Heterolysis of Dihydrogen by Nucleophilic Calcium Alkyls

Abstract

Abstract β‐Diketiminato (BDI) calcium alkyl derivatives undergo hydrogenolysis with H 2 to regenerate [(BDI)CaH] 2 , allowing the catalytic hydrogenation of a wide range of 1‐alkenes and norbornene under very mild conditions (2 bar H 2 , 298 K). The reactions are deduced to take place with the retention of the dimeric structures of the calcium hydrido‐alkyl and alkyl intermediates via a well‐defined sequence of Ca−H/C=C insertion and Ca−C hydrogenation events. This latter deduction is strongly supported by DFT calculations (B3PW91) performed on the 1‐hexene/H 2 system, which also indicates that the hydrogenation transition states display features which discriminate them from a classical σ‐bond metathesis mechanism. In particular, NBO analysis identifies a strong second order interaction between the filled α‐methylene sp 3 orbital of the n‐hexyl chain and the σ* orbital of the H 2 molecule, signifying that the H−H bond is broken by what is effectively the nucleophilic displacement of hydride by the organic substituent.