Synthesis, Characterization, and Bromine Substitution of Diamine Complexes of Carboxyborane and Methoxycarbonylborane. Diazabora Rings Containing B-Carboxyl and B-Carboxylato Groups

Article abstract of DOI:10.1021/ic9713442

DA·2BH₂CN (1) [DA = N,N,N′N,N′-tetramethylethanediamine (TMEDA, a), N,N,N′,N′-tetramethylpropanediamine (TMPDA, b), and N,N,N′,N′-tetramethylbutanediamine (TMBDA, c)], (DA·2BH₂CNEt)(BF₄)₂ (2), and DA·2BH₂COOH (3) complexes were prepared from Me₂S solution of (BH₂CN)_n (→ 1 → 2 rarr; 3) in fast procedures, differently from those usually applied, in good yields, Dimethyl esters (4) were prepared from 3 with methanol in very fast reactions catalyzed by HBr. 3 and 4 were transformed into DA·BH₂COOH (5a,b) and DA·BH₂COOMe (6) with DA in THF. Bromination of 3 in aqueous HBr yielded diamine-bis(monobromocarboxyboranes) (7), in vigorous conditions diamine-bis(dibromocarboxyboranes) (9) were produced. N-Bromosuccinimide in methanol transformed 3a,b directly into diamine-bis(bromomethoxycarbonylboranes) (8a,b). BrH·DA·BH(Br)COOH (10a,b) could be prepared from 5a,b with bromine in strongly acidic medium. N-Deprotonated 10a quickly transformed into a five-membered cyclic cation [TMEDA·BHCOOH]⁺ (cation in 12a) in both water and methanol. Thus, this ion was formed by dissolution of 10a in water, bromination of 5a or destroying one of the borane moieties in 7a by a base. N-Deprotonated 10b in water decomposed with a pH-dependent rate, but in methanol, in the presence of base, it transformed into a six-membered cyclic cation [TMPDA·BHCOOH]⁺ (cation in 12b). 8a,b could also be transformed into cyclic cations by destroying one of their borane moieties in methanol, (8b in absence of acids only). All cations were prepared as bromide, hexafluorophosphate and tetraphenylborate salts. pK_a values of 12a,b (pK_a = 5.5-5.6) are 3 units lower than those of amine-carboxyboranes. Zwitterions (18a,b) formed by their deprotonation were prepared. Cyclic cation [TMEDA·B(Br)COOH]⁺ (cation in 19a) formed on destroying one dibromocarboxyborane of 9a. This cation, quite a strong acid (pK_a ≈ 4.2), was prepared as hexafluorophosphate and tetraphenylborate salt. The latter was transformed with aqueous KOH into the corresponding zwitterion (21a).

Full text of DOI:10.1021/ic9713442

Inorg. Chem. 1998, 37, 5131-5141
5131
Synthesis, Characterization, and Bromine Substitution of Diamine Complexes of
Carboxyborane and Methoxycarbonylborane. Diazabora Rings Containing B-Carboxyl and
B-Carboxylato Groups
Be´la Gyo1ri,*,† Zolta´n Berente,† and Zolta´n Kova´cs‡
Department of Inorganic and Analytical Chemistry, Lajos Kossuth University,
H-4010 Debrecen, Hungary, and Department of Chemistry, The University of Texas at Dallas,
Richardson, Texas 75083-0688
ReceiVed October 23, 1997
DA‚2BH2CN (1) [DA ) N,N,N′,N′-tetramethylethanediamine (TMEDA, a), N,N,N′,N′-tetramethylpropanediamine
(TMPDA, b), and N,N,N′,N′-tetramethylbutanediamine (TMBDA, c)], (DA‚2BH2CNEt)(BF4)2 (2), and DA‚2BH2-
COOH (3) complexes were prepared from Me2S solution of (BH2CN)n (f 1 f 2 f 3) in fast procedures, differently
from those usually applied, in good yields. Dimethyl esters (4) were prepared from 3 with methanol in very fast
reactions catalyzed by HBr. 3 and 4 were transformed into DA‚BH2COOH (5a,b) and DA‚BH2COOMe (6) with
DA in THF. Bromination of 3 in aqueous HBr yielded diamine-bis(monobromocarboxyboranes) (7), in vigorous
conditions diamine-bis(dibromocarboxyboranes) (9) were produced. N-Bromosuccinimide in methanol transformed
3a,b directly into diamine-bis(bromomethoxycarbonylboranes) (8a,b). BrH‚DA‚BH(Br)COOH (10a,b) could be
prepared from 5a,b with bromine in strongly acidic medium. N-Deprotonated 10a quickly transformed into a
five-membered cyclic cation [TMEDA‚BHCOOH]+ (cation in 12a) in both water and methanol. Thus, this ion
was formed by dissolution of 10a in water, bromination of 5a or destroying one of the borane moieties in 7a by
a base. N-Deprotonated 10b in water decomposed with a pH-dependent rate, but in methanol, in the presence of
base, it transformed into a six-membered cyclic cation [TMPDA‚BHCOOH]+ (cation in 12b). 8a,b could also
be transformed into cyclic cations by destroying one of their borane moieties in methanol, (8b in absence of acids
only). All cations were prepared as bromide, hexafluorophosphate and tetraphenylborate salts. pKa values of
12a,b (pKa ) 5.5-5.6) are 3 units lower than those of amine-carboxyboranes. Zwitterions (18a,b) formed by
their deprotonation were prepared. Cyclic cation [TMEDA‚B(Br)COOH]+ (cation in 19a) formed on destroying
one dibromocarboxyborane of 9a. This cation, quite a strong acid (pKa ≈ 4.2), was prepared as hexafluorophosphate
and tetraphenylborate salt. The latter was transformed with aqueous KOH into the corresponding zwitterion
(21a).
Introduction
tiosteoporotic,11 and antiinflammatory9,12 activities of amine
carboxyboranes and their ester, amide, and peptide derivatives.
The biological role of these complexes is still being ex-
plored.3,13,14 With respect to these biological and pharmacologi-
cal activities we focused our efforts on the synthesis and
characterization of boron-substituted derivatives, a type of
compounds, which have attracted less attention so far. To date
only few derivatives of the type amine‚BH(R)COX (R ) alkyl,
X ) OR, NR2)15,16 and a couple of carboxylated diazadibori-
nanes and diazadiborolidines17,18 have been prepared. We
planned to attach different substituents to boron via the
brominated derivatives of carboxyboron compounds, as inter-
mediates, taking advantage of the good leaving character of the
A large number of amine carboxyboranes and their derivatives
substituted on the carbon atom (amine‚BH2X, where X )
COOH,1-5 COOR,5-7 CONR2,1,2,5 C(O)NHOH,8 CSNHR,4
C(OR)dNR,5 C(CN)dNR,4 etc.) have been synthesized in the
past twenty years. Extensive biological and pharmacological
studies revealed remarkable hypolipidemic,9 anticancer,9,10 an-
* Corresponding author. Tel.: +36 52 316 666, ext. 2002. Fax: +36
52 489 667. E-mail: bgyori@tigris.klte.hu.
† Lajos Kossuth University.
‡ The University of Texas at Dallas.
(1) Spielvogel, B. F.; Wojnowich, L.; Das, M. K.; McPhail, A. T.;
Hargrave, K. D. J. Am. Chem. Soc. 1976, 98, 5702.
(2) Spielvogel, B. F.; Harchelroad, J. F.; Wisian-Neilson, P. J. Inorg. Nucl.
Chem. 1979, 41, 1223.
(3) Spielvogel, B. F.; Das, M. K.; McPhail, A. T.; Onan, K. D.; Hall, I.
H. J. Am. Chem. Soc. 1980, 102, 6343.
(4) Gyo¨ri, B.; La´za´r, I.; Emri, J. J. Organomet. Chem. 1988, 344, 29.
(5) Mittakanti, M.; Morse, K. W. Inorg. Chem. 1990, 29, 554.
(6) Mittakanti, M.; Feakes, D. A.; Morse, K. W. Synthesis 1992, 380.
(7) Gyo¨ri, B.; Berente, Z.; Emri, J.; La´za´r, I. Synthesis 1995, 191.
(8) Norwood, V. M., III; Morse, K. W. Inorg. Chem. 1988, 27, 302.
(9) Spielvogel, B. F.; Sood, A.; Hall, I. H. WO Patent 93 09,123, 1993;
Chem. Abstr. 1993, 119, 203618f.
(11) Rajendran, K. G.; Chen, S. Y.; Sood, A.; Spielvogel, B. F.; Hall, I.
H. Biomed. Pharmacother. 1995, 49, 131.
(12) Hall, I. H.; Rajendran, K. G.; Chen, S. Y.; Wong, O. T.; Sood, A.;
Spielvogel, B. F. Arch. Pharm. (Weinheim, Ger.) 1995, 328, 39.
(13) Miller, M. C., III; Shrewsbury, R. P.; Elkins, A. L.; Sood, A.;
Spielvogel, B. F.; Hall, I. H. J. Pharm. Sci. 1995, 84, 933.
(14) Murphy, M. E.; Elkins, A. L.; Shrewsbury, R. P.; Sood, A.; Spielvogel,
B. F.; Hall, I. H. Met.-Based Drugs 1996, 3, 31.
(15) Sood, A.; Spielvogel, B. F. Main Group Met. Chem. 1989, 12, 143.
(16) Sutton, C. H.; Baize, M. W.; Todd, L. J. Inorg. Chem. 1994, 33, 4221
and references therein.
(17) Miller, N. E. Inorg. Chem. 1991, 30, 2228.
(10) Karthikeyan, S.; Sood, A.; Tomasz, J.; Spielvogel, B. F.; Hall, I. H.
Amino Acids 1995, 8, 323.
(18) Ferrie, D. J.; Miller, N. E. Synth. React. Inorg. Met.-Org. Chem. 1994,
24, 1139.
S0020-1669(97)01344-X CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/12/1998

5132 Inorganic Chemistry, Vol. 37, No. 20, 1998
Gyo¨ri et al.
bromine. Our earlier paper19 reported the synthesis of many
aminecomplexesofbromocarboxyboraneandbromomethoxycarbon-
ylborane. Through a Br- f amine exchange, similarly to the
method discovered by No¨th,20 these complexes afforded the
previously unknown bis(amine)carboxyboronium cations [AA′B-
(H)COOR]+.21,22 Furthermore, our preliminary studies showed
that bromination of TMEDA‚BH2COOR (R ) H, Me) yields
[TMEDA‚B(H)COOR]+ cyclic cation directly.21 Relying upon
these findings we have investigated the bromination of diamine
complexes of carboxyborane and methoxycarbonylborane.
These studies yielded numerous bromocarboxyboron complexes
and several carboxyboronium cations, which represent a new
type of amine-borane complexes. The biological screening of
these compounds is planned. Our observations showing that
amines readily expel bromide encourage us to attempt the
synthesis of A‚BH(X)COOR (X ) R′, OR′, SR′, NR′2)
complexes from bromocarboxy compounds for further (includ-
ing biological) studies.
Acidity constants of 5a and 14a were determined pH-metrically at
25 ( 0.1 °C at 0.2 M KCl ionic strength using a Radiometer PH M 52
pH-meter referenced to a saturated calomel electrode, and the data were
evaluated by PSEQUAD.25 Approximate acidity constants of 14b and
19a were estimated from the half-neutralization pH.
The boron and bromine content of the samples was determined with
acid-base titration in the presence of mannitol, or by using the Volhard
method, respectively, after fusion with NaOH and KOH. Analyses of
BF4 and PF6 salts were performed in the presence of large excess of
CaCl2.
Safety Note. Dihydrocyanoborane oligomer was always handled
in methyl sulfide solution because explosions were experienced with
neat (BH2CN)n.26
Syntheses. TMPDA‚2BH2CN (1b). A methyl sulfide solution of
cyanodihydroborane (33.5 mL, 2.29 M; 76.8 mmol BH2CN) was added
dropwise to an ice-cooled and stirred methyl sulfide solution (35 mL)
of TMPDA (5.00 g, 38.4 mmol) over 30 min, and the mixture was
stirred for a further 20 min at room temperature. The precipitate was
filtered off, washed with methyl sulfide (3 × 10 mL) and dried in a N2
stream. Yield: 7.86 g (98%). Anal. Calcd (found) for C9H22B2N4:
B, 10.40 (10.31). 1H NMR (DMSO-d6, δ): 2.83 (m, 4H, NCH2), 2.61
(s, 12H, NCH3), 2.01 (m, 2H, CH2CH2CH2). 13C{1H} NMR (DMSO-
d6, δ): 59.56 (NCH2), 49.23 (NCH3), 17.34 (CH2CH2CH2). 11B NMR
(DMSO-d6, δ): -14.6 (br t). IR (KBr, cm-1): ν(B-H), 2431, 2417;
ν(CtN), 2194.
