Catalytic action alkenes

A large variety of codimerization reactions under the catalytic action of copper complexes is known. Usually, these reactions proceed via carbene intermediates and provide substituted ethenylcyclopropanes. Most of the catalysts for these reactions consist of copper(I) chloride and a phosphorus ligand, such as triphenylphosphane or triphenyl phosphite. Under the influence of these catalysts, carbenes are presumably formed from various substituted cyclopropenes at temperatures ranging from —40 to - -20°C, and these carbenes can be trapped by reaction with alkenes. ... 

Under the catalytic action of palladium(O) complexes, (2-siloxyallyl) acetates and the vinylogous [4-(trimethylsiloxy)penta-2,4-dienyl] acetate (Table 18), (2-oxo-3-silylpropyl) acetates (1-acetoxy-3-silylpropan-2-ones), ° and (2-oxoalkyl) carbonates react with the strained, but otherwise nonactivated double bonds in norbornene, norbornadiene, and dicyclopentadiene to form polycyclic cyclopropyl ketones, see also formation of 1, 2 °and 3. In contrast, the (2-siloxyallyl) acetates failed to react with simple alkenes such as dec-1-ene and cyclohexene. " With the substrates mentioned, the exo anti) diastereomers were obtained exclusively. 

Palladium(II) acetate and palladium(II) chloride (often applied as the soluble dibenzonitrile complex) are especially suited for cyclopropanation of strained double bonds as well as styrene and its ring-substituted derivatives.152,154,155 The good coordinating abilities of these palladium ) compounds, however, somewhat complicate the catalytic action and may even limit it. Thus, the presence of phosphane ligands in palladium(II) halides causes a significant induction period for decomposition of the diazocarbonyl compound,156 and the formation of stable palladium diene complexes may even prevent the cyclopropanation reaction.155,157 Furthermore, alkenes such as 4-dimethylaminostyrene and 4-vinylpyridine cannot be cyclo-propanated since their basic center deactivates the catalyst.155... 

The less bulky ligand (71) studied by Gladfelter leads to dimeric complexes [Rh2(71)2(CO)2] and even tetramers.222 Transformations of rhodium carbonyl complexes in alkene hydroformylation are discussed from the standpoint of the catalytic system self-control under the action of reaction... 

Another piece of mechanistic evidence was reported by Snapper et al. [14], who describe a ruthenium catalyst caught in action . During studies on ring opening metathesis, these authors were able to isolate and characterize carbene 5 in which a tethered alkene group has replaced one of the phosphines originally present in Id. Control experiments have shown that compound 5 by itself is catalytically active, thus making sure that it is a true intermediate of a dissociative pathway rather than a dead-end product of a metathetic process. 

Alcohols may be prepared (1) by hydration of alkenes (1) in presence of an acid and (11) by hydroboratlon-oxidatlon reaction (2) from carbonyl compounds by (1) catalytic reduction and (11) the action of Grignard reagents. Phenols may be prepared by (1) substitution of (1) halogen atom In haloarenes and (11) sulphonic acid group In aiyl sulphonic acids, by -OH group (2) by hydrolysis of diazonium salts and (3) industrially from cumene. 

The importance of this reaction also lies in the fact that asymmetric epoxidation of alkenes other than allylic alcohols is possible with this catalytic system (see Section 9.3.4). The third reaction relates to catalysts developed by Unilever for improved detergent action in the presence of hydrogen peroxide. The important point to note is that catalytic intermediates with metal-oxo groups play a pivotal role in all these reactions. 

Dioxiranes are extremely useful reagents for the epoxidation of alkenes under neutral conditions. Since the oxygen atom transfer to the alkene regenerates the initial ketone, this epoxidation is catalytic. Dioxiranes are easily generated by the action of an oxone (potassium persulfate) on a ketone (usually acetone) either in a biphasic mixture or in a homogeneous aqueous organic solution. 

