Synthesis of a Versatile Tridentate Anthracene Ligand and its Application for the Synthesis of Hypervalent Pentacoordinate Boron Compounds (10-B-5)

✍ Scribed by Makoto Yamashita; Yohsuke Yamamoto; Kin-ya Akiba; Shigeru Nagase

• Publisher: John Wiley and Sons
• Year: 2000
• Tongue: English
• Weight: 116 KB
• Volume: 39
• Category: Article
• ISSN: 0044-8249
• DOI: 10.1002/1521-3773(20001117)39:22<4055::aid-anie4055>3.0.co;2-n

✦ Synopsis

Hypervalent pentacoordinate boron compounds (10-B-5) [1] 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 [3] 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 ± 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 1). 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-