TMBDA‚2BH2CN (1c). A methyl sulfide solution of cyanodihy-
droborane (21.8 mL, 2.29 M; 49.8 mmol BH2CN) was added dropwise
to an ice-cooled and stirred methyl sulfide solution (12 mL) of TMBDA
(3.59 g, 24.9 mmol) over 20 min and the mixture was stirred at room
temperature for a further 20 min. The precipitate was filtered off,
washed with methyl sulfide (4 × 5 mL), and dried in a N2 stream.
Yield: 5.12 g (93%). Anal. Calcd (found) for C10H24B2N4: B, 9.74
(9.60). 1H NMR (CDCl3, δ): 2.93 (m, 4H, NCH2), 2.69 (s, 12H,
NCH3), 1.75 (m, 4H, CH2(CH2)2CH2). 13C{1H} NMR (CDCl3, δ):
62.63 (NCH2), 50.26 (NCH3), 21.04 (CH2(CH2)2CH2). 11B NMR
(CDCl3, δ): -15.4 (br t). IR (KBr, cm-1): ν(B-H), 2429, 2395; ν(Ct
N), 2196.
[TMPDA‚2BH2CNEt](BF4)2 (2b). Et3OBF4 (5.46 g, 28.7 mmol)
was added to the stirred suspension of 1b (2.90 g, 14.0 mmol) in
dichloromethane (30 mL), and the mixture was refluxed for 3 h. The
solvent was distilled off, the residual oilswhich slowly transformed
into crystalsswas kept under vacuum to reach a constant weight. It
was then stirred with ether (40 mL) for 20 min, then filtered, washed
with ether (3 × 20 mL), and dried in a N2 stream. Yield: 6.05 g (99%).
Anal. Calcd (found) for C13H32B4F8N4: B, 9.84 (9.76). 1H NMR (CH2-
Cl2, δ(TMS)): 4.11 (q, 4H, NCH2CH3), 3.12 (m, 4H, (CH3)2NCH2),
2.81 (s, 12H, NCH3), 2.13 (m, 2H, CH2CH2CH2), 1.54 (t, 6H,
NCH2CH3). 13C{1H} NMR (CH2Cl2, δ(TMS)): 60.28 ((CH3)2NCH2),
50.52 (NCH3), 41.60 (NCH2CH3), 17.87 (CH2CH2CH2), 13.32
(NCH2CH3). 11B NMR (CH2Cl2, δ): -0.1 (s, BF4-), -14.1 (br,
complex). IR (KBr, cm-1): ν(B-H), 2476, 2436; ν(CtN), 2317.
[TMBDA‚2BH2CNEt](BF4)2 (2c). A dichloromethane solution (20
mL) of Et3OBF4 (7.07 g, 37.2 mmol) was added to the stirred solution
of 1c (3.93 g, 17.7 mmol) in dichloromethane (30 mL), and the mixture
was refluxed for 2 h and then left at room temperature for an hour.
The gelly precipitate was filtered off, compressed with a glass rod,
washed with dichloromethane (4 × 10 mL), and dried in a N2 stream.
Yield: 5.68 g (71%). Anal. Calcd (found) for C14H34B4F8N4: B, 9.53
(9.37). 1H NMR (CH2Cl2, δ(TMS)): 4.13 (q, 4H, NCH2CH3), 3.15
(m, 4H, (CH3)2NCH2), 2.77 (s, 12H, NCH3), 1.82 (m, 4H, NCH2CH2),
1.54 (t, 6H, NCH2CH3). 13C{1H} NMR (CH2Cl2, δ(TMS)): 61.12
((CH3)2NCH2), 50.93 (NCH3), 41.57 (NCH2CH3), 20.65 (NCH2CH2),
13.37 (NCH2CH3). 11B NMR (CH2Cl2, δ): -0.1 (s, BF4-), -14.5 (br,
complex). IR (KBr, cm-1): ν(B-H), 2462, 2434; ν(C(N) 2317.
TMPDA‚2BH2COOH (3b). 2b (5.85 g, 13.3 mmol) was dissolved
in water (25 mL) and the solution was gently shaken at 55, 70, and 80
°C for consecutive 5 min periods. The solution was then quickly cooled
Experimental Section
Methods and Materials. All reactions, except those involving water
or noted otherwise, were performed under an oxygen- and water-free
N2 atmosphere using the general Schlenk techniques in flamed or oven-
dried glassware with absolutized solvents freshly distilled prior to use.
Methyl sulfide was dried over sodium, then fractionally distilled.
Dichloromethane was distilled from CaH2 and then refluxed with
NaBH4/diglyme and fractionally distilled. Methanol was distilled from
Mg(OCH3)2. THF and ether were distilled from sodium benzophenone.
Chloroform was distilled from P2O5 after shaking with concd H2SO4
and drying with CaCl2. Acetonitrile and DMSO were distilled from
CaH2. Acetone was distilled from a 1.5 m Raschig packed column.
TMEDA was distilled from KOH. TMPDA and TMBDA (Aldrich)
were kept over 4 Å molecular sieves (Aldrich), which were activated
by keeping at 320 °C for 12 h, and stored under dry N2. NBS was
recrystallized from water and dried in an N2 stream before use. Bromine
(Ferak), 48% aq HBr solution, KPF6 (Fluka), KBrO3, KBr, and NaBPh4
(Reanal) were used as received.
Methyl sulfide solution of cyanodihydroborane oligomer was
prepared by known procedure.23 Et3OBF4 was synthesized by the
Meerwein protocol.24 Methanolic HBr solution was prepared by
absorbing gaseous HBr (liberated from 48% aq HBr solution by P2O5
and dried by Granusic A (J. T. Baker)) in methanol under N2.
Compounds 1a, 2a, 3a, 5a,4 and 4a7 were prepared as previously
described.
NMR spectra were recorded on a Bruker AM 360 instrument in 5
mm o.d. tubes at room temperature. 1H spectra were referred to DSS
in D2O and TMS in other solvents. 13C (90.5 MHz) spectra were
referred to deuterated solvent signals (CDCl3, 77.0 ppm; acetone-d6,
29.9 ppm; DMSO-d6, 39.5 ppm), and DSS in D2O as external reference.
Ambiguities in assigning 1H and 13C signals were cleared with
homonuclear decoupling and shift correlation (1H-1H and 13C-1H)
experiments. Carbons directly attached to boron could not be observed.
11B (115.5 MHz) spectra were referred to Et2O‚BF3 in a capillary
inserted into the tube. In cases when multiplicities could only be
revealed by applying Gaussian multiplication, multiplets are marked
“broad” and coupling constants are not given.
IR spectra were recorded on a Perkin-Elmer Paragon PC 1000 FT-
IR spectrometer.
(19) Gyo¨ri, B.; Kova´cs, Z.; Emri, J.; Berente, Z. J. Organomet. Chem. 1994,
484, 225.
(20) (a) No¨th, H., Beyer, H., Vetter, H.-J. Chem. Ber. 1964, 97, 110. (b)
No¨th, H., Schweizer, P.; Ziegelgansberger, F. Chem. Ber. 1966, 99,
1089.
(21) Emri, J.; Kova´cs, Z.; Gyo¨ri, B.; La´za´r, I. Inorg. Chim. Acta 1994,
223, 155.
(22) Emri, J.; Kova´cs, Z.; Gyo¨ri, B. Inorg. Chim. Acta 1995, 233, 119.
(23) Gyo¨ri, B.; Kova´cs, Z.; Emri, J.; La´za´r, I. Inorg. Chim. Acta 1994,
218, 21.
(25) Ze´ka´ny, L.; Nagypa´l, I. In: Computational Methods for the Deter-
mination of Formation Constants; Leggett, D. J., Ed.; Plenum Press:
New York, 1985; p 291.
(24) Meerwein, H. Org. Synth. 1966, 46, 113.

Diazabora Rings with B-Carboxyl and B-Carboxylato Groups
Inorganic Chemistry, Vol. 37, No. 20, 1998 5133
to 60 °C left to cool to room temperature, and allowed to stand for an
hour, while a large amount of white needles appeared. The mixture
was then cooled in an ice-water bath for 30 min, the crystals were
collected on a filter, washed with 0 °C water (3 × 10 mL), and dried
in a N2 stream. Yield: 2.87 g (88%). Anal. Calcd (found) for C9H24-
B2N2O4: B, 8.79 (8.77). 1H NMR (DMSO-d6, δ): 10.16 (s, 2H,
COOH), 2.89 (m, 4H, NCH2), 2.63 (s, 12H, NCH3), 1.95 (m, 2H,
CH2CH2CH2). 13C{1H} NMR (DMSO-d6, δ): 58.94 (NCH2), 48.56
(NCH3), 17.06 (CH2CH2CH2). 11B NMR (DMSO-d6, δ): -14.6 (br).
immediately evaporated in vacuo and kept in a vacuum until a
microcrystalline material (4a) appeared in the oil (6a). In a typical
experiment the product consisted of 85 mol % 6a, 12 mol % TMEDA,
and 3 mol % 4a. This composition remained constant for a few hours
in a refrigerator (4 °C). Yield: 1.53 g (contains 7.18 mmol 6a). 1H
NMR (CDCl3, δ): 3.54 (s, 3H, OCH3), 3.13 (m, 2H, BNCH2), 2.76 (s,
6H, BNCH3), 2.60 (m, 2H,:NCH2), 2.24 (s, 6H,:NCH3). 13C{1H} NMR
(CDCl3, δ): 59.13 (BNCH2), 53.52 (:NCH2), 49.63 (BNCH3), 47.62
(OCH3), 45.27 (:NCH3). 11B NMR (CDCl3, δ): -10.7 (br t). IR (neat,
cm-1): ν(B-H), 2394, 2302 sh; νas(CdO), 1672.
IR (KBr, cm-1):
νassoc(O-H), 2720, 2630, 2574; ν(B-H), 2406;
νas(CdO), 1648.
TMPDA‚BH2COOMe (6b). TMPDA (0.96 g, 7.4 mmol) was
added to the solution of 4b (0.81 g, 2.96 mmol) in THF (5.7 mL). The
mixture was refluxed for 1.5 h and then cooled to room temperature.
The solvent was evaporated in vacuo and the residue was kept in a
vacuum (<0.1 mmHg) to reach a constant weight. The product is a
clear oil consisting of 6b (97 mol %) and 4b (3 mol %). This
composition remains constant for ca. 1 day in refrigerator (4 °C). The
increase of the amount of 4b is indicated by appearance of white
crystals. Yield: 1.15 g (96%). Anal. Calcd (found) for C9H23-
BN2O2: B, 5.35 (5.50). 1H NMR (CDCl3, δ): 3.54 (s, 3H, OCH3),
3.02 (m, 2H, BNCH2), 2.71 (s, 6H, BNCH3), 2.27 (m, 2H,:NCH2), 2.21
(s, 6H,:NCH3), 1.83 (m, 2H, CH2CH2CH2). 13C{1H} NMR (CDCl3,
δ): 60.69 (BNCH2), 56.56 (:NCH2), 49.18 (BNCH3), 47.48 (OCH3),
44.95 (:NCH3), 21.17 (CH2CH2CH2). 11B NMR (CDCl3, δ): -11.0
(br t). IR (neat, cm-1): ν(B-H), 2393, 2300 sh; νas(CdO), 1673.
TMBDA‚BH2COOMe (6c). THF (4 mL) and TMBDA (0.887 g,
6.15 mmol) were added to 4c (0.354 g, 1.229 mmol), and the mixture
was refluxed for 11 h, then allowed to cool to room temperature and
to stand overnight. The precipitated crystals (4c) were collected on
filter, and the filtrate was evaporated to dryness in vacuo and kept at
0.1 mmHg to give a dense, slightly opalescent oil of constant weight.