Allylic alcohols r iesent a small fi-action of the total population of alkenes found in organic molecules. Asymmetric epoxidation of allylic alcohols therefore taps only a small portion of the synthetic potential inherent in a completely general asymmetric epoxidation of isolated (nonfunctionalized) alkenes. A partial solution to this problem now exists. The recent development of a catalytic asymmetric process for the dihydroxylation of alkenes provides an indirect route to epoxides or epoxide-like functionalization of alkenes. The stereochemistry of the process, the scope of enantioselectivity and chemical yield and a summary of key chemical transformations are presented in this section. Since this approach to alkene functionalization is at an early stage of development, the results sutnmarized here are certain to benefit fix>m extensions and improvements as research in this area progresses. 

The mode of action of alkene metathesis is still under investigation, but Chauvin in 1971 was the first to postulate the now accepted catalytic cycle. In the catalytic cycle, first a metal alkylidene complex 6 reacts formally in a [2+2]-cycloaddition with one alkene 7 to form a metalla-cyclobutane 8, which undergoes a formal [2+2]-cycloreversion in the next step to form a new metal alkylidene complex 9 and one product alkene 10. The newly formed metal alkylindene complex 9 then reacts with the second substrate alkene 11 to the metalla-cyclobutane 12 which gives upon formal [2+2]-cycloreversion the second product alkene 13 and again the metal alkylidene 6 to start a new catalytic cycle. 

About the only catalytic use for silver is in the conversion of ethylene to ethylene oxide by the action of oxygen.42 This reaction is very specific. Other alkenes are oxidized to CO2 and H2O with no epoxide formation observed. Gold is generally inactive as a catalyst for most reactions but it has found some use in catalytic oxidations. 43... 

The O-benzylated aldehyde 517 was also coiu erted into the a,p-unsaturated ester 522 through Wadsworth-Emmons reactioiT-- with ethyl 2-(diethoxyphosphono)acetate5 ° in excellent yield (Scheme 88). The intermediate alkene 522 was subjected to Sharpless asymmetric dihydroxylalion " to afford the diol ester 523 in excellent yield and with a diastereoselectivity in excess of 95 5. Subsequent to alkali-catalyzed hydrolysis of 523, the carboxylic acid obtained was condensed with the p-lolucncsulfonalc salt of glycine benzyl ester or phenylalanine benzyl ester, by the action of 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC), to afford the benzyl-protected amide derivatives catalytic... 

Aniline that is orf/io-substituted with a hexa-2,5-dienyl side-chain undergoes catalytic, palladium-assisted cyclization to 2-propylquinoline (Scheme 34) this is only one of a series of ring-closure reactions involving palladium-promoted nucleophilic attack on an alkene. A tetrahydroquinoline (55) is produced by the action of trifluoroacetic acid on the hydroxylamine (54). This method has also been used to prepare l,4-benzoxazines. ... 

Catalytic reactions involving secondary alcohols on metal oxides are thought to proceed through mechanisms involving a cooperative action of acidic and basic sites. For the studied zeolites, a quasi-linear correlation was established between the alkene selectivities and the ratio of basic to acidic sites, as determined by adsorption calorimetry (Figure 13 see also Figure 6). 

Oxidative Cleavage of Alkenes. An alternative to the oxidative cleavage of alkenes using ozone or the Lemieux-Johnson protocol has been reported recently. Under the action of catalytic osmium tetroxide, with oxone as a reoxidant, a variety of substituted alkenes were cleaved efficiently to furnish carbonyl compounds (eq 37). Any of the aldehydes that are produced via this sequence are immediately oxidized in situ to give the corresponding acid clearly this does not happen for any ketones so produced. Even electron deficient alkenes such as o , -unsaturated carbonyl compounds could be conveniently oxidized, although the products then underwent a decarboxylation reaction to produce the corresponding diacid. 

Herisson and Chanvin proposed that metathesis reactions are catalyzed by carbene (alkyl-idene) complexes that react with alkenes via the formation of a cyclic intermediate, a metallacyclobutane, as shown in Figure 14.24c. In this mechanism, a metal carbene complex first reacts with an alkene to form the metallacyclobutane. This intermediate can either revert to reactants or form new products because all steps in the process are in equilibria, an equilibrium mixture of alkenes results. This non-pairwise mechanism would enable the statistical mixture of products to form from the start by the action of catalytic amounts of the necessary carbene complexes, with both R and R groups, as shown in Figure 14.24c. 

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