Regioselective Ortho Palladation of Stabilized Iminophosphoranes in Exo Positions: Scope, Limitations, and Mechanistic Insights

Abstract

The reactivity of the stabilized iminophosphoranes R 3 PdNC(O)Ph (R ) p-tolyl, m-tolyl), Ph 3 PdNC(O)CH 2 Ph, and Ph 2 P(CH 2 ) n PPh 2 )NC(O)Ph (n ) 1, 2) toward Pd(OAc) 2 has been examined. These substrates undergo palladation at the ortho C(sp 2 )-H bond of the benzamide ring, giving exo complexes with complete regioselectivity. The mechanism of this reaction has been studied using DFT calculations and the regioselectivity explained on a kinetic basis, since the activation barrier is lower for the exo pathway than for the endo. The origin of this lower energy for the exo barrier seems to reside mainly on the interaction energy between the metal center and the ligand at the transition state. On the other hand, the palladation of the related keto-stabilized systems Ph 3 PdNC(O)FG (FG ) NC 4 H 8 , NC 4 H 8 O) gives selectively the endo isomers through CsH bond activation at the Ph ring of the PPh 3 unit.

Organometallic Palladium Complexes with a Water-Soluble Iminophosphorane Ligand As Potential Anticancer Agents

“…The main distances Pd-N(1) and Pd-C(18) of 2.037(4) and 1.966(4) Å found for 3 are a little shorter or similar than those found in related carbonyl-stabilized iminophosphorane complexes where the cyclometalation occurs in the benzyl ring like [Pd{C 6 H 4 (C(O)N=P(p-tol) 3 -kC,N)-2}{acac-O,O’)] (2.0695(16) and 1.961(2) Å), 38 [Pd(μ-Cl){C 6 H 4 (C(O)N=P(m-tol) 3 -kC,N)-2}] 2 (2.085(2) and 1.979(2) Å) 38 or in {Pd(μ-Cl){C 6 H 3 (Me-3)(C(O)N=PPh 3 -2}] 2 (2.043(2) and 1.962(3) Å). 39 The Pd1-O1 (2.023(3) Å) and Pd1-O2 (2.094(3) Å) bond distances fall in the usual range of distances found for chelating acac ligands.…”

“…39 The Pd1-O1 (2.023(3) Å) and Pd1-O2 (2.094(3) Å) bond distances fall in the usual range of distances found for chelating acac ligands. 38 The P(1)-N(1) bond length in 3 of 1.643(4) Å is very similar to that in [Pd{C 6 H 4 (C(O)N=P(p-tol) 3 -kC,N)-2}{acac-O,O’)] (1.6457(18) Å), 38 [Pd(μ-Cl){C 6 H 4 (C(O)N=P(m-tol) 3 -kC,N)-2}] 2 (range 1.644-1.610 Å) 28l (1.644(2) Å), 38 [Pd(C 6 H 4 -2-PPh 2 =N-C(O)-2NC 5 H 4 -k-C,N,N)Cl] (1.649(2) Å) 37 or in [PdCl 2 (TPA=N-C(O)-2-NC 5 H 4 )] (1.658(2) Å) 34 but considerably longer than in other semi-stabilized cyclopalladated complexes. 37 This elongation of the P-N bond is due not only to the coordination of the N atom to the palladium center but to the delocalization of charge density through the P-N-C-O system due to the presence of a CO group in the IM ligand.…”

Computational study of the cyclopalladation mechanism of azobenzene with PdCl2 in N,N-dimethylformamide

“…They also compared several possible reacting species with the findings that correlate well with those presented here e e.g. the highest barrier was associated with the agostic complex related to 4-R. Related and concordant studies were carried out by Fagnou et al to explain relative reactivities of fluorobenzenes in a direct intermolecular arylation catalyzed by Pd acetate [32] and by Lledós and Urriolabeitia to understand regioselectivity in cyclopalladation of iminophosphoranes with Pd acetate [37].…”

Dehydrogenative (Hetero)arene Alkoxylations Triggered by PdII‐Catalyzed C(sp2)–H Activation and Coordinating Substituent: PdII,III or PdIV Complex as Key Intermediate?

“…Although the ortho ‐palladation of benzamides N ‐protected with an iminophosphorane functionality afforded dimeric palladium chelates [Equation ], catalyzed methoxylation of such starting materials with use of the Pd(OAc) 2 /PhI(OAc) 2 system did not take place, whatever the temperature . Switching to Oxone as oxidant, however, led to reaction, especially with PdCl 2 (MeCN) 2 as catalyst and at room temperature [Equation ].…”