Synthesis of C-substituted t-BuNH-8,9-R,R′-nido-7,8,9-C3B8H9 (R,R′ = H,H; MeH; Me,Me; Ph,H and Ph,Ph) tricarbollide compounds and their tautomeric conversions. Effect of substituents on tautomeric equilibria between neutral and zwitterionic forms

Abstract

Treatment of C-substituted nido dicarbadecaboranes 5,6-R′,R-5,6-C₂B₈H₁₀ (1) (where R′,R = H,H (1a); H,Me, (1b); Me,Me, (1c); H,Ph, (1d) and Ph,Ph, (1e) with 1,8-bis-(dimethylamino)naphthalene (proton sponge = PS) and t-BuNC in CH₂Cl₂, followed by acidification, generated a series of pure neutral compounds 7-t-BuNH-8,9-R,R′-nido-7,8,9-C₃B₈H₉ (N2) (where R,R′ = H,H (N2a); H,Me (N2b); Me,Me (N2c); H,Ph (N2d), and Ph,Ph (N2e)), each of which exhibits tautomerism. Dissolution of the substituted compounds (N2b–N2e) in protic solvents (PRS), such as MeCN and Me₂CO, leads to tautomeric equilibrium with the zwitterionic tautomers 7-t-BuNH₂-8,9-R,R′-nido-7,8,9-C₃B₈H₈ (Z2) (where R,R′= H,H (Z2a); H,Me (Z2b); Me,Me (Z2c); H,Ph (Z2d) and Ph,Ph (Z2e)), while the unsubstituted compound N2a exhibits absolute tautomerism – a complete conversion into the zwitterionic tautomer Z2a. The tautomeric behaviour of individual compounds is therefore strongly affected by the nature of the substituent, as assessed via NMR spectroscopy in terms of tautomerisation constants K_T = C_Z2/C_N2 (where C_Z2 and C_N2 are equilibrium concentrations of Z2 and N2 forms in a given solvent). Individual tautomers were characterised by ¹¹B and ¹H NMR spectroscopy and the structure of the monomethylated N2b tautomer was determined by an X-ray diffraction study.