Metal catalysts, addition reaction

Transesterification of methyl methacrylate with the appropriate alcohol is often the preferred method of preparing higher alkyl and functional methacrylates. The reaction is driven to completion by the use of excess methyl methacrylate and by removal of the methyl methacrylate—methanol a2eotrope. A variety of catalysts have been used, including acids and bases and transition-metal compounds such as dialkjitin oxides (57), titanium(IV) alkoxides (58), and zirconium acetoacetate (59). The use of the transition-metal catalysts allows reaction under nearly neutral conditions and is therefore more tolerant of sensitive functionality in the ester alcohol moiety. In addition, transition-metal catalysts often exhibit higher selectivities than acidic catalysts, particularly with respect to by-product ether formation. 

Alkynes are reduced to alkanes by addition of TI2 over a metal catalyst. The reaction occurs in steps through an alkene intermediate, and measurements indicate that the first step in the reaction is more exothermic than the second step. 

Phosphonium salts can be synthesized by the transition-metal-catalyzed addition reaction of triaryphosphines and acids to unsaturated compounds. The reaction of PPh3, CH3SO3H, and alkynes in the presence of a palladium or rhodium catalyst gave alkenylphosphonium salts. Although Pd(PPh3)4 directed the C-P bond formation at the internal carbon atom of aliphatic 1-alkynes (Markovnikov mode), [RhCl(cod)]2... 

These critical aspects of the classical fluorous biphasic catalysis led in recent works to the development of protocols for the conversions with modified catalyst systems in non-fluorinated hydrocarbons as solvents. As part of the BMBE lighthouse project, Gladyzs and coworkers appHed this concept to C - C coupHng reactions (Suzuki reaction) and other metal-catalyzed addition reactions (hydrosilylation, selective alcoholysis of alkynes), which have direct relevance for the synthesis of fine chemicals and specialties [74]. 

Catalytic reduction of olefins by heavy metal catalysts probably involves metal hydride addition reactions also. If this is correct, the observed inhibition of the reduction by carbon monoxide, phosphines, sulfur compounds, and other materials with unshared electrons is exactly what would be expected if a vacant orbital on the hydride is required before addition can take place. 

There are transition-metal catalyzed addition reaction of alkyl units to alkenes, often proceeding with metal hydride elimination to form an alkene. An intramolecular cyclization reaction of an A-pyrrolidino amide alkene was reported using an iridium catalyst for addition of the carbon ot to nitrogen to the alkene unit. OS I, 229 IV, 665 VII, 479. 

With the platinum-metal catalysts, this reaction can be suppressed by conducting the hydrogenation in acid solution or in acetic anhydride, which removes the amine from the equilibrium as its salt or as its acetate. For reactions with Raney nickel, where acid cannot be used, secondary amine formation is prevented by addition of ammonia. Hydrogenation of nitriles containing other functional groups may lead to cyclic compounds. For example, indoUzidine and quinoUzidine derivatives have been obtained in certain cases (7.21). 

Ranu et al. have developed a mild and efficient method for the synthesis of a-dehydro-p-amino esters and nitriles by nucleophilic addition of amines to MBH acetates in water at room temperature without using any basic, acidic or metal catalyst. The reactions yielded only y-addition products, providing, with high stereoselectively, ( )-isomers in the case of MBH adducts bearing a carboxylic ester moiety and (Z)-isomers for adducts containing CN functionality (Scheme 3.123). An interesting stereoselective transformation of MBH adducts into cinna-mylamines via treatment with DMF-DMA has been described by Kim and coworkers (Scheme 3.124). ... 

The mechanisms of these reactions are varied, but can still be categorized. The hydrocyanations, hydrosilylations, and many of the hydroborations, occur with late-metal catalysts. These reactions occur by oxidative addition of the H-X bond, followed by migratory insertion of the olefin into the M-H or M-X bond, and reductive elimination to form the final product. Hydrocyanation occurs by insertion of the unsaturated reagent into the M-H bond, while hydrosilylation and hydroboration have been shown to occur by insertion of the olefin into the M-H bond in some cases and into the M-X bond in others. HydrosUy-lations and hydroborations of alkenes and alkynes catalyzed by (P transition metal complexes and by lanthanides follow a different pathway because these complexes caimot undergo oxidative addition. The mechanism of the reactions catalyzed by these complexes involves u-bond metatheses. 

It has been shown that the stereochemistry of the hydrosilylation of 1-aUcynes giving 1-silyl-l-alkenes depends on the catalysts or promoters used. For example, the reactions under radical conditions give the cis-product predominantly via trans-addition , while the platinum-catalyzed reactions afford the trans-product via exclusive cts-addition. In the reactions catalyzed by rhodium complexes, thermodynamically unfavorable c/s-1-silyl-l-alkenes are formed via apparent trans-addition as the major or almost exclusive product. Since the trans-addition of HSiEts to 1-alkynes catalyz by RhCl(PPh3)3 was first reported in 1974 , there have been controversy and dispute on the mechanism of this mysterious trans-addition that is vray rare in transition-metal-catalyzed addition reactions to aUtynes. Recently, iridium 4i6 mthenium complexes were also found to give the ds-product with extremely high selectivity (vide supra). 

