A ferrocenium promoted method for the chemoselective oxidation of benzylic amines to imines has been developed with yields varying from 53% to 93%. Use of 5 mol % ferrocenium hexafluorophosphate as a promoter was sufficient for this radical based oxidation reaction. Air was bubbled into the reaction mixture, which worked as the oxidant for this transformation. Most importantly, the reaction was carried out in pure aqueous medium at 65 °C. A 10 gram scale reaction with benzylamine was successfully carried out to show the practical applicability of this methodology.
Oxidative coupling of primary amines to imines has been achieved by using a water soluble cobalt complex as catalyst and air as the oxidant at near ambient conditions. Aromatic, heteroaromatic and aliphatic amines were successfully converted to the corresponding imines with yields of up to 96 %. A 20 gram scale reaction for the synthesis of imine from benzylamine in good yield is also demonstrated with this method. The catalyst has been found to be reusable with up to five cycles. It is highly efficient, gives a turnover number (TON) of up to 128, and shows chemoselectivity with the only byproducts being water and ammonia. Control experiments and mechanistic studies indicate that the Co /Co catalytic cycle is responsible for these oxidative transformations. Some of the reactive intermediates of this reaction have also been isolated and structurally characterized.
Reactions of {η 5 -C 5 H 4 [C(O)Cl]}Co(η 4 -C 4 Ph 4 ) and {η 5 -C 5 H 4 [C(O)Cl]}Fe(η 5 -Cp) with 8-aminoquinoline resulted in cobalt and iron sandwich derived carboxamides.The reaction of these carboxamides with Pd(OAc) 2 in acetonitrile resulted in αC−H activation of the Cp rings of the sandwich compounds and formation of novel palladacycles 3 and 4, having both N−H and one α-C−H hydrogen atom of the Cp ring displaced and palladium forming a square planar complex with acetonitrile as the fourth ligand. These air-stable palladacycles reacted with MeI and EtI in acetic acid, resulting in monoand 2,5-di-α-alkylated sandwich carboxamides, thereby providing a new method to realize Cp-multisubstituted sandwich compounds. Selectivity in α-substitution was observed in the presence of NaHCO 3 . The cobalt sandwich carboxamide 1, the new palladacycles 3 and 4, and the 2,5-dimethylated cobalt sandwich carboxamide 5 have also been structurally characterized using single-crystal X-ray structural studies.
Palladium
catalyzed bis-arylations, -alkylations, and -allylations on the Cp
ring of iron and cobalt sandwich compounds have been achieved using
the bidentate picolinamide directing group. This directing group along
with catalytic Pd(OAc)2 was found to be highly efficient
for C–H functionalization, giving up to 87% yields. The palladacyclic
intermediate for the C–H activation of the Cp ring has been
isolated and structurally characterized for the cobalt sandwich compound
[η5-C5H5]Co(η4-C4Ph4). Attempted C–H annulation reactions
using picolinamide-derived sandwich compounds did not yield the expected
annulated products and instead oxidized the Cp- and picolinamide-bound
CH2 unit to aldehydes. Detailed studies on this novel and
unprecedented oxidation indicated that this happens only with the
assistance of the picolinamide directing group. We have also shown
that the sp2 C–H functionalization and the sp3 C–H oxidation can be effectively carried out as a
one-pot reaction.
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