Boronic acid triisopropyl ester

Br-C6H2(OR)2-Br-p is converted to 4-bromo-2,5-dialkoxyphenylboronic acid via reactions with BuLi and boronic acid triisopropyl ester. Thus, Monomer-1 (Br-C6H2(OBu)2-B(OH)2 (Bn = butyl)), shown in Chart 4.3 is obtained in 73% yield. 

Synonyms Boric acid, triisopropyl ester Boric acid, tris (1-methylethyl) ester Boron isopropoxide Boron triisopropoxide Isopropyl borate... 

However, the application of these classical procedures for 1-alkenylboronic acid or ester synthesis may suffer from the formation of small amounts of the opposite stereoisomers, or from bis-alkenylation leading to the boronic acid derivatives. Also, formation of trialkylboranes may occur. A recent useful variant utilizes organolithium reagents and triisopropyl borate, followed by acidification with HCl to give directly alkyl-, aryl-, 1-alkynyl-, and 1-alkenylboronic esters in high yields, often over 90% (Scheme 2-6) [27]. Triisopropyl borate was shown to be the best of the available alkyl borates to avoid multiple alkylation of the borates. 

A variety of alkyl- and arylboronic acids can be obtained in large quantity by the above general procedure. The reaction of organolithiums with triisopropyl borates at -78 °C is an alternative and convenient method to achieve high yields of boronic acids or their esters [9, 10] (eq (8)). 

Simple (halomediyl)boronic esters are not accessible by methods used for higher homologs. The discovery that (chloromethyl)lithium can be generated at -78 C from chloroiodomethane by treatment widi butyllithium in the presence of triisopropyl borate and captured immediately to form the (chloromethyl)borate salt and, after acidification, the boronic ester has provided practical access to these useful reagents [33]. The less expensive reagent dibromomethane under similar conditions generates (bromomethyl)lithium and yields (bromomethyl)(triisopropyl)borate anion (42), which is converted into diisopropyl (bromomethyl)boronate (43) by treatment with anhydrous hydrogen chloride or, more conveniently, methanesulfonic acid while the reaction mixture is still cold (Scheme 8.8) [34]. 

In the aromatic and heterocyclic series, no hydroboration is possible. In general, an organoalkali intermediate is prepared by metalation or halogen/metal permutation before being treated with a boric acid derivative such as trimethyl borate or, because of its cleaner reaction, triisopropyl borate. The resulting ate complex sets free the oxidizable boronate upon addition of water or a stoichiometric amount of diluted hydrochloric acid. The boronate can be accessed directly when fluorodimethoxyborane is used instead of a boric acid ester as the adduct eliminates lithium fluoride spontaneously. The oxidation relies on the same procedures and principles as outlined above. 

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