Picolinamide as a Directing Group on Metal Sandwich Compounds: sp² C–H Bond Activation and sp³ C–H Bond Oxidation

Abstract

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.

Aerobic Oxidation of Primary Amines to Imines in Water using a Cobalt Complex as Recyclable Catalyst under Mild Conditions

“…Recent report from our group has shown the utility of picolinamide as a useful functionality for sp 3 C−H bond oxidation of organometallic compounds . Complexes of early transition metals with the bispicolinamide ligand [ N , N ′‐bis(2′‐pyridinecarboxamide)‐1,2‐benzene] (bpb) were first reported by Vagg and co‐workers in 1979 and many of its complexes have been structurally characterized as well .…”

“…On the basis of above experiments and supported by previous literature, [7][8][9][10] ap lausible reaction mechanism for the oxidative coupling of benzylamine with the catalysthas been proposed as shown in Scheme5. Compound A,[ Co II (bpb)] reacts with oxygen to generate the active intermediate B,w hich coordinates with benzylamine to form the intermediate D. [7,9] Intermediate D can also be generated via intermediate C'.I ti ss ubsequently converted to a Schiff base intermediate E and H 2 O. [7a, 9] We would also like to mention that formation of as uperoxo intermediate that abstractsahydrogen atom from the amine to form an imine could not be rule out.…”

“…The oxidation of amines to imines have been reported to proceed through a radical mechanism . Therefore, to find out the possibility of a radical pathway, we carried out the reactions with 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO) free radical and butylated hydroxytoluene (BHT; Scheme , entry 5).…”

“…[10] Complexes of early transition metalsw ith the bispicolinamide ligand [N,N'-bis(2'-pyridinecarboxamide)-1,2-benzene] (bpb) were first reported by Vagg andc o-workersi n1 979 and many of its complexes have been structurally characterized as well. [10] Complexes of early transition metalsw ith the bispicolinamide ligand [N,N'-bis(2'-pyridinecarboxamide)-1,2-benzene] (bpb) were first reported by Vagg andc o-workersi n1 979 and many of its complexes have been structurally characterized as well.…”

“…[7][8][9][10] Therefore, to find out the possibility of ar adicalp athway,w ec arriedo ut the reactions with 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) free radicala nd butylated hydroxytoluene (BHT;S cheme 4, entry 5). [7][8][9][10] Therefore, to find out the possibility of ar adicalp athway,w ec arriedo ut the reactions with 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) free radicala nd butylated hydroxytoluene (BHT;S cheme 4, entry 5).…”