Multiple CO insertion

H.iii. Multiple CO Insertion Processes Promoted by Other Transition Metai Complexes... 

The factors that control the strictly alternating copolymer chain with no detectable errors (e. g., microstructures involving double insertion of ethene) have been the object of detailed studies since the discovery of the first Pd" catalysts for the alternating alkene/CO copolymerisation [11]. Sen was the first to demonstrate that double carbonylation is thermodynamically unfavorable and to suggest that the higher binding affinity of Pd" for CO relative to ethene inhibits multiple ethene insertions, even in the presence of very low concentrations of CO [12]. Therefore, once a palladium alkyl is formed, CO coordination ensures that the next monomer will be a CO molecule to generate the acyl complex. 

In 1963, Heck reported the ring opening of propylene oxide by the carbonylating reagent tetracarbonylhydridocobalt(I) in the presence of carbon monoxide, which results in a stable acyl cobalttetracarbonyl compound (Fig. 15). However, no polymeric products were reported, which would result from multiple ring opening and CO insertion processes [58, 59]. 

Reaction of CpM(CO)3R (R = Me, CH2Ph) with 2-butyne produces an alkenyl ketone metallacycle, thermally for Mo, photochemically for W (202). Addition of CNBu or PPh3 drives CO insertion into the metal position of the metallacycle, and lactone products form [Eq. (59)]. With trifluoromethyl as the original metal alkyl group multiple alkyne and CO insertions lead to an eight-membered oxymetallacycle ring [Eq. (60)]. 

A model for such a reaction sequence is (21-XLIV).184 In the case of Pd11 complexes of rigid bidentate nitrogen ligands, products of multiple successive insertions of alkenes and CO have proved isolable.185 The insertion of an alkene into the Pd—acyl bond of a neutral acyl chloro complex leads to displacement of the halide ligand and formation of a chelate-stabilized product (21-XLV).186... 

In contrast to the case of CO insertion that usually allows insertion of only one CO unit into a metal-carbon bond, isocyanides undergo multiple insertions sometimes leading to polyisocyanides [53,54]. Since the inserted isocyanide units may be regarded as imines derived from carbonyl groups, the insertion products can be regarded as polycarbonyl compounds where CO units are multiply inserted into the metal carbon bonds. The multiple insertion products of isocyanides have found applications both in organic synthesis and polymer synthesis [55]. 

The insertion ability of isocyanides differs from that of CO. In contrast to the carbon monoxide, for which consecutive CO insertion is disfavored, isocyanides readily undergo the multiple insertion processes especially with more nucleophilic metals such as Ni and Pd (Eq. 7.14). The multiple insertion may lead to new types of organic compounds difficult to prepare by other synthetic methods. 

Multiple insertions are possible. Systems have been designed that involve two alkene insertions and three carbon monoxide insertions, which proceed in good yield, such as the conversion of 4.95 to 4.96 (Scheme 4.39). The efficiency of the reaction is notable, as the tandem sequence can go astray at various points (Scheme 4.40). Several intermediates are capable of undergoing 3-hydride elimination (4.99,4.101) or direct alkene insertion (4.97,4.99), but do not CO insertion occurs instead. The high CO pressure (40 atm) is likely to be responsible in part for this selectivity. 

Other three samarium enolates which can be considered formally as the double CO insertion into multiple bonds of PhC CPh [92], PhN=NPh [105] and < CH=CH-[93] accor ng to the equation ... 

In a carbonylation reaction applied to polymer synthesis, a number of cationic Pd(II) complexes, such as [Pd(dipy)Me(CO)][BAr 4], convert ethylene—CO mixtures to a perfectly alternating copolymer (-C0-CH2-CH2-) that allows for easy subsequent functionalization of the carbonyl group. The mechanism involves alternating insertions of CO and ethylene, to account for which, the alkyl must prefer to insert CO and the acyl must prefer to insert ethylene. We have already seen that multiple insertion of CO is not favored (Section 7.2), but multiple insertion of ethylene is seen for Zr(IV) in cases where there is no CO to compete (Section 12.2). As expected, if alternation is to occur, the reaction barrier for the CO insertion into Pd-alkyl must be the lowest (calculated as 15 kcal/mol), followed by ethylene into a Pd-acyl (17 kcal/mol), followed by ethylene into a Pd-alkyl (19 kcal/mol). 