In a typical experiment the product was contaminated with 8 mol %
TMBDA. This composition remained constant for a few days in a
refrigerator. Yield: 0.540 g (contains 2.36 mmol 6c). 1H NMR
(CDCl3, δ): 3.54 (s, 3H, OCH3), 3.01 (m, 2H, BNCH2), 2.69 (s, 6H,
BNCH3), 2.31 (m, 2H, BCH2CH2), 2.24 (s, 6H,:NCH3), 1.67 (m, 2H,
BNCH2CH2), 1.45 (m, 2H,:NCH2CH2). 13C{1H} NMR (CDCl3, δ):
62.31 (BNCH2), 59.04 (:NCH2), 49.18 (BNCH3), 47.96 (OCH3), 45.32
(:NCH3), 25.10 (:NCH2CH2), 21.03 (BNCH2CH2). 11B NMR (CDCl3,
δ): -10.7 (br t). IR (KBr, cm-1): ν(B-H), 2393; νas(CdO), 1672.
TMEDA‚2BH(Br)COOH (7a). 3a (1.00 g, 4.31 mmol) was
suspended in water (5 mL), then dissolved by addition of NaOH solution
(15.3 mL, 0.295 M, 4.51 mmol). To this solution was added a mixture
of bromine (1.43 g, 8.96 mmol), 48% aq HBr (10 mL), and water (20
mL) in seconds, under extremely vigorous stirring. After 20 min 48%
aq HBr (6 mL) was added, and the resulting mixture was heated to 60
°C over 10 min. The mixture was then allowed to cool to room
temperature, and the precipitate was filtered, washed with 2 M HBr (3
× 6 mL) then water (5 × 5 mL), predried in a fast N2 stream, washed
with ether (3 × 10 mL), and dried in a N2 stream. Yield: 1.35 g (80%).
Anal. Calcd (found) for C8H20B2Br2N2O4: B, 5.55 (5.74); Br, 41.01
(40.17). 1H NMR (DMSO-d6, δ): 11.16 (s, 2H, COOH), 3.49 (m,
4H, CH2), 2.86, 2.85 (2 × s, 2 × 6H, NCH3). 13C{1H} NMR (DMSO-
d6, δ): 53.73, 53.65 (NCH2), 47.10, 46.35, 46.21 (NCH3). 11B NMR
(DMSO-d6, δ): -4.5 (br). IR (KBr, cm-1): νassoc(O-H), 2744, 2598;
ν(B-H), 2474; νas(CdO), 1659.
TMBDA‚2BH2COOH (3c). 2c (4.89 g, 10.8 mmol) was dissolved
in water (20 mL), and the solution was heated to 85 °C in 2 min, then
kept and gently shaken at 85 °C for 10 min. The solution was then
cooled to room temperature and slowly stirred with a magnetic stir bar
for 15 min. The crystals were filtered off, washed with water (3 × 7
mL), and dried in a N2 stream. Yield: 2.71 g (97%). Anal. Calcd
(found) for C10H26B2N2O4: B, 8.32 (8.43). 1H NMR (DMSO-d6, δ):
10.11 (s, 2H, COOH), 2.93 (m, 4H, NCH2), 2.58 (s, 12H, NCH3), 1.53
(m, 4H, CH2(CH2)2CH2). 13C{1H} NMR (DMSO-d6, δ): 60.91 (NCH2),
48.61 (NCH3), 20.39 (CH2(CH2)2CH2). 11B NMR (DMSO-d6, δ):
-11.4 (br). IR (KBr, cm-1): νassoc(O-H), 2720, 2630, 2568; ν(B-
H), 2430, 2380; νas(CdO), 1644.
TMPDA‚2BH2COOMe (4b). Methanolic solution of HBr (0.90
mL, 0.276 M, 0.248 mmol) was added to the stirred suspension of 3b
(1.02 g, 4.15 mmol) in methanol (25 mL), and the clear solution, formed
immediately upon the addition of HBr, was stirred for 10 min. The 4
Å molecular sieves (1.85 g) were added to the solution, which was
allowed to stand for several hours. The molecular sieves were filtered
off, and the filtrate was evaporated to dryness in vacuo. The residue
was collected on a filter with ether (30 mL) and extracted into ether to
a point when only a small amount of noncrystalline material remained
on the filter (ca. 45 times). The precipitate was filtered from the extract,
washed with ether (2 × 3 mL) and dried in a N2 stream. Yield: 1.01
g (89%). Anal. Calcd (found) for C11H28B2N2O4: B, 7.89 (7.89). 1H
NMR (CDCl3, δ): 3.54 (s, 6H, OCH3), 2.98 (m, 4H, NCH2), 2.72 (s,
12H, NCH3), 2.08 (m, 2H, CH2CH2CH2). 13C{1H} NMR (CDCl3, δ):
59.69 (NCH2), 49.93 (OMe), 47.99 (NMe), 18.17 (CH2CH2CH2). 11B
NMR (CDCl3, δ): -11.2 (br t). IR (KBr, cm-1): ν(B-H), 2403;
νas(CdO), 1670.
TMBDA‚2BH2COOMe (4c). Methanolic solution of HBr (1.14
mL, 0.276 M, 0.314 mmol) was added to the stirred suspension of 3c
(0.68 g, 2.62 mmol) in methanol (15.5 mL), and the stirred mixture
was then placed into a 65 °C water bath for 6 h, while the slurry
transformed into a clear solution. On standing at room-temperature
large plates precipitated. The product was filtered at 0 °C, washed
with methanol (3 × 2 mL), and dried in a N2 stream. Yield: 0.61 g
(81%). Anal. Calcd (found) for C12H30B2N2O4: B, 7.51 (7.63). 1H
NMR (CDCl3, δ): 3.54 (s, 6H, OCH3), 3.03 (m, 4H, NCH2), 2.69 (s,
12H, NCH3), 1.65 (m, 2H, CH2(CH2)CH2). 13C{1H} NMR (CDCl3,
δ): 61.66 (NCH2), 49.66 (OCH3), 47.91 (NCH3), 21.09 (CH2(CH2)-
CH2). 11B NMR (CDCl3, δ): -11.1 (t, 1J(B,H) ) 101 Hz). IR (KBr,
cm-1): ν(B-H), 2406; νas(CdO), 1674.
TMPDA‚BH2COOH (5b). TMPDA (3.02 g, 23.2 mmol) was added
to a supension of 3b (1.90 g, 7.73 mmol) in THF (15 mL), and the
mixture was refluxed for 6 h. TMPDA and THF were then removed
in vacuo, and the solid residue was kept in a vacuum to reach a constant
weight. It was then suspended in ether (25 mL) and the suspension
was filtered. The filtered solid was extracted with the filtrate to a point
when only a flimsy gel remains (ca. 45-50 times). The crystals were
filtered from the extract, washed with ether (5 mL), and dried in a N2
stream. Yield: 2.03 g (70%). Anal. Calcd (found) for C8H21BN2O2:
B, 5.75 (5.80). 1H NMR (CDCl3, δ): 3.02 (m, 2H, BNCH2), 2.70 (s,
6H, BNCH3), 2.29 (m, 2H, CH2CH2CH2), 2.22 (s, 6H,:NCH3), 1.84
(m, 2H,:NCH2). 13C{1H} NMR (CDCl3, δ): 60.83 (BNCH2), 56.78
(:NCH2), 49.48 (BNCH3), 45.11 (:NCH3), 21.33 (CH2CH2CH2). 11B
NMR (CDCl3, δ): -11.3 (br). IR (KBr, cm-1): νassoc(O-H), 2682,
2560; ν(B-H), 2380; νassoc(N-H), 1944 br; νas(CdO), 1654.
TMEDA‚BH2COOMe (6a). TMEDA (0.92 g, 7.92 mmol) was
added to the solution of 4a (1.03 g, 3.96 mmol) in THF (8 mL). The
mixture was refluxed for 1 h, cooled to 0 °C, and the solvent was
TMPDA‚2BH(Br)COOH (7b). 3b (0.82 g, 3.33 mmol) was
dissolved in a mixture of acetone (17 mL), water (8 mL), and 48% aq
HBr (0.4 mL). The solution was cooled in an ice-water bath and,
prior to the precipitation of a solid, a solution of bromine (1.21 g, 7.57
mmol) in aq HBr (18 mL, 5 m/m %) was added over 10-15 min at
steadily decreasing speed under stirring. Acetone was removed in
vacuo, and the precipitated crystals were collected on a filter, washed
with water (4 × 6 mL), and dried in a N2 stream. Yield: 1.10 g (82%).
Anal. Calcd (found) for C9H22B2Br2N2O4: B, 5.36 (5.48); Br, 39.59
(39.12). 1H NMR (DMSO-d6, δ): 11.12 (s, 2H, COOH) 3.05 (m, 4H,
NCH2), 2.79 (s, 12H, NCH3), 2.05 (m, 2H, CH2CH2CH2). 13C{1H}
NMR (DMSO-d6, δ): 57.54 (NCH2), 46.29, 46.24, 45.81 (NCH3), 15.83
(CH2CH2CH2). 11B NMR (DMSO-d6, δ): -4.4 (br). IR (KBr, cm-1):
ν
assoc(O-H), 2751, 2658, 2598; ν(B-H), 2470; νas(CdO), 1653.
TMBDA‚2BH(Br)COOH (7c). 3c (0.80 g, 3.08 mmol) was

5134 Inorganic Chemistry, Vol. 37, No. 20, 1998
Gyo¨ri et al.
suspended in water (8 mL) and dissolved by addition of aq NaOH
solution (6.52 mL, 1.00 M, 6.52 mmol). This solution was cooled in
an ice-water bath, and before precipitation of any solid a bromine
solution (prepared by adding 48% aq HBr (5.4 mL) to a 0 °C solution
of KBrO3 (0.353 g, 2.11 mmol) in water (10 mL)) was added in a
second under extremely vigorous stirring. The mixture was warmed to
ca. 30 °C and filtered after the disappearance of the light yellow color.
The microcrystalline product (which contains 4 mol % 3c and 5 mol
% TMBDA‚BH(Br)COOH‚BBr2COOH and TMBDA‚2BBr2COOH)
was washed with water (4 × 6 mL), predried with air suction, washed
with ether (4 × 5 mL), and dried with air suction. Yield: 1.22 g (95%).
Anal. Calcd (found) for C10H24B2Br2N2O4: B, 5.18 (5.30); Br, 38.26
(39.06). 1H NMR (DMSO-d6, δ): 11.08 (br s, COOH), 3.09 (m, 4H,
NCH2), 2.76, 2.77 (2 × s, 2 × 6H, NCH3), 1.61 (m, 4H, CH2CH2CH2-
CH2). 13C{1H} NMR (DMSO-d6, δ): 59.88 (NCH2), 46.32, 45.75
(NCH3), 19.63 (CH2(CH2)2CH2). 11B NMR (DMSO-d6, δ): -4.5 (br).
colorless. The permeability of the precipitate against water was
constantly decreasing as the filtration advanced. The filtered material
was dried with air suction, washed with ether (3 × 6-8 mL) in the
course of drying. Yield: 1.33 g (58%). Anal. Calcd (found) for
C9H20B2Br4N2O4: B, 3.85 (3.89); Br, 56.92 (55.16). 1H NMR (DMSO-
d6, δ): 11.80 (s, 2H, COOH), 3.39 (m, 4H, NCH2), 3.02 (s, 12H, NCH3),
2.22 (m, 2H, CH2CH2CH2). 13C{1H} NMR (DMSO-d6, δ): 56.16
(NCH2), 45.70 (NCH3), 15.83 (CH2CH2CH2). 11B NMR (DMSO-d6,
δ): -0.2 (br). IR (KBr, cm-1):
νas(CdO), 1667.
νassoc(O-H), 2790, 2683, 2621;
TMBDA‚2BBr2COOH (9c). 2c (0.80 g, 3.08 mmol) was dissolved
in aq NaOH solution (6.34 mL, 1.00 M, 6.34 mmol), and aq HBr (2.8
mL, 30 m/m%) and then bromine solution in 48% aq HBr (8.0 mL,
6.0 M, 48.0 mmol Br2) were added at 0 °C and under vigorous stirring.
The mixture was then placed into a 35 °C water bath. The bath was
warmed to 70 °C over 30 min, then to 85 °C over 40 min for 8 h.
