ϫ60 °C for 1 h, allowed to warm to 15 °C within 1 h and hydro-
lysed with 5 % HCl (100 ml). The organic layer was separated, the
aqueous layer was extracted with ether (2 x 10 ml) and the com-
bined extracts were dried over MgSO4. The solvent was removed
under reduced pressure and the solid residue was stored over water
at 15 °C for 1 month to give acid 8 (2.69 g, 77 %).
Tris(4-fluorophenyl)boroxin (C6H4FBO)3 (9a)
(Method C) Compound 9a (1.96 g, 40 %) was obtained by azeo-
tropic treatment of acid 9 in toluene followed by crystallisation
from boiling toluene. Solid 9a was stored over P4O10. 9 in a mixture
with 9a was prepared by the reaction of 4-fluorophenylmagnesium
bromide [from 1-bromo-4-fluorobenzene (7 g, 41 mmol) and mag-
nesium (1 g, 41.1 mmol)] with B(OMe)3 (6.7 g, 64.5 mmol).
C18H12B3F3O3 (365.73): calculated C 59.12, H 3.31, F 15.58; found
C 58.8, H 3.24, F 15.7 %.
IR ν¯ (cmϪ1): (film on Si) 3631 m, 3224 vs, b (O-H), 1614 m, 1589 w, 1473 s,
1443 s, 1409 s, 1369 s, 1339 s, 1295 w, 1264 m, 1231 w, 1196 s, 1150 vw,
1131 w, 1100 vw, 1045 m, 1005 m, 995 m, 883 vw, 858 m, 814 m, 742 m,
725 m, 686 vw, 677 vw, 645 vw, 629 m, 595 w, 568 w, 546 s, 512 w, sh, 453 w.
Tris(pentafluorophenyl)boroxin (C6F5BO)3 (1a)
(Method A) A trap with pentafluorophenyl(dihydroxy)borane
(63 mg, 0.30 mmol) was connected to a drying tube filled with
Sicapent (P4O10/SiO2). The borane was kept at 100 °C until a con-
stant weight was achieved (2 Ϫ 2.5 h). The loss of mass was 5.4 mg
(0.30 mmol of H2O). The arylboroxin 1a was obtained quantitat-
ively.
MS (70 eV): calc. for C18H12B3F3O3: 366.10177 (Mϩ); found Mϩ 366.10252
m/z.
IR ν¯ (cmϪ1): (KBr pellet) 3050 w (C-H), 2429 w, 2235 w, 2150 w, 2031 w,
1913 w, 1783 w, 1599 s, 1511 w, 1405 s, 1368 s, 1350 s, 1306 m, 1219 m,
1155 m, 1094 m, 1016 w, 945 w, 835 m, 814 w, 742 m, 683 w, 627 w, 527 m,
504 w.
(Method B) Thionyl chloride (100 mg, 0.84 mmol) was added to
the solution of acid 1 (20 mg, 0.09 mmol) in ether (0.3 ml). Within
a few minutes compound 1a was precipitated. After removing the
volatile products under reduced pressure 1a was isolated in quanti-
tative yield.
We gratefully acknowledge financial support by the Deutsche For-
schungsgemeinschaft, the Russian Foundation for Basic Research,
and the Fonds der Chemischen Industrie.
MS (70 eV): calc. for C18B3F15O3: 581.98874 (Mϩ); found Mϩ 581.98869
m/z.
References
IR ν¯ (cmϪ1): (film on Si) 1675 vw, 1649 m, 1525 m, 1487 s, 1441 w, 1349 s,
1281 w, 1263 w, 1228 m, 1093 w, 984 s, 887 w, 860 vw, 768 w, 759 w, 709 w,
627 m, 577 w, 482 w.
[1] B. M. Mikhailov, Y. U. Bubnov, Organoboron compounds in
organic synthesis. /; Bell Bain, Glasgow, 1984.
[2] Methoden der organischen Chemie (Houben-Weyl). Bd 13/3b.
Organobor-Verbindungen II; Thieme, Stuttgart, 1983.
[3] K. Ishihara, S. Ohara, H. Yamamoto, J. Org. Chem. 1996,
61, 4196.
Tris(2,3,5,6-tetrafluorophenyl)boroxin (2,3,5,6-C6HF4BO)3 (4a)
(Method A) Compound 4a (98 % yield) was prepared by the ther-
mal dehydration of acid 4 (98 mg, 0.51 mmol).
2832
Z. Anorg. Allg. Chem. 2002, 628, 2827Ϫ2833