Ancillary Ligand and Ketone Substituent Effects on the Rate of Ketone Insertion into Zr−C Bonds of Zirconocene−1-Aza-1,3-diene Complexes

Abstract

Zirconocene−1-aza-1,3-diene complexes, [Me2Si(C5H4)2]Zr[N(Ar)CHCHCH(Ph)] (Ar = Ph, 2a; Ar = p-MeOC6H4, 2b) and Cp2Zr[N(Ar)CHCHCH(Ph)] (Ar = Ph, 3a; Ar = p-MeOC6H4, 3b), have been synthesized and characterized by NMR spectroscopy. X-ray crystal structure determinations of compounds 2a and 3a,b reveal folded five-membered-ring moieties for the zirconacycles. DFT calculations and variable-temperature NMR experiments for complex 2a establish a rapid ring-flipping process at room temperature, with the conformation bearing a pseudoequatorial Ph group more stable by 5.6 kcal/mol. Kinetic studies on ketone insertion into these zirconocene complexes show second-order reactions, and the insertion is more favorable in the presence of a [Me2Si] ansa bridge, a less electron-rich substituent on the nitrogen, and a more basic ketone. One of the insertion products, namely Cp2Zr[N(Ph)CHCHCH(Ph)CPh2O] (5a), has also been characterized by X-ray crystallography.

Insertion of Nitriles into Zirconocene 1‐aza‐1,3‐diene Complexes: Chemoselective Synthesis of N–H and N‐Substituted Pyrroles

“…in 1991,2a are attractive synthetic precursors for selective transformations. Based on X‐ray crystal analysis, these complexes are described as σ 2 ,π‐azazirconacyclopentenes ( A ) with a folded five‐membered‐ring moiety rather than η 4 ‐azadiene complexes ( B ) 2b,i. In some cases, a rapid ring‐flipping process between the two conformers has been verified by variable‐temperature NMR experiments and DFT calculations (Figure 1).…”

“…In some cases, a rapid ring‐flipping process between the two conformers has been verified by variable‐temperature NMR experiments and DFT calculations (Figure 1). 2a,i Because of the existence of highly polarized ZrC and ZrN bonds, they can be viewed as N,C dianion or homoenolate equivalents,4 which engage in a series of addition reactions. In spite of the potential synthetic utility of these zirconacycles in the development of novel methodologies, it is somewhat surprising that unlike the zirconocene–butadiene complexes,5 so far only the reactions with tert ‐butylisocyanide,2a ketones,2b,d,i special imine,6 and PhBCl 2 2h have been achieved.…”

“…2a,i Because of the existence of highly polarized ZrC and ZrN bonds, they can be viewed as N,C dianion or homoenolate equivalents,4 which engage in a series of addition reactions. In spite of the potential synthetic utility of these zirconacycles in the development of novel methodologies, it is somewhat surprising that unlike the zirconocene–butadiene complexes,5 so far only the reactions with tert ‐butylisocyanide,2a ketones,2b,d,i special imine,6 and PhBCl 2 2h have been achieved. During our ongoing research on the chemistry of early‐transition‐metal complexes of conjugated organic substrates,7 we found that the cross‐coupling reactions of 1‐azabuta‐1,3‐dienes with nitriles could be efficiently promoted by low‐valent zirconocene species.…”

“…Zirconocene‐1‐aza‐1,3‐diene complexes 2 can be conveniently prepared by a ligand‐exchange reaction of azadienes 1 with zirconocene–butene complexes2a,i generated from the Negishi reagent 9. Thus, treatment of 1,4‐diphenyl‐substituted azadiene ( 1 a ) with the Negishi reagent [Cp 2 ZrBu 2 ] led to azazirconacyclopentene 2 a (R 1 =Ph), which was rapidly and cleanly coupled with three equivalents of PhCN at room temperature to give 2,3‐diphenyl‐1 H ‐pyrrole ( 3 a ) in 72 % yield after hydrolysis with 3 M HCl (Table 1, entry 1).…”

A Novel ZnII Complex Bearing Two Monodentate (4-Methoxyphenyl)[(1E, 2E)-3-phenylprop-2-en-1-ilidene] Schiff Bases: Crystal Structure and DFT Study

“…Recently, Kürşat and Yunus [13] used ab initio HF and DFT methods (B3LYP with 6-31G * basis set) and investigated the optimized geometrical structure, vibrational frequencies, and NMR shifts of (4-methoxyphenyl)[(1E, 2E)-3-phenylprop-2-en-1-ilidene], based on the crystal structure of the molecule, previously reported by Li et al [14]. e reactivity of the (4-methoxyphenyl)[(1E, 2E)-3phenylprop-2-en-1-ilidene] molecule to transition metals was also previously investigated by Knölker et al [15], Mashima et al [16], and Zhang et al [17], who obtained organometallic compounds of iron, titanium, and zirconium, respectively. However, to our knowledge, there is no report about coordination compounds of Zn II of that molecule.…”