After cooling to room temperature, the precipitate was collected on a
filter, washed with water (5 × 10 mL), 1% aq NaHSO3 (3 × 5 mL),
and then with water (3 × 6 mL), and dried with air suction while being
washed with ether (3 × 5 mL) in the course of drying. The product is
contaminated with TMBDA‚(BBr2COOH)‚(BHBrCOOH) and TMBDA‚
2BH(Br)COOH (7-8 mol % altogether). Yield: 0.40 g (23%). Anal.
Calcd (found) for C10H22B2Br4N2O4: B, 3.76 (3.84); Br, 55.54 (54.09).
1H NMR (DMSO-d6, δ): 11.70 (s, 2H, COOH), 3.43 (m, 4H, NCH2),
2.97 (s, 12H, NCH3), 1.73 (m, 4H, CH2(CH2)2CH2). 13C{1H} NMR
(DMSO-d6, δ): 58.61 (NCH2), 45.51 (NCH3), 19.52 (CH2(CH2)2CH2).
11B NMR (DMSO-d6, δ): - 0.4 (br). IR (KBr, cm-1): νassoc(O-H),
2770, 2664, 2596; νas(CdO), 1655.
TMEDA‚BH(Br)COOH‚HBr (10a). Aq HBr (15 mL, 30 m/m%)
was added to 5a (0.90 g, 5.17 mmol) in an ice-water bath. To the
vigorously stirred suspension a solution of bromine in 30 m/m% aq
HBr (1.72, mL 3.00 M, 5.16 mmol) was added dropwise, while the
mixture turned into a clear solution. When a microcrystalline precipitate
appeared, the addition of the bromine was interrupted (at ca. 1.15 mL),
and aq HBr (1.5 mL, 30 m/m%) was added. The addition of the
bromine solution was then continued. After stirring at 0 °C for 30
min the product was collected on a filter, washed with acetone (3 × 6
mL) and dried in a N2 stream. Yield: 1.26 g (73%). Anal. Calcd
(found) for C7H19BBr2N2O2: B, 3.24 (3.38); Br, 47.87 (47.55). 1H
NMR (20% DCl, δ): 3.75-3.50 (m, 4H, NCH2), 3.02, 3.01, 2.99, 2.97
(4 × s, 12H, NCH3). 11B NMR (20% DCl, δ): -5.7 (br).
IR (KBr, cm-1):
νassoc(O-H), 2740, 2657, 2591; ν(B-H), 2464;
νas(CdO), 1654.
TMEDA‚2BH(Br)COOMe (8a). To a suspension of 3a (0.62 g,
2.67 mmol) in methanol (10 mL) was added NBS (0.96 g, 5.39 mmol)
over 30 min in small portions. After 30 min the precipitated crystals
were separated on a filter, washed with methanol (3 × 4 mL), and
dried in a N2 stream. Yield: 0.83 (74%). Anal. Calcd (found) for
C10H24B2Br2N2O4: B, 5.18 (5.15); Br, 38.26 (38.57). 1H NMR (CDCl3,
δ): 3.63 (s, 6H, OCH3), 3.63, 3.45 (2 × m, 3+1H, NCH2) 3.02, 2.98,
2.97, 2.96 (4(s, 4(3H, NCH3). 13C{1H} NMR (CDCl3, δ): 55.97, 55.19
(NCH2), 49.50 (OCH3), 49.10, 48.77, 48.69, 47.72 (NCH3). 11B NMR
(CDCl3, δ): -7.0 (br d). IR (KBr, cm-1): ν(B-H), 2501 sh, 2484;
νas(CdO), 1676.
TMPDA‚2BH(Br)COOMe (8b). To a solution of 3b (0.63 g, 2.56
mmol) in methanol (12 mL) was added solid NBS (0.91 g, 5.12 mmol)
in small portions over 30 min. The solution was evaporated to dryness,
and the residue was kept under high vacuum (<0.1 mmHg) to afford
a semisolid. It was then dissolved in dichloromethane (10 mL) and
the succinimide was removed by extraction with aq HBr solution (7 ×
3 mL, 0.005 M). The CH2Cl2 layer was dried with Na2SO4, the solvent
was evaporated with a N2 stream, and the residue was kept in a vacuum
to give a colorless oil of constant weight. Yield: 0.87 g (79%). Anal.
Calcd (found) for C11H26B2Br2N2O4: B, 5.01 (4.86); Br, 37.01 (36.10).
1H NMR (CDCl3, δ): 3.62 (s, 6H, OCH3), 3.25, 3.15, 2.99 (3 × m,
1+2+1H, NCH2), 2.95, 2.92 (2 × s, 3+9H, NCH3), 2.28, 2.19, 2.05
(3 × m, 0.5+1+0.5H, CH2CH2CH2). 13C{1H} NMR (CDCl3, δ):
59.53, 59.13 (NCH2), 49.39 (OCH3), 47.99, 47.72, 47.48, 47.13 (NCH3),
17.15 (CH2CH2CH2). 11B NMR (CDCl3, δ): -6.4 (br). IR (neat,
cm-1): ν(B-H), 2473; νas(CdO), 1674.
TMEDA‚2BBr2COOH (9a). To a stirred suspension of 3a (0.94
g, 4.05 mmol) in aq HBr solution (9.0 mL, 3.5 m/m%) was added a
solution of bromine in 48% aq HBr (4.32 mL, 4.5 M, 19.44 mmol
Br2) over 4-5 min at 0 °C. The flask was equipped with a reflux
condenser, and the bath temperature was raised to 40 °C over 10 min,
then to 70 °C over 40 min, then to 80 °C over 20 min for 2 h, and then
to 95 °C over 1.5 h for 2.5 h. After cooling to room temperature the
crude product was filtered, washed with water (5 × 5 mL), and dried
with air suction. Crude 9a was dissolved in DMSO (8.0 mL) and kept
at 50-55 °C for 5 h. The solution was concentrated to ca. 2 mL in
vacuo employing a 60 °C bath and 0.1 M aq HBr solution was slowly
added. The precipitate was collected on a filter, washed with water (4
× 5 mL), dried with air suction, washed with ether (3 × 6-8 mL) in
the course of drying. Yield: 1.24 g (56%). Anal. Calcd (found) for
C8H18B2Br4N2O4: B, 3.95 (4.06); Br, 58.38 (56.77). 1H NMR (DMSO-
d6, δ): 11.85 (s, 2H, COOH), 3.95 (s, 4H, NCH2), 3.08 (s, 12H, NCH3).
13C{1H} NMR (DMSO-d6, δ): 53.79 (NCH2), 46.97 (NCH3). 11B NMR
(DMSO-d6, δ): -0.6 (br). IR (KBr, cm-1): νassoc(O-H), 2786, 2676,
2618; νas(CdO), 1654.
The filtrate combined with the washing liquor was evaporated in
vacuo, and aq KPF6 (0.320 g, 1.74 mmol in 4.0 mL) was added. 14a
was obtained as described in method B for 14a. Yield: 0.32 g (17%).
TMPDA‚BH(Br)COOH‚HBr (10b). To a stirred 0 °C suspension
of 5b (0.83 g, 4.41 mmol) in acetone (5 mL) was added aq HBr (0.66
mL, 30 m/m%), then bromine dissolved in 30 m/m% aq HBr (1.60
mL, 3.0 M, 4.80 mmol). After 5 min the solution was diluted with
acetone (15 mL), and then it was concentrated in vacuo to a viscous
oil. Addition of acetone (15 mL) and evaporation were repeated three
times and increasing amount of solid was obtained. The product was
filtered off, washed with acetone (3 × 5 mL) and dried in a N2 stream.
Yield: 1.13 g (74%). Anal. Calcd (found) for C8H21BBr2N2O2: B,
3.11 (3.15); Br, 45.94 (44.86). 1H NMR (1 M DCl, δ): 3.21 (m, 4H,
NCH2), 2.94, 2.89 (2 × s, 12H, NCH3), 2.22 (m, 2H, CH2CH2CH2).
11B NMR (1 M DCl, δ): -5.3 (br).
2-Carboxy-1,1,3,3-tetramethyl-1,3,2λ4-diazaborolidinium Bro-
mide (12a). 10a (0.81 g, 2.43 mmol) was dissolved in water (6 mL),
and then the solvent was removed in vacuo. Acetone (10 mL) was
added to the residue, and then the solvent was evaporated; acetone (5
mL) was added again, and the product was filtered off, washed with
acetone (2 × 2 mL), and dried in a N2 stream. Yield: 0.48 g (78%).
Anal. Calcd (found) for C7H18BBrN2O2: B, 4.27 (4.26); Br, 31.59
(31.83). 1H NMR (D2O, δ): 3.64 (m, 4H, NCH2), 3.04, 3.03 (2 × s,
2 × 6H, NCH3). 13C{1H} NMR (D2O, δ): 62.12 (NCH2), 56.56, 51.15
TMPDA‚2BBr2COOH (9b). To a stirred suspension of 3b (1.01
g, 4.11 mmol) in water (8.0 mL) was added a solution of bromine in
48% aq HBr (7.20 mL, 4.5 M, 32.4 mmol Br2) over 4-5 min at 0 °C.
The flask was equipped with a reflux condenser. The bath was warmed
to 45 °C over 10 min, then to 80 °C over 30 min for 1 h, then to 95 °C
for 10 h. After cooling to room temperature the crude product was
filtered, washed with water (ca. 6 × 10-15 mL) until the filtrate was
1
(NCH3). 11B NMR (D2O, δ): 0.9 (d, J(B,H) ) 110 Hz). IR (KBr,
cm-1): νassoc(O-H), 2710, 2584; ν(B-H), 2454; νas(CdO) 1684.
2-Carboxy-1,1,3,3-tetramethyl-1,3,2λ4-diazaborinanium Bromide
(12b). To a suspension of 10b (0.88 g, 2.53 mmol) in methanol (8
mL) was added methanolic KOH solution (7.76 mL, 0.342 M, 2.65
mmol). The solution was evaporated to a constant weight. Acetonitrile

Diazabora Rings with B-Carboxyl and B-Carboxylato Groups
Inorganic Chemistry, Vol. 37, No. 20, 1998 5135
(80 mL) was added to the residue and the suspension stirred for 20
min in a 40 °C water bath. The insoluble parts (KBr) were filtered off
and washed with 40 °C acetonitrile (3 × 4 mL). The filtrate was
evaporated in vacuo, and ether (20 mL) and methanol (8 mL) were
slowly added to the stirred residue until only a small amount of jelly
material remained undissolved. The mixture was then filtered, the
filtrate was evaporated with a N2 stream, and the residue was kept in
a vacuum for 15 min. The residue was suspended in ether (15 mL)
and filtered, and the product was dried in a N2 stream. Yield: 0.62 g
(92%). Anal. Calcd (found) for C8H20BBrN2O2: B, 4.05 (4.00); Br,
29.93 (31.13). 1H NMR (D2O, δ): 3.23, 3.19 (overlapping multiplets,
4H, NCH2), 2.97, 2.87 (2 × s, 2 × 6H, NCH3), 2.37 (m, 1H, axial
CH2CHHCH2), 2.02 (m, 1H, equatorial CH2CHHCH2). 13C{1H} NMR
(D2O, δ): 63.52 (NCH2), 56.37, 48.12 (NCH3), 21.46 (CH2CH2CH2).
then it was cooled to 0 °C in an ice-water bath. After 20 min the
crystals were collected on a filter, washed with cold water (4 × 1.5
mL), and dried by air suction. Yield: 1.16 g (79%). Anal. Calcd
(found) for C28H72B4F30KN8O8P5: C, 23.10 (23.13), H, 4.98 (4.96); B,
2.97 (2.94); K, 2.69 (2.62); N, 7.70 (7.70). 1H NMR (acetone-d6, δ):
10.74 (s, 1H, COOH), 3.84 (m, 4H, NCH2), 3.20, 3.15 (2 × s, 2 × 6H,
NCH3). 13C{1H} NMR (acetone-d6, δ): 60.37 (NCH2), 54.68, 49.20
1
(NCH3). 11B NMR (acetone-d6, δ): 1.9 (d, J(B,H) ) 113 Hz). IR
(KBr, cm-1): νassoc(O-H), 2757, 2664, 2532; ν(B-H), 2464; νas(Cd
O), 1664.
