Tetraarylborates

Complex C (Scheme 21) seems to shows stable penta-coordination for apparently very different reasons. The compound is cationic, but the counterion is a non-coordinating tetraarylborate. Interestingly, neither the dichloromethane solvent nor the diethylether present in the reaction mixture seem to coordinate to the open site in solution. The compound was crystallized from a tetrahydrofuran/pentane mixture as the tetrahy-drofuran (THF) solvate, but in the crystal structure, the THF is remote from the open site at platinum. The open site is shielded somewhat by the methyl groups of the protonated TpMe2 ligand, but it does not appear completely inaccessible. A reasonable explanation for the... 

The carbonylative cross-coupling reactions of haloazines are usually run under an ambient to moderate carbon monoxide pressure. Arylboronic acids or tetraarylborates are usually the reagent of choice due to their robustness and availability. The coupling of 2-iodopyridine and phenylboronic acid under ambient CO pressure, for example, led to the formation of 2-benzoylpyridine in good yield (7.66.),87... 

Photochemical ring closures to give system (114) have been performed, whereby the photochemical step gives rise to a new carbon-carbon bond. Apparently, (114 R = Ph) is photochemically stable similar to arylboranes but in contrast to tetraarylborates (69JOC1675). 

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]. 

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]. 

Treatment of the cyclotrigermene 87a with trityl tetraarylborates yields salts containing the corresponding cyclotrigermenium cation Its strnctural parameters are... 

Cobaltacarboranes also have been used in sensor technology. The sandwich complex [Co(B9C2Hu)2] can be used as a hpophilic coreceptor instead of tetraarylborate ion additives in ion selective electrodes, producing electrodes that are more selective for specific alkah metal ions. ... 

A related triarylboron-carbene adduct (64) was isolated from the attempted recrystallization of the 1-butyl-3-methylimidazolium tetraarylborate salt (63) in warm MeOH/H20 (equation 7)." The solid-state structure of (64) comprises a distorted tetrahedral boron center with all four B-C bond distances similar, as observed in compound (62). ... 

The development of cationic catalysts necessitated the development of unreactive counteranions, which typically had been PFg and BF4. Unfortunately, a F can be transferred from either of these counteranions to a highly reactive cationic catalyst [102]. An initial response was to use the tetraphenylborate as a counterior, which eliminated the nucleophilic F" problem, but had the problem of electrophilic addition to the aromatic ring [103]. A solution to these problems is fluorinated tetraarylborates that prevent nucleophilic attack of the F anion as well as electrophilic addition to the aromatic ring [104-107]. Structures 1 and 2 [105], are some examples of these counteranions. 

Cationic oxazaborinane 59 (Figure 3.10) is a chiral super-Lewis-acidic catalyst recently described by Corey and coworkers [61]. The catalyst is in equilibrium with 59a and the oxazaborinane system 59 5 59a is unstable and undergoes gradual decomposition at temperatures above -60 °C. A more active catalyst system is the tetraarylborate salt 59+B[C6H3 - 3,5 - (CF3)2]4 which allows the cycloadditions of cyclopentadiene (18) and a./i-unsaturated aldehydes to occur with a high level of stereoselectivity and enantioselectivity (Equations 3.16 and 3.17). 

Of related interest to the foregoing photomigrations is the production of biaryls by the oxidation of tetraarylborate salts [Eq. (30)]. For this purpose, electrooxidation (54), halogen 81), ceric (54), ferric 83),... 

Apart from arylboronic acids, tetraarylborates, under photochemical conditions [24] or in the presence of a stoichiometric amount of diphenylsilicon dichloride, under an oxygen atmosphere, give biaryls in good to high yields [25], Namely, tetramethyl ammonium tetra(4-tolyl)borate (441) was coupled to 4,4 -dimethylbiphenyl (442) in 96% yield, respectively, Scheme 10. 

Recent innovations have involved the use of trifluoroborate [29], tetraarylborate [30], and trihydroxyborate [31] salts, some originating from DoM reactions, as coupling partners in the Suzuki-Miyaura cross-couplings. 

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... 

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