Metal activity, catalytic cyclization

Unsaturated metallacycles have also had a fascinating history. Often these are readily formed by the coupling of alkyne units and may be involved in the catalytic cyclization of alkynes. There are numerous examples of structurally characterized metallacyclopentadienes, and even some examples of metal-lacycloheptatrienes and 4-benzene complexes (77). More recently metal-lacyclobutadienes have been implicated as the active intermediates in alkyne... 

A review on the industrial applications of homogeneous catalysts is particularly welcome. The proceedings of recent symposia and a general text have been published in addition to reviews on metal cluster catalysts, activation of saturated hydrocarbons by metal complexes in solution, catalysis by arene Group-VIB tricarbonyls, titanocene-catalysed reactions ofalkenes, transition-metal hydrides in catalysis, the mechanisms of the catalytic cyclization of aliphatic, hydrocarbons, asymmetric hydrosilylation and asymmetric synthesis. A n.m.r. study of the conformations of chelated Diop and a MO study of organo-metallic migration reactions are also of interest. Polymer supported catalysts have been reviewed and the relationship between cross-linking of the polymer and catalytic activity has been discussed. ... 

Other Heteroatom Nucleophiles. Alcohols and carboxylic acids also add to metal-activated alkenes, and processes for the industrial conversion of ethylene to vinyl acetate and acetals are well established. However, these processes have not been extensively used with more cort5)lex alkenes. In contrast, a number of intramolecular versions of the processes have been developed, a few examples of which are given here. Allylphenols cyclize readily in the presence of palladium(II) to form benzofurans (eq 4). Catalytic amounts of palladium acetate can be used if the reaction is carried out under 1 atm of molecular oxygen with copper diacetate as cooxidant, or in the presence of tert-butyl hydroperoxide. If instead of palladium acetate a chiral jr-allylpalladium acetate complex is used, the cyclization proceeds to yield 2-vinyl-2,3-dihydrobenzofuran with up to 26% ee. ... 

In an extension of this work, the Shibasaki group developed the novel transformation 48—>51 shown in Scheme 10.25c To rationalize this interesting structural change, it was proposed that oxidative addition of the vinyl triflate moiety in 48 to an asymmetric palladium ) catalyst generated under the indicated conditions affords the 16-electron Pd+ complex 49. Since the weakly bound triflate ligand can easily dissociate from the metal center, a silver salt is not needed. Insertion of the coordinated alkene into the vinyl C-Pd bond then affords a transitory 7t-allylpalladium complex 50 which is captured in a regio- and stereocontrolled fashion by acetate ion to give the optically active bicyclic diene 51 in 80% ee (89% yield). This catalytic asymmetric synthesis by a Heck cyclization/ anion capture process is the first of its kind. 

Catalytic quantities of transition- or non-transition metals promote the cyclization of 2-alkynynylbiphenyl analogs to phenanthrene or fulvene analogs. The mechanism is thought to involve activation of the alkyne by metal coordination, prior to cyclization (Equations (179) and (180)).146... 

The proposed catalytic mechanism for intramolecular McMurry reaction begins with the reduction of TiCl3 by zinc metal to generate the activated titanium species A-19. Reductive cyclization of the dicarbonyl substrate forms the McMurry coupling product, along with titanium oxide complex B-15. To close the catalytic cycle, the oxide complex B-15 is converted to TiCl3 by Me3SiCl (Scheme 63).8d,8e... 

With respect to the large number of unsaturated diazo and diazocarbonyl compounds that have recently been used for intramolecular transition metal catalyzed cyclopropanation reactions (6-8), it is remarkable that 1,3-dipolar cycloadditions with retention of the azo moiety have only been occasionally observed. This finding is probably due to the fact that these [3+2]-cycloaddition reactions require thermal activation while the catalytic reactions are carried out at ambient temperature. A7-AUyl carboxamides appear to be rather amenable to intramolecular cycloaddition. Compounds 254—256 (Scheme 8.61) cyclize intra-molecularly even at room temperature. The faster reaction of 254c (310) and diethoxyphosphoryl-substituted diazoamides 255 (311) as compared with diazoacetamides 254a (312) (xy2 25 h at 22 °C) and 254b (310), points to a LUMO (dipole) — HOMO(dipolarophile) controlled process. The A -pyrazolines expected... 

C at pressures of about 250—400 kPa (36—58 psi). The two types of catalysts, the amorphous silica—alumina (52) and the crystalline aluminosilicates called molecular sieves or zeolites (53), exhibit strong carboniumion activity. Although there are natural zeolites, over 100 synthetic zeolites have been synthesized and characterized (54). Many of these synthetic zeolites have replaced alumina with other metal oxides to vary catalyst acidity to effect different type catalytic reactions, for example, isomerization. Zeolite catalysts strongly promote carboniumion cracking along with isomerization, disproportionation, cyclization, and proton transfer reactions. Because butylene yields depend on the catalyst and process conditions, Table 7 shows only approximations. 

The cyclopropanation of alkenes, alkynes, and aromatic compounds by carbenoids generated in the metal-catalyzed decomposition of diazo ketones has found widespread use as a method for carbon-carbon bond construction for many years, and intramolecular applications of these reactions have provided a useful cyclization strategy. Historically, copper metal, cuprous chloride, cupric sulfate, and other copper salts were used most commonly as catalysts for such reactions however, the superior catalytic activity of rhodium(ll) acetate dimer has recently become well-established.3 This commercially available rhodium salt exhibits high catalytic activity for the decomposition of diazo ketones even at very low catalyst substrate ratios (< 1%) and is less capricious than the old copper catalysts. We recommend the use of rhodium(ll) acetate dimer in preference to copper catalysts in all diazo ketone decomposition reactions. The present synthesis describes a typical cyclization procedure. 

Catalytic activity in zeolitic materials is strongly influenced by the type of alkali metal cations, and maximum catalytic activity, e.g, in isomerization reactions, is explained by the formation of an imide species EuNH [305]. Synergetic effects were observed in bimetallic supported Si02 which showed considerable hydrogen uptake during hydrogenation reactions [307]. The formation of Ln-NH2, -NH, -N species seemed to be suppressed in the presence of transition metal powders and precipitation of elemental lanthanides is favored [309]. Lanthanide imides were favored as active species in the Ln/AC-mediated cyclization of ethyne and propyne [310]. 

The use of hydrogen as terminal reductant has been accomplished by its activation with transition metal complexes. The resulting weak M-H bonds can be used in both radical generation and reduction through HAT. In this manner, conceptually novel radical chain reactions, such as hydrogen mediated cyclizations, or metal catalyzed processes with coupled catalytic cycles for radical generation and reduction, have been realized. The latter transformations are especially attractive for enantioselective synthesis. 

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