2-Carboxy-1,1,3,3-tetramethyl-1,3,2λ4-diazaborinanium Hexa-
fluorophosphate (14b). To a suspension of 10b (0.55 g, 1.58 mmol)
in methanol (5 mL) was added methanolic KOH solution (4.85 mL,
0.342 M, 1.66 mmol KOH), and the solution was evaporated to dryness
in vacuo. Aqueous KPF6 (0.44 g, 2.39 mmol in 5.5 mL) was added to
the residue, and the mixture was acidified with H2SO4 (0.5 mL, 2 M).
The precipitate was redissolved by warming and the clear solution was
allowed to cool to room temperature for a day. After shaking and
leaving the mixture for 1 h in an ice-water bath, the crystals were
collected on a filter, washed with cold water (4 × 1 mL), and dried by
air suction. Yield: 0.34 g (65%). Anal. Calcd (found) for C8H20-
BF6N2O2P: B, 3.26 (3.19). 1H NMR (acetone-d6, δ): 10.82 (s, 1H,
COOH), 3.47 (m, 2H, equatorial NCHH), 3.34 (m, 2H, axial NCHH),
3.14, 3.08 (2 × m, 2 × 6H, NCH3), 2.59 (m, 1H, axial CH2CHHCH2),
2.12 (m, 1H, equatorial CH2CHHCH2). 13C{1H} NMR (acetone-d6,
δ): 62.95 (NCH2), 55.48, 45.99 (NCH3), 19.47 (CH2CH2CH2). 11B
NMR (acetone-d6, δ): -0.8 (d, 1J(B,H) ) 112 Hz). IR (KBr, cm-1):
ν(O-H), 3430; ν(B-H), 2477; νas(CdO), 1713.
1
11B NMR (D2O, δ): -1.9 (d, J(B,H) ) 113 Hz). IR (KBr, cm-1):
ν(B-H), 2464; νas(CdO), 1680.
2-Methoxycarbonyl-1,1,3,3-tetramethyl-1,3,2λ4-diazaborolidini-
um Bromide (13a). To the solution of 6a (7.24 mmol, freshly prepared
from 4a (0.97 g, 3.73 mmol)) in methanol (8 mL) was added dropwise
a freshly prepared solution of bromine (1.17 g, 7.32 mmol) in methanol
(13 mL). The first 5 mL portion was added in 5 min, the second 5
mL over 30 min, and the remaining bromine solution was added over
80 min as the rate of discoloration decreased. The addition of bromine
was accompanied by the appearance of a precipitate, and the color of
the mixture turned permanently pale yellow. The mixture was then
stirred at room temperature. After 1 h methanolic NaOH solution (3.03
mL, 2.42 M, 7.33 mmol) was added dropwise over 5 min. The slightly
opalescent solution was evaporated at room temperature in vacuo to
give a solid residue of constant weight. It was moved onto a filter and
was extracted (ca. 20-22 times) into acetone (20 mL). The crystals
precipitated from the extract were collected on a filter, washed with
acetone (2 × 2 mL), and dried in a N2 stream. The product was washed
off from the filter using chloroform (10 mL in 4 decreasing portions)
and the filtrate was then evaporated to dryness employing a N2 stream.
The solid residue was suspended in ether (4 mL) and then the
suspension was evaporated to dryness. The residue was suspended in
ether (8 mL), and the product was filtered, washed with ether (2 × 5
mL), and dried in a N2 stream. Yield: 1.29 g (67%). Anal. Calcd
(found) for C8H20BBrN2O2: B, 4.05 (3.97); Br, 29.93 (30.54). 1H NMR
(CDCl3, δ): 4.21, 3.99 (2 × m, 2 × 2H, NCH2), 3.68 (s, 3H, OCH3),
3.21, 3.15 (2 × s, 2 × 6H, NCH3). 13C{1H} NMR (CDCl3, δ): 60.12
(NCH2), 54.62, 49.39 (NCH3), 49.23 (OCH3). 11B NMR (CDCl3, δ):
2-Carboxy-1,1,3,3-tetramethyl-1,3,2λ4-diazaborolidinium Tet-
raphenylborate (15a). To a 0 °C solution of 3a (0.405 g, 2.33 mmol)
in water (5 mL) was added bromine water (11.45 mL, 0.210 M, 2.40
mmol Br2) dropwise over a few minutes. The solution was warmed to
room temperature, and aq NaBPh4 solution (19.0 mL, 0.128 M, 2.43
mmol) was added. After 30 min stirring the precipitate was filtered
and washed with water (3 × 5 mL). The wet crude 15a was dissolved
off the filter with acetone, water (10 mL) was added to the solution,
and then the acetone was removed by gently warming and bubbling
N2 through the solution. The precipitated crystals were filtered, washed
with water (2 × 3 mL), and dried by air suction. Yield: 1.05 g (91%).
Anal. Calcd (found) for C31H38B2N2O2: B, 4.39 (4.37). 1H NMR
(acetone-d6, δ): 7.34 (m, 8H, o-CH), 6.94 (m, 8H, m-CH), 6.82 (m,
4H, p-CH), 3.71 (m, 4H, NCH2), 3.13, 3.08 (2 × s, 2 × 6H, NCH3).
1
1.1 (d, J(B,H) ) 115 Hz). IR (KBr, cm-1): ν(B-H), 2517; νas(Cd
1
13C{1H} NMR (acetone-d6, δ): 164.97 (q, B-C(phenyl), J(C,11B) )
O), 1692.
48.8 Hz), 137.04 (o-phenyl), 126.07 (m-phenyl), 122.29 (p-phenyl),
60.31 (NCH2), 54.68, 49.17 (NCH3). 11B NMR (acetone-d6, δ): 1.9
(d, cation, 1J(B,H) ) 114 Hz), -5.7 (s, BPh4). IR (KBr, cm-1):
2-Methoxycarbonyl-1,1,3,3-tetramethyl-1,3,2λ4-diazaborinani-
um Bromide (13b). To a stirred solution of freshly prepared 6b (0.67
g, 3.32 mmol) in methanol (4 mL) was added a freshly prepared solution
of bromine (0.58 g, 3.63 mmol) in methanol (7 mL) over 25 min at 0
°C. After 10 min methanolic NaOH solution (1.50 mL, 2.42 M, 3.63
mmol) was added dropwise over 20 min. The solution was then
evaporated to give an oil that solidified. The residue was moved to a
filter with acetone (10 mL) and extracted into the filtrate (10 times).
The solid that precipitated from the extract was filtered off, washed
with acetone (2 × 1 mL), and dried in a N2 stream. Yield: 0.60 g
(64%). Anal. Calcd (found) for C9H22BBrN2O2: B, 3.85 (3.78); Br,
28.44 (28.81). 1H NMR (CDCl3, δ): 3.68 (s, 3H, OCH3), 3.60 (m,
2H, equatorial NCHH), 3.27 (m, 2H, axial NCHH), 3.18, 3.04 (2 × s,
2 × 6H, NCH3), 2.89 (m, 1H, axial CH2CHHCH2), 2.24 (m, 1H,
equatorial CH2CHHCH2). 13C{1H} NMR (CDCl3, δ): 62.17 (NCH2),
ν
assoc(O-H), 2880, 2750, 2622; ν(B-H), 2474; νas(CdO), 1668.
2-Carboxy-1,1,3,3-tetramethyl-1,3,2λ4-diazaborinanium Tet-
raphenylborate (15b). To a solution of 12b (0.258 g, 0.966 mmol)
in water (6 mL) was added acetone (12 mL) and aq NaBPh4 solution
(8.4 mL, 0.128 M, 1.08 mmol). The precipitate was redissolved by
addition of acetone (ca. 20 mL). Acetone was removed in vacuo. The
crystals of 15b were collected on a filter, washed with water (5 × 5
mL), and dried by air suction. Yield: 0.463 g (95%). Anal. Calcd
(found) for C32H40B2N2O2: B, 4.27 (4.26). 1H NMR (acetone-d6, δ):
7.34 (m, 8H, o-CH), 6.93 (m, 8H, m-CH), 6.79 (m, 4H, p-CH), 3.32
(m, 2H, equatorial NCHH), 3.23 (m, 2H, axial NCHH), 3.07 and 3.02
(2 × s, 2 × 6H, NCH3), 2.49 (m, 1H, axial CH2CHHCH2), 2.01 (m,
1H, equatorial CH2CHHCH2). 13C{1H} NMR (acetone-d6, δ): 164.94
(q, B-C(phenyl), 1J(C,11B) ) 48.8 Hz), 137.01 (o-phenyl), 126.01 (m-
phenyl), 122.26 (p-phenyl), 62.87 (NCH2), 55.54, 45.89 (NCH3), 19.47
(CH2CH2CH2). 11B NMR (acetone-d6, δ): -0.8 (d, complex, 1J(B,H)
) 117 Hz); -5.7 (s, BPh4). IR (KBr, cm-1): νassoc(O-H), 2736, 2610;
ν(B-H), 2484; νas(CdO), 1666.
54.95 (NCH3), 49.34 (OCH3), 46.29 (NCH3), 19.28 (CH2CH2CH2). 11
B
NMR (CDCl3, δ): -1.6 (d, 1J(B,H) ) 113 Hz). IR (KBr, cm-1): ν(B-
H), 2479; νas(CdO), 1683.
2-Carboxy-1,1,3,3-tetramethyl-1,3,2λ4-diazaborolidinium Hexa-
fluorophosphate-Potassium Hexafluorophosphate (4/1) (14a). 5a
(0.70 g, 4.02 mmol) and KPF6 (1.01 g, 5.50 mmol) were dissolved in
water (12 mL). To this solution was addded over 2-3 min an aq
bromine solution (4.14 mmol Br2) prepared by the reaction of KBrO3
(0.230 g, 1.38 mmol), KBr (0.980 g, 8.23 mmol) and aq HBr (1.52
mL, 48 m/m%) in water (6.5 mL) at 0 °C. After addition of bromine
was complete, the precipitate was redissolved by warming the mixture
to 65 °C, then the solution was allowed to cool to room temperature,
2-Methoxycarbonyl-1,1,3,3-tetramethyl-1,3,2λ4-diazaborolidini-
um Hexafluorophosphate (16a). To the suspension of 8a (0.412 g,
0.956 mmol) in methanol (6.0 mL) was added TMEDA (0.57 g, 4.91
mmol), and the mixture was refluxed for 6 h during which period a
clear solution was obtained. It was evaporated to dryness in vacuo.
The residue was dissolved in water (2.0 mL), and aq KPF6 (0.144 g,
0.78 mmol in 1.80 mL) was added. The crystalline precipitate was

5136 Inorganic Chemistry, Vol. 37, No. 20, 1998
Gyo¨ri et al.
redissolved by heating to 60 °C, and the clear solution was allowed to
cool to room temperature. It was kept in an ice-water bath for 30
min and the colorless needles were collected on a filter, washed with
cold water (3 × 1 mL), and dried by air suction. Yield: 0.279 g (44%).
Anal. Calcd (found) for C8H20BF6N2O2P: B, 3.26 (3.21). 1H NMR
(acetone-d6, δ): 3.87 (m, 4H, NCH2), 3.68 (s, 3H, OCH3), 3.19 and
3.16 (2s, 2 × 6H, NCH3). 13C{1H} NMR (acetone-d6, δ): 60.39
(NCH2), 54.65 (NCH3), 49.23 (NCH3 and OCH3). 11B NMR (acetone-
the solvent was evaporated. The residue was solidified by addition of
dichloromethane (10 mL) and, if it was necessary, trituration. The solid
was filtered and dried in a N2 stream. The solid was slurried in
acetonitrile (60 mL) and stirring was continued at room temperature
for 1 h. The insoluble parts (KBr) were filtered off and the filtrate
was evaporated to dryness in vacuo. The residual solid was collected
to a filter with ether (10 mL) and dried in a N2 stream. Yield: 0.34 g
(60%).
1
d6, δ): 2.1 (d, J(B,H) ) 113 Hz). IR (KBr, cm-1): ν(B-H), 2521;
Anal. Calcd (found) for C7H17BN2O2: B, 6.28 (6.06). 1H NMR
(D2O, δ): 3.53 (m, 4H, NCH2), 2.98, 2.96 (2 × s, 2 × 6H, NCH3).
