Phosphine, catalysis 3 + 2-cycloaddition

Ketene [2 + 2] cycloaddition with disubstituted ketones or aldehydes with phosphine catalysis gives P-lactone formation with a preference for the traws-diastereomer (Scheme 4.32). The formation ofphosphonium eno-late intermediates was monitored using NMR, and based on the NMR observation of the intermediates a mechanism was proposed in which the phosphonium enolate 158 reacted with a second molecule of the ketene forming intermediate 159, which then was acylated by the aldehyde followed by loss of 158 and formation of the product p-lactone. The use of the chiral phosphine catalyst BINAPHANE gave enantioselective product formation. 

Michael-aldol reaction as an alternative to the Morita-Baylis-Hillman reaction 14 recent results in conjugate addition of nitroalkanes to electron-poor alkenes 15 asymmetric cyclopropanation of chiral (l-phosphoryl)vinyl sulfoxides 16 synthetic methodology using tertiary phosphines as nucleophilic catalysts in combination with allenoates or 2-alkynoates 17 recent advances in the transition metal-catalysed asymmetric hydrosilylation of ketones, imines, and electrophilic C=C bonds 18 Michael additions catalysed by transition metals and lanthanide species 19 recent progress in asymmetric organocatalysis, including the aldol reaction, Mannich reaction, Michael addition, cycloadditions, allylation, epoxidation, and phase-transfer catalysis 20 and nucleophilic phosphine organocatalysis.21... 

Co2(CO)g has been used in numerous reactions in addition to the plethora of substitution reactions that it undergoes with phosphines, nitrosyls, alkynes, and so forth. Many of these substituted carbonyl complexes and their reactions have been discussed elsewhere in this report. One area in which Co2(CO)g has received much attention is catalysis two classes of catalytic reactions of which Co2(CO)g plays a major role are hydroformylation (see Hydroformylation) and Pauson Khand cycloaddition (see Pauson-Khand Reaction). 

A method has been developed that combines the advantages of solid-supported catalyst extraction and solution-phase reactivity. By preparing a palladium complex bearing an anthracene tag, this can then be attached to a solid support via a chemoselective Diels-Alder cycloaddition to sequester the palladium catalyst along with any dissociated phosphine or phosphine oxide at the end of the reaction, leaving the desired catalysis product in the solution. The basis of the methodology is shown in Scheme 15. 

Some data are available about catalysis in 1,2-cycloadditions. Tributyl phosphine catalyses dimerisation of phenyl isocyanate to uretidinedione in toluene . The reaction is kinetically of first order with respect to catalyst and overall third order the reverse process is first order with respect to catalyst and overall second order. The mechanism is complex, as revealed by the value of the apparent activation energy of the forward reaction (E= l.l 0.7 kcal.mole" ), which presumably results from the combined temperature dependence of two or more steps, including formation of an isocyanate-phosphine complex (see eqn. (13), p. 113). 

The development of new routes for the synthesis of phosphines is crucial for the synthesis of organophosphorus ligands for use in transition metal catalysis. Chemical transformations involving triple C-C bonds, such as nucleophilic addition and cycloaddition, are widely used for the synthesis of a diverse range of phosphines, which cannot be synthesized by other means [100], On the other hand, the [2- -2- -2] cycloaddition of alkynes catalyzed by transition metals are attracting much attention due to their use in the synthesis of various substituted aryl phosphines. This methodology can also be applied with other methods to synthesize various aryl phosphines [100, 101]. 

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