1-Hy­droxy-1H-benzo[d][1,2,6]oxaza­borinin-4(3H)-one

The structure of the title compound, C₇H₆BNO₃, a new boron heterocycle, prepared by the condensation of (2-eth­oxy­carbonyl­phen­yl)boronic acid and hydroxyl­amine, reveals the specific mode of intra­molecular condensation between a phenyl­boronic acid and an ortho hydroxamic acid substituent. The crystal structure shows that dehydration occurs to form a planar oxaza­borinine ring possessing both phenol-like B-O-H and lactam functional groups. In the extended structure, inter­molecular hydrogen bonding generates a 14-membered ring. To our knowledge, this is the first crystal structure determination involving a six-membered ring that exhibits consecutive B-OH, O, NH, and C=O functional groups.

S1. Comment

The title compound, (Ia), was obtained by the condensation of (2-ethoxycarbonylphenyl)boronic acid and hydroxylamine. Based on previous work, we knew that the arylboronic acid and ortho-hydroxamic acid substituents had many possible modes of interaction, as summarized in Scheme 1. They could dehydrate to form a planar six-membered B—O heterocycle (Groziak et al., 1997; Robinson et al., 1998) existing in one of two prototropic forms, viz. (Ia) and (Ib). They could dehydrate to a five-membered boron B—N heterocycle, (II), or a B—O heterocycle in the form of either the (E) or (Z) oxime, viz. (IIIa) and (IIIb). They could interact by N···B chelation, (IV) (Groziak & Robinson, 2002), or form a covalent chelate (Groziak et al., 1994), or perhaps form a planar intramolecular hydrogen bond, (V) (Scouten et al., 1994; Robinson & Groziak, 1999). Alternatively, the boronic acid could dehydratively trimerize and interact with two hydroxamic acid groups to form an internally chelated boroxine (Robinson et al., 1996). Our results show that the boronic acid group has dehydratively condensed with the ortho-hydroxamic acid group to form a planar six-membered boron heterocycle ring, viz. (Ia).

In (Ia) (Fig. 1), the bicyclic ring system is essentially planar, with a mean deviation of 0.041 (1) Å from the least-squares plane defined by the ten constituent atoms. The cyclized hydroxamic acid group is clearly in a lactam rather than a lactim form. The heterocyclic periphery of (Ia) may be compared to those of the previously prepared (VI), (VII), and (VIII) (see Scheme 2) (Groziak et al. 1997; Robinson et al. 1998). The length of the newly formed B1—O2 bond in (Ia) [1.3930 (9) Å] compares favorably to that of the analogous bond in (VI) [1.388 (6) Å]. On the other hand, the O2—N1 bond length in (Ia) [1.3935 (7) Å] is shorter than that in (VI) [1.419 (6) Å]. The exocyclic B—O bond length in oxazaborine (VI) is 1.350 (6) Å, similar to the bond length of 1.3449 (8) Å in (Ia). Somewhat longer B—O distances (ca 1.37 Å) occur in diazaborines (VII) and (VIII).

The crystal structure of (Ia) is characterized by intermolecular B—O—H···O═C and N—H···O═C hydrogen bonding (Table 1). The intermolecular hydrogen-bonding scheme features a bifurcated interaction to atom O1 and an R₄²(14) graph set, as shown in Fig. 2. The crystal packing places the centroid of the 6:6 ring junction in line with the center of the hydrogen-bonded ring at a distance of 3.384 (1) Å, as shown in Fig. 3.

The molecule of (Ia) is a true B–O for C═N replacement analogue of 4-hydroxyphthalazin-1(2H)-one, (IX) (see Scheme 3), the ambident lactam/lactim form of 2,3-dihydrophthalazine-1,4-dione, but to our knowledge this structure has not been determined previously by crystallographic analysis. Instead, the closest nitrogen heterocycle reference compound appears to be 6-hydroxypyridazin-3(2H)-one, (X), the ambident lactam/lactim form of maleic hydrazide (Cradwick, 1976).

Related literature

For related literature, see: Cradwick (1976); Groziak & Robinson (2002); Groziak et al. (1994, 1997); Hauser & Renfrow (1939); Robinson & Groziak (1999); Robinson et al. (1996, 1998); Scouten et al. (1994).

