Insertion, protonolysis

This is a reaction with high atom economy and without the use of organometallic reagents, additives, or redox systems. Such an acetoxypalladation-initiated carbon-heteroatom multiple bond insertion-protonolysis system may extend the scope of transition metal-catalysed reactions pertaining to the insertion of carbon-heteroatom multiple bonds into metal-carbon bonds, and provide a new methodology in organic synthesis. The generality of the present catalytic system is shown in Table 10.2.[3]... 

The catalyst precursor Ca(nacnac)(N(SiMe3)2)(THF) reacted with diphenylpho-sphine to yield the phosphide complex 12 [29], which was proposed to undergo alkene insertion. Protonolysis of the resulting Ca-alkyl complex with diphenylpho-sphine via c-bond metathesis then completes the catalytic cycle (Scheme 18). 

Alkynols 4.105 may be cyclized to a-methylene lactones 4.106 using palladium-catalysed carbonylation (Scheme 4.42)." The reaction is proposed to proceed via formation of an acyl palladium species 4.107, which undergoes intramolecular alkyne insertion. Protonolysis of the carbon-palladium bond of the vinyl complex 4.108 yields the product. 

However, some experimental data, in particular the observatimi of sequential hydroamination/bicyclization sequences (see Sect. 4) catalyzed by organolan-thanide [20-23] and organolithium [24] species is in conflict with this scenario, as the sequential reaction requires a finite lifetime for the rare earth metal alkyl intermediate. Therefore, the intermediacy of the metal-alkyl species and its potential lifetime is unclear at present and probably strongly depends on catalyst and substrate structure. Unfortunately, involvement of concerted insertion/protonolysis... 

P-H oxidative addition followed by alkyne insertion into a Pd-P bond gives the re-gio-isomeric alkenyl hydrides 15 and 16. Protonolysis with diaUcyl phosphite regenerates hydride 17 and gives alkenylphosphonate products 18 and 19. Insertion of alkene 18 into the Pd-H bond of 17 followed by reductive eUmination gives the bis-products, but alkene 19 does not react, presumably for steric reasons. P-Hydride elimination from 16 was invoked to explain formation of trace product 20. 

Like alkynes, a variety of mechanistic motifs are available for the transition metal-mediated etherification of alkenes. These reactions are typically initiated by the attack of an oxygen nucleophile onto an 72-metalloalkene that leads to the formation of a metal species. As described in the preceding section, the G-O bond formation event can be accompanied by a wide range of termination processes, such as fl-H elimination, carbonylation, insertion into another 7r-bond, protonolysis, or reductive elimination, thus giving rise to various ether linkages. 

In addition to /3-H elimination, olefin insertion, and protonolysis, the cr-metal intermediate has also proved to be capable of undergoing a reductive elimination to bring about an alkylative alkoxylation. Under Pd catalysis, the reaction of 4-alkenols with aryl halides affords aryl-substituted THF rings instead of the aryl ethers that would be produced by a simple cross-coupling mechanism (Equation (126)).452 It has been suggested that G-O bond formation occurs in this case by yy/z-insertion of a coordinated alcohol rather than anti-attack onto a 7r-alkene complex.453... 

Isonitrile insertion into zirconacycles to afford iminoacyl complexes 28 is fast, but rearrangement to q2-imine complexes 30 is slow. In the case of tBuNC, the rearrangement does not occur. Amines 32 are formed on protonolysis of the q2-imine complex. The q2-imine complexes 30 readily undergo insertion of Ti-components (alkenes, alkynes, ketones, aldehydes, imines, isocyanates) to provide a wide variety of products 37 via zirconacycles 36. The overall sequence gives a nice demonstration of how a number of compo-... 

A Pd(II)-catalyzed sequential cyclization-coupling reaction of allenyl N-tosylcar-bamates and acrolein has been developed (Scheme 16.97) [103]. The proposed mechanism involves intramolecular aminopalladation of an allene, followed by insertion of acrolein and carbon-Pd bond protonolysis. 

However, this is not always the case. Excess of K - K has been found to occur during the initial stage of the copolymerisation when the cooligomer chain bound to the metal is still soluble and catalysis occurs in the homogeneous phase [36]. This may also occur when protonolysis involves H20 in place of MeOH, with formation of a Pd - OH+ species, which regenerates Pd - H+ by insertion of CO to Pd - COOH+ followed by C02 evolution. Thus in each catalytic cycle one molecule of CO is not incorporated into the polymer chain, but is consumed as C02 ... 

In the other mechanism, the catalytic cycle initiates through the insertion of CO into a Pd-alkoxy bond, with formation of a Pd-carboalkoxy intermediate, which inserts the olefin with formation of an alkylcarboalkoxy /i-chelate, which undergoes protonolysis by the alkanol through the intermediacy of its enolate isomer (see Sect. 2.3.1), yielding the ester and the Pd-alkoxy species, which then initiates a new catalytic cycle [122-125]. 

Because of its nature, DEK must form via a hydride mechanism. Up to the formation of a Pd-acyl intermediate, the paths leading to MP or DEK are similar. DEK forms if the insertion of a second molecule of ethene into the Pd-acyl bond is followed by protonolysis of the Pd - C bond of the resulting Pd-alkylacyl intermediate. 

The chain transfer by protonolysis represents the predominant termination step in homogeneous ethene/CO copolymerisation, and involves the reaction between a propagating Pd-alkyl species and MeOH or adventitious water (Scheme 7.15a). As a result, the propagation is terminated with formation of a polymeric chain with a ketone-end group and Pd-OMe (or Pd-OH) species, which can re-enter the catalytic cycle by CO insertion. 

The following discussion deals not only with this reaction, but related reactions in which a transition metal complex achieves the addition of carbon monoxide to an alkene or alkyne to yield carboxylic acids and their derivatives. These reactions take place either by the insertion of an alkene (or alkyne) into a metal-hydride bond (equation 1) or into a metal-carboxylate bond (equation 2) as the initial key step. Subsequent steps include carbonyl insertion reactions, metal-acyl hydrogenolysis or solvolysis and metal-carbon bond protonolysis. 

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