2-(6-Bromo­pyridin-2-yl)-6-methyl-[1,3,6,2]­diox­aza­borocane, a new stable (pyridin-2-yl)­boronic acid derivative

The crystal structure of the first reported non-substituted N-methyl­diox­aza­borocane confirms that the presence of a methyl group attached to the N atom introduces an NB bond length that is longer than that in a simple diox­aza­borocane ring. The presence of more N atoms in the vicinity of the B atom in the title compound [systematic name: 6a-(6-bromo­pyridin-2-yl)-3a-methyl-2,3,4,5-tetra­hydro-1,6-dioxa-3a-aza-6a-borapentalene], C₁₀H₁₄BBrN₂O₂, does not modify significantly any structural parameter in the diox­aza­borocane ring. On the other hand, a small asymmetry appears in the bond angles of the pyridine C atom next to the B atom.

S1. Comment

As part of our study of pyridinylboron derivatives, we have recently focused on dioxazaborocanes. These compounds are very interesting since they can undergo an internal B← N dative bonding, which leads to a tetracoordinated B atom. Such boron derivatives are expected to be more stable and easier to purify than the corresponding acids or dialkyl esters. To date, only a few crystallographic studies have been carried out on such compounds, and those that have been published do not relate to N-methyldioxazaborocanes (Iwanek et al., 2002; Höpfl et al., 1998a, 1998b).

Such studies? are of particular interest in the field of pyridin-2-ylboronic derivatives, since the stability of such compounds is not clearly established. On the one hand, different authors have shown the instability of some pyridin-2-ylboronic acids and esters (Fischer & Havinga, 1974; Ishiyama et al., 2001). On the other hand, we have prepared, isolated and crystallized some stable pyridin-2-ylboron derivatives (Bouillon et al., 2003; Sopkova et al., 2003).

The aim of the present study is to understand the effect of a second N atom in the vicinity of the B atom. According to the literature (Murafaji et al., 1996), the N atom in pyridine can also undergo dative bonding, a fact that has often been found to be the cause of instability of pyridin-2-yl boron compounds. In the title compound, (I), the pyridine N atom is situated close to the B atom and could influence the dative B←N dioxazaborocane bond.

In the crystal structure of (I) (Fig. 1), the N atom of the dioxazaborocane cycle lies next to the B atom, as expected. The dioxazaborocane moiety comprises two fused five-membered rings, an arrangement that is reported for all structures containing a dioxazaborocane ring deposited in the Cambridge Structural Database to date (CSD; Version 5.24; Allen & Kennard, 1993). For the comparison below, structures containing only non-substituted dioxazaborocane cycle were selected (Rettig & Trotter, 1975; Thadani et al., 2001; Doidge-Harrison et al., 1998; Thadani et al., 2002; Caron & Hawkins, 1998; Howie et al., 1997).

Among these structures, two kinds of conformations for the double ring can be observed, viz. either `boat-like' or `chair-like'. The dioxazaborocane ring in (I) is in a `chair-like' conformation. In the structure of B-phenyl-diptychboroxazolidine (Rettig and Trotter, 1974), the most closely related compound, the dioxazaborocane ring is also in a `chair-like' conformation, but oriented differently from that in (I); in (I), the part oriented towards the pyridine ring is that situated on the side of the pyridine ring containing the N atom, contrary to what was observed in B-phenyl-diptychboroxazolidine. The distance between dioxazaborocane atom O2 and pyridine atom N2 (O—N = 2.923 Å) in (I) is similar to the distance between the corresponding O atom and phenyl C atom in B-phenyl-diptychboroxazolidine (O—C = 2.964 Å).

