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

Treatment of [η 6 ‐(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 [η 6 ‐(2‐R‐bicyclo[4.2.0]octa‐1,3,5‐triene)]tricarbonylchromiurn complexes 3 and 5 – 9 in moderate (RI, 50%; RCHO, 67%) to good (RMe, D, SiMe 3 , CO 2 Me, > 80%) yield ( Scheme 1 ). Analogous reactions with tricarbonyl (η 6 ‐indane)chromium ( 10 ) give mixtures of complexes substituted at C(4) and C(5) ( Scheme 2 ). In 10 , deprotonation β to the ring junction is strongly favoured with the bulky base TMPLi. Double lithiation/electrophile additions to 2 give access to [η 6 ‐(2‐R′‐5‐R″‐bicyclo[4.2.0]octa‐1,3,5‐triene)]tri‐carbonylchromium complexes (e.g. 13 (R′R″Me 3 Si) and 14 (R′Me 3 Si, R″CHO)) as single products. The Cr(CO) 3 group can be easily removed by oxidation (I 2 , Ce(IV), O 2 /light; 2 examples each) to give the free arenes. Base‐catalyzed (C s F, DMF/D 2 O) deuterodesilylation of 13 yields the [(2,5‐ 2 H 2 )bicyclo[4.2.0]octa‐1,3,5‐triene]chromium complex 15 , and treatment of 2,5‐bis(trimethylsilyl) compound 16 with CF 3 COOD 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 (D 6 )DMSO, but benzylic functionalization via this route remains elusive.