Aryl borates, alkylation with

Procedures for shipping boric acid esters depend on the particular compound. Aryl borates produce phenols when in contact with water and are therefore subject to shipping regulations governing such materials and must carry a Corrosive Chemical label. Lower alkyl borates are flammable, flash points of methyl, ethyl, and butyl borates are 0, 32, and 94°C, respectively, and must be stored in approved areas. Other compounds are not hazardous, and may be shipped or stored in any convenient manner. Because borate esters are susceptible to hydrolysis, the more sensitive compounds should be stored and transferred in an inert atmosphere, such as nitrogen. 

A regioselective and highly syn-stereoselective catalyst-free intermolecular alkylation of aryl borates with aryl epoxides under mild, neutral conditions has been reported.27 The reaction of /ra .s-stilbene oxide with tri(3,5-dimethylphenyl)borate gave a 38% yield (>95% syn) of the C-alkylated product (13), easily separated from (g) the O-alkylated product(s). Triflic anhydride has been used to activate enones to nucleophilic attack by electron-rich arenes in the presence of a sterically hindered base.28 Resorcinol dimethyl ether, for example, reacted with cyclohex-2-en-l-one to... 

Aryl borates can provide both C-alkylation and O-alkylation products upon reaction with epoxides <07JOC7761>. The reaction shown below exemplifies a general reaction of arylborates with vinyl epoxides <07SL3011>. A number of examples were reported, the yield shown is typical. The ratio of SN2 to SN2 is generally >95 5 and the ratio of C-alkylation O-alkylation is also excellent (usually >95 5). 

The oxidation of carbon-boron bond converts boranes into alkyl or aryl borates, which may be hydrolyzed subsequently to alcohols and boric acid [991], The oxidation is carried out with hydrogen peroxide [183,1201, 1202] or trimethyiamine oxide [991, 992]. Phenylboronic acids are oxidized to phenols biochemically [1034]. 

This last example succeeds partly because nickel catalyses the union of the three reagents in the second step. Similar things happen in the combination of nickel allyls with zinc borates 76. The allyls are derived from unsymmetrical allylic acetates 75 and react at the end remote from the phenyl group (R1 is an alkyl group and R2 is an aryl or alkyl group).19... 

Anodic stability limits, for example, of vinyl compounds, are linearly correlated with a slope of — 1 eV-V [269] to their Uhomo-values. However, for alkyl and aryl borates a much larger value of—3 eV-V is obtained [252], in accordance with our values for the chelatoborates. Ii[B(C6H4 xF x02)2]. x = 0 (1), x = 1 (2), x = 4 (3) and Hthium bis[2,3-naphthalenediolato(2—)-0,0 ]borate, Li[B(02CioH6)2] (4) [211] of Figure 17.6 show that the underlying oxidation mechanisms are closely related. 

All examples mentioned so far correspond to reactions between two aromatic groups, however, couplings in which one or both partners are alkyl groups can be achieved using electron-rich boron-based nucleophiles. Fiirstner has reported the use of B-alkyl or 5-allyl methoxy-9-BBN anions for the efficient coupling with some aryl chlorides using an in situ prepared IPr HCl/Pd(OAc)j system [118], Some of the results obtained with these easy-to-handle borate-based nucleophiles are shown below (Scheme 6.34). 

Quaternary salts of 4,4 -bipyridine other than 1-alkyl or 1-aryl quaternary salts have also been synthesized. Thus l,l -dialkoxy diquaternary salts are formed by reaction of 4,4 -bipyridine l,l -dioxide (see next section) with dialkyl sulfates," and 1-alkyl-I -alkoxy diquaternary salts are obtained similarly. l,r-Bis(2,4-dinitrophenoxy)-4,4 -bipyridinium bisffluoro-borate) has been prepared from 4,4 -bipyridine l,l -dioxide by reaction with 2,4-dinitrobenzenediazonium fluoroborate in sulfolane. Interestingly,... 

The borate intermediate was also coupled with aryl halides. When a ynyl group is present, coupling is followed by a tandem cyclization which leads to 2-[ t-(cycloalkylidene)alkyl]indoles. <95CC409>... 

Some other important aspects of boric acid chemistry are summarized in Fig. 5-26. Among these is the formation of borate esters [B(OR)3, R = alkyl or aryl], usually obtained as colorless liquids, on treatment with alcohols and H2S04. The vast literature on these compounds falls generally within the purview of organic chemistry, and will not be developed here however, it will be noted that a well-known qualitative test for boron involves treatment of the sample with methanol to form B(OMe)3, which produces a bright green color in a Bunsen burner flame. A major early discovery in this area was the synthesis of boronic acids by E. Frankland in 1862, via partial oxidation of trialkylboranes, with subsequent hydrolysis of the ester ... 

Most nonmetal borates reported in the literature are ammonium and substituted ammonium salts, hi general, boric acid forms salts with alkyl- or aryl-amines that are roughly as or more basic than pyridine. Depending on the reaction conditions, a number of different crystalline borates might be isolated from a given amine. However, the chemistry of substituted amine borates is dominated by the occurrence of pentaborates, and relatively few compounds in this class have been reported containing more basic borates anions. 

The system consisting of gaseous BFs/EtsSiH has been reported "- to reduce aldehydes and ketones thus benzaldehyde was reduced to toluene in 52% yield. In alkyl carbonyl compounds, aldehydes afford the corresponding alcohol products, whereas ketones, such as undecan-2-one and cyclohexanone, are readily converted to the hydrocarbons in high yields. On reduction with limited amounts of both reagents these alkyl ketones were reduced to the alcohols, indicating the intermediacy of the corresponding borate ester. In the case of aromatic compounds, aryl aldehydes and diaryl and alkyl aryl ketones are reduced to... 

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