5503-74-2Relevant articles and documents
On the directing effect of boronate groups in the lithiation of boronated thiophenes
Borowska, Elena,Durka, Krzysztof,Lulinski, Sergiusz,Serwatowski, Janusz,Wozniak, Krzysztof
experimental part, p. 2208 - 2218 (2012/06/01)
An investigation of thiophene boronates has revealed the usefulness of a metalation reaction in the synthesis of various lithiated thiophene boronates, which were further converted to functionalized thiopheneboronic derivatives. The lithiation of 2- and 3-thienylboronic N-butyldiethanolamine (BDEA) esters with lithium diisopropylamide and lithium 2,2,6,6-tetramethylpiperidide showed that both boronated thiophenes were readily deprotonated. In the latter case, lithiation at the 2-position adjacent both to sulfur and the borocanyl group is thermodynamically favoured due to the significant stabilizing effect of the borocanyl group. Further derivatization with a range of electrophiles followed by hydrolysis afforded various 2-substituted 3-thiopheneboronic acids. Lithiation of the corresponding thiopheneboronic "ate" complexes of the type [ThB(OR)3]Li revealed that the 2-thienyl derivatives could not be effectively deprotonated, whereas the "ate" complex, [3-ThB(OEt)3]Li, was selectively lithiated with nBuLi at C-2. This points to a directing effect of the anionic boronate moiety. The resulting bimetallic species, [(2-Li-3-Th)B(OEt)3]Li, underwent ring-closing dimerization upon heating to give, after subsequent hydrolysis, 4,8-dihydro-4,8-dihydroxy-p-diborino[2,3-b:5,6-b′]dithiophene - a cyclic diborinic acid. A computational study of the lithiation of boronated thiophenes and furans proved that boronation decreases ring-proton acidity. This effect is much stronger for the boronic "ate" complexes than for the corresponding neutral BDEA esters. Calculations of the transition states have shown that the specific directing effect of boronate groups in 3-thienyl derivatives is due to intramolecular oxygen-lithium coordination.