13C{1H} NMR (D2O, δ): 61.69 (NCH2), 56.38, 50.85 (NCH3). 11B
νas(CdO), 1699.
2-Methoxycarbonyl-1,1,3,3-tetramethyl-1,3,2λ4-diazaborinani-
um Hexafluorophosphate (16b). To a solution of 8b (0.45 g, 1.11
mmol) in methanol (6.0 mL) was added TMPDA (0.72 g, 5.53 mmol).
The mixture was refluxed for 3 h, and then evaporated to dryness in
vacuo. The residue was dissolved in water (2 mL), filtered, and H2-
SO4 (0.10 mL, 2 M) and aq KPF6 (0.22 g, 1.20 mmol in 2.6 mL) were
added to the filtrate. The precipitate was redissolved by warming the
mixture to 70-80 °C, and the clear solution was allowed to cool to
room temperature. It was kept in an ice-water bath for 30 min, and
the crystals were collected on a filter, washed with cold water (3 ×
1.5 mL), and dried by air suction. Yield: 0.33 g (86%). Anal. Calcd
(found) for C9H22BF6N2O2P: B, 3.12 (3.20). 1H NMR (acetone-d6,
δ): 3.68 (s, 3H, OCH3), 3.43 (m, 2H, equatorial NCHH), 3.34 (m, 2H,
axial NCHH), 3.13, 3.03 (2 × s, 2 × 6H, NCH3), 2.58 (m, 1H, axial
CH2CHHCH2), 2.15 (m, 1H, equatorial CH2CHHCH2). 13C{1H} NMR
(acetone-d6, δ): 62.71 (NCH2), 55.24 (NCH3), 49.53 (OCH3), 46.07
(NCH3), 19.47 (CH2CH2CH2). 11B NMR (acetone-d6, δ): -0.7 (d,
1J(B,H) ) 113 Hz). IR (KBr, cm-1): ν(B-H), 2479, νas(CdO), 1673.
2-Methoxycarbonyl-1,1,3,3-tetramethyl-1,3,2λ4-diazaborolidini-
um Tetraphenylborate (17a). To a solution of 13a (0.223 g, 0.835
mmol) in water (12 mL) was added aq NaBPh4 solution (6.5 mL, 0.140
M, 0.91 mmol). Acetone was added to the slurry to obtain a clear
solution. The acetone was then removed in vacuo, when the product
separated from the aqueous solution as small plates. It was filtered,
washed with water (3 × 2 mL), and dried by air suction. Yield: 0.412
g (97%). Anal. Calcd (found) for C32H40B2N2O2: B, 4.27 (4.24). 1H
NMR (acetone-d6, δ): 7.34 (m, 8H, o-CH), 6.94 (m, 8H, m-CH), 6.79
(m, 4H, p-CH), 3.67 (m, 4H, NCH2), 3.65 (s, 3H, OCH3), 3.08, 3.05
(2s, 2 × 6H, NCH3). 13C{1H} NMR (acetone-d6, δ): 164.89 (q, B-Ph,
1J(B,H) ) 48.8 Hz); 136.96, 125.98, 122.21 (Ph); 60.12 (NCH2), 54.51
(NCH3) 49.09 (NCH3 and OCH3). 11B NMR (acetone-d6, δ): 1.9 (d,
complex, 1J(B,H) ) 110 Hz); -5.7 (s, BPh4). IR (KBr, cm-1): ν(B-
H), 2445, νas(CdO), 1695.
1
NMR (D2O, δ): 1.3 (d, J(B,H) ) 117 Hz). IR (KBr, cm-1): ν(B-
H), 2422; νas(CO2-), 1502; νs(CO2-), 1400.
1,1,3,3-Tetramethyl-1,3,2λ4-diazaborinanium 2-Carboxylate (18b).
The pH of a solution of 12b (0.64 g, 2.40 mmol) in water (10 mL)
was adjusted between 9.5 and 9.7 with aq KOH (ca. 2.33 mL, 1.012
M, ca. 2.36 mmol). The solution was evaporated in vacuo, the vitreous
residue was suspended in acetonitrile (35 mL) and the slurry was stirred
at room temperature until the insoluble parts consisted of white
microcrystalline only. The slurry was then filtered, the solid was
washed into the filtrate with acetonitrile (2 × 1 mL) and the filtrate
was evaporated in vacuo. The solid residue was collected on a filter
with ether (10 mL) and dried in a N2 stream. Yield: 0.378 g (85%).
Anal. Calcd (found) for C8H19BN2O2: B, 5.81 (5.67). 1H NMR (D2O,
δ): 3.18 (m, 2H, axial NCHH), 3.10 (m, 2H, equatorial NCHH), 2.95,
2.76 (2 × s, 2 × 6H, NCH3), 2.33 (m, 1H, axial CH2CHHCH2), 1.98
(m, 1H, equatorial CH2CHHCH2). 13C{1H} NMR (D2O, δ): 63.51
(NCH2), 56.34, 48.10 (NCH3), 21.43 (CH2CH2CH2). 11B NMR (D2O,
δ): -1.0 (d, 1J(B,H) ) 96 Hz). IR (KBr, cm-1): ν(B-H), 2442;
-
ν(CO2 )as, 1490; ν(CO2-)s, 1406.
2-Bromo-2-carboxy-1,1,3,3-tetramethyl-1,3,2λ4-diazaborolidini-
um Hexafluorophosphate (19a). Methanol (3.0 mL) and TMEDA
(0.35 g, 3.01 mmol) were added to 9a (0.63 g, 1.15 mmol) at 0 °C.
The suspension was stirred at room temperature for 4 h, then evaporated
in vacuo. The residue was dissolved in water (2.80 mL), and the
solution was acidified with H2SO4 (0.20 mL, 2 M). The insoluble parts
were filtered off, washed with water (2 × 0.4 mL), and aq KPF6 (0.164
g, 0.87 mmol in 2.0 mL) was added to the filtrate. The precipitate
was redissolved by warming, and the solution was allowed to cool to
room temperature. After 30 min in an ice-water bath the crystals were
collected on a filter, washed with 0 °C water (3 × 1 mL), and dried by
air suction. Yield: 0.261 g (29%). Anal. Calcd (found) for C7H17-
BBrF6N2O2P: B, 2.72 (2.67); Br, 20.13 (20.41). 1H NMR (D2O, δ):
3.86 (m, 4H, NCH2), 3.22 and 3.19 (2 × s, 2 × 6H, NCH3). 13C{1H}
NMR (D2O, δ): 61.74 (NCH2), 57.48 and 53.89 (NCH3). 11B NMR
(D2O, δ): 4.2 (s). IR (KBr, cm-1): ν(O-H)monomeric, 3413; νassoc(O-
H), 2767, 2618; νas(CdO)monomeric, 1693; ν(CdO)dimeric, 1656.
2-Methoxycarbonyl-1,1,3,3-tetramethyl-1,3,2λ4-diazaborinani-
um Tetraphenylborate (17b). To a solution of 13b (0.156 g, 0.555
mmol) in water (3 mL) was added aq NaBPh4 solution (4.4 mL, 0.140
M, 0.62 mmol). Acetone was added to the slurry to obtain a clear
solution. The acetone was then removed in vacuo, when the product
crystallized from the aqueous solution. It was filtered, washed with
water (3 × 2 mL) and dried by air suction. Yield: 0.283 g (98%).
Anal. Calcd (found) for C33H42B2N2O2: B, 4.16 (4.16). 1H NMR
(acetone-d6, δ): 7.34 (m, 8H, o-CH), 6.94 (m, 8H, m-CH), 6.79 (m,
4H, p-CH), 3.65 (s, 3H, OCH3), 3.25 (m, 2H, equatorial NCHH), 3.17
(m, 2H, axial NCHH), 3.01, 2.93 (2 × s, 2 × 6H, NCH3), 2.44 (m,
1H, axial CH2CHHCH2), 1.97 (m, 1H, equatorial CH2CHHCH2). 13C-
2-Bromo-2-carboxy-1,1,3,3-tetramethyl-1,3,2λ4-diazaborolidini-
um Tetraphenylborate (20a). To an aqueous solution of 19a (0.316
g, 0.796 mmol in 60 mL) was added dropwise a NaBPh4 solution (0.286
g, 0.836 mmol in 5 mL water) over a few minutes at 36 °C. The
mixture was then allowed to cool to room temperature, and the
precipitate was collected on a filter, washed with water (4 × 3 mL),
and dried with air suction. Yield: 0.435 g (96%). Anal. Calcd (found)
for C31H37B2BrN2O2: B, 3.79 (3.72); Br, 13.99 (13.68). 1H NMR
(acetone-d6, δ): 7.35 (m, 8H, o-CH), 6.94 (m, 8H, m-CH), 6.80 (m,
4H, p-CH), 3.83 (s, 4H, NCH2), 3.28, 3.25 (2 × s, 2 × 6H, NCH3).
1
{1H} NMR (acetone-d6, δ): 164.94 (q, B-Ph4, J(C,B) ) 48.8 Hz),
137.01, 126.04, 122.32 (Ph), 62.63 (NCH2), 55.21 (NCH3), 49.55
(OCH3), 46.07 (NCH3), 19.44 (CH2CH2CH2). 11B NMR (acetone-d6,
13C{1H} NMR (acetone-d6, δ): 165.00 (q, B-C(phenyl), J(C,11B) )
1
1
δ): -0.8 (d, complex, J(B,H) ) 115 Hz); -5.7 (s, BPh4). IR (KBr,
48.8 Hz), 137.07 (o-phenyl), 126.12 (m-phenyl), 122.37 (p-phenyl),
60.10 (NCH2), 55.78 and 52.17 (NCH3). 11B NMR (acetone-d6, δ):
4.8 (br s, cation), -5.7 (s, BPh4). IR (KBr, cm-1): νassoc(O-H), 2777,
2624; νas(CdO), 1668.
cm-1): ν(B-H), 2465; νas(CdO), 1684.
1,1,3,3-Tetramethyl-1,3,2λ4-diazaborolidinium 2-Carboxylate (18a).
Methanol (11.5 mL) and TMEDA (0.57 g, 4.91 mmol) were added to
7a (0.64 g, 1.64 mmol). After stirring at room temperature for 2.5 h
a clear solution was obtained. It was evaporated in vacuo to give an
oily residue. It was dissolved in methanol (3 mL) which was in turn
thoroughly evaporated. Ether (12 mL) was added to the resinous
residue to furnish a solid. The mixture was filtered, and the solid was
washed with ether (2 × 5 mL) and dried in a N2 stream. This solid
was dissolved in water (1.0 mL), aq KOH solution (3.20 mL, 1.012
M, 3.24 mmol) was added, and the solution was evaporated to dryness.
Dichloromethane (10 mL) was added to the resinous residue, and then
2-Bromo-1,1,3,3-tetramethyl-1,3,2λ4-diazaborolidinium 2-Car-
boxylate (21a). To a solution of 20a (0.368 g, 0.644 mmol) in acetone
(6.5 mL) was added water (2.0 mL) and over 1 min KOH solution
(1.14 mL 0.567 M, 0.646 mmol). Acetone was removed by bubbling
N2 through the solution and the water loss was compensated (2 × 1
mL). KBPh4 was then filtered off and washed with water, and the
filtrate combined with the washing liquid was evaporated to dryness
in vacuo. The residue was scratched off the wall of the flask in the
presence of acetone (5 mL), collected on a filter, washed with acetone

Diazabora Rings with B-Carboxyl and B-Carboxylato Groups
Inorganic Chemistry, Vol. 37, No. 20, 1998 5137
Scheme 1
(2 × 1 mL), and dried in a N2 stream. Yield: 0.147 g (91%). Anal.
Calcd (found) for C7H16BBrN2O2: B, 6.28 (6.06). 1H NMR (D2O, δ):
3.81 (m, 4H, NCH2), 3.17, 3.13 (2 × s, 2 × 6H, NCH3). 13C{1H}
NMR (D2O, δ): 61.46 (NCH2), 57.36, 53.86 (NCH3). 11B NMR (D2O,
δ): 4.9 (s).
This improvement is probably due to the soft character of Me2S,
which renders “BH2CN” more susceptible to base exchange.
