Intramolecular borylation reaction catalyzed by Lewis acid: Preparation of 1H-2,1-benzazaborole derivatives

Article abstract of DOI:10.1039/b003999n

It has been found that 1H-2,1-benzazaboroles can be prepared by the interaction of substituted benzylamino-chloroboranes with Al₂Cl₆ in CH₂Cl₂ at 0 °C, the ¹³C NMR spectroscopy data obtained being in

Full text of DOI:10.1039/b003999n

Intramolecular borylation reaction catalyzed by Lewis acid: preparation of
1H-2,1-benzazaborole derivatives
Alexander M. Genaev,a Sandor M. Nagy,*b Georgii E. Salnikova and Vyacheslav G. Shubina
a Novosibirsk Institute of Organic Chemistry, Siberian Division of Russian Academy of Sciences, Novosibirsk 630090,
Russia
b Equistar Chemicals, L.P., Equistar Technology Center, 11530 Northlake Drive, Cincinnati, Ohio, USA
Received (in Corvallis, USA) 17th May 2000, Accepted 6th July 2000
Published on the Web 3rd August 2000
It has been found that 1H-2,1-benzazaboroles can be
prepared by the interaction of substituted benzylamino-
chloroboranes with Al2Cl6 in CH2Cl2 at 0 °C, the 13C NMR
spectroscopy data obtained being in favour of an electro-
philic substitution mechanism involving formation of cati-
onic complexes as reactive intermediates.
The p-delocalized anions accessible through deprotonation of
1H-2,1-benzazaboroles (1) are isoelectronic with indenyl
anions (Scheme 1), making them attractive precursors for single
site olefin polymerization catalysts based on heterocyclic
analogues of metallocenes1 (cf. ref. 2).
Scheme 1
Scheme 5
The two well known approaches towards benzazaborole
derivatives involve intramolecular condensation of o-(amino-
methyl)benzene boronic acids3 (Scheme 2) and intramolecular
cyclization of benzylaminoboranes4 (Scheme 3). The first
method is applicable for preparation of derivatives with B–OH
or B–O–B fragments only, while the second route requires high
process temperatures and is not applicable to compounds in
which there is no B–H bond.
The starting materials, halogen substituted aminoboranes
2a–d,† have been prepared by the procedures analogous to
those described in refs. 6 and 7 (Scheme 5). There are two sets
1
of signals in the H and 13C NMR spectra of each of these
compounds indicating the presence of mixtures of cis- and
trans-isomers, obviously due to partially double bond character
of the B–N bond‡ (cf. ref. 6).
Aminoboranes 2a and 2b, when treated with AlCl3 under
Dewar’s conditions5 (without solvent, 150 °C) yielded poly-
meric products only.
It has been found that the interaction of compounds 2b and 2c
with equimolar amounts of Al2Cl6 in CH2Cl2 at 0 °C results in
formation of the target products, 3b and 3c, respectively, the
yields being 75% (Scheme 6).§ In contrast, the compounds 2a
and 2d do not react at 0 °C, while at rt they give multicomponent
mixtures of unidentified products.
Scheme 2
Scheme 3
As part of our program to develop synthetically attractive
approaches towards precursors for heterocyclic analogues of
cyclopentadienyl ligands, we studied the utility of Lewis acid
catalysed borylation reactions for the preparation of benzobor-
azoles 1. To the best of our knowledge there have been no data
on their preparation by this reaction, the closest related data
being those of M. J. S. Dewar on the AlCl3 catalysed
preparation of (10R)-9-aza-10-boraphenanthrene and related
heteroaromatics at high temperature and without solvent5
(Scheme 4).
Scheme 6
The mechanism of the cyclization reaction has been studied
by NMR using 2b as starting material, CD2Cl2 solvent and
Al2Br6 as a Lewis acid which is more soluble in this solvent than
Al2Cl6.¶ These experiments indicate that upon the interaction of
2b with Al2Br6 at 290 °C a mixture of 4 and 5 is formed, their
ratio being approximately 1+1 (Scheme 7).∑ When the tem-
perature rises to 0 °C, the structure 4 transforms entirely into
5.
The results reported here suggest that intramolecular electro-
philic borylation is a viable approach toward 1H-2,1-benzaza-
Scheme 4
DOI: 10.1039/b003999n
Chem. Commun., 2000, 1587–1588
This journal is © The Royal Society of Chemistry 2000
1587

