Sulfones transition metal catalysts

Not only heteroatom-H bonds but also activated C-H bonds can add to the jr-system of an allene. Since carbon lacks a free electron pair, the transition metal catalyst must first activate the C-H bond the new species formed will then react with the C=C double bond. For efficient activation of that kind, two acceptors (typically esters, nitriles and/or sulfones) are necessary. In accord with this mechanistic picture is the fact that the reaction does not benefit from an additional base (which would deproto-nate the pronucleophile). Hence neutral conditions are even better. 

In contrast to the free-radical polymerizations, there have been relatively few studies on transition metal catalysed polymerization reactions in water. This is largely due to the fact that the early transition metal catalysts used commercially for the polymerization of olefins tend to be very water-sensitive. However, with the development of late transition metal catalysts for olefin polymerizations, water is beginning to be exploited as a medium for this type of polymerization reaction. For example, cationic Pd(II)-bisphosphine complexes have been found to be active catalysts for olefin-CO copolymerization [21]. Solubility of the catalyst in water is achieved by using a sulfonated phosphine ligand (Figure 10.5) as described in Chapter 5. 

Treatment of halides or sulfonates with hydride donors such as tetrabutylammonium borohydride,38 lithium aluminium hydride,39 lithium triethylborohydride46 or sodium borohydride generate deoxy sugar derivatives (Scheme 3.8c).41 When sodium borohydride is employed, a transition metal catalyst (PdCk or NiC-b) may be added. 

The transition metal catalysts themselves are known to be stabilized by tertiary phosphines in their low oxidation states at one or more stages along the catalytic cycle. For this reason the most studied ligands in aqueous phase catalysis are derived from water-insoluble tertiary phosphines by installing on them ionic groups, typically sulfonate, but also sulfate, phosphonate, carbox-ylate, quaternary ammonium, phosphonium, as well as neutral polyether or polyamide substituents, etc. 

Many of the reactions are catalyzed by transition metal catalysts. A variety of water-soluble ligands have been devised to make the catalysts soluble in water. The most common is a sulfonated phosphine made by the sulfonation of triphenylphosphine (8.30) with fuming sulfuric acid.229... 

Sulfides react faster with hydrogen peroxide and alkyl hydroperoxides than do alkenes. For this reason, transition metal catalysts are rarely necessary, but these reactions are acid catalyzed and first order in both sulfide and peroxide. The acid (HX) can be as weak as alcohol or water but the "effectiveness (of the oxidation) is determined by the pXa of the acid. Sulfides also react faster with peroxides than do ketones (see the Baeyer-Villiger reaction, sec. 3.6). Formation of the sulfone in these reactions is straightforward, but requires more vigorous reaction conditions. It is usually easy to isolate the sulfoxide from oxidation of a sulfide. Direct conversion of a sulfide to a sulfone requires excess peroxide and vigorous reaction conditions (heating, long reaction times, more concentrated peroxide). 

Electron-deficient as well as electron-rich aryl boronic acids proved to be competent partners in the reaction, but electron-deficient boronic acids required higher temperatures. Boronic acids containing aryl halides (I, Cl) were also competent partners, providing a functional handle for further elaboration. Both primary and secondary amines have been utilised as coupling partners. A limitation of this chemistry is the inability to use nitrogen-based heterocycles due to either protodeboronation or the instability of the electron-poor sulfonyl chloride intermediate. Buchwald and coworkers later found that pyridylzinc reagents could be coupled with 2,4,6-trichlorophenyl chlorosulfate (TCPC) to access pyridine sulfonates without a transition metal catalyst." The pyridine sulfonates were subsequently treated with amines to generate sulfonamides. 

Polymerization of olefins such as styrene is promoted by acid or base or sodium catalysts, and polyethylene is made with homogeneous peroxides. Condensation polymerization is catalyzed by acid-type catalysts such as metal oxides and sulfonic acids. Addition polymerization is used mainly for olefins, diolefins, and some carbonyl compounds. For these processes, initiators are coordination compounds such as Ziegler-type catalysts, of which halides of transition metals Ti, V, Mo, and W are important examples. 

Colona and coworkers oxidized a variety of alkyl aryl and heterocyclic sulfides to the sulfoxides using t-butyl hydroperoxide and a catalytic amount of a complex (97) derived from a transition metal and the imines of L-amino acids. Of the metals (M = TiO, M0O2, VO, Cu, Co, Fe), titanium gave the highest e.e. (21%), but molybdenum was the most efficient catalyst. The sulfoxides were accompanied by considerable sulfone. ... 

Homogeneous catalysis by transition metal complexes almost always involves processes in which product-catalyst separation and catalyst recycling are important issues. For years, researchers have worked to find effective ways to isolate metal-complex catalysts in phases separate from those containing the catalyst, usually by anchoring the metal complex to a solid surface. As summarized by Driessen-Holscher, it is now evident that the method that has met with most practical success in this direction involves the use of multiple liquid phases. For example, rhodium complexes with water-soluble sulfonated ligands are used to catalyze alkene hydroformyla-tion, and the aqueous-phase catalyst and the organic products are easily separated as insoluble liquid phases. 

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