Carbonylative Coupling of 4 , 4 ’-Diiodobiphenyl Catalyzed by Pd ( NHC ) Complex

To develop a luminescent material with high color purity, luminous efficiency, and stability, we synthesized diketone by carbonylative Suzuki coupling in the presence of Pd(NHC) complex as the catalyst. Carbonylative coupling of 4,4’-diiodobiphenyl and phenylboronic acid was investigated to study in detail the catalytic ability of the Pd(NHC) complex. Reactions were carried out using both CO and metal carbonyls. Bis-(1,3-dihydro-1,3-dimethyl-2H-imidazol2-ylidene) diiodo palladium was used as the catalytic complex. Reaction products biphenyl-4,4’-diylbis (phenylmethanone) 3 and (4’-iodobiphenyl-4-yl)(phenyl) methanone 4 were obtained as a result of CO insertion into the palladium(II)-aryl bond. However, when pyridine-4-yl boronic acid was used in place of phenylboronic acid as the starting reagent, synthetic reaction yielding 3 and 4 were found not to occur.

During the course of an on-going synthetic project for preparing aryl ketones, we decided to evaluate the applicability of N-heterocyclic carbene (NHC) ligands.NHC ligands have gained popularity in metal-catalyzed crosscoupling reactions for several reasons [32][33][34][35]: 1) the steric bulk that they introduce around the metal center facilitates reductive elimination; 2) their strong σ-donating character enables facile oxidative addition; and 3) their greater stability at elevated temperatures relative to phosphineligands enables their use under a broader range of reaction conditions.Carbonylative Suzuki coupling using the synthesized NHC-Pd complex was carried out under a balloon of CO or metal carbonyls.To study the scope of the process, the reaction conditions were optimizied for the cross-coupling of 4,4'-diiodobiphenyl and phenylboronic acid with N-heterocyclic carbene (NHC) ligand under a balloon (1 atm) of CO or metal carbonyls.4,4'-diiodobiphenyl 1) and phenylboronic acid 2) were reacted under CO (1 bar, a balloon) atmosphere in the presence of the Pd(NHC) complex catalyst formed in situ [36,37].
The reaction mixture was kept at 80˚C for 24 h.After elimination of Pd(NHC) complex by filteration, the reaction mixture was diluted water (10 mL) and CH 2 Cl 2 (20 mL).The neutralized solution was extracted with CH 2 Cl 2 .

Conclusions
When metal carbonyl was used in place of CO, we achieved the same reaction products.
We assume that the two reactions needed to obtain 3 require a longer reaction time as 4 is formed as a reaction intermediate.When pyridine-4-ylboronic acid is used in place of phenylboronic acid,  synthesize various diketones by using heteroaromatic boronic acid to develop a luminescent material.

Scheme 1 .
Scheme 1. Simplified catalytic cycle showing the formation of 3.