Cleavage of ethers, acetals, and esters

The initial products of ether cleavage are the alkyl halide and a borate ester, (R0)3B. The borate esters are usually inert to further displacement but, because the iodide is more nucleophilic than the other halides, warming the borate esters (60-80 °C) in the presence of BI3, will result in the complete conversion of all the alkyl residues to iodides (Eq. 6) [12]. 

BBr3 also coordinates to ethereal oxygens and promotes C-O bond cleavage to an alkyl bromide and an alkoxyborane that is hydrolyzed to an alcohol during work-up (Eq. 7) [13]. 

BBr3 has been widely used to cleave ethers because the reaction proceeds completely under mild conditions. In a special case, BBra has been used to cleave acetals that cannot be deprotected by usual acidic conditions [14]. Because alkyl aryl ethers are cleaved at the alkyl-oxygen bond to give ArOH and alkyl bromides, BBr3 has been most generally used for the demethylation of methyl aryl ethers [13,15]. The presence of a carbonyl substituent facilitates selective deprotection of polymeth-oxyaryl compounds (Eq. 8) [16]. 

The cleavage of mixed dialkyl ethers occurs at the more substituted carbon-oxygen bond. Methyl ethers of secondary or tertiary alcohols give methanol and secondary or tertiary alkyl bromides selectively by reacting with BBra [17], although the addition of Nal and 15-crown-5 ether can change this selectivity (Eq. 9) [18]. In contrast, methyl ethers of primary alcohols are generally cleaved at the Me-O bond [19]. 

Kabalka and co-workers have shown that an attenuated form of BI3, BI3 NEt2Ph, will cleave a variety of compounds containing C-O single bonds at elevated temperatures [20]. Solutions of this reagent are prepared by reacting the commercially available amine-borane complex with I2 in benzene at 80 °C for several hours. This reagent cleaves ethers [21], esters [20], and geminal diacetates [21]. Esters [20] are cleaved to an activated acyl intermediate RCOX which can be used to prepare acids, other esters, and amides (Eq. 10). 

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