Previous papers in the literature describe the ethylation
reactions (Scheme 1, ii) employing 50-100% excess of Et3OBF4
and 24-72 h reaction times.1,2,29-31 In contrast, we have found
that diamine-bis(cyanoboranes) 1 could be completely converted
into their bis(ethylnitrilium dihydroborane) tetrafluoroborates
2 in 2-3 h using a small (3-5%) excess of Et3OBF4.
Diamine-bis(carboxyboranes) 3 were conveniently prepared
from the corresponding nitrilium salts 2 (Scheme 1, iii) using a
short (5-15 min), high-temperature (80-100 °C) hydrolysis
with 85,4 88, and 97% yields, respectively. This high-
temperature hydrolysis has already been applied in our labora-
tory for the synthesis of other amine-carboxyboranes.4,7 It is
Results and Discussion
Diamine-bis(carboxyboranes) and Diamine-bis(methoxy-
carbonylboranes) (3, 4). Diamine-bis(carboxyboranes) 3a-c
were prepared by the usual route for the synthesis of amine
carboxyboranes (Scheme 1, i-iii). Diamine-bis(cyanoboranes)
(1) were synthesized from a Me2S solution of cyanoborane
(Scheme 1, i) in nearly quantitative (ca. 3-fold compared to
ether, THF, and glyme28 solutions) yields, similarly to our
synthesis of cyanoborane complexes of monobasic amines.7,27
(26) Gyo¨ri, B.; Emri, J.; Fehe´r, I. J. Organomet. Chem. 1984, 262, C7.
(27) Gyo¨ri, B.; Emri, J.; Fehe´r, I. J. Organomet. Chem. 1983, 255, 17.
(28) Martin, D. R.; Chiusano, M. A.; Denniston, M. L.; Dye, D. J.; Martin,
E. D.; Pennington, B. T. J. Inorg. Nucl. Chem. 1978, 40, 9 and
references therein.
(29) Spielvogel, B. F.; Ahmed, F. U.; McPhail, A. T. Inorg. Synth. 1989,
25, 79.
(30) Kemp, B.; Kalbag, S.; Geanangel, R. A. Inorg. Chem. 1984, 23, 3063.
(31) Mittakanti, M.; Charandabi, M. R. M. D.; Morse, K. W. Inorg. Chem.
1990, 29, 3218.

5138 Inorganic Chemistry, Vol. 37, No. 20, 1998
Gyo¨ri et al.
superior to those described in the literature2,29,30 that afford lower
yields in long reaction times (1-3 days) at room temperature.
Diamine-bis(methoxycarbonylboranes) 4 were readily syn-
thesized from the appropriate carboxyborane complexes 3
(Scheme 1, iv) employing an esterification procedure elaborated
earlier in our laboratory.7 Unlike complexes of monofunctional
amines, the observed order of reactivities of 3 toward esterifi-
cation is TMPDA > TMEDA . TMBDA, where the complex
of the latter showed orders of magnitude slower esterification.
Diamine-carboxyboranes and Diamine-methoxycarbon-
ylboranes (5, 6). The first mono(carboxyborane) complex of
a diamine has been prepared in our laboratory by treating
TMEDA‚2BH2COOH with excess TMEDA in acetonitrile.4 The
TMPDA complex 5b was obtained similarly in THF (Scheme
1, ix). Analogous reactions involving 3c and TMBDA did not
result in formation of significant quantities of the expected
mononuclear complex 5c either in THF, or in TMBDA. Quite
surprisingly, the reaction between Me3N‚BH2COOH and a large
excess of TMBDA yielded the binuclear complex 3c exclusively.
It may be due to the much lower solubility of the binuclear
derivative 3c compared to that of the mononuclear complex 5c.
A similar phenomenon, observed for TMEDA‚2BH3 and
TMEDA‚BH3, was explained by No¨th32 as due to lattice
stabilization effects.
Reactions between diamine-bis(methoxycarbonylboranes) 4
and the appropriate amines reached nearly complete conversion.
All the expected diamine-methoxycarbonylboranes 6 were
prepared in good yields. The products were, however, con-
taminated with some starting material, even when large excess
of amine was employed. After removal of the amine the
equilibrium (Scheme 1, vi) slowly shifted to reformation of 4.
It should be noted that mononuclear complexes 6 are much more
stable in CDCl3 solutions (in which 4 are also highly soluble)
than in neat form.
7a was also formed under similar conditions, but its isolated
yield was low due to extensive transformation into 12a.
As with monofunctional amine complexes, diamine-bis-
(bromocarboxyboranes) 7 undergo decomposition in aqueous
media, which is accompanied by partial formation of the cyclic
cation in the case of 7a (see next section). The decomposition
was instantaneous in alkaline and fast in neutral media. In acid
solution the rate of the decomposition decreased as the
concentration of the acid increased, similarly to that observed
for Me3N‚BH(Br)COOH.19 Preparations were therefore per-
formed in media strongly acidified by HBr. In addition, the
use of HBr allowed us to prepare more concentrated bromine
solutions than in water alone.
Preparation of diamine-bis(bromomethoxycarbonylboranes)
8 was attempted by esterification of the bromocarboxyborane
complexes 7 (Scheme 1, x). The rates of these reactions showed
the same order as seen for esterification of nonbrominated
carboxyborane complexes 3, but appeared much lower even in
the presence of 10 mol % HBr per carboxylic groups. A similar
difference has been reported for monofunctional amine com-
plexes of bromocarboxyborane compared to corresponding
amine-carboxyboranes.7 Formation of TMEDA‚2BH(Br)-
COOMe (8a) was accompanied by extensive transformation into
the cyclic cation 13a (see below). In contrast, esterification of
7b took place without observable formation of cyclic cation
13b, thereby making this route synthetically useful. Slow
esterification of TMBDA‚2BH(Br)COOH (7c) was accompanied
by extensive decomposition to TMBDA‚2HBr.
The diamine-bis(carboxyboranes) 3a and 3b could conve-
niently be converted into the corresponding brominated esters
8a and 8b in one step with NBS in methanol (Scheme 1, vii),
(NBS was added as solid in small portions, since it slowly reacts
with methanol). After introduction of the first portion (ca. 5
mol %), the starting materials dissolved in minutes forming of
the corresponding esters (4a,b). A study of the effect of NBS
as esterification catalyst is currently under way in our laboratory.
A possible explanation may be the generation of acyl hypo-
bromites,33 which would react with methanol affording the
corresponding esters and HOBr and the latter may be able to
maintain the catalytic cycle. The bromination reactions were
very fast at room temperature.. Formation of cyclic cations
13a,b was negligible, and the products were less contaminated
than those obtained from 7a,b by esterification.
Further bromination of all studied BH(Br)COOH complexes
(7) could be carried out only in heterogeneous phase owing to
their low solubility in each studied solvent, and required
vigorous conditions. Formation of 9c was accompanied by
extensive decomposition and an impure product was obtained
in low yield.
Bromination of diamine-carboxyboranes 5a,b in strongly
acidic medium afforded diamine-bromocarboxyborane-hydro-
bromides 10a,b (Scheme 1, xiv) in a fast reaction. In the
reaction of the TMEDA complex 5a, a considerable amount of
cyclic cation 12a formed as a byproduct. These N-protonated
mononuclear complexes (10a,b) proved to be valuable inter-
mediates in the syntheses of cyclic cations 12a,b.
2-Carboxyl, 2-Methoxycarbonyl, and 2-Carboxylate De-
rivatives of 1,1,3,3-Tetramethyl-1,3,2λ4-diazaborolidine, 1,1,3,3-
Tetramethyl-1,3,2λ4-diazaborinane and 2-Bromo-1,1,3,3-
tetramethyl-1,3,2λ4-diazaborolidine Rings (12-21). BrH‚
TMEDA‚BH(Br)COOH (10a) on dissolution in water or in
dilute acids transformed into cyclic cation 12a in a fast reaction
Diamine-bis(bromocarboxyboranes) (7), Diamine-bis(bro-
momethoxycarbonylboranes) (8), Diamine-bis(dibromocar-
boxyboranes) (9), and Diamine-bromocarboxyborane-hy-
drobromides (10). Bromination of diamine-bis(carboxyboranes)
3 takes place in two steps with decreasing rates (eq 1a,b).
DA‚2BH2COOH + 2Br2 fast8 DA‚2BH(Br)COOH + 2HBr
(1a)
DA‚2BH(Br)COOH + 2Br2 slow8
(1b)
DA‚2BBr2COOH + 2HBr
DA ) TMEDA, TMPDA, TMBDA
Diamine-bis(bromocarboxyboranes) 7, which contain two chiral
boron atoms, could be obtained in good yields using slightly
more than 1 molar equiv of bromine per carboxyborane groups
(Scheme 1, viii). Since all brominated derivatives possess very
low solubilities in water, the isolated complexes 7 were
contaminated with byproducts containing BH2COOH and BBr2-
COOH groups. The amount of impurities could be minimized
by starting from the clear solution of the sodium salt of 3a, and
freshly precipitated 3c with large specific surface area. Bro-
mination of the TMPDA complex 3b was more conveniently
accomplished in homogeneous phase employing a water-
acetone mixture, where the relatively slow reaction between
bromine and acetone prevented the formation of overbrominated
byproducts containing BBr2COOH groups. TMEDA complex
(32) No¨th, H. In: Progress in Boron Chemistry; Brotherton, R. J., Steinberg,
(33) Ramachandran, M. S.; Easwaramoorthy, D.; Rajasingh, V.; Vive-
H., Eds.; Pergamon Press: Oxford, 1970; Vol. 3, p 215.
kanandam, T. S. Bull. Chem. Soc. Jpn. 1990, 63, 2397.

Diazabora Rings with B-Carboxyl and B-Carboxylato Groups
Inorganic Chemistry, Vol. 37, No. 20, 1998 5139
O
Scheme 2
group (pKa ). Based on the pKa values determined for
TMEDA‚BH2COOH (pKaO ) 7.0, pKaN ) 8.9)34 and consider-
ing that H‚Br exchange caused ca. 2.5 units decrease in pKa
value of Me3N‚BH2COOH,19 the equilibrium concentrations of
the corresponding species are expected as follows: [10a] >
[10′′a] > [10′a]. However, our experiments leading to 10a
yielded 12a almost exclusively. Consequently, the ring closure
of 10′a must be extremely fast in comparison with the
decomposition of 10a and 10′′a. The state of the protonation
equilibrium for analogous 10b is expectably similar. Therefore,
the failure to form ring 12b in water is caused mainly by the
sloth of the ring closure step, which can be explained by the
general trend established for intramolecular nucleophilic sub-
stitutions. Namely, five-membered rings are formed two or 3
orders of magnitude faster than six-membered rings.35 Rysch-
kewitsch has also found the formation of CnN2B rings much
more favored for n ) 2, compared to n ) 3.36
The pKa values in methanol of protonated amines and acetic
acid, respectively are about 1 and 5 log units higher compared
to water.37 Assuming similar changes for pKaN and pKaO values
of 10b, the proportion [10′b]/[10′′b] becomes much larger in
methanol than in water. This can explain the predominant ring
formation (12b) on deprotonation of 10b by NaOH in methanol.
Processes taking place in the course of the bromination of
diamine-methoxycarbonylboranes 6a,b, as well as the decom-
position of 8a,b in methanol, are probably analogous to those
shown in Scheme 2, i.e., the first step of these processes is the
formation of HBr‚DA‚BH(Br)COOMe (11a,b) (though com-
plexes of this composition were not prepared). However, there
is one essential difference: species containing BH(Br)COO-
groups obviously do not form, so the equilibria (iv) and (v)
can be simplified to the equilibrium between ester derivatives
of 10a,b and 10′a,b, where N-protonated species slowly
decompose via Br‚MeOH/OH- exchange and N-deprotonated
species yield cyclic cations 13a,b. Bromination of 6a and 6b
afforded cyclic cations 13a and 13b, after neutralization by
methanolic NaOH solution. The decomposition of one of the
borane moieties in 8a in methanol was fairly slow, yielding
cyclic cation 13a (Scheme 1, xii and xv), similar to the
decomposition of 7a described above. The process attained
g95% conversion, despite the presence of HBr evolved from
the decomposed borane moiety, which once more demonstrates
the rapidity of the formation of the five membered ring. On
the other hand, decomposition of one of the borane moieties in
8b cannot result in ring closure unless the evolved HBr was
continuously removed by 4 Å molecular sieves or NaOH. In
the case of 4 Å molecular sieves the reaction took 12 h at 50
°C, after removal of the molecular sieves the decomposition
went on at a similar rate, but the ring formation was not
prolonged. In the presence of one molar equivalent of NaOH
the decomposition was 3-4 times faster (probably owing to
the Br- f OH- exchange).