The introduction of the N atom does not seem to influence either the bond lengths or the bond angles in the vicinity of the B atom. All observed values are close to the mean calculated from the CSD structures (Table 2). Only one significant difference was observed for the title compound; the B← N distance is 1.696 (7) Å, significantly longer than the mean value, 1.661 Å, calculated from the data available (Table 2). As the calculated parameter describing the tetrahedral character of boron (THC_DA; Höpfl et al., 1999) is the same for (I) (THC_DA=66%) and for B-phenyl-diptychboroxazolidine (THC_DA=66%), the reason for the longer B—N bond length might be attributed to the presence of a methyl group bound to the dioxazaborocane N atom. To confirm this hypothesis, a second search in the CSD was carried out and structures containing an N-methyldioxazaboracane ring were selected (Table 2). These structures do not contain a simple motif; they are either dioxazaborocane-4,8-dione (Mancilla et al., 1997) or dioxazaborocane-4-one (Farfan et al., 1990). The B←N bond length in these structures varies from 1.66 to 1.719 Å, close to the value observed in our structure. Futhermore, a dissymmetry in the bond angles around atom C3, the pyridine C atom attached to the B atom, is observed (Table 1).

The dioxazaborocane cycle is positioned approximately perpendicularly with respect to the pyridine ring, as in all of the CSD structures. The angle between the plane of the pyridine ring and the plane through the atoms O1, O2 and N10 is about 87.69 (18)°, similar to that observed in B-phenyl-diptychboroxazolidine (_89.05°).

In the crystal packing, two layers can be distinguished along the c axis, the first containing pyridine rings and the other formed by dioxazaborocane rings. No hydrogen bonds or stacking interactions between pyridine rings were detected in the crystal packing (Fig. 2).

S2. Experimental

The title compound was synthesized from 2,6-dibromopyridine using the method described by Bouillon et al. (2003). Crystals of (I) suitable for X-ray analysis were obtained by slow evaporation from acetonitrile at room temperature.

S3. Refinement

H atoms were treated as riding, with C—H distances in the range 0.93–0.96 Å (Sheldrick,1997). Owing to the lack of sufficient Friedel pairs, the absolute structure could not be determined unambiguously, and the Flack data were removed from the CIF.

Computing details

Data collection: CAD-4-PC (Enraf–Nonius, 1996); cell refinement: CAD-4-PC; data reduction: JANA2000 (Petříček & Dušek, 2000 ); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 (Farrugia, 1997); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. A view of (I), showing the labelling of the non-H atoms. Displacement ellipsoids are shown at the 30% probability level and H atoms are drawn as small circles of arbitrary radii.
	Fig. 2. A general view of the crystal packing of (I), projected along the b axis. Displacement ellipsoids are drawn at the 20% probability level.

6a-(6-bromopyridin-2-yl)-3a-methyl-2,3,4,5-tetrahydro1,6-dioxa-3a-aza-6a- borapentalene

Crystal data

C10H14BBrN2O2	Dx = 1.611 Mg m^−3
Mr = 284.95	Melting point: 433 K
Orthorhombic, Pca21	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2c -2ac	Cell parameters from 25 reflections
a = 10.7716 (12) Å	θ = 18–24°
b = 7.0104 (7) Å	µ = 3.49 mm^−1
c = 15.5546 (14) Å	T = 293 K
V = 1174.6 (2) Å^3	Prism, translucent light colourless
Z = 4	0.65 × 0.63 × 0.29 mm
F(000) = 576

Data collection

Enraf–Nonius CAD-4 diffractometer	1408 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.000
Graphite monochromator	θmax = 30.0°, θmin = 2.6°
θ/2θ scans	h = 0→15
Absorption correction: gaussian JANA2000 (Petříček & Dušek, 2000)	k = 0→9
Tmin = 0.250, Tmax = 0.707	l = −21→0
1767 measured reflections	3 standard reflections every 60 min
1767 independent reflections	intensity decay: 5.5%