0 °C. The reaction mixture was allowed to warm to rt, decanted and
evaporated. The residue was extracted with 5 mL of hexane, the solution
was filtered and evaporated. 1H NMR (CD2Cl2) d 2.32 (s, 3H, 5- or 7-CH3),
2.49 (s, 3H, 7- or 5-CH3), 3.13 (s, 3H, N–CH3), 4.36 (s, 2H, CH2), 7.00 (m,
1H, H4 or H6), 7.18 (m, 1H, H6 or H4), 7.6–7.7 (m, 5H, Ph); 13C NMR
(CD2Cl2) d 21.3 (q), 21.6 (q), 34.4 (q), 60.5 (t), 119.9 (d), 127.7 (d, 2C),
127.8 (d), 129.0 (d), 132.6 (d, 2C), 138.7 (s), 141.4 (s), 151.7 (s); 11B NMR
(CD2Cl2) d 40.4; MS 235 (M+). Analogous procedure was used for the
preparation of 3c. 1H NMR (CD2Cl2) d 3.14 (s, 3H, N–CH3), 5.44 (s, 1H,
CH), 7.28–7.34 (m, 1H), 7.36–7.42 (m, 1H), 7.42–7.55 (m, 5H), 7.60–7.66
(m, 1H), 7.66–7.72 (m, 2H), 7.94–7.99 (m, 1H), 8.00–8.06 (m, 2H); 13C
NMR (CD2Cl2) d 32.6 (q), 74.9 (d), 122.9 (d), 126.8 (d), 127.8 (d, 2C),
127.9 (d), 128.2 (d, 2C), 129.00 (d, 2C), 129.03 (d), 129.1 (d), 130.7 (d),
133.9 (d, 2C), 140.4 (s), 155.7 (s); 11B NMR (CD2Cl2) d 40.4; MS 283
(M+).
¶ Taking into account that AlCl3 used in the preparative experiments
obviously contains some proton donating impurities (AlCl2OH and the like)
we did not use ‘extra dry’ Al2Br6 in mechanistic experiments.
∑ Structure 4: 1H NMR (215 °C, CD2Cl2) d 2.25 (s, 6H, 3- and 5-CH3), 3.45
(d, JHNCH 5 Hz, 3H, N–CH3), 4.69 and 4.73 (m, JHCH 13, JHNCH 8 and 4 Hz,
2H, N–CH2), 7.04 (br s, 1H, N–H), 7.00 (s, 1H, H4), 7.08 (s, 2H, H2 and H6),
7.5–8.0 (m, 5H, Ph); 13C NMR (242 °C, CD2Cl2) d 20.8 (q, 2C), 39.6 (q),
60.5 (t), 126.7 (s), 127.6 (d, 2C), 129.1 (d, 2C), 132.6 (d), 137.0 (d, 2C),
138.8 (d), 139.9 (s, 2C). Structure 5: 1H NMR (215 °C, CD2Cl2) d 2.53 (s,
3H, 5- or 7-CH3), 2.60 (s, 3H, 7- or 5-CH3), 3.11 (d, JHNCH 6 Hz, 3H, N–
CH3), 4.55 (dd, JHCH 16, JHNCH 2 Hz, 1H, N–CH2), 5.32 (dd, JHCH 16 Hz,
JHNCH 6 Hz, 1H, N–CH2), 6.80 (br s, N–H), 7.28 (s, 1H, H4 or H6), 7.29 (s,
1H, H6 or H4), 7.5–8.0 (m, 5H, Ph); 13C NMR (242 °C, CD2Cl2) d 22.3 (q),
22.5 (q), 40.9 (q), 60.4 (t), 121.5 (d), 128.7 (d, 2C), 132.5 (d), 133.8 (d),
134.5 (d, 2C), 148.7 (s), 151.6 (s), 154.1 (s).
Scheme 7
boroles, starting from readily available and inexpensive
chemicals.
Notes and references
† Satisfactory spectral data have been obtained for all the new compounds.
The signals of the carbon atoms bearing B-centred fragments were not
observed in the 13C NMR spectra.
1 S. Nagy, R. Krishnamurti and B. P. Etherton, WO 96/34021, (Chem.
Abstr., 1996, 126, 19432j).
2 G. Schmid, S. Amirkhalili, U. Höhner, D. Kampmann and R. Boese,
Chem. Ber., 1982, 115, 3830.
‡ For example, the mixture of 2b and 2bA (approx. 1+1, numeration as for
structure 4): 1H NMR (CD2Cl2) d 2.35 (s, 6H, 3- and 5-CH3), 2.37 (s, 6H,
3- and 5-CH3), 2.99 (s, 3H, N–CH3), 2.65 (s, 3H, N–CH3), 4.59 (s, 2H, N–
CH2), 4.40 (s, 2H, N–CH2), 6.99 (s, 1H, H4), 6.96 (s, 1H, H4), 7.01 (s, 2H,
H2 and H6), 6.88 (s, 2H, H2 and H6), 7.3–7.8 (m, 10H, Ph); 13C NMR
(CD2Cl2) d 21.28 (q, 2C), 21.30 (q, 2C), 38.0 (q), 37.3 (q), 56.0 (t), 56.4 (t),
125.7 (d, 2C), 125.0 (d, 2C), 127.9 (d, 2C), 127.8 (d, 2C), 129.07 (d), 129.10
(d), 129.28 (d), 129.32 (d), 133.1 (d, 2C), 132.5 (d, 2C), 138.12 (s), 138.14
(s), 138.3 (s, 2C), 138.4 (s, 2C).
§ Preparation of 3b. Chloroaminoborane 2b (2 mmol) was added to the
suspension of Al2Cl6 (2 mmol) in 5 mL of CH2Cl2 at 0 °C whilst stirring in
an argon atmosphere. The dark-red solution formed after stirring for 0.5 h
was added dropwise to the solution of 1 mL NEt3 in 30 mL of hexane at
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Chem. Commun., 2000, 1587–1588