Synthesis and crystal structures of boryl ortho-silylaryl trifluoromethanesulfonates

The preparation and solid-state structures of three borylated ortho-silylaryl trifluoromethanesulfonates are reported. All compounds show the expected connectivity and unexceptional metric parameters as well as weak intramolecular interactions.


Chemical context
Arynes are remarkably versatile intermediates in organic synthesis (Anthony et al., 2021;Takikawa et al., 2018;Tadross et al., 2012).Their generation from ortho-silylaryl triflates (Shi et al., 2021) using fluoride salts (Himeshima et al., 1983) or other mild bases (Idiris & Jones, 2017) has enabled the development of many otherwise impossible transformations.However, ortho-silylaryl triflates can themselves be challenging to introduce in many chemical contexts, which has limited their usefulness.We previously showed (Demory et al., 2015) that simple ortho-silylaryl triflate aryne precursors can be diversified in a straightforward manner by leveraging the versatility of organoboronate groups introduced via Ir-catalysed C-H borylation (Bisht et al., 2022;Mkhalid et al., 2010).Hosoya and co-workers published a closely related study showcasing a complementary reaction scope (Yoshida et al., 2015).In the course of our studies, we prepared crystals of several boryl aryne precursors.

Structural commentary
Compound 1a crystallizes in the orthorhombic space group Pna2 1 with one molecule in the asymmetric unit (Z = 4).The central ring and the directly attached heteratoms form a nearly planar motif (average deviation from the least squares plane = 0.062 A ˚).The C-B, C-Si, and C-O bond distances are within the expected values for single bonds: 1.572 (4), 1.909 (3) and 1.450 (3) A ˚, respectively.Compound 1b crystallizes in the monoclinic space group C2/c as colourless blocks with one molecule in the asymmetric unit.The central benzene ring and its direct heteroatom (O, Si, and B) form a nearly perfect plane (mean deviation from the least squares plane = 0.021 A ˚).The pinacolate moiety is disordered over two sites with site occupancy factors of 0.905 and 0.095, attached to one pivot borane atom.The C-B, C-Si, and C-O bonds are 1.599 (3), 1.908 (2), and 1.443 (2) A ˚, respectively, in the typical range for Csp 2 -E single bonds.Compound 2 crystallizes in the monoclinic space group P2 1 /n (Z = 8) with two molecules in the asymmetric unit of very similar metric parameters, except for the orientation of the triflate group [C2-O1-S1-C10 = 95.7 (3) and 150.1 (3) � ], as shown in Fig. 4. The C-B, C-Si, and C-O bonds are 1.558 (5)/ 1.553 (5), 1.451 (4)/1.450(4), and 1.908 (4)/1.899(4) A ˚, respectively.It is noteworthy that the variation of the C-B bond length is the largest in this series, albeit still within the expected bond length for a carbon-boron single bond and within the respective standard deviations (see Table 1 and Figs.1-4).

Supramolecular features
The supramolecular arrangement of 1a, 1b, and 2 is unexceptional and shows only very weak intermolecular aryl/ methyl-H� � �O (>2.58A ˚) and aryl/methyl-H� � �F (>2.60A ˚) interactions, the latter being slightly below the sum of their van der Waals radii.In compound 1a, the molecular motifs arrange in a slipped manner giving a stair-like arrangement.Besides these weak Si(CH 3 ) 3 � � �O interactions [2.797 (2) A ˚], further aryl-H� � �O interactions [2.683 (2) A ˚] dominate the packing.The crystal structure of 1b is characterized by inter- Overlay of the two independent molecules of compound 2.
The molecular structure of compound 1a.Displacement ellipsoids are drawn at the 50% probability level.

Figure 2
The molecular structure of compound 1b.Displacement ellipsoids are drawn at the 50% probability level.

Figure 3
The two independent molecules in compound 2. Displacement ellipsoids are drawn at the 50% probability level.

Database survey
A database survey (Cambridge Structural Database, WEBCSD v.1.9.40; Groom et al., 2016) shows that, despite the large interest in these aryne precursors, only a limited number of ortho-silylaryl triflates have been structurally characterized, including precursors for complex natural products (Guo et al., 2023: BEVBIR)

Synthesis and crystallization
Aryl boronates 1-2 were synthesised via Ir-catalysed C-H borylation according to a previously reported protocol (Demory et al., 2015).Crystals of 1-2 were grown according to the following procedures: A 100 mL round-bottom flask containing a suspension of 1a and 1b (4.00 g, 9.42 mmol, 1a/1b ' 2.5:1) in n-pentane (15 mL) was heated gently to 313 K and filtered through a sintered frit.The filtrate was concentrated under reduced pressure, giving a colourless solid.This procedure was repeated twice, yielding a viscous colourless oil, storage of which under air for six weeks at RT afforded cubic crystals of 1b.

Refinement
Crystal data, data collection and structure refinement details are summarized in  BF 3 O 5 SSi, C 16 H 24 BF 3 O 5 SSi and C 17 H 26 BF 3 O 5 SSi 145   Figure 7 Solid-state packing of compound 2 showing short inter-and intramolecular interactions.

Figure 5
Solid-state packing of compound 1a showing short inter-and intramolecular interactions.

Figure 6
Solid-state packing of compound 1b showing short inter-and intramolecular interactions.
positional disorder of the pinacolborane moiety pivoting around the boron atom.The site occupancy factors were freely refined to give a 0.097 (4):0.903(4) occupancy.
Attempts to model the trifluoromethanesulfonate group in 1b with a positional disorder using two (and three) parts, did not produced a satisfactory model.Hence the refinement with somewhat large ellipsoids for the trifluoromethanesulfonate group was finally used.Hydrogen atoms were refined with common isotropic displacement parameters for the H atoms of the same group and idealized geometries with methyl groups allowed to freely rotate about the C-C bond.The distances for methyl and aromatic C-H groups were set to 0.98 A ˚and 0.95 A ˚, respectively.

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.

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.

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.

Table 2
Experimental details.