As organosulfur compounds have been widely believed to be catalyst poisons, examples of the transition metal catalyzed reaction of these sulfur compounds have been limited. After the development of transition metal catalyzed addition of organosulfur compounds such as disulfides and thiols to carbon-triple bonds [9], many types of transition metal catalyzed addition reactions of organosulfur compounds have been developed. As to allenes, for example, the addition of thiols to terminal allenes successfully proceeds regioselectively at the internal double bonds of the allenes by the action of palladium acetate catalyst [10a,10b], while the disulfide addition to terminal allenes takes place at the terminal double bond in the presence of tetrakis (triphenylphosphine) palladium catalyst (Scheme 11.6) [10c]. 

Ketones a-Olefins bearing keto functionalities show also only weak interactions with aluminum compounds resulting in insuffident proteaion for the successful polymerization by transition metal catalysts. Additionally, undesired side reactions, for example, the keto-enol tautomerization of 2,2-dimethyl-11-dodecen-3-one in combination with MAO were reported. ... 

Silylcuprate reagents add across alkynes without any additional transition metal-catalysts. The reaction with 1,2-dienes shows divergent regioselectivity depending on substituents on the silicon center to give either of-substituted vinylsilanes or y substituted... 

In the transition-metal-catalyzed addition reactions of thiols to terminal alkynes, several addition products, i.e., Markovnikov-type adduct 1, Markovnikov addition and then double-bond-isomerization product 2, a n -Markovnikov adduct 3, double hydrothiolation product 4, and bisthiolation product 5, may be formed (Scheme 2). Controlling the product selectivity can be attained by the selection of transition metal complexes as catalysts, the use of additives, and/or the optimization of the reaction conditions (solvent, temperature, molar ratios of the starting materials, and so on). 

The unit has virtually the same flow sheet (see Fig. 2) as that of methanol carbonylation to acetic acid (qv). Any water present in the methyl acetate feed is destroyed by recycle anhydride. Water impairs the catalyst. Carbonylation occurs in a sparged reactor, fitted with baffles to diminish entrainment of the catalyst-rich Hquid. Carbon monoxide is introduced at about 15—18 MPa from centrifugal, multistage compressors. Gaseous dimethyl ether from the reactor is recycled with the CO and occasional injections of methyl iodide and methyl acetate may be introduced. Near the end of the life of a catalyst charge, additional rhodium chloride, with or without a ligand, can be put into the system to increase anhydride production based on net noble metal introduced. The reaction is exothermic, thus no heat need be added and surplus heat can be recovered as low pressure steam. 

The addition of alcohols to form the 3-alkoxypropionates is readily carried out with strongly basic catalyst (25). If the alcohol groups are different, ester interchange gives a mixture of products. Anionic polymerization to oligomeric acrylate esters can be obtained with appropriate control of reaction conditions. The 3-aIkoxypropionates can be cleaved in the presence of acid catalysts to generate acrylates (26). Development of transition-metal catalysts for carbonylation of olefins provides routes to both 3-aIkoxypropionates and 3-acryl-oxypropionates (27,28). Hence these are potential intermediates to acrylates from ethylene and carbon monoxide. 

The most common catalysts are sodium hydroxide and calcium hydroxide, generally used at a modest excess over the nominal stoichiometric amount to avoid formaldehyde-only addition reactions. Calcium hydroxide is cheaper than NaOH, but the latter yields a more facile reaction and separation of the product does not require initial precipitation and filtration of the metal formate (57). 

In addition to the processes mentioned above, there are also ongoing efforts to synthesize formamide direcdy from carbon dioxide [124-38-9J, hydrogen [1333-74-0] and ammonia [7664-41-7] (29—32). Catalysts that have been proposed are Group VIII transition-metal coordination compounds. Under moderate reaction conditions, ie, 100—180°C, 1—10 MPa (10—100 bar), turnovers of up to 1000 mole formamide per mole catalyst have been achieved. However, since expensive noble metal catalysts are needed, further work is required prior to the technical realization of an industrial process for formamide synthesis based on carbon dioxide. 

A modification of the direct process has recentiy been reported usiag a ckculating reactor of the Buss Loop design (11). In addition to employing lower temperatures, this process is claimed to have lower steam and electricity utihty requirements than a more traditional reactor (12) for the direct carbonylation, although cooling water requirements are higher. The reaction can also be performed ia the presence of an amidine catalyst (13). Related processes have been reported that utilize a mixture of methylamines as the feed, but require transition-metal catalysts (14). 

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

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