1,2-dihydro-1,2-oxaborine: A boron-oxygen heterocycle isoelectronic with benzene

Article abstract of DOI:10.1021/om070113s

1,2-Dihydro-2-phenyl-1,2-oxaborine (1b) has been prepared by the reaction of (2,3-dihydro-1,2-oxaborol-3-yl)-potassium (8) with methylene chloride and KN(SiMe₃)₂. Structural characterization of the phenyl-Cr(CO)₃ complex. 9 suggests that the 1,2-dihydro-1,2-oxaborine ring has a π-delocalized structure.

Full text of DOI:10.1021/om070113s

Organometallics 2007, 26, 1563-1564
1563
1,2-Dihydro-1,2-oxaborine: A Boron-Oxygen Heterocycle
Isoelectronic with Benzene
Jinhui Chen, Zoltan Bajko, Jeff W. Kampf, and Arthur J. Ashe, III*
Department of Chemistry, The UniVersity of Michigan, Ann Arbor, Michigan 48109-1055
ReceiVed February 5, 2007
Summary: 1,2-Dihydro-2-phenyl-1,2-oxaborine (1b) has been
prepared by the reaction of (2,3-dihydro-1,2-oxaborol-3-yl)-
potassium (8) with methylene chloride and KN(SiMe3)2. Struc-
tural characterization of the phenyl-Cr(CO)3 complex 9
suggests that the 1,2-dihydro-1,2-oxaborine ring has a π-de-
localized structure.
oxaborine (1b). The availability of 1b has allowed us to prepare
its phenyl-Cr(CO)3 complex 9, which has been structurally
characterized. These data clarify the potential aromaticity of
the 1,2-dihydro-1,2-oxaborine ring system.
Our synthesis of 1b involves an extension of the carbenoid
ring-expansion route recently used to prepare the analogous
boron-nitrogen heterocycle 5.10 The appropriate 1,2-oxaborolide
(8) needed for the ring expansion was prepared in two steps
from the readily available 2,2-dibutyl-2,5-dihydro-1,2-oxastan-
nole (6), as illustrated in Scheme 1.11 The reaction of 8 with
excess methylene chloride and KN(SiMe3)2 gave a 35% yield
of 1b as an air-sensitive colorless liquid.12 When the reaction
was performed using methylene chloride-d2, the deuterium in
1b was exclusively at the 3-position. The reaction is consistent
with an in situ formation of chlorocarbene, followed by addition
of the carbene to the position adjacent to boron of 8 and ulti-
mately by ring expansion and loss of chloride as illustrated.10,13
The 1H NMR spectrum of 1b in THF-d8 shows a first-order
pattern, which is consistent with the assigned structure. The 1H,
The boron-oxygen heterocycle 1,2-dihydro-1,2-oxaborine (1)
is a potentially aromatic six-π-electron compound. Several
(10) Ashe, A. J., III; Fang, X. D.; Fang, X. G.; Kampf, J. W.
Organometallics 2001, 20, 5413.
(11) Chen, J.; Kampf, J. W.; Ashe, A. J., III. Organometallics 2004, 23,
5088.
fused-ring derivatives of 1, isoelectronic with naphthalene,1,2
e.g., 2, and phenanthrene,3-6 e.g., 3, have been prepared.
Structural data on 2 and 3 show that there are no significant
differences between the B-O bond lengths which are exocyclic
and endocyclic to the ring.1,5,6 These B-O bond distances are
also not significantly different from those of PhB(OH)2.7 Thus,
the arylboronic acid derivatives 2 and 3 seem to have little
π-delocalization over the heterocyclic rings. Unfortunately there
are no structural data for compound 4, which is the only reported
non-fused-ring 1,2-dihydro-1,2-oxaborine.8 Recently ab initio
and DFT calculations on 1a have suggested that 1,2-dihydro-
1,2-oxaborines have considerable aromatic stabilization.9 To
experimentally test this hypothesis, a good synthesis of mini-
mally substituted derivatives of 1 would be highly desirable.
We report here on a synthesis of 1,2-dihydro-2-phenyl-1,2-
(12) Experimental procedures and characterization of new compounds
are as follows. 1b: THF (20 mL) at -78 °C was added slowly via a cannula
