Transition metal catalysts, initiation

Another method of manufacture involves the oxidation of 2-isopropylnaphthalene ia the presence of a few percent of 2-isopropylnaphthalene hydroperoxide/i)ti< 2-22-(y as the initiator, some alkaU, and perhaps a transition-metal catalyst, with oxygen or air at ca 90—100°C, to ca 20—40% conversion to the hydroperoxide the oxidation product is cleaved, using a small amount of ca 50 wt % sulfuric acid as the catalyst at ca 60°C to give 2-naphthalenol and acetone in high yield (70). The yields of both 2-naphthalenol and acetone from the hydroperoxide are 90% or better. 

Addition. Chlorine adds to vinyl chloride to form 1,1,2-trichloroethane [79-00-5] (44—46). Chlorination can proceed by either an ionic or a radical path. In the Hquid phase and in the dark, 1,1,2-trichloroethane forms by an ionic path when a transition-metal catalyst such as ferric chloride [7705-08-0], FeCl, is used. The same product forms in radical reactions up to 250°C. Photochernically initiated chlorination also produces... 

Between the 1920s when the initial commercial development of mbbery elastomers based on 1,3-dienes began (5—7), and 1955 when transition metal catalysts were fkst used to prepare synthetic polyisoprene, researchers in the U.S. and Europe developed emulsion polybutadiene and styrene—butadiene copolymers as substitutes for natural mbber. However, the tire properties of these polymers were inferior to natural mbber compounds. In seeking to improve the synthetic material properties, research was conducted in many laboratories worldwide, especially in the U.S. under the Rubber Reserve Program. 

Another interesting feature of these azirine reactions is that many of the thermally initiated reactions can be effected at room temperature in the presence of a suitable transition metal catalyst. Some typical examples are displayed in Scheme 95. 

Studies on the dimerization and hydrogenation of olefins with transition metal catalysts in acidic chloroaluminate(III) ionic liquids report the formation of higher molecular weight fractions consistent with cationic initiation [L7, 20, 27, 28]. These... 

Transition metal catalysts arc characterized by their redox ehemistry (catalysts can be considered as one electron oxidants/reductants). They may also be categorized by their halogen affinity. While in the initial reports on ATRP (and in most subsequent work) copper266,267 or ruthenium complexes267 were used, a wide range of transition metal complexes have been used as catalysts in ATRP. 

Figure 11.15. Promotional effect of different transition metals on initial NOx storage/reduction activity for 0.5Pt/7.5Ba/2.5 Promoter catalyst [90].

Based on an early process discovered by Natta in the 1950s, soluble transition metal catalysts like metallocenes were developed mainly in the 1950s as initiators for polyolefin syntheses. Others are still now under investigation, like the so-called LTM (for "late transition metal") catalysts. Metallocenes seem... 

Another remarkable property of iodorhodium(III) porphyrins is their ability to decompose excess diazo compound, thereby initiating carbene transfer reactions 398). This observation led to the use of iodorhodium(III) me.vo-tetraarylporphyrins as cyclopropanation catalysts with enhanced syn anti selectivity (see Sect. 2.2.3) s7, i°o) as wep as catalysts for carbenoid insertion into aliphatic C—H bonds, whereby an unusually high affinity for primary C—H bonds was achieved (see Sect. 6.1)287). These selectivities, unapproached by any other transition metal catalyst,... 

A direct addition of cydoethers to terminal alkynes has been discovered by Zhang and Li (Scheme 6.136) [271]. The best results were obtained when the reactions were run without additional solvent and in the absence of additives such as transition metal catalysts, Lewis acids, or radical initiators. Typically, the cycloether was used in large excess (200 molar equivalents) as solvent under sealed-vessel conditions. At a reaction temperature of 200 °C, moderate to good yields of the vinyl cycloether products (as mixtures of as and trans isomers) were obtained. The reaction is proposed to follow a radical pathway. 

Some transition metal catalysts induce the living polymerization of various acetylenic compounds.68,69 Such polymerizations of phenylacetylene catalyzed by rhodium complexes are used in conjunction with a quantitative initiation and introduction of functional groups at the initiating chain end (Scheme 16).70 The catalyst is prepared from an [RhCl(nbd)]2/Ph2C=C(Ph)Li/PPh3 mixture and proceeds smoothly to give quantitatively the polymer 54 with a low polydispersity ratio. 

Our initial studies focused on the transition metal-catalyzed [4+4] cycloaddition reactions of bis-dienes. These reactions are thermally forbidden, but occur photochemically in some specific, constrained systems. While the transition metal-catalyzed intermole-cular [4+4] cycloaddition of simple dienes is industrially important [7], this process generally does not work well with more complex substituted dienes and had not been explored intramolecularly. In the first studies on the intramolecular metal-catalyzed [4+4] cycloaddition, the reaction was found to proceed with high regio-, stereo-, and facial selectivity. The synthesis of (+)-asteriscanoHde (12) (Scheme 13.4a) [8] is illustrative of the utihty and step economy of this reaction. Recognition of the broader utiHty of adding dienes across rc-systems (not just across other dienes) led to further studies on the use of transition metal catalysts to facilitate otherwise difficult Diels-Alder reactions [9]. For example, the attempted thermal cycloaddition of diene-yne 15 leads only... 

The C-H insertion of alkyl sulfonamides using hypervalent iodine reagents in the presence of a transition metal catalyst was initially disclosed by Dauban and Dodd <20000L2327>. In this report, sulfonamide 204 was treated with PhI(OAc)2 and base to form an intermediate iminoiodinane 205 (Scheme 28). The material 205 was first... 

A series of internal alkynes has been investigated, revealing that the presence of an alkyl or aryl group does not appear to change the course of the reaction. Internal alkynes, however, do not undergo germylformylation in this system. In the case of tin, two reports exist of the stannylformyla-tion of unsaturated carbon substrates, but both proceed by a free radical mechanism initiated by AIBN and do not require a transition metal catalyst.128... 

The initiation of thermo-oxidation of polyethylene films by transition metal catalysts during composting proceeded slowly compared to oxidation at the same temperature in an oven (60-70 °C) [56]. The starch-filled polyethylene bags exposed on the surface of the compost broke down into small pieces while the buried bags remained intact after 49 days of exposure [55,... 

Catalytic hydroboration of vinylic ethers, acetals, and esters with pinacolborane takes place smoothly in the presence of transition metal catalysts. However, a noticeable exception is the catalytic hydroboration of vinyl bromides 59 which do not furnish the expected hydroborated product under these conditions. The reaction of vinyl bromides with pinacolborane initially affords the expected /3-boronoalkylbromide 60. A fast. -elimination ensues to furnish the terminal alkene 61 and 7 -bromopinacolborane 63. The alkene 61 undergoes hydroboration with unreacted pinacolborane to provide the debrominated boronate 62. The intermediate 5-bromopinacolborane 63 cleaves the ethereal C-O bond in the solvent (THF) to provide 4-bromobutyl borate 64 as a side product (Scheme 11) <1996JA909, 2000CSP14505>. 

Polybutadiene made by alkyllithium initiators is essentially free of the gel and color sometimes associated with the transition-metal catalysts and thus occupies a large market as an impact modifier in plastics. 

Polybutadiene (transition-metal catalyst) Polyisoprene (transition-metal catalyst) Polybutadiene and SBR (alkyllithium initiator)... 

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