3,5-Trifluoromethyl benzeneboronate

Bronlc acids containing electron-capturing subsitituents were developed by Poole and co-workers. Table 8.19 (451,535,536). In terms of volatility, stability of derivatives, and response to the electron-capture detector the 3,5-bis(trifluoromethyl)benzeneboronic acid, 2,4-dichlorobenzeneboronic acid, and 4-bromo-benzeneboronic acid were recommended for general applications. In particular, the 3,5-bis(trifluoromethyl)benzeneboronate derivatives are remarkably volatile, more so than the benzeneboronates, and are suitable for the analysis of bifunctional compounds of low volatility. All the benzeneboronate derivatives are susceptible to solvolysis which is the primary limitation to their general use for trace analysis. 

Enantioselective D-A reactions of acrolein are also catalyzed by 3-(2-hydroxyphenyl) derivatives of BINOL in the presence of an aromatic boronic acid. The optimum boronic acid is 3,5-di-(trifluoromethyl)benzeneboronic acid, with which more than 95% e.e. can be achieved. The TS is believed to involve Lewis acid complexation of the boronic acid at the carbonyl oxygen and hydrogen bonding with the hydroxy substituent. In this TS tt-tt interactions between the dienophile and the hydroxybiphenyl substituent can also help to align the dienophile.114... 

Acyl aryl boron species also react with amines to yield amides with mixed results. Eor example, catecholborane was used to generate lactams successfully (46). Aryl boronic acids with electron-withdrawing groups, such as 3,4,5-trifluorobenzene-boronic acid and 3,5-bis-(trifluoromethyl)benzeneboronic acid, can act efficiently as an amidation catalyst when added to a mixture of acid and amine (47). 

Recently, catalytic asymmetric Diels-Alder reactions have been investigated. Yamamoto reported a Brdnsted-acid-assisted chiral (BLA) Lewis acid, prepared from (R)-3-(2-hydroxy-3-phenylphenyl)-2,2 -dihydroxy-1,1 -binaphthyl and 3,5-Z7ri(trifluoromethyl)-benzeneboronic acid, that is effective in catalyzing the enantioselective Diels-Alder reaction between a,P-enals and various dienes. The interesting aspect is the role of water, THE, and MS 4A in the preparation of the catalyst (Eq. 12.19). To prevent the trimerization of the boronic acid during the preparation of the catalyst, the chiral triol and the boronic acid were mixed under aqueous conditions and then dried. Using the catalyst prepared in this manner, a 99% ee was obtained in the Diels-Alder reaction... 

The kinetics of protodeboronation of a range of substituted benzeneboronic acids in aqueous sulphuric acid mixtures were also examined. Good first-order kinetics were obtained in all cases except for the 3-fluoro compound (due to the sulphonation side reaction) and the 3-trifluoromethyl compound, which hydrolysed, hydrogen fluoride being produced the rates with this latter compound were only followed to 30 % reaction. The kinetic details are summarised in Table 190, from... 

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