to a mixture of KN(SiMe3)2 (1.54 g, 7.70 mmol) and 8 (1.33 g, 7.33 mmol)
with stirring. Methylene chloride (10 mL) was added slowly at -78 °C.
The mixture was stirred at -78 °C for 2 h and then warmed slowly to 25 °C
for 10 h. The solvent was removed under reduced pressure, and the residue
was extracted with pentane (2 × 25 mL). Removal of the solvent from the
extracts gave a dark red oil, which was distilled (32 °C at 0.01 Torr) to
afford 1b as a colorless liquid (0.40 g, 35% yield). IR (film; cm-1): 3075,
3014, 1617, 1504, 1435, 1388, 1266, 740, 697, 678. UV (hexane; λmax
,
1
nm): 288, 231, 198. H NMR (500 MHz, THF-d8): δ 8.01 (dd, J ) 7.5,
1.4 Hz, 2H, Ar H), 7.69 (d, J ) 6.0 Hz, 1H, H(6)), 7.62 (dd, J ) 11.0, 6.0
Hz, 1H, H(4)), 7.39 (m, 3H, Ar H), 7.08 (d, J ) 11.0 Hz, 1H, H(3)), 6.35
(t, J ) 6.0 Hz, 1H, H(5)). 11B NMR (160.4 MHz, CDCl3): δ 39.6. 13C
NMR (125.7 MHz, c-C6D12): δ 150.0 (C(6)), 147.7 (C(4)), 134.6 (Co),
131.6 (Cp), 128.8 (Cm), 125.8 (br, C(3)), 112.5 (C(5)). HRMS (EI; m/z):
calcd for C10H911BO, 156.0746 (M+); found, 156.0753. Anal. Calcd for
C10H9BO: C, 77.00; H, 5.82. Found: C, 76.97; H, 5.96. 1b-d: When the
above reaction was performed using methylene chloride-d2, the isolated
product had a deuterium atom at C(3), as shown by the 1H NMR: no signal
at δ 7.08 (H(3)), δ 7.62 signal now d (J ) 6.0 Hz, H(4)), the rest of the
spectrum unchanged. 9: A THF (3 mL) solution of 1b (62.6 mg, 0.40 mmol)
was added to Cr(CO)3(CH3CN)3 (104 mg, 0.40 mmol). The resulting red
solution was heated to 70 °C for 12 h. After removal of the solvent the
crude product was extracted with hexanes to give a bright yellow solution.
The solvent was removed, leaving a crystalline product (117 mg). The
product was recrystallized from ether/hexanes to give yellow crystals. Mp:
116 °C. IR (hexane, film; cm-1): 1981, 1916. 1H NMR (400 MHz,
CDCl3): δ 7.68 (dd, J ) 11.2, 6.2 Hz, 1H, H(4)), 7.62 (d, J ) 4.4 Hz, 1H,
H(6)), 6.82 (d, J ) 11.2 Hz, 1H, H(3)),6.40 (dd, J ) 6.2, 4.4 Hz, 1H,
H(5)), 5.96 (d, J ) 6.4 Hz, 2H, Ph H), 5.61 (t, J ) 6.4 Hz, 1H, Ph H), 5.30
(t, J ) 6.4 Hz, 2H, Ph H). 13C NMR (100.6 MHz, CDCl3): δ 233.0, 148.9,
147.8, 124 (br), 112.2, 99.3, 95.6, 91.4. 11B NMR (160.4 MHz, CDCl3): δ
46.5. HRMS (EI; m/z): calcd for C13H911BCrO4 (M+), 291.9987; found,
291.9999. Anal. Calcd for C13H9BCrO4: C, 53.47; H, 3.11. Found: C, 53.37;
H, 3.03.
* To whom correspondence should be addressed. E-mail: ajashe@
umich.edu.
(1) Arcus, V. L.; Main, L.; Nicholson, B. K. J. Organomet. Chem. 1993,
460, 139.
(2) Paetzold, P.; Stanescu, C.; Stubenrauch, J. R.; Bienmu¨ller, M.; Englert,
U. Z. Anorg. Allg. Chem. 2004, 630, 2632.
(3) (a) Dewar, M. J. S.; Dietz, R. J. Chem. Soc. 1960, 1344. (b) Davis,
F. A.; Dewar, M. J. S. J. Org. Chem. 1968, 33, 3324.
(4) (a) Grisdale, P. J.; Williams, J. L. R. J. Org. Chem. 1969, 34, 1675.
(b) Zhou, J. Q.; Worm, K.; Dolle, R. E. J. Org. Chem. 2004, 69, 5147.
(5) Maringgele, W.; Meller, A.; Noltemeyer, M.; Sheldrick, G. M. Z.
Anorg. Allg. Chem. 1986, 536, 24.
(6) Greig, L. M.; Kariuki, B. M.; Habershon, S.; Spencer, N.; Johnston,
R. L.; Harris, K. D. M.; Philip, D. New J. Chem. 2002, 26, 701.
(7) Rettig, S. J.; Trotter, J. Can. J. Chem. 1977, 55, 3071.
(8) Ko¨ster, R.; Pourzal, A.-A. Synthesis 1973, 674.
(9) Minyaev, R. M.; Minkin, V. I.; Gribanova, T. N.; Starikov, A. G.
MendeleeV Commun. 2001, 2, 43.
(13) For a similar mechanistic proposal see: Nakadaira, Y.; Sato, R.;
Sakurai, H. Organometallics 1991, 10, 435.
10.1021/om070113s CCC: $37.00 © 2007 American Chemical Society
Publication on Web 03/03/2007