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.049	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.144	H-atom parameters constrained
S = 1.06	w = 1/[σ^2(Fo^2) + (0.1027P)^2] where P = (Fo^2 + 2Fc^2)/3
1767 reflections	(Δ/σ)max < 0.001
146 parameters	Δρmax = 0.74 e Å^−3
1 restraint	Δρmin = −0.78 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Br1	0.14758 (6)	0.37870 (8)	0.46149 (7)	0.0547 (2)
C1	0.0867 (4)	0.1310 (7)	0.4958 (3)	0.0344 (9)
N2	0.1213 (4)	0.0718 (7)	0.5715 (3)	0.0338 (8)
C3	0.0760 (4)	−0.0989 (7)	0.5990 (4)	0.0343 (10)
C4	−0.0047 (5)	−0.1975 (8)	0.5473 (4)	0.0433 (11)
H4	−0.0388	−0.3112	0.5671	0.052*
C5	−0.0367 (5)	−0.1318 (8)	0.4665 (6)	0.0488 (14)
H5	−0.0887	−0.2031	0.4312	0.059*
C6	0.0094 (5)	0.0398 (9)	0.4389 (3)	0.0459 (13)
H6	−0.0102	0.0909	0.3854	0.055*
B7	0.1182 (5)	−0.1698 (10)	0.6934 (4)	0.0380 (12)
O1	0.0424 (4)	−0.3261 (8)	0.7226 (4)	0.0584 (13)
O2	0.1348 (4)	−0.0174 (7)	0.7549 (3)	0.0493 (10)
C8	0.1154 (6)	−0.4584 (11)	0.7699 (6)	0.061 (2)
H8A	0.0749	−0.5818	0.7729	0.073*
H8B	0.1305	−0.4127	0.8278	0.073*
C13	0.2610 (7)	0.0283 (8)	0.7620 (5)	0.0532 (15)
H13A	0.2768	0.0962	0.8153	0.064*
H13B	0.2871	0.1076	0.7143	0.064*
C12	0.3288 (6)	−0.1582 (10)	0.7610 (5)	0.0494 (14)
H12A	0.3236	−0.2213	0.8164	0.059*
H12B	0.4154	−0.1406	0.7460	0.059*
N10	0.2616 (4)	−0.2696 (5)	0.6930 (3)	0.0336 (9)
C14	0.3314 (5)	−0.2641 (10)	0.6112 (5)	0.0462 (13)
H14A	0.3437	−0.1339	0.5941	0.069*
H14B	0.2853	−0.3301	0.5676	0.069*
H14C	0.4105	−0.3247	0.6189	0.069*
C9	0.2353 (7)	−0.4707 (7)	0.7194 (5)	0.0503 (16)
H9A	0.3018	−0.5206	0.7549	0.060*
H9B	0.2255	−0.5522	0.6695	0.060*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Br1	0.0762 (4)	0.0398 (3)	0.0482 (3)	0.0011 (2)	0.0008 (4)	0.0057 (3)
C1	0.033 (2)	0.036 (2)	0.034 (2)	0.0037 (18)	0.0016 (18)	−0.0048 (19)
N2	0.0313 (17)	0.0366 (19)	0.033 (2)	0.0040 (16)	−0.0014 (15)	0.0010 (18)
C3	0.0256 (19)	0.036 (2)	0.041 (2)	0.0074 (18)	0.0018 (17)	0.002 (2)
C4	0.039 (2)	0.039 (3)	0.052 (3)	−0.002 (2)	−0.001 (2)	0.002 (3)
C5	0.044 (2)	0.055 (3)	0.048 (3)	−0.003 (2)	−0.009 (3)	−0.014 (3)
C6	0.044 (3)	0.061 (4)	0.033 (2)	0.012 (3)	−0.0069 (18)	−0.009 (2)
B7	0.027 (2)	0.045 (3)	0.042 (3)	0.009 (2)	0.006 (2)	0.013 (3)
O1	0.0302 (17)	0.071 (3)	0.074 (3)	−0.0049 (18)	0.0010 (19)	0.032 (3)
O2	0.051 (2)	0.061 (3)	0.0362 (19)	0.0232 (18)	0.0032 (16)	−0.0039 (19)
C8	0.047 (3)	0.054 (4)	0.082 (5)	−0.008 (3)	−0.001 (3)	0.030 (4)
C13	0.065 (4)	0.036 (3)	0.059 (3)	0.009 (3)	−0.017 (3)	−0.011 (3)
C12	0.045 (3)	0.046 (3)	0.057 (4)	0.006 (2)	−0.014 (3)	−0.008 (3)
N10	0.0265 (15)	0.0314 (16)	0.043 (2)	0.0015 (16)	−0.0024 (17)	0.0034 (17)
C14	0.034 (2)	0.050 (3)	0.054 (3)	0.002 (2)	0.014 (2)	−0.004 (3)
C9	0.045 (3)	0.035 (2)	0.071 (4)	0.001 (2)	−0.003 (3)	0.014 (3)