S2. Experimental

(2-Ethoxycarbonylphenyl)boronic acid (Aldrich) was condensed with excess NH₂OH in aqueous KOH according to a procedure reported for the preparation of benzohydroxamic acid from ethyl benzoate (Hauser & Renfrow, 1939). The solid potassium hydroxamate salt obtained in the first step was isolated as a white solid, and the solid was dissolved in H₂O and passed through a column of Dowex 50WX8 cation exchange resin (H⁺ form). Slow evaporation of the eluate gave the title compound (iield 83%, from the K⁺ salt) as colorless crystals (m.p. 482–484 K, H₂O). ¹H NMR [(CD₃)₂SO]: δ 11.82 (bs, exchanges, 1H, OH), 9.49 (bs, exchanges, 1H, NH), 8.09 (d, J = 7.6 Hz, 1H, ArH), 7.98 (d, J = 7.1 Hz, 1H, ArH), 7.81 (pseudo-t, 1H, ArH), 7.75 (pseudo-t, 1H, ArH). ¹³C NMR [(CD₃)₂SO]: δ 161.2 (C═O), 135.1, 132.8, 132.7, 132.3, 128.0 (br, C—B), 126.3. FT–IR, ν (cm^-1): 3314 (br, OH), 1625 (C═O), 1409, 1292, 1235, 1125. UV λ max (CH₃OH): 231, 273 nm.

S3. Refinement

H atoms bonded to C atoms were refined as riding on their parent atoms, with C—H = 0.95 Å and U_iso(H) = 1.2U_eq(C). H atoms bonded to N and O atoms were located in a difference Fourier map and were subsequently freely refined with no restraints.

Computing details

Data collection: APEX2 (Bruker, 2011); cell refinement: SAINT (Bruker, 2011); data reduction: SAINT (Bruker, 2011); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: XP in SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. A view of (Ia), with displacement parameters drawn at the 50% probability level.
	Fig. 2. A view of the hydrogen-bonded ring and bifurcated nature of atom O1. [Symmetry codes: (iii) x, -y-1/2, z-1/2; (iv) -x+1, y+1/2, -z+1/2; (v) -x+1, -y, -z.]
	Fig. 3. A view of the packing that emphasizes the manner in which the planar rings overlap. Hydrogen-bond interactions are shown with dashed lines.

1-Hydroxy-1H-benzo[d][1,2,6]oxazaborinin-4(3H)-one

Crystal data

C7H6BNO3	F(000) = 336
Mr = 162.94	Dx = 1.537 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 8670 reflections
a = 9.0848 (3) Å	θ = 3.3–32.9°
b = 6.2043 (2) Å	µ = 0.12 mm^−1
c = 13.1207 (5) Å	T = 90 K
β = 107.792 (2)°	Block, colourless
V = 704.17 (4) Å^3	0.50 × 0.33 × 0.20 mm
Z = 4

Data collection

Bruker APEXII diffractometer	2564 independent reflections
Radiation source: fine-focus sealed tube	2420 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.012
Detector resolution: 8.3 pixels mm^-1	θmax = 33.1°, θmin = 2.4°
ω scans	h = −13→13
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	k = −9→9
Tmin = 0.943, Tmax = 0.977	l = −20→20
11658 measured reflections

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)] = 0.034	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.103	H atoms treated by a mixture of independent and constrained refinement
S = 1.04	w = 1/[σ^2(Fo^2) + (0.0626P)^2 + 0.1884P] where P = (Fo^2 + 2Fc^2)/3
2564 reflections	(Δ/σ)max = 0.001
117 parameters	Δρmax = 0.54 e Å^−3
0 restraints	Δρmin = −0.24 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
O1	0.40195 (6)	−0.00533 (8)	0.20721 (4)	0.01483 (12)
O2	0.40143 (6)	−0.24104 (8)	0.45339 (4)	0.01368 (12)
O3	0.26801 (6)	−0.16815 (9)	0.57746 (4)	0.01599 (12)
H3	0.316 (2)	−0.283 (3)	0.6146 (14)	0.049 (4)*
N1	0.42848 (7)	−0.19124 (10)	0.35710 (5)	0.01385 (13)
H1	0.4867 (16)	−0.292 (2)	0.3376 (11)	0.030 (3)*
C1	0.22797 (7)	0.08701 (11)	0.41957 (5)	0.01179 (13)
C2	0.12594 (8)	0.22893 (12)	0.44704 (5)	0.01483 (14)
H2	0.0931	0.1998	0.5078	0.018*
C3	0.07270 (9)	0.41218 (12)	0.38565 (6)	0.01623 (14)
H3A	0.0026	0.5067	0.4042	0.019*
C4	0.12176 (8)	0.45789 (12)	0.29689 (6)	0.01533 (14)
H4	0.0870	0.5853	0.2566	0.018*
C5	0.22112 (8)	0.31811 (11)	0.26719 (5)	0.01364 (13)
H5	0.2536	0.3480	0.2063	0.016*
C6	0.27254 (7)	0.13246 (11)	0.32847 (5)	0.01141 (13)
C7	0.37126 (7)	−0.02250 (11)	0.29437 (5)	0.01168 (13)
B1	0.29826 (9)	−0.11392 (12)	0.48663 (6)	0.01219 (14)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0210 (2)	0.0145 (2)	0.0122 (2)	−0.00363 (18)	0.00992 (18)	−0.00168 (16)
O2	0.0180 (2)	0.0140 (2)	0.0112 (2)	0.00282 (17)	0.00770 (18)	0.00227 (16)
O3	0.0221 (3)	0.0160 (2)	0.0123 (2)	0.00440 (19)	0.00883 (19)	0.00346 (17)
N1	0.0184 (3)	0.0133 (3)	0.0131 (3)	0.0023 (2)	0.0096 (2)	0.00100 (18)
C1	0.0141 (3)	0.0123 (3)	0.0094 (3)	0.0010 (2)	0.0042 (2)	−0.00001 (19)
C2	0.0183 (3)	0.0156 (3)	0.0116 (3)	0.0035 (2)	0.0061 (2)	−0.0001 (2)
C3	0.0193 (3)	0.0147 (3)	0.0146 (3)	0.0043 (2)	0.0051 (2)	−0.0005 (2)
C4	0.0178 (3)	0.0124 (3)	0.0143 (3)	0.0011 (2)	0.0028 (2)	0.0011 (2)
C5	0.0162 (3)	0.0128 (3)	0.0115 (3)	−0.0011 (2)	0.0036 (2)	0.0012 (2)
C6	0.0131 (3)	0.0115 (3)	0.0099 (3)	−0.0007 (2)	0.0039 (2)	−0.00071 (19)
C7	0.0138 (3)	0.0116 (3)	0.0107 (3)	−0.0024 (2)	0.0053 (2)	−0.00133 (19)
B1	0.0146 (3)	0.0126 (3)	0.0103 (3)	0.0006 (2)	0.0050 (2)	0.0001 (2)

