Hypervalent pentacoordinate boron compounds (10-B-5) have been postulated as transition states in S N 2-type reactions at a boron atom. For example, the reaction of the [BH 3 -CO] complex with NMe 3 [2] as well as the intramolecular bond switch at the boron atom in compounds bearing a van Koten type ligand have been reported. There has been only one report of isolable hypervalent boron compounds (10-B-5 and 12-B-6), [4] but the compounds bearing tridentate pyridine diol ligand(s) were characterized by 1 H, 13 C, 19 F, and 11 B NMR spectra in solution. The X-ray analysis of the compounds has not been reported. Here we report the synthesis and X-ray structures of 1,8-dimethoxy-9-borylanthracene (1 a Β± c, see Scheme 2): the first fully characterized hypervalent 10-B-5 compounds.
Recently, we reported the synthesis and the X-ray structure of the hypervalent five-coordinate carbon compound (10-C-5) 2 through the use of an 1,8-dimethoxy-9-anthracenyl ligand. [5] Ester 3 was synthesized from the 9-OTf derivative 4 (OTf trifluoromethanesulfonate) by carbon monoxide insertion in
methanol mediated by [Pd(PPh 3 ) 4 ]. However, several attempts to synthesize 1 from 4 were not successful. Thus, we designed a novel versatile precursor 1,8-dimethoxy-9-bromoanthracene (8). The synthetic pathway for 8 is illustrated in Scheme 1. After conversion of 1,8-dimethoxy-9-hydroxyanthracene (5) [6] into the corresponding phosphate (6), reaction conditions for the reduction of 6 and the subsequent treatment of the resulting anion 7 with BrCF 2 CF 2 Br to yield 8 were examined (Table ). Only the 9-H compound was obtained using Birch reduction conditions (entry 1) and only a trace amount of 8 was obtained by using lithium naphthalenide (entry 2). Fortunately, the reduction of 6 with lithium 4,4'-di-Koizumi, T. Yamamoto, Organometallics 1999, 18, 1349. [9] 1 H NMR (400 MHz, [D 6 ]benzene, 25 8C): d 8.21 (d, 12 H, 3 J H,H 6 Hz; ortho), 7.23 (m, 18 H; meta and para), 1.09 (m, 27 H, 3 J Pt,H 43, 4 J Pt,H 25 Hz; PCH 3 ); 31 P{ 1 H} NMR (160 MHz, [D 6 ]benzene, 25 8C, relative to 85 % H 3 PO 4 ): d 27.0 ( 1 J P,Pt 2959, 2 J P,Pt 418, 3 J P,P 86 Hz); 13 C{ 1 H} NMR (100 MHz, CD 2 Cl 2 , 258C): d 152.5 (ipso, 2 J C,Pt 48 Hz), 136.4 (ortho), 128.4 (para), 128.0 (meta), 21.9 (m, 2 J Pt,C 82 Hz); 195 Pt{ 1 H} NMR (64.1 MHz, [D 6 ]benzene, 25 8C): d Γ 3980 relative to K 2 [PtCl 4 ]; 29 Si{ 1 H} NMR (79 MHz, [D 6 ]benzene, 25 8C): d 279.4 (J Si,Pt 945 Hz); elemental analysis calcd for C 45 H 57 Si 3 P 3 Pt 3 (%): C 39.73, H 4.22; found: C 39.67, H 4.03. [10] Complex 2 gave satisfactory elemental analyses, NMR data, and X-ray crystallographic results. Analogous complexes [(dppe)Pt(R 2 SiO-SiR 2 )] (R Me, iPr) were prepared by reaction of O 2 with disilene Pt complexes [(dppe)Pt(R 2 SiSiR 2 )] (dppe 1,2-bis(diphenylphosphanyl)ethane):