Metal oxides possess multiple functional properties, such as acid-base, redox, electron transfer and transport, chemisorption by a and 71-bonding of hydrocarbons, O-insertion and H-abstract, etc. which make them very suitable in heterogeneous catalysis, particularly in allowing multistep transformations of hydrocarbons1-8 and other catalytic applications (NO, conversion, for example9,10). They are also widely used as supports for other active components (metal particles or other metal oxides), but it is known that they do not act often as a simple supports. Rather, they participate as co-catalysts in the reaction mechanism (in bifunctional catalysts, for example).11,12... 

Cobalt, as its CpCo(CO)2 complex, has proven to be especially suited to catalyze [2 + 2 + 2] cycloadditions of two alkyne units with an alkyne or alkene. These cobalt-mediated [2 + 2 + 2] cycloaddition reactions have been studied in great detail by Vollhardt337. The generally accepted mechanism for these cobalt mediated cycloadditions, and similar transition metal mediated cycloadditions in general, has been depicted in equation 166. Consecutive co-ordination of two triple bonds to CpCo(CO)2 with concomitant extrusion of two molecules of carbon monoxide leads to intermediates 578 and 579 via monoalkyne complex 577. These react with another multiple bond to form intermediate 580. The conversion of 578 to 580 is said to be kinetically favored over that of 579 to 580. Because intermediates like 580 have never been isolated, it is still unclear whether the next step is a Diels-Alder reaction to form the final product or an insertion to form 581. The exact circumstances might determine which pathway is followed. 

In this chapter we will discuss some aspects of the carbonylation catalysis with the use of palladium catalysts. We will focus on the formation of polyketones consisting of alternating molecules of alkenes and carbon monoxide on the one hand, and esters that may form under the same conditions with the use of similar catalysts from alkenes, CO, and alcohols, on the other hand. As the potential production of polyketone and methyl propanoate obtained from ethene/CO have received a lot of industrial attention we will concentrate on these two products (for a recent monograph on this chemistry see reference [1]). The elementary reactions involved are the same formation of an initiating species, insertion reactions of CO and ethene, and a termination reaction. Multiple alternating (1 1) insertions will lead to polymers or oligomers whereas a stoichiometry of 1 1 1 for CO, ethene, and alcohol leads to an ester. 

The (co)polymerization of dienes can be a good method for the preparation of polymers with reactive vinyl groups, a method that enables the preparation of polymers possessing plural vinyl groups per polymer chain. A fluorinated bis(phenoxy-imine) Ti complex was shown by Coates and co-workers to convert 1,5-hexadiene to poly(methylene-l,3-cyclopentane-fti-3-vinyl tetramethylene), which contained multiple vinyl groups. As already discussed, Saito et al. and others revealed that bis(phenoxy-imine) Ti complexes favored secondary insertion. " This is probably responsible for the formation of 3-vinyl tetramethylene units. Likewise, the same catalyst system can form sPP-/ -poly(methylene-l,3-cyclopentane-z -3-vinyl tetramethylene) from propylene and 1,5-hexadiene. Very recently. 

Because of its low acidity, hydrogen cyanide seldom adds to nonactivated multiple bonds. Catalytic processes, however, may be applied to achieve such additions. Metal catalysts, mainly nickel and palladium complexes, and [Co(CO)4]2 are used to catalyze the addition of HCN to alkenes known as hydrocyanation.l67 l74 Most studies usually apply nickel triarylphosphites with a Lewis acid promoter. The mechanism involves the insertion of the alkene into the Ni—H bond of a hydrido nickel cyanide complex to form a cr-alkylnickel complex173-176 (Scheme 6.3). The addition of DCN to deuterium-labeled compound 17 was shown to take place... 

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