Transition-metal-promoted cyclization

Balme G, Bouyssi D, Monteiro N (2006) The Virtue of Michael-Type Addition Processes in the Design of Transition Metal-Promoted Cyclizative Cascade Reactions. 19 115-148 Barluenga J, Rodriguez F, Fanands FJ, Fldrez J (2004) Cycloaddition Reaction of Group 6 Fischer Carbene Complexes. 23 59-121 Basset J-M, see Candy J-P (2005) 16 151-210... 

Over the last twenty years, the tandem process involving transition metal-promoted cyclization reactions initiated or determined by a Michael addition has attracted an increasing interest in organic chemistry. The diversity of examples discussed in this work demonstrates the high potential of these tandem reactions for the efficient one-pot synthesis of complex structures with limiting catalysts and remarkable atom economy. It is expected that the development of further useful new sequences founded on this concept will be developed in the near future. 

The Virtue of Michael-Type Addition Processes in the Design of Transition Metal-Promoted Cyclizative Cascade Reactions... 

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... 

In 2010, Toste s group reported the first example of a highly enantioselective polyene cyclization reaction in which transition metal-promoted alkyne activation serves as the cyclization initiating event [34], The (MeO-DTBM-BIPHEP)gold(I)-catalyzed reaction offers an efficient method for the stereoselective synthesis of polycyclic compounds... 

Scheme 43 Synthesis of enantiopure ring-substituted 1,2-diaminocyclohexanes by transition metal catalyzed and promoted cyclization of 1,7-octadienes...

Hegedus et al. have thoroughly studied the homogeneous hydroamination of olefins in the presence of transition metal complexes. However, most of these reactions are either promoted or assisted, i.e. are stoichiometric reactions of an amine with a coordinated alkene [98-101] or, if catalytic, give rise to the oxidative hydroamination products, as for example in the cyclization of o-allylanilines to 2-alkylindoles [102, 103], i.e. are relevant to Wacker-type chemistry [104]. 

Asymmetric synthesis of tricyclic nitro ergoline synthon (up to 70% ee) is accomplished by intramolecular cyclization of nitro compound Pd(0)-catalyzed complexes with classical C2 symmetry diphosphanes.94 Palladium complexes of 4,5-dihydrooxazoles are better chiral ligands to promote asymmetric allylic alkylation than classical catalysts. For example, allylic substitution with nitromethane gives enantioselectivity exceeding 99% ee (Eq. 5.62).95 Phosphi-noxazolines can induce very high enatioselectivity in other transition metal-catalyzed reactions.96 Diastereo- and enantioselective allylation of substituted nitroalkanes has also been reported.9513... 

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. 

Metal complexes of heterocyclic compounds display reactivities changed greatly from those of the uncomplexed parent systems. All of the -electron system(s) of the parent heterocycle can be tied up in the complex formation, or part can be left to take part in alkenic reactions. The system may be greatly stabilized in the complex, so that reactions, on a heteroatom, for example, can be performed which the parent compound itself would not survive. Orbital energy levels may be split and symmetries changed, allowing hitherto forbidden reactions to occur. In short, a multitude of new reaction modes can be made possible by using complexes dimerization of azirines with a palladium catalyst serves as a typical example (Scheme 81). A variety of other insertion reactions, dimerizations, intramolecular cyclizations, and intermolecular addition reactions of azirines are promoted by transition metals. 

The cyclizations of unsaturated precursors 128 can be induced by an electrophile or by a transition metal catalyst as illustrated by the iodonium ion promoted cyclization of allyloxybenzenes 130 (Scheme 76) <2004JA3416> and the preparation of 4,5,7-trimethylchroman 132 from 131 using a RuCl3/AgOTf catalytic system (Scheme 77) <20040L581, CHEC-III(7.08.11.1.2)517>. 

Abstract This review gives an insight into the growing field of transition metal-catalyzed cascades. More particularly, we have focused on the construction of complex molecules from acyclic precursors. Several approaches have been devised. We have not covered palladium-mediated cyclizations, multiple Heck reactions, or ruthenium-catalyzed metathesis reactions because they are discussed in others chapters of this book. This manuscript is composed of two main parts. In the first part, we emphasize cascade sequences involving cycloaddition, cycloisomerization, or ene-type reactions. Most of these reaction sequences involve a transition metal-catalyzed step that is either followed by another reaction promoted by the same catalyst or by a purely thermal reaction. A simple change in the temperature of the reaction mixture is often the only technical requirement to go from one step to another. The second part covers the cascades relying on transition metalo carbenoid intermediates, which have recently undergone tremendous... 

For transition metals and Zn(II), how might metal complexes be designed to promote rapid cleavage of RNA One approach is the functionalization of ligands to participate in catalysis. The N-methyl-CR ligand was modified to contain a basic group for bifunctional catalysis (21). A Zn(II) complex of one of the modified macrocycles was shown to accelerate cyclization of the RNA model substrate 1 (1 = 4-nitrophenylphosphate ester of propylene glycol) 20-fold more rapidly than the Zn(II) complex of N-methyl-CR. 

As is clear from the introductory discussion, most, if not all, of the d-block transition metals are expected to participate in reactions that are related to those discussed here. In addition to the Co-based methodology mentioned earlier, some related reactions of Pd and are known. Also related are the cyclization reactions of metal-carbene complexes containing Cr, Mo, W and other transition metals with alkynes and alkenes and a recently reported Nb- or Ta-promoted diyne-alkyne cyclization reaction, which appears to be closely related to a number of previously developed alkyne cyclotrimerization reactions, such as those catalyzed by Co. Investigations of reactions involving other transition metals may prove to be important especially from the viewpoint of developing asymmetric and catalytic procedures. 

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