Predominant 1,2-Insertion of Styrene in the Pd-Catalyzed Alternating Copolymerization with Carbon Monoxide

Abstract

The regioselectivity of styrene insertion to an acyl-Pd bond was studied by NMR in (i) a stoichiomeric reaction and (ii) a copolymerization with CO. In the stoichiometric reaction of styrene with [(CH(3)CO)Pd(CH(3)CN)[(R,S)-BINAPHOS]].[B[3,5-(CF(3))(2)C(6)H(3)](4)], both 1,2- and 2,1-products were given. To mimic the real polymerization conditions, a polyketone-substituted complex [[CH(3)(CH(2)CHCH(3)CO)(n)]Pd[(R,S)-BINAPHOS]].[B(3,5-(CF(3))(2)C(6)H(3))(4)] (n approximately 14) was prepared. When this polymer-attached Pd species was treated with styrene, the 1,2-insertion product was the only detectable species. Thus, exclusive 1,2-insertion is demonstrated to be responsible for the styrene-CO copolymerization, in sharp contrast to the predominant 2,1-insertion with conventional nitrogen ligands. Chain-end analysis revealed that beta-hydride elimination took place from the 2,1-complex but not from the 1,2-complex. Thus, once 2,1-insertion occurs, rapid beta-hydride elimination proceeds to terminate the polymerization, as is common to the other phosphorus-ligand systems. The resulting Pd-H species re-initiates the copolymerization, as was proven by MALDI-TOF mass analysis of the product copolymers.

Catalyst activity or stability: the dilemma in Pd-catalyzed polyketone synthesis

“…63,64 In the latter case it was demonstrated that the CO insertion into the Pd-allyl bond is much less efficient than into the Pd-alkyl bond. 65 The observation in the NMR-2D-NOESY spectrum of 1e of a strong cross-peak between H a of styrene unit and H ortho of the CH 3 -substituted aryl ring, together with the lack of such interaction with the H ortho of the CF 3 -substituted aryl ring, probes evidence that only the isomer featured by the Pd-C bond trans to the Pd-N bond of the CF 3 -substituted aryl ring is formed.…”

Phosphine Ligands in the Palladium‐Catalysed Methoxycarbonylation of Ethene: Insights into the Catalytic Cycle through an HP NMR Spectroscopic Study

“…The uncoordinated anion and acid molecule are hydrogen bonded together. Table 3 compares the bond lengths, and interbond angles in 32-35 with those of the [1,2-bis(di-tert-butylphosphinomethyl)benzene] palladium(II) complex (36). [55] Bond lengths and angles are broadly comparable across the five complexes.…”

Ligand-Dependent Diastereoselectivity in the Palladium-Catalyzed Copolymerization of Styrene with Carbon Monoxide

“…In contrast, with catalyst 5b, most of the chains begin by styrene insertion into a L n PdÀCOOMe intermediate, the formation of which is promoted by a higher concentration of the oxidant [1]. A MALDI high-resolution mass spectrum of a copolymer obtained with this catalyst precursor under 20 bar CO enabled us to identify, in the low-mass range, the copolymer chains shown below [15] [20]. To obtain more information about the factors involved in the copolymerization with 4b and 5b, we carried out NMR model studies for comonomer insertion with the two ligand systems.…”