Geometric parameters (Å, º)

Br1—C1	1.931 (5)	C8—C9	1.514 (10)
C1—N2	1.303 (7)	C8—H8A	0.9700
C1—C6	1.373 (7)	C8—H8B	0.9700
N2—C3	1.361 (7)	C13—C12	1.498 (8)
C3—C4	1.371 (8)	C13—H13A	0.9700
C3—B7	1.615 (8)	C13—H13B	0.9700
C4—C5	1.383 (11)	C12—N10	1.501 (8)
C4—H4	0.9300	C12—H12A	0.9700
C5—C6	1.370 (10)	C12—H12B	0.9700
C5—H5	0.9300	N10—C14	1.479 (7)
C6—H6	0.9300	N10—C9	1.496 (6)
B7—O1	1.440 (8)	C14—H14A	0.9600
B7—O2	1.446 (9)	C14—H14B	0.9600
B7—N10	1.696 (7)	C14—H14C	0.9600
O1—C8	1.421 (9)	C9—H9A	0.9700
O2—C13	1.400 (9)	C9—H9B	0.9700
N2—C1—C6	127.4 (5)	O2—C13—C12	105.8 (5)
N2—C1—Br1	116.0 (4)	O2—C13—H13A	110.6
C6—C1—Br1	116.5 (4)	C12—C13—H13A	110.6
C1—N2—C3	117.5 (5)	O2—C13—H13B	110.6
N2—C3—C4	119.1 (5)	C12—C13—H13B	110.6
N2—C3—B7	117.1 (5)	H13A—C13—H13B	108.7
C4—C3—B7	123.8 (5)	C13—C12—N10	103.1 (5)
C3—C4—C5	121.6 (5)	C13—C12—H12A	111.1
C3—C4—H4	119.2	N10—C12—H12A	111.1
C5—C4—H4	119.2	C13—C12—H12B	111.1
C6—C5—C4	119.1 (6)	N10—C12—H12B	111.1
C6—C5—H5	120.4	H12A—C12—H12B	109.1
C4—C5—H5	120.4	C14—N10—C9	110.9 (4)
C5—C6—C1	115.3 (6)	C14—N10—C12	110.4 (5)
C5—C6—H6	122.4	C9—N10—C12	112.8 (5)
C1—C6—H6	122.4	C14—N10—B7	117.1 (4)
O1—B7—O2	115.1 (5)	C9—N10—B7	102.4 (4)
O1—B7—C3	111.2 (5)	C12—N10—B7	102.9 (4)
O2—B7—C3	114.1 (5)	N10—C14—H14A	109.5
O1—B7—N10	101.8 (4)	N10—C14—H14B	109.5
O2—B7—N10	101.2 (4)	H14A—C14—H14B	109.5
C3—B7—N10	112.4 (4)	N10—C14—H14C	109.5
C8—O1—B7	110.2 (4)	H14A—C14—H14C	109.5
C13—O2—B7	110.0 (4)	H14B—C14—H14C	109.5
O1—C8—C9	104.0 (6)	N10—C9—C8	104.5 (5)
O1—C8—H8A	111.0	N10—C9—H9A	110.9
C9—C8—H8A	111.0	C8—C9—H9A	110.9
O1—C8—H8B	111.0	N10—C9—H9B	110.9
C9—C8—H8B	111.0	C8—C9—H9B	110.9
H8A—C8—H8B	109.0	H9A—C9—H9B	108.9