Synthesis and reactivity of subvalent compounds

Abstract

Abstract The reactivities of the two stable diamino carbenes 1,3-di-tert-butylimidazol-2-ylidene (1) and 1,3-di-tert-butylimidazolin-2-ylidene (2) toward hydrogen, oxygen, water and carbon monoxide were investigated. The carbenes do not react with O2 or CO but are attacked by water to give the respective hydrolysis products tBu–NCHCH2–N(CHO)tBu (7) and tBu–NH–CH2CH2–N(CHO)tBu (8). While 2 is hydrolyzed instantaneously, the aromatically stabilized 1 reacts only very slowly and can be handled in air for brief periods of time. The carbenes 1 and 2 are unreactive towards H2 alone but can be catalytically hydrogenated to the respective aminals 2,3-dihydro-1,3-di-tert-butylimidazole (5) and 1,3-di-tert-butylimidazolidine (6). The reaction products were characterized by single crystal X-ray crystallography, multinuclear NMR spectroscopy (1H, 13C, 17O), IR spectroscopy and DFT methods. The extent of aromatic delocalization in 1 was investigated through isodesmic hydrogenation reactions at the B3LYP/6-31G(d)//B3LYP/6-31G(d) level. At this level of theory, the ‘carbene oxide’ (2,3-dihydro-imidazol-2-one) retains ca. 50% of the aromatic delocalization energy of the carbene. The oxidation of the diamino carbenes to the ‘carbene oxides’ (ureas) is calculated to be exothermic by −79.1 kcal mol−1 (1) and −86.4 kcal mol−1 (2). The oxygen affinity of the carbenes resembles that of trimethylphosphine (−79.7 kcal mol−1) and triethyl phosphite (−88.6 kcal mol−1) and is significantly higher than that of CO (−67.6 kcal mol−1). The aminals 5 and 6 are structurally related to Thauer's hydrogenase (TH2/TH+) but do not show hydrogenase activity.