Heterolysis of Dihydrogen by Nucleophilic Calcium Alkyls

Abstract

b-Diketiminato (BDI) calcium alkyl derivatives undergo hydrogenolysis with H 2 to regenerate [(BDI)CaH] 2 , allowing the catalytic hydrogenation of aw ide range of 1alkenes 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 aw ell-defined sequence of CaÀH/ C = Ci nsertion and Ca À Ch ydrogenation events.T his 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 whichd iscriminate them from ac lassical s-bond metathesis mechanism. In particular,N BO analysis identifies as trong second order interaction between the filled amethylene sp 3 orbital of the n-hexyl chain and the s*o rbital of the H 2 molecule,signifying that the H À Hbond is broken by what is effectively the nucleophilic displacement of hydride by the organic substituent.Thecatalytichydrogenationofalkenesandalkynesbymidto late transition metal complexes has been developed to ahigh degree of sophistication and is typically predicated on the cooperative binding and activation of both unsaturated hydrocarbons and H 2 at ar edox-active metal center. [1][2][3][4][5][6][7][8][9] Albeit less well developed, similar catalysis was achieved by group 3and organolanthanide complexes more than aquarter of ac entury ago. [10][11][12][13][14] In these latter cases,ad 0 electron configuration mitigates against redox-based reactivity and turnover is derived from alkene insertion into ap olarised metal-hydrogen bond to form an alkyl intermediate,which is itself reactive toward H 2 through s-bond metathesis to release the hydrocarbon and regenerate the active hydride catalyst.