Cyclization reactions carbene transition metal complexes

One of the earliest enantioselective carbon-carbon bond-forming processes catalyzed by chiral transition-metal complexes is asymmetric cyclopropanation discussed in Chapter 5, which can proceed via face-selective carbometallation of carbene-metal complexes. Some other more recently developed enantioselective carbon-carbon bond forming reactions, such as Pd-catalyzed enantioselective alkene-CO copolymerization (Chapter 7) and Pd-catalyzed enantioselective alkene cyclization (Chapter 8.7), are thought to involve face-selective carbometallation of acy 1-Pd and carbon-Pd bonds, respectively (Scheme 4.4). Similarly, the asymmetric Pauson-Khand reaction catalyzed by chiral Co complexes most likely involves face-selective cyclic carbometallation of chiral alkyne-Co complexes (Chapter 8,7). 

The term intramolecular enyne metathesis describes two types of processes. One involves a [2+2] cycloaddition of a multiple bond and a transition-metal carbene complex and the other is an oxidative cyclization catalyzed by low-valent transition-metal complexes, for example, Pt, Pd and Ru. The latter reaction is also called a skeletal reorganization. Both processes lead to similar products (Eq. 84). 

Enyne metathesis is unique and interesting in synthetic organic chemistry. Since it is difficult to control intermolecular enyne metathesis, this reaction is used as intramolecular enyne metathesis. There are two types of enyne metathesis one is caused by [2+2] cycloaddition of a multiple bond and transition metal carbene complex, and the other is an oxidative cyclization reaction caused by low-valent transition metals. In these cases, the alkyli-dene part migrates from alkene to alkyne carbon. Thus, this reaction is called an alkylidene migration reaction or a skeletal reorganization reaction. Many cyclized products having a diene moiety were obtained using intramolecular enyne metathesis. Very recently, intermolecular enyne metathesis has been developed between alkyne and ethylene as novel diene synthesis. 

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... 

Alkenes and alkynes coordinate to transition metals and undergo a variety of reactions, and are very important substrates for transition metal-catalysed reactions. Their reactions with halides, cyclization via carbene complexes, hydrogenation, and oxidative reactions with Pd(II) are treated in Sections 3.2, 8.2, 10.1 and 11.1, respectively. The many other reactions of alkenes and alkynes as main reactants are treated in this chapter. 

The iodide complexes, [CpMo(CO)3 (CH2)3I ] [M = Mo (96) or W (97)], were originally prepared by Winter and co-workers (162). Both 96 and 97 react with excess iodide to form [CpMfCOJjIfCOfCH CHj ]. Winter and co-workers found that transition metal anions may induce similar cyclization reactions. Hence, reaction of excess Na[CpMo(CO)3] with Br(CH2)3Br yields 89, which reacts with further Na[CpMo(CO)3] to form an intermediate anionic acyl complex, which in turn undergoes elimination of Br- to form the carbene ligand in a manner similar to that... 

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. 

Besides the conventional methods, the metallo-carbene route to access cyclic compounds has become a versatile tool in sugar chemistry. Synthesis of stavudine 112, an antiviral nucleoside, from an allyl alcohol [101] is realized by a Mo(CO)5-mediated cyclization reaction (O Scheme 26). Molybdenum hexacarbonyl smoothly reacts with the triple bond of 113 to generate the intermediate Mo-carbene, which undergoes a clean cyclorearrangement to yield the furanoid glycal 114. Alkynol isomerization is effected by group-6 transition metal carbonyl complexes [102]. 

Transition metal-catalyzed reactions using alkynes as precursors of carbene and vinylidene complexes used in cyclizations leading to N-het-erocycles 05CL1068. 

The theme of this chapter is that transition metals let you do things to organic molecules which are unthinkable without them. Nowhere is this more true than in metathesis reactions, and we finish the chapter with a reminder of the power of the ruthenium catalysts we introduced in Chapter 38. There we discussed the carbene-based mechanism of the reaction, and we showed you some simple examples such as this cyclization of a symmetrical amine to give a five-membered heterocycle using a catalytic amount of the ruthenium complex known as Grubbs 1 catalyst. 

Unusual type of P-based cyclophane 154 containing NH,NH-functionalized carbene heterocyclic fragment coordinated to the transition metal was obtained by metal template assisted cyclization reactions of the diphosphine hgand bearing reactive 2-fluoro substiments at phenyl groups on phosphorus atoms and carbene heterocycle in coordination sphere of 153 (Scheme 12.57). Aerial oxidation of the complexes of the cyclophane hgand resulted in the isolation of the free macrocycle as di(phosphine oxide)/imidazolidinium salt [142, 143]. 

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