Geometric parameters (Å, º)

O1—C7	1.2616 (8)	C2—C3	1.3918 (10)
O2—B1	1.3930 (9)	C2—H2	0.9500
O2—N1	1.3935 (7)	C3—C4	1.3974 (10)
O3—B1	1.3449 (8)	C3—H3A	0.9500
O3—H3	0.898 (18)	C4—C5	1.3906 (10)
N1—C7	1.3350 (9)	C4—H4	0.9500
N1—H1	0.905 (15)	C5—C6	1.4008 (9)
C1—C6	1.4029 (9)	C5—H5	0.9500
C1—C2	1.4031 (9)	C6—C7	1.4745 (9)
C1—B1	1.5468 (10)
B1—O2—N1	118.70 (5)	C5—C4—H4	119.8
B1—O3—H3	118.9 (11)	C3—C4—H4	119.8
C7—N1—O2	126.12 (6)	C4—C5—C6	118.89 (6)
C7—N1—H1	121.2 (9)	C4—C5—H5	120.6
O2—N1—H1	112.6 (9)	C6—C5—H5	120.6
C6—C1—C2	118.56 (6)	C5—C6—C1	121.46 (6)
C6—C1—B1	118.05 (6)	C5—C6—C7	119.28 (6)
C2—C1—B1	123.37 (6)	C1—C6—C7	119.22 (6)
C3—C2—C1	120.27 (6)	O1—C7—N1	118.21 (6)
C3—C2—H2	119.9	O1—C7—C6	122.92 (6)
C1—C2—H2	119.9	N1—C7—C6	118.85 (6)
C2—C3—C4	120.34 (6)	O3—B1—O2	118.36 (6)
C2—C3—H3A	119.8	O3—B1—C1	123.02 (6)
C4—C3—H3A	119.8	O2—B1—C1	118.59 (6)
C5—C4—C3	120.44 (6)
B1—O2—N1—C7	4.21 (10)	O2—N1—C7—O1	−177.11 (6)
C6—C1—C2—C3	−0.99 (10)	O2—N1—C7—C6	1.54 (10)
B1—C1—C2—C3	177.02 (7)	C5—C6—C7—O1	−5.99 (10)
C1—C2—C3—C4	−0.77 (11)	C1—C6—C7—O1	171.87 (6)
C2—C3—C4—C5	1.68 (11)	C5—C6—C7—N1	175.42 (6)
C3—C4—C5—C6	−0.78 (10)	C1—C6—C7—N1	−6.71 (9)
C4—C5—C6—C1	−1.02 (10)	N1—O2—B1—O3	177.37 (6)
C4—C5—C6—C7	176.79 (6)	N1—O2—B1—C1	−4.60 (9)
C2—C1—C6—C5	1.90 (10)	C6—C1—B1—O3	177.52 (6)
B1—C1—C6—C5	−176.22 (6)	C2—C1—B1—O3	−0.50 (11)
C2—C1—C6—C7	−175.92 (6)	C6—C1—B1—O2	−0.41 (10)
B1—C1—C6—C7	5.97 (9)	C2—C1—B1—O2	−178.43 (6)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H3···O1^i	0.898 (18)	1.797 (18)	2.6864 (7)	170.4 (16)
N1—H1···O1^ii	0.905 (15)	1.866 (15)	2.7707 (8)	177.2 (13)

Symmetry codes: (i) x, −y−1/2, z+1/2; (ii) −x+1, y−1/2, −z+1/2.