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Bicyclo[4.2.0]octa-1,3,5-triene: 2-Mono- and 2,5-Disubstituted Derivatives via Highly Regioselective Lithiation of Its Cr(CO)3 Complex and via Reductive Silylation/Oxidation

Authors
Perret, Celia
Rudolph, Bruno
Published in Helvetica Chimica Acta. 1990, vol. 73, no. 7, p. 1970-1979
Abstract Treatment of [η6-(bicyclo[4.2.0]octa-1,3-5 triene)]tricarbonylchromium(0) (2) with BuLi or lithium 2,2,6,6-tetramethylpiperidinide (TMPLi) gives rise to a highly regioselective deprotonation at C(2). Subsequent reaction with electrophiles (6 examples) gives [η6-(2-R-bicyclo[4.2.0]octa-1,3,5-triene)]tricarbonylchromiurn complexes 3 and 5-9 in moderate (R==I, 50%; R==CHO, 67%) to good (R==Me, D, SiMe3, CO2Me, > 80%) yield (Scheme 1). Analogous reactions with tricarbonyl (η6-indane)chromium (10) give mixtures of complexes substituted at C(4) and C(5) (Scheme 2). In 10, deprotonation to the ring junction is strongly favoured with the bulky base TMPLi. Double lithiation/electrophile additions to 2 give access to [η6-(2-R-5-R-bicyclo[4.2.0]octa-1,3,5-triene)]tri-carbonylchromium complexes (e.g. 13 (R'==R"==Me3Si) and 14 (R'==Me3Si, R"==CHO)) as single products. The Cr(CO)3 group can be easily removed by oxidation (I2, Ce(IV), O2/light; 2 examples each) to give the free arenes. Base-catalyzed (CsF, DMF/D2O) deuterodesilylation of 13 yields the [(2,5-2H2)bicyclo[4.2.0]octa-1,3,5-triene]chromium complex 15, and treatment of 2,5-bis(trimethylsilyl) compound 16 with CF3COOD gives the 2,4-dideuterated 17. Compound 16 is also accessible more directly via reductive silylation/oxidation of bicyclo[4.2.0]octa-1,3,5-triene (1). Stereoselective base-catalyzed (t-BuOK.) H/D exchange of the benzylic H-atoms. opposite to the Cr(CO)3 moiety in 2 takes place rapidly in (D6)DMSO, but benzylic functionalization via this route remains elusive.
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KUNDIG, Ernst Peter, PERRET, Celia, RUDOLPH, Bruno. Bicyclo[4.2.0]octa-1,3,5-triene: 2-Mono- and 2,5-Disubstituted Derivatives via Highly Regioselective Lithiation of Its Cr(CO)3 Complex and via Reductive Silylation/Oxidation. In: Helvetica Chimica Acta, 1990, vol. 73, n° 7, p. 1970-1979. https://archive-ouverte.unige.ch/unige:6221

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