1564 Organometallics, Vol. 26, No. 7, 2007
Communications
Figure 1. Comparison of the 1H NMR, 13C NMR (in parentheses),
and 11B NMR (arrows) chemical shift values of 1b and 5b in THF-
d8.
Scheme 1
Figure 2. Solid-state structure of 9 (ORTEP). Thermal ellipsoids
are set at the 50% probability level. Hydrogen atoms have been
omitted for clarity. Selected distances (Å): B(1)-C(5), 1.567(7);
B(1)-C(4), 1.481(8); C(4)-C(3), 1.391(8); C(3)-C(2), 1.433(9);
C(2)-C(1), 1.308(9); C(1)-O(1), 1.359(7); O(1)-B(1), 1.384(6).
Scheme 3
Scheme 2
significantly shorter than the exocyclic B-C bond (1.567(7)
Å). Thus, the structural data for 9 are consistent with a
delocalized π-bonding in the 1,2-dihydro-1,2-oxaborine ring.
In order to explore possible electrophilic hydrogen/deuterium
exchange, 1b was treated with CD3CO2D/CF3CO2D at 25 °C.
This reaction led only to the formation of phenylboronic anhy-
dride 12 and other unidentified products. Under identical condi-
tions treatment of 5a with CF3CO2D led to H/D exchange. The
reaction of 1b with dimethyl acetylenedicarboxylate (DMAD)
in benzene at 90 °C gave 12 and dimethyl phthalate (15). These
products are probably formed via a Diels-Alder reaction to
give 13,18 followed by an Alder-Rickert cleavage, as illustrated
in Scheme 3. Phenyloxyborane (14) may be the precursor of
12.19 Under identical conditions 5 and DMAD do not react.
In summary, we have developed a new synthesis which allows
the preparation of a minimally substituted 1,2-dihydro-1,2-
oxaborine. Preliminary investigation of the chemistry of 1b
reveals that the 1,2-dihydro-1,2-oxaborine ring is readily cleaved
under mild conditions. However, the molecular structure of 9
suggests that the 1,2-dihydro-1,2-oxaborine ring is aromatic, as
had been predicted by DFT calculations.
11B, and 13C NMR chemical shift values of 1b are very similar
to those of 5b,14 as illustrated in Figure 1. There are only small
differences in chemical shift values of the atoms near oxygen/
nitrogen, which are consistent with the different electronega-
tivities of those atoms. Overall the similarity of the spectra is
consistent with a similarity in electronic structure of the two com-
pounds. In this context it is important to emphasize that 1,2-
dihydro-1,2-azaborine (5a) has classical aromatic properties.15
The reaction of 1b with Cr(CO)3(CH3CN)3 in THF at 70 °C
gave yellow crystals of 9 (Scheme 2). The molecular structure
of 9,16 illustrated in Figure 2, resembles that of 11, the phenyl-
Cr(CO)3 complex of 5b.17 Apparently the 1,2-dihydro-1,2-
oxaborine ring is a poorer ligand than phenyl. Interestingly, the
corresponding reaction of 5b with Cr(CO)3(CH3CN)3 initially
forms 10. Compound 10 is only converted to the phenyl-
coordinated 11 on subsequent heating.17 The uncoordinated 1,2-
oxaborine ring of 9 is completely planar ((0.004 Å) and is
canted by 5.7° relative to the phenyl ring. The intra-ring bond
distances of the oxaborine ring are close to those calculated by
DFT for 1a (average difference, 0.02 Å). It is particularly
noteworthy that the endocyclic B-C bond (1.481(8) Å) is
Acknowledgment. We thank the National Science Founda-
tion for partial support.
(14) Pan, J.; Kampf, J. W.; Ashe, A. J., III. Organometallics 2004, 23,
5626.
Supporting Information Available: A CIF file giving crystal-
lographic data for 9. This material is available free of charge via
the Internet at http:pubs.acs.org.
(15) Pan, J.; Kampf, J. W.; Ashe, A. J., III. Org. Lett. 2007, 9, 679.
(16) Crystal data for 9: C13H9BCrO4, monoclinic, P21/c, a ) 11.672(2)
Å, b ) 7.113(1) Å, c ) 15.635(2) Å, â ) 107.503(8)°, V ) 1238.0(3) Å3,
Z ) 4, Dc ) 1.567 g cm-3, T ) 150(2) K, λ(Mo ΚR) ) 0.710 73 Å. Data
were collected on a Siemans SMART CCD instrument. Final R indices (I
> 2σ(I)): R1 ) 0.0606, wR2 ) 0.1292. R indices (all data): R1 ) 0.1122,
wR2 ) 0.1408. GOF on F2 ) 1.088.
OM070113S
(18) Heating 1b with DMAD in benzene-d6 to 65 °C for 16 h resulted
in partial conversion to 12 and 15. However, no intermediate products were
1
(17) Pan, J.; Kampf, J. W.; Ashe, A. J., III. Organometallics 2006, 25,
197.
detected by H NMR spectroscopy.
(19) See: Pachaly, B.; West, R. J. Am. Chem. Soc. 1985, 107, 2987.