Double CO insertion

An alternative reaction path could be a double CO insertion (Uquaiion (35)) followed by hydrogenation. 

The carbyne complex 319 reacts with CO to give 320 °, which results from a double CO insertion and PMes migration, while 321 reacts with the manganese carbonyl complex 322 in refluxing toluene to give 323 °°. 

Depending on the catalyst system and the reaction conditions, especially at elevated CO pressure it is possible to obtain selectively double carbonylation reactions to 1-keto carboxylic derivatives [25]. Recent mechanistic investigations have shown that double CO insertion into the palladium-carbon bond does not occur directly instead, the terminal step of double carbonylation is generally a coupling reaction between metal-bonded acyl, alkoxycarbonyl or amidocarbonyl groups and CO. 

Chain propagation of CO/ethylene copolymerization proceeds by a strictly alternating insertion of CO and olefin monomers in the growing chain. It is safe to assume that double CO insertion does not occur for thermodynamic reasons [Ic]. However, the complete absence of double ethylene insertions is remarkable because ethylene insertion in a Pd-alkyl species must be exothermic by about 20 kcal/mol (84 kJ mol). The observation of strict alternation is the more surprising since the same palladium catalysts also efficiently dimerize ethylene to butenes [25]. The perfect alternation is maintained even in the presence of very low concentrations of carbon monoxide. When starting abatch polymerization at a high ethylene/CO ratio, error-free copolymer is produced until all the CO is consumed then the system starts forming butenes (with some catalyst systems at about twice the rate of copolymerization ). 

In summary, chain propagation involves alternating reversible carbon monoxide insertion in Pd-alkyl species and irreversible insertion of the olefin in the resulting Pd-acyl intermediates. The overall exothermicity of the polymerization is caused predominantly by the olefin insertion step. Internal coordination of the chain-end s carbonyl group of the intermediate Pd-alkyl species, together with CO/olefin competition, prevents double olefin insertion, and thermodynamics prevent double CO insertions. The architecture of the copolymer thus assists in its own formation, achieving a perfect chemoselectivity to alternating polyketone. 

Figure 43. Single and double CO insertion between two [Rh (OEP)] complexes.

Catalysis works best when some amount of phosphine particularly 1,2-bis(diphenylphosphino)ethane (DPPE) is used as additive along with [Co2(CO)gj. Yield of the thio lactone is susceptible to the amoxmt of solvent used. Reaction goes very well in higher dilution. Yields of double CO inserted products vary with type of thiols and acetylenes used. Reaction works best with ethanethiol (70% yields). Aromatic thiols were less effective and monocarbonylation product forms in major byproduct [43]. 

If the acylpalladium species can undergo further CO insertion to give a-ketoacylpalla-dium species and the complex should be attacked by a nucleophile, the double carbonylation process might be realized. However, the CO insertion into the acyl-transition metal bond seems to be a thermodynamically unfavorable process and the double CO insertion process is considered not operative for Pd-catalyzed double carbonylation of aryl halides. ... 

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 ... 

Of the possible erroneous insertions, double CO insertion is forbidden for the thermodynamic reasons discussed in Section 7.2, and double alkene insertion is rare because of its much slower intrinsic rate and the high affinity of the catalyst for CO, together amounting to a rate enhancement of 2000 versus CO insertion into M-R. 

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