Enantioselective Alternating Olefin‐Carbon Monoxide Copolymerization: A new concept for activity and stereoselectivity

Abstract

(7.111.95) Cationic Pd-complexes modified by dicyclohexyl{(R)-l-[(S)-2-(diphenylphosphino)ferrocenyl]ethyl}phos-phine (la)give very active catalytic systems for the regioregular isotactic specific copolymerization of propene with CO. Other alk-I-enes also give stereoregular and regioregular copolymers, even if with lower productivity. The copolymers are isolated as poly(4-alkyl-tetrahydrofuran-2,2,5,5-tetrayl-2-oxy-2-methylenes) B in the solid state and give the isomeric poly(2-alkyl-1 -oxopropane-l,3-diyls) A by dissolution in (CF,),CHOH. Solid polymer A (R = Et) is formed back at least partially when the dissolved material is reprecipitated from MeOH. The use of the related (ferroceny1)diphosphine ligands lbe and 2 as the catalyst modifier shows that the presence of both elements of chirality and of large substituents on the P-atoms of the ligand is necessary to achieve good stereocontrol, and that the large difference in basicity between the two P-atoms is probably the reason for the good catalytic activity.Diphosphine-containing cationic Pd-catalysts of the type [PdX,(P-P)] [I] (X = weakly coordinating or non-coordinating anion, P-P = diphosphine) led to the commercial application of the alternating copolymerization of ethene with CO [2], at least on a pilote-plant scale [3]. To extend the scope of the reaction to other aliphatic olefins, problems related to the regioselectivity [4] and to the stereoselectivity [5] of the copolymerization process must be solved. Even though the requirements of the ligands to achieve the above goals seem to be reasonably well identified [4-71, the catalytic activity of the stereoselective systems investigated remains low, thus hampering potential applications. The most active system for the copolymerization of propene reported so far seemingly contains propane-l,3-diylbis(diethylphosphine) [7] as the modifying ligand. The absence of chirality in this ligand, however, causes formation of a material having a low stereoregularity. Surprisingly, the copolymers of propene [7-101 were not of type A (Fig. I), e.g. poly(2-methyl-l-oxopropane-l,3-diyl) (R = Me), but of type B, e.g. poly-(tetrahydro-4-methylfuran-2,2,5,5-tetrayl-2-oxy-2-methylene (R = Me)).The stability of the spiro structure B for the olefin-carbon monoxide copolymers seems to be influenced by the degree of substitution of the C=C bond of the olefin substrate [l 11 [12]. These results imply that the copolymerization mechanism is less simple [8] [9] than expected [l] [13]. In spite of different working hypothesis that may be

Copolymerization of styrene with CO catalysed by palladium‐substituted 1,10‐phenanthroline bis(methoxycarbonyl) complexes [Pd(L—L)(COOMe) 2 ]

“…Alkoxycarbonyl complexes of transition metals have been largely studied, because of their relevance as intermediates in several processes involving carbon monoxide and alcohols as reagents 25) . The alternating copolymerization of styrene-CO catalysed by the palladium phenanthroline bis(methoxycarbonyl) complexes, Pd(L1L)(COOMe) 2 , together with 1,4-naphthoquinone (1,4-NQ) as oxidant and (L1L)HPF 6 as acid cocatalyst, is now reported. The non-conventional acid phenanthrolinium hexafluorophosphate (phenHPF 6 ) was chosen due to the importance of the presence of a nonesterifyable (by the alcoholic solvent) acid cocatalyst, whose conjugated base was a weakly coordinating anion.…”

“…The alternating copolymerization of styrene-CO catalysed by the palladium phenanthroline bis(methoxycarbonyl) complexes, Pd(L1L)(COOMe) 2 , together with 1,4-naphthoquinone (1,4-NQ) as oxidant and (L1L)HPF 6 as acid cocatalyst, is now reported. The non-conventional acid phenanthrolinium hexafluorophosphate (phenHPF 6 ) was chosen due to the importance of the presence of a nonesterifyable (by the alcoholic solvent) acid cocatalyst, whose conjugated base was a weakly coordinating anion. The phenHPF 6 cocatalyst meets both these requirements being composed from the acid phenH + (pK a = 4.86) and from the weakly coordinating counterion, PF 6 -.…”

“…The first catalysts were generated in situ by reaction of Pd(OCOCH 3 ) 2 2) or Pd(OCOCF 3 ) 2 3) and nitrogen or phosphorus ligands, in the presence of an acid cocatalyst and, usually, an oxidant. Actually, preformed palladium catalytic precursors, such as Pd(L1L) 2 (X) 2 (L1L = nitrogen or phosphorus bischelating ligands and X = PF 6 -, CF 3 COO -), allow one to reach good yields in ethylenecarbon monoxide copolymerization 4,5) . Many efforts have been also devoted to copolymerize CO with a-olefins, such as propene [6][7][8][9][10] and styrene [11][12][13][14][15][16][17][18][19][20][21] .…”

“…Actually, preformed palladium catalytic precursors, such as Pd(L1L) 2 (X) 2 (L1L = nitrogen or phosphorus bischelating ligands and X = PF 6 -, CF 3 COO -), allow one to reach good yields in ethylenecarbon monoxide copolymerization 4,5) . Many efforts have been also devoted to copolymerize CO with a-olefins, such as propene [6][7][8][9][10] and styrene [11][12][13][14][15][16][17][18][19][20][21] . Since in the latter case, the initation step is hypothesized to be a 2,1-insertion of styrene in a palladium carbomethoxy bond 22) , followed by CO insertion and successive 2,1-insertion of styrene in Pd-acyl bonds formed 23) , a new class of palladium bis(methoxycarbonyl) complexes, Pd(L1L)(COOMe) 2 , bearing substituted phenanthrolines as ligands, have been prepared 24) to prove their involvement in styrene-CO copolymerization.…”

“…The phenHPF 6 cocatalyst meets both these requirements being composed from the acid phenH + (pK a = 4.86) and from the weakly coordinating counterion, PF 6 -. The cocatalyst (L1L)HPF 6 has been recently used in styrene-CO copolymerization catalyzed by [Pd(N1N) 2 ] [X] 2 (N1N = phenanthroline and bipyridine, and X = PF 6 -, BF 4 -) 11) . The influence of the substituents at the nitrogen chelating ligand on the tacticity and the thermal behaviour of alternating copolymer are also reported, as investigated by NMR and by DSC/TGA, respectively.…”