Hydrogen Isotope Exchange with Superbulky Alkaline Earth Metal Amide Catalysts

Heavier alkaline earth (Ae) metal amide complexes Ae­(NR2)2 (Ae = Ca, Sr, Ba) were found to be highly active catalysts for hydrogen isotope exchange (HIE). The activities for D/H exchange between C6D6 and H2 strongly increase with metal size (Ca < Sr < Ba) and with amide bulk: N­(SiMe3)2 < N­(DIPP)­(SiiPr3)< N­(SiiPr3)2, DIPP = 2,6-diisopropylphenyl. At 120 °C and pressures of 10–50 bar, no hydrogenation side-products are produced, and TONs of 205 and TOFs of 268, competitive with those for precious metal catalysts, have been achieved. The reverse H/D exchange between C6H6 and D2 is even faster by a factor 1.5–2. Substrates also include a range of substituted arenes. Alkyl-substituted aromatic rings are preferably deuterated in acidic benzylic positions, and this tendency increases with the number of alkyl-substituents. Although unactivated (sp3)­C–H units could not be deuterated, the (sp3)­Si–H function in primary, secondary, and tertiary alkylsilanes could be converted. Two different pathways for C6H6/D2 isotope exchange have been evaluated by DFT calculations: (A) a deprotonation/protonation mechanism and (B) direct nucleophilic aromatic substitution. Although the exact nature of the catalyst(s) is unclear, the first step is the conversion of Ae­(NR2)2 with D2 into R2NAeD which can aggregate to larger clusters. Energy profiles with model catalysts (iPr3Si)2NAeD and [(Me3Si)2NAeD]2 (Ae = Ca or Ba) show that the direct nucleophilic aromatic substitution is the most likely mechanism for deuteration of arenes. The key to this unusual reaction is the initial formation of a π-arene···Ae complex which is followed by the generation of an intermediate with a Meisenheimer anion. Heavier Ae metal amide complexes are, despite the lack of partially filled d-orbitals for substrate activation, potent catalysts for HIE.