Crystal structure of a 1,1-dibutyl-1H,3H-naphtho[1,8-cd][1,2,6]oxastannaborinin-3-ol

A novel oxastannaborininol, 1,1-dibutyl-1H,3H-naphtho[1,8-cd][1,2,6]oxastannaborinin-3-ol, has been synthesized and crystallized. It is the first reported compound with a heterocycle containing an Sn–O–B unit.


Chemical context
Both tin and boron organic compounds are widespread reagents for cross-coupling reactions in organic synthesis (Negishi, 2002). The combination of tin-and boron-containing groups in one molecule can be advantageous, as they can undergo cross-coupling under different conditions. While the stannyl group easily undergoes transmetalation at elevated temperatures, a boronic acid will not do so with an additional activator, usually a base (Cá rdenas, 2003). However, those groups are not usually connected. The only reported use of esters of stannanols and boronic acids lies in their increased Lewis acidity compared to the free boronic acid (Beckett et al., 1999). They have been otherwise mentioned only in one publication, although no applications were reported (Murphy et al., 1993).
Heterocycles containing an E-O-B unit (E = Si, Ge, Sn, Pb) have so far only been reported for silicon (Fig. 1). Benzosiloxaboroles, containing a five-membered ring with an Si-O-B  unit, have shown promising properties for medical applications, being strong antimicrobial (Durka et al., 2019) and antifungal agents (Brzozowska et al., 2015).
Oxasilaboroninols have only been described in two cases. Sumida and co-workers accidentally stumbled upon 3 while trying to synthesize an oxasilole. They showed that both organometallic moieties can be replaced successively through Suzuki-Miyaura and Hiyama coupling (Sumida et al., 2018). Su and Hartwig on the other hand synthesized oxasiliaboroninol 4 using ruthenium catalysis (Su et al., 2018). In their report, they describe multiple transformations for this product, being able to replace selectively the boronic acid group while leaving a silanol group behind.

Structural commentary
The title molecule (1) is a cyclic intramolecular ester of a boronic acid and a stannanol. The asymmetric unit contains one molecule (Fig. 2). It shows notable disorder of the tin atom and the butyl groups. They occupy two sets of positions with site-occupancy factors of 0.295 (6) and 0.705 (6). It is furthermore planar, pointing towards electron delocalization over almost the whole molecule. The C-C bond lengths in the naphthalene structure are between 1.352 (4) and 1.439 (3) Å . This is in line with the bond lengths in naphthalene ranging from 1.350 to 1.421 Å (Abrahams et al., 1949). The Sn-O bond distance is 2.0041 (17) and 2.040 (3) Å and the Sn-C bond connecting the tin atom to the aromatic ring has a length of 2.151 (6) Å and 2.210 (4) Å , varying due to disorder. The B-C bond has a length of 1.594 (3)

Supramolecular features
In the crystal, the molecules form dimers through pairs of hydrogen bonds between the ring oxygen atom and the hydroxyl group with a distance of 2.805 (2) Å between the two involved oxygen atoms (Fig. 3, Table 1).

Figure 3
Structure of the dimer formed through hydrogen bonding. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms are drawn as fixed-size spheres with a radius of 0.15 Å .

Database survey
Searching the Cambridge Structural Database (CSD, version 5.41, update of November 2019; Groom et al., 2016), not a single ester of a boronic acid and a stannanol has been crystallized. The same is true for the corresponding germanol and plumbanol derivatives. One ester of a boronic acid and trimethylsilanol (5) has been crystallized (Ito et al., 2011). Two additional crystal structures containing the C-B-O-Sn-C motif have been reported. However, in those cases, either the O-Sn bond in 6 (Braunschweig et al., 2017) or the O-B bond in 7 (Boese et al., 1996) are not covalent, but rather coordinative bonds. Those three molecules are shown in Fig. 4. The CSD lists three stannanols, all of which are triaryl stannanols (Rů žička et al., 2013;Barbul et al., 2012). For those compounds, the Sn-O bond has a length of 1.981 to 2.057 Å , agreeing with the bond length of 2.0041 (17) Å found for the title compound. The Sn-C Ar bond length varies between 2.143 and 2.208 Å , matching the corresponding bond in the title compound.

Synthesis and crystallization
8-Iodo-1-naphthylboronic acid was prepared according to literature (Katz, 1986). Under argon, 122.6 mg (0.412 mmol, 1 eq.) of 8-iodo-1-naphthylboronic acid and 0.13 mL (0.459 mmol, 1.1 eq.) of tributyltin methoxide were heated to 373 K for 22.5 h; 0.2 mL (0.706 mmol, 1.7 eq.) of tributyltin methoxide were added and stirring was continued for 21 h at 373 K. Then 0.5 mL (1.764 mmol, 4.3 eq.) of tributyltin methoxide were added and the mixture was heated to 403 K for an additional 23 h. The mixture was cooled to RT and diluted by the addition of hexane. It was washed with equal volume 1 M aq. NaOH, dried (Na 2 SO 4 ), filtered and concentrated in vacuo. The residue was purified by column chromatography (pure hexane to hexane:ethyl acetate 1:1) to obtain a yellowish solid that was crystallized by slow evaporation of a solution in 1,2-dimethoxyethane at 258 K and washed with pentane to obtain 27.3 mg (0.068 mmol, 15%) of colorless crystals suitable for X-ray crystallography.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Data were collected at 200 K, as a phase transition leads to breaking crystals at lower temperatures. The disordered tin atom and butyl groups were restrained using rigid body (RIGU) restraints with for 1-3 distances and 1-2 distances of 0.004 and same-distance (SADI) restrains were applied to equivalent 1,2-and 1,3distances within the disorder. Ellipsoids of four atoms and 182 Breitwieser and Chen [Sn(C 4 H 9 ) 2 (C 10 H 7 BO 2 )] Acta Cryst.

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

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (