P-Polyketone

Structural and magnetic studies of polynuclear transition metal p-polyketonates. M. D. Glick and R. L. Lintvedt, Prog. Inorg. Chem., 1976, 21, 233-260 (51). 

In 1907, Collie proposed that polymers of ketene (CH2=C=O) might be precursors of such compounds as orsellinic acid, a common constituent of lichens. The hypothesis was modernized in 1953 by Birch and Donovan, who proposed that several molecules of acetyl-CoA are condensed (Eq. 21-19) but without the two reduction steps required in biosynthesis of fatty acids (Fig. 17-12).329 As we now know they were correct in assuming that the condensation occurs via malonyl-CoA and an acyl carrier group of an enzyme. The resulting p-polyketone can react in various ways to give the large group of compounds known as polyketides. 

In their simplest form, polyketides are natural compounds containing alternating carbonyl and methylene groups ( p-polyketones ). The biosynthesis of polyketides begins with the condensation of a starter unit (typically, acetyl-CoA or propionyl-CoA) with an extender unit (commonly malonyl-CoA or methylmalonyl-CoA, followed by decarboxylation of the extender unit (/, 2) (Fig. 1). Repetitive decarboxylative condensations result in lengthening of the polyketide carbon chain, and additional modifications such as ketoreduction, dehydratation, and enoylreduction may also occur (discussed below). 

Most of the pigments of flowers arise from a single polyketide precursor. Phenylalanine is con-verfed to trans-cinnamic acid (Eq. 14-45) and then to cirmamoyl-CoA. The latter acts as the starter piece for chain elongation via malonyl-CoA (step a in the accompanying scheme). The resulting p-polyketone derivative can cyclize in two ways. The aldol condensation (step b) leads to stilbenecar-boxylic acid and to such compounds as pinosylvin of pine trees. The Claisen condensation (step c) produces chalcones, flavonones, and flavones. These, in turn, can be converted to the yellow fla-vonol pigments and to the red, purple, and blue anthocyanidins. ... 

This unusual biosynthetic pathway—the synthesis and regio-controlled cyclization of p-polyketones, optionally leading to tetrahydro-, dihydro-, or fully conjugated isoquinolines, or to naphthalenes and respective naphthoquinones— could be imitated effieiently in vitro. The smooth course of these chemical model reaetions underlines the chemieal plausibility of the proposed biogenetie scheme and simultaneously provides the basis for a first and variable total synthesis of naphthyl isoquinoline alkaloids. 

It has already been shown (e.g. Chapters 20 and 21) that the insertion of a p-phenylene into the main chain of a linear polymer increased the chain stiffness and raised the heat distortion temperature. In many instances it also improved the resistance to thermal degradation. One of the first polymers to exploit this concept commercially was poly(ethylene terephthalate) but it was developed more with the polycarbonates, polysulphone, poly(phenylene sulphides) and aromatic polyketones. 

Devasagayam, T.P.A., Werner, T., Ippendorf, H., Martin H.-D., and Sies, H. 1992. Synthetic carotenoids, novel polyene polyketones and new capsorubin isomers as efficient quenchers of singlet molecular oxygen. 

Drent, E. van Broekhoven, J. A. M. Doyle, M. J. Wong, P. K. Palladium Catalyzed Copolymerization of Carbon Monoxide with Alkenes to Alternating Polyketones and Polyspiroketals. Fink, G. Muelhaupt, R. Brintzinger, H. H. Eds. Ziegler Catal. Springer, Berlin, 1995, pp 481 496. 

The active species in the catalytic cycles are square-planar Pd complexes of the formula [Pd (X)(S)(L-L)]Y where L-L is a chelating ligand with the same or different donor atoms among P, N, O and S X is the growing polyketone chain or hydride S may be a solvent molecule, a co-monomer, or a keto group from the chain. Finally, Y is a counter-anion of weak nucleophilicity in order to avoid competition with the co-monomer for coordination to palladium (Scheme 7.1). 

Notably, this HP NMR investigation showed the formation of a transient binuc-lear p-H-p-CO complex, [Pd2(p-H)(p-CO)(dppf)2]OTs (5), and of the termination product [Pd(p-OH)(dppf)]2(OTs)2 (6) (Chart 7.1). Based on the in situ study, these compounds could be isolated, characterised and used to catalyse copolymerisation reactions. Both complexes proved to be active in batch copolymerisation reactions. However, the productivities in polyketone were significantly lower than those... 

Besides proving the formation of p-chelates [Pd(CH7CH7C(0)Me)(P-P)] at room temperature, the spectra showed the occurrence of chain-transfer by protonolysis with adventitious water to give the p-hydroxo compounds cis/trans [Pd(p-OH)(P-P)]2 as well as the conversion of the latter compounds into cis/trans bis-chelates [Pd(P-P)2] (Chart 7.2) [5f]. Independent experiments with isolated compounds showed that the p-OH and bis-chelate complexes are not dead ends, and can reenter the catalysis cycle to give alternating polyketones. 

NMR analysis of the polyketone end groups and in situ NMR studies have shown that two transfer mechanisms in MeOH may occur simultaneously (a) methano-lysis of Pd-acyl and (b) protonolysis of Pd-alkyl (Scheme 7.15). The eventual presence of water in MeOH, even in trace amounts, gives rise to two similar terminations, yielding different end groups (-COOH) and metal re-initiator (Pd-OH) [5e-g, 13, 36]. Termination by P-H transfer (c) is typical for reactions performed in organic solvents. 

In anhydrous organic solvents, ethene/CO copolymerisation termination occurs exclusively by P-H transfer to give vinyl terminated polyketone and Pd-H (Scheme 7.15c). On the other hand, traces of water are very difficult to eliminate and consequently chain transfer by protonolysis is often observed, together with p-H transfer. Experimental evidence in this sense has been straightforwardly obtained by an in situ NMR study of the chemical stability of the p-chelate [Pd(CH7CH7C(0)-Me)(dppe)]PF5 (7) in wet and anhydrous CD2CI2 [5ej. Figure 7.13 reports a sequence of P H NMR spectra taken after dissolution of the p-chelate in the wet solvent already the first spectrum at room temperature showed the formation of the p-hydroxo binuclear complex [Pd(OH)(dppe)]2(PF )2 (8), that was the only detectable species after 15 h. 

FIGURE 6.16 Formation of polyketones in the OH reaction with p-xylene (adapted from Wiesen et al 1995). 

Palladium-catalyzed a-arylation of ketones is performed with arylene dihalides and bifunctional aromatic ketones 148 to result in the bond formation at the r/) -a-carbon of the ketone, leading to polyketone 149. The reaction is carried out in the presence of Pd(0) and various phosphines. Several bidentate phosphines and bulky alkylphosphines such as dppf, BINAP, PCys, and P Bu3 are shown to be effective, while PPh3 results in no reaction. Arylene dibromide and diiodide are applicable as the co-monomers. The polymerization reaction is carried out in THE in the presence of NaO Bu at 75 °C under N2, and polymers 149 are isolated in 60-80% yields (M = 7000-15 000). Polyketone 149 is further transformed to conjugated polymer PPV by reduction of the ketone moiety with LiAlH4 followed by dehydration with an acid (Equation (69)). 

A key impetus in the study of these materials was the pursuit of catalytic CO/olefin copolymerization, reactivity to which 431-438 and 447 and their parent alkyls are entirely inert. Though 448 has been found to react with norbomadiene, affording the insertion product Tp PdC7H8C (O)p-Tol) (449), it does so slowly (>1 day). However, when the parent p-Tolyl complex 440 is simultaneously exposed to both CO and norbor-nadiene, catalytic copolymerization ensues, affording the polyketone within hours (Section IV.A).140 141... 

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