Transition metals promotion, isomerization

TABLE 2. Transition metal promoted isomerization of 1,2,2-trimethylbicyclobutane (33) to 75 and 76 in CHCU ... 

The primary allene adducts of general structure 99 as introduced in Section 8.2.2.1 generally are able to undergo (Baldwin disfavored) S-endo-trig cyclizations to furnish products of type 100. These isomerizations are either promoted by bases or catalyzed by Lewis acids or transition metal complexes. 

The sulfate promoted transition metal oxides focussed considerable attention in recent years due to attractive catalytic properties. Most of the research carried out to date centered on sulfated zirconias,1 5 not surprisingly perhaps, as they exhibit the highest surface acidity (Ho <-16.04) among the members of this family of materials and appear to be able to initiate isomerization reactions in temperatures as low as 298 K. Far less interest attracted sulfated porous titanias, mainly owing to a lower surface acidity,6 although it may be a useful property in many catalytic situations. Thus closer inspection of the preparation procedures for sulfated titanias may be of interest, in particular as the reports on preparation and properties of these materials are scarce and we are not familiar with any work dealing with titania-sulfate aerogels. 

The trans cis isomerization of [Cr(C204)(H20)2] is also catalyzed by metal ions and a detailed study of catalysis by Mg11 has been published.540 Dissociation of oxalato complexes of Crin of the type [Cr(ox)n(OH2)6 2n](3 2n)+ (n = l, 3) is also promoted by a series of transition metal ions.541... 

Moreover, MPVO reactions are traditionally performed with stoichiometric amounts of Al(III) alkoxides. Some improvements came from the use of dinuclear AI(III) complexes that can be used in catalytic amount [6, 7]. This is why there has been an ever-increasing interest in catalytic MPVO reactions promoted by lanthanides and transition-metal systems [8]. In these cases, it is believed that reaction proceeds via formation of a metal hydride, in contrast with the mechanism accepted for traditional aluminum alkoxide systems, which involves direct hydrogen transfer by means of a cyclic intermediate [9]. As well as La, Sm, Rh and Ir complexes, Ru complexes have been found to be excellent hydrogen transfer catalysts. The high flexibility of these systems makes them very useful not only for MPVO-type reactions, but also for isomerization processes [10]. 

Very recent work (60b) has confirmed that Ir films do not isomerize neopentane most of the transition metals as well as palladium (60c) rearrange isobutane to k-butane but are also inactive for the former conversion. This clearly indicates that isomerization of neopentane on Pt is mechanistically rather special and, in view of the known propensity of Pt to promote ay exchange with deuterium of paraffins (5,49), refocuses attention on the ay species diadsorbed on one metal atom as the precursor for bond shift in simple alkanes. The following mechanism for neopentane isomerization on Pt is feasible, where the shifting... 

Abstract. Tungstated zirconia catalysts are stable and highly selective catalytic materials for the isomerization of alkanes when promoted by platinum and a transition metal oxide and when dihydrogen is present in the feed. Physical properties and the catalytic performance of these solids for the isomerization of n-pentane are discussed. 

Kulasegaram S, Kulawiec RJ (1997) J Org Chem 62 6547. For the isomerization of epoxides promoted by various transition metal complexes see the references cited therein... 

The inability of most transition metals to promote the isomerization of acyclic alkanes, especially by the cyclic type mechanism, may be understood if one considers in some detail the mechanisms of hydrocracking. In Tables VIII and IX (39,102,125-128) are reported the distributions of the products... 

Medium pore aluminophosphate based molecular sieves with the -11, -31 and -41 crystal structures are active and selective catalysts for 1-hexene isomerization, hexane dehydrocyclization and Cg aromatic reactions. With olefin feeds, they promote isomerization with little loss to competing hydride transfer and cracking reactions. With Cg aromatics, they effectively catalyze xylene isomerization and ethylbenzene disproportionation at very low xylene loss. As acid components in bifunctional catalysts, they are selective for paraffin and cycloparaffin isomerization with low cracking activity. In these reactions the medium pore aluminophosphate based sieves are generally less active but significantly more selective than the medium pore zeolites. Similarity with medium pore zeolites is displayed by an outstanding resistance to coke induced deactivation and by a variety of shape selective actions in catalysis. The excellent selectivities observed with medium pore aluminophosphate based sieves is attributed to a unique combination of mild acidity and shape selectivity. Selectivity is also enhanced by the presence of transition metal framework constituents such as cobalt and manganese which may exert a chemical influence on reaction intermediates. 

In 2009, Lambert reported a mild, efficient, and stereoselective synthesis of fused cyclopentenes via Mgl2-promoted isomerization of activated VCPs. The VCPs were prepared by Pd-catalyzed cyclopropanation of 1,3-dienyl p-ketoesters in moderate to high yields and good diastereoselectivities. Typical standard reaction conditions for the VCP-CP rearrangement (i.e., pyrolysis, transition metal catalysis, standard Lewis acid catalysis) were found to be ineffective for these substrates. However, the use of 1.5 equiv. of Mgl2 led to conplete conversion, providing bicyclic cyclopentenes in high yields fScheme 11.15L... 

A new protecting group for amines has been studied and used in the first chiral synthesis of anticapsin. The route involves initial bis-allylation to give a base-stable group resistant to nucleophiles, but which can be easily deprotected using the known allyl-to-propenyl isomerization reaction promoted by transition-metal complexes (Scheme 11). 

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