Mechanisms of the CO₂ Insertion into (PCP) Palladium Allyl and Methyl σ-Bonds. A Kinetic and Computational Study

Abstract

The reaction of the σ-bonded (PCP)Pd-Me complex (PCP = 2,6-bis [(di-tert-butylphosphino)methyl]phenyl) with CO 2 is first-order in palladium and first-order in CO 2 with a rate constant k s = 8.9 ( 0.8 M -1 s -1 at 353 K. Activation parameters are ΔH q = 73 ( 7 kJ/mol and ΔS q = -118 ( 19 J/K mol. Based on this and theoretical calculations we propose an S E 2 mechanism where the coordinated methyl group attacks a completely noncoordinated carbon dioxide molecule in a bimolecular reaction. The PCPPd-crotyl complex was synthesized in an 65:35 E:Z mixture, and it was shown to react with CO 2 to give the complex PCPPd-O(CO)CH(CH 3 )CHCH 2 as a single isomer, where the former γ-carbon has been carboxylated. Theoretical calculations again suggest an S E 2 mechanism with a noncoordinated carbon dioxide reacting with the terminal carbon on the allyl group, forming an η 2 -bonded olefin complex as an intermediate. The rearrangement of this intermediate to the O-bonded product is concluded to be rate determining. The crystal structure of PCPPd-O(CO)C(CH 3 ) 2 CHCH 2 is reported and as well as the solubility of carbon dioxide in benzene-d 6 at different pressures and temperatures.

Applications of Five-Membered Ring Products in Cyclometalation Reactions for Other Purposes

“…CO 2 inserted into a palladium metal and methyl group bond gives the acetate product 9.49 , as shown in Eq. ( 9.15 ) [ 97 ]. …”

Acrylates from Alkenes and CO2, the Stuff That Dreams Are Made of

“…47 Carboxylation of Ni(II) and Pd(II) allyl complexes has also been proposed to proceed via zwitterionic intermediates resulting from the attack of CO 2 at the terminal carbon of the η 1 -coordinated allyl group. [48][49][50][51][52][53] B1 is isoelectronic to the thermodynamic insertion product in cationic olefin polymerization such as in the Brookhart systems, 54 and is fairly stable, if solvation is taken into account. This means that a second, lower barrier G { s ¼ 82kJ=mol À Á needs to be overcome for recoordination of the carboxylate unit to nickel.…”

Making C–C Bonds from Carbon Dioxide via Transition-Metal Catalysis

“…Taking advantage of the wealth of literature pertaining to stoichiometric carboxylations [24][25][26][27][28][29][30][31], CO 2 incorporation was examined in a catalytic context in 1997 when Nicholas demonstrated the carboxylation of allyltin reagents in the presence of Pd(0) [47]. Wendt's subsequent mechanistic study confirms a metallo-ene type mechanism for carbon dioxide allylation [33], and Hazari's extension of this work reveals that well-defined palladium carboxylates supported by either phosphine or NHC ligands promote carboxylation of both allyltin and allylboron reagents [48]. A related copper-catalyzed method was reported by Huong [49].…”

“…Although the ligands supporting the transition metal directly influence the energetics of CO 2 insertion, mechanisms involving cyclic four-or six-membered transition states, as well as outer sphere attacks to carbon dioxide are reasonable to consider (Fig. 3) [26,[32][33][34][35][36].…”