(Scheme 1, xvi). This reaction was accompanied by decom-
position affording TMEDA‚2HBr, B(OH)3, CO, and H2, to a
small extent depending on reaction conditions. In case of BrH‚
TMPDA‚BH(Br)COOH (10b) in water, exclusively this slow
decomposition took place and formation of the cyclic cation
1
12b was not observed by H NMR. On the other hand, in
methanol the decomposition of 10b and the ring closure took
place concurrently, while reaction between 10b and NaOH
afforded 12b instantaneously and in good yield.
Apparently, ring closure took place whenever 10a was in
water, or 10b was in methanol, i.e., bromination of 5a in water
(Scheme 1, xiv and xvi) and thermal or alkaline decomposition
of one of the borane moieties in 7a or 7b in water or methanol,
respectively (Scheme 1, xiii and xvi).
The rate of the decomposition of 7a,b in water increased with
pH, similarly to that established for Me3N‚BH(Br)COOH in our
earlier paper.19 The hydrolysis of this complex was explained
by the combination of fast decarbonylation of the deprotonated
B-COO- group and slow decomposition of Me3N‚BH(Br)-
COOH via a bromide f water exchange on the boron. The
rate-determining step of the transformations of 7a,b into either
12a,b or DA‚2HBr, appears to be the formation of intermediates
10a,b as they could not be detected during monitoring of the
In all previously mentioned reactions, one of the two borane
moieties in 7a,b and 8a,b was destroyed. This loss inspired us
to find a more economical path to 12a,b and 13a,b. Base
exchange reactions of DA‚2BH(Br)COOR + DA‚2DA‚BH(Br)-
COOR type would result in the cyclic cations in 2-fold yield.
However, the yields were found close to those attained by the
simple decomposition of 7a,8a or that of 7b,8b effected by
1
reactions by H NMR.
10a,b in both water and methanol are in equilibrium with
their N-deprotonated 10′a,b and O-deprotonated 10′′a,b forms
(Scheme 2, iv and v). It seems certain, that of the three species,
only 10′a,b is capable of forming a ring, while the other two
undergo decomposition (Scheme 2, iii and vi) in the manner
described in the previous paragraph. The state of the equilib-
rium is governed by the magnitude and the proportion of acidity
(34) Gyo¨ri, B.; Berente, Z.; Kova´cs, Z. In preparation.
(35) Carey, F. A.; Sundberg, R. J. In AdVanced Organic Chemistry; Plenum
Press: New York, 1990; Part B, p 165.
(36) Ryschkewitsch, G. E.; Sullivan, T. E. Inorg. Chem. 1970, 9, 899.
(37) Rorabacher, D. B.; MacKellar, W. J.; Shu, F. R.; Bonavita, S. M.
Anal. Chem. 1971, 43, 561.
N
constants of the ammonium group (pKa ) and the carboxylic

5140 Inorganic Chemistry, Vol. 37, No. 20, 1998
Gyo¨ri et al.
NaOH, ruling out taking place of considerable base exchange.
This consequence was unequivocally checked in reactions of
7a and 8a with TMPDA, and conversely, of 7b and 8b with
TMEDA, where the base exchange should be indicated by the
appearance of the cyclic cation formed with the added amine.
1H NMR monitoring of these reactions showed noticeable base
exchange only in the reaction of 8a and TMPDA (26-28%).
12a,b and 13a,b are hygroscopic crystalline solids and very
soluble in either water or methanol. 13a,b possess considerable
solubility also in acetone and chloroform. All cyclic cations
are quite stable in acidic aqueous media. 12a,b undergo
deprotonation with bases and transform into 18a,b (Scheme 1,
xx). The protonation constants (pKa) are 5.59 for the five-
membered cyclic cation and approximately 5.55 for the six-
membered cyclic cation. These values are 3 units lower than
those of amine-carboxyboranes.38,39 13a,b in alkaline media
also transform into 18a,b via the hydrolysis of the ester group
(Scheme 1, xix). In the presence of equimolar NaOH the half-
lives were 4 h for 13a and 72 days for 13b, so the difference
between the reaction rates was a few 100-fold. Hydrolysis
experiments with 13a in the presence of 5-20 mol % NaOH
showed that NaOH did not act as a catalyst.
logues of R-amino acids, or often named them simply the boron
analogues of amino acids.1,3,15,29,40,41 This concept has been
widely accepted in the literature.6,16,38,42,43 This analogy inspired
wide-ranging studies of the biological and pharmacological
activities of these compounds. However, this analogy, which
is based on the C+ T B isoelectronic relationship, obviously
does not denote chemical similarity between amine-carboxybo-
ranes and R-amino acids, since the charges, which naturally have
a major influence on chemical properties, of the isoelectronic
analogue species are different.
In contrast to protonated glycine (NH3+-CH2COOH), am-
monia-carboxyborane NH3‚BH2COOH, regarded as its isoelec-
tronic analogue, could not be deprotonated on the nitrogen
atom.39 All metal complexes prepared so far have been
carboxylato complexes, and chelate formation characteristic to
R-amino acids has not been observed.39,42 Another important
difference between protonated glycine and NH3‚BH2COOH is
that peptide bond could be formed only on the carboxyl group
of NH3‚BH2COOH; amine carboxyboranes are in the N-terminal
position in all dipeptides and a tripeptide41 known so far. Ab
initio calculations investigating the amine-carboxyborane T
R-amino acid analogy revealed considerable differences in the
geometry, charge distribution, dipole moment and electrostatic
potential, which, according to the authors, were due, at least
partly, to the presence of an additional hydrogen attached to
nitrogen.44 The difference of 6 units between the pKa value of
NH3‚BH2COOH (8.33)39 and pKa1 value of glycine (2.35),45
which is assigned to the deprotonation of the isoelectronic
protonated glycine, does not refer to strict analogy between the
two species. When regarding amine-carboxyboranes as the
boron analogues of R-amino acids in general, one should
consider that the former, except ammonia-carboxyborane, are
complexes of alkyl or aromatic amines. Alkylamine complexes
are analogues of N-alkylamino acids, while amino acid analogy
for aromatic amine complexes cannot be interpreted.
-
Both cyclic cations form poorly water soluble salts with PF6
-
and BPh4 anions (Scheme 1, xvii and xviii). The hexafluo-
rophosphates appeared valuable from preparative aspect as they
could be precipitated in pure form from reaction mixtures, even
when considerable amounts of the corresponding ammonium
ions were present. The hexafluorophosphate salt of [TMEDA‚
B(H)COOH]+ ion crystallized as a double salt with potassium
hexafluorophosphate, [TMEDA‚B(H)COOH]PF6‚KPF6 (4/1),
while no precipitate fell out on addition of NaPF6 solution. The
tetraphenylborates could be obtained in pure form only from
reaction mixtures containing no ammonium ions.
Zwitterionic species 18a,b, formed from cyclic cations of
12a,b and 13a,b with KOH, could be prepared in pure form
after separation from KBr. Moreover, pure aqueous solution
of 18a,b could be obtained in the reactions between the
appropriate tetraphenylborate salts (15a,b) and KOH (Scheme
1, xxi). Both zwitterions are hygroscopic and possess extremely
high solubility in water.
TMEDA-bis(dibromocarboxyborane) (9a) could be trans-
formed into a cyclic cation containing the >B(Br)COOH group
(Scheme 1, xxii), which was prepared as a hexafluorophosphate
salt (19a), in two different ways. These were the reactions of
9a with TMEDA in methanol and with NaOH in water. These
reactions showed rates similar to those affording 12a from 7a,
but the yields were much lower. The cyclic cation [TMEDA‚
B(Br)COOH]+ was found to be the strongest known acid in
the area of carboxyboron compounds with an approximate pKa
value of 4.17. The zwitterionic species 21a formed upon
deprotonation of 19a undergoes slow decomposition in water
(with a half-life of ca. 40 h), but is stable in solid form. 21a
was prepared with equimolar KOH from [TMEDA‚B(Br)-
COOH]BPh4 (20a) (Scheme 1, xxiv), which was in turn
produced from 19a (Scheme 1, xxiii). The formation of the
analogous six-membered ring could not be observed in similar
reactions.
In the chemistry of boron-nitrogen compounds, analogies
are sought based on not only C+ T B isoelectronic relationship,
but more frequently on the B-N T C-C relationship, where
isoelectronic species possess the same charge (see for ex-
ample: borazine-benzene, hexagonal BN-graphite, cubic BN-
diamond). The latter relationship was called isoelectronic and
isosteric by Wiberg.46 There has been some confusion about
these two terms in the literature since the classical paper of
Langmuir.47 We use the term “isoelectronic” as recommended
by IUPAC,48 and ignore the term “isosteric”, as to our
knowledge IUPAC has not yet defined. Considering the
isoelectronic relationship between BN and CC groups, ammonia-
carboxyborane is isoelectronic with propionic acid, and amine-
carboxyboranes are isoelectronic with carboxylic acids. Using
this approach an isoelectronic boron analogue of glycine does
not exist, while isoelectronic boron analogue of alanine would
be H3N‚BH(NH2)COOH.
The compounds DA‚2BH2COOH (DA ) TMEDA, TMPDA,
(40) (a) Spielvogel, B. F.; Ahmed, F. U.; Morse, K. W.; McPhail, A. T.
Inorg. Chem. 1984, 23, 1776. (b) Spielvogel, B. F. Phosphorus Sulfur
1994, 87, 267.
(41) Sood, A.; Sood, C. K.; Spielvogel, B. F.; Hall, I. H. Eur. J. Med.
Chem. 1990, 25, 301.
(42) Norwood, V. M., III; Morse, K. W. Inorg. Chem. 1986, 25, 3690.
(43) (a) Charandabi, M. R. M. D.; Feakes, D. A.; Ettel, M. L.; Morse, K.
W. Inorg. Chem. 1991, 30, 2433. (b) Morin, C. Tetrahedron 1994,
50, 12521.
Analogy between Amine-carboxyboranes and r-Amino
Acids or Carboxylic Acids. Spielvogel, the pioneer of the area,
regarded amine-carboxyboranes as the protonated boron ana-
(44) Laurence, P. R.; Thomson, C. J. Mol. Struct. 1982, 88, 37.
(45) Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum
Press: New York, 1975; Vol. 1, p 1.
(46) Wiberg, E. Naturwissenschaften 1948, 35, 182, 212.
(47) Langmuir, I. J. Am. Chem. Soc. 1919, 41, 1543.
(48) Muller, P. Pure Appl. Chem. 1994, 66, 1077.
(38) (a) Das, M. K.; Mukherjee, P. J. Chem. Res. 1985, 66. (b) Das, M.
K.; Mukherjee, P. J. Chem. Res. 1987, 368.
(39) Scheller, K. H.; Martin, R. B.; Spielvogel, B. F.; McPhail, A. T. Inorg.
Chim. Acta 1982, 57, 227.

Diazabora Rings with B-Carboxyl and B-Carboxylato Groups
Inorganic Chemistry, Vol. 37, No. 20, 1998 5141
TMBDA) reported in this paper are isoelectronic analogues of
methylated R,ω-dicarboxylic acids (namely adipic, pimelic, and
suberic acid, respectively). Compounds DA‚BH2COOH are
isoelectronic analogues of methylated derivatives of ω-amino
acids. 18a is isoelectronic with 1,1,3,3-tetramethylproline-
betaine, and such relationship can be established between 18b
and 1,1,3,3-tetramethylpipecolinate betaine, as well as between
12a,b and the protonated forms of the corresponding betaines.
Acknowledgment. This work was supported by the Hungar-
ian Scientific Research Fund (OTKA T014985/1995).
Supporting Information Available: Description of further syn-
thetic methods for 8b, 13a, 13b, 14a, 14b, 16a, 16b, 18a, 19a, and
textual presentation of IR and NMR data (4 pages). Ordering
information is given on any current masthead page.
IC9713442