Sodium tetraarylborates

Not many catalyzed processes involving free radicals are known with these metals. Some vanadium-catalyzed pinacol coupling reactions were developed (reviews [129, 171], [172, 173] and cited ref, [174]). Niobium and tantalum complexes were applied in pinacol coupling reactions [130]. Vanadium(IV) [175-179] and vanadium(V) ([129], reviews [180-186]) complexes are known to catalyze asymmetric oxidative dimerizations of phenols and naphthols in moderate to excellent ees applying oxygen as the terminal oxidant. Biaryls are accessible by intramolecular coupling of sodium tetraarylborates, catalyzed by EtOVOCl2 in the presence of air [187]. 

This group also investigated the use of (/ ,/ )- ,2-bis(terf-butylsulfinyl)benzene (see Figure 5.13) [48]. Gratifyingly, on using sodium tetraarylborates instead of PhB(OH)2, a family offlavanone derivatives was obtained in moderate to good yields and in up to >99% ee (Scheme 5.13). 

Scheme 5.13 Rh-catalyzed asymmetric 1,4-addition of sodium tetraarylborates to chromenones [48].

For these rhodium-cattilyzed arylations, once again Hayashi s group [26] studied other azomethine imines as substrates for this reaction type. The application of sodium tetraarylborate as nucleophile and a chirtil diene ligand similar to the one used previously (see Scheme 6.20) resulted in the preparation of a family of chirtil l-(diarylmethyl)-pyrtizolidin-3-ones in excellent yields and enantioselec-tivities (Scheme 6.21). 

Rhodium catalysts could also promote the addition of arylborates and arylboronic acids to and CTO bonds. The Rh-catalyzed addition-cyclization of sodium tetraarylborates 80 to 3-(2-alkynylethyl)-2-cycloalk en-l-ones 79, furnishing spiro-carbocycles 81 was reported by Shintani et al. (Scheme 5.53) [51]. These tetraarylborates function as surrogates of 1,2-dimetalloarenes, forming two new C—C bonds sequentially through the cascade process. 

SCHEME 5.53 Rh-catalyzed addition-cyclization of sodium tetraarylborates 80 3-(2-alkynylethyl)-2-cycloalk en-l-ones 79. 

Aryl and heteroaryl (furyl, thienyl) boronic acids are especially suitable for the preparation of their iodonium salts, having the added advantage of better yields and lack of toxicity [108]. Tetraarylborates (sodium or potassium) reacted with (diacetoxyiodo)arenes in acetic acid to afford diaryliodonium salts in excellent yield (Scheme 37). It appears that triarylboranes formed upon reaction of the borates with acetic acid serve actually as the real arylating agents [109]. 

Tetraarylborate salts and readily available sodium tetraphenylborate can be used in aoss-coupling with aromatic halides. AU four aryl groups are transferred if the reaction is carried... 

Tetraarylborate salts, and in the first place readily available sodium tetraphenylborate, can be used in cross-coupling with aromatic halides. All four aryl groups are transferred if the reaction is carried out in aqueous media in the presence of inorganic bases, in sharp contrast with the process in anhydrous systems, in which it is usually possible to use only one aryl group. 

Barlow et al. reported the synthesis of cationic (T) -cyclopentadienyl)(r -fulvene)ruthenium(ll) salts from ruthenocenylmethanol (57) either by treatment with trimethylsilyl triflate to achieve 58 in 79% yield. Alternatively, the reaction of 57 with tetrafluoroboric acid or with hexafluorophosphoric acid gave the tetrafluoroborate or hexafluorophosphate salts in 59 and 60 in 92% and 52% yield, respectively. Treatment of 59 with sodium tetrakis[3,5-bis(trifluoromethyl)]borate afforded the tetraarylborate salt 61 in 69% yield. 

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