Transition-metal-coordinated alkenes reactions with

Reactions of transition-metal coordinated olefins with diazo compounds as a route to cyclopropane products have not yet been rigorously established. Catalysts that should be effective in this pathway are those that are more susceptible to olefin coordination than to association with a diazo compound and also those whose coordinated alkene is sufficiently electrophilic to react with diazo compounds, especially diazomethane. Pd(II), Pt(II), and Co(II) compounds appear to be capable of olefin coordination-induced cyclopropanation reactions, but further investigations will be required to unravel this mechanistic possibility. 

The reaction occurs with some enantioselectivity and requires the presence of pivaldehyde (which is also oxidized)29,30. The reaction occurs for many other alkenes using transition metals coordinated to 1,3-diketone type ligands31-34. Use of a cobalt(II) complex and aldoacetal in place of the Mn(III) compound and pivaldehyde gives a novel method for the synthesis of acid-sensitive epoxides35. 

Considering the mechanistic rationales of the transition metal-catalyzed enyne cycloisomerization, different catalytic pathways have been proposed, depending on the reaction conditions and the choice of metal catalyst [3-5, 45], Complexation of the transition metal to alkene or alkyne moieties can activate one or both of them. Depending on the manner of formation of the intermediates, three major mechanisms have been proposed. The simultaneous coordination of both unsaturated bonds to the transition metal led to the formation of metallacydes, which is the most common pathway in transition metal-catalyzed cycloisomerization reactions. Hydrometalation of the alkyne led to the corresponding vinylmetal species, which reacts in turn with olefins via carbometalation. The last possible pathway involves the formation of a Jt-allyl complex which could further react with the alkyne moiety. The Jt-allyl complex could be formed either with a functional group at the allylic position or via direct C-H activation. Here the three major pathways will be discussed in a generalized form to illustrate the mechanisms (Scheme 8). 

New Applications of TCNE in Organometallic Chemistry, A. J. Fatiadi (1987). Selected reactions used in organometallic synthesis are reviewed. 311 references are given. Structure and bonding of metal-TCNE complexes as well as reactions of TCNE with main-group organometallics, with transition-metal complexes, with metal-coordinated alkenes and alkynes, and reactions of platinum-family complexes are discussed. 

For this review we limit the definition of G-F bond activation at transition metal centers to reactions involving cleavage of C-F bonds mediated by transition metals. The majority of such reactions involve a change in the primary coordination sphere of the metal. Nevertheless, the definition allows for enhancement of reactivity of G-F bonds even if the immediate coordination sphere of the metal is unchanged. Section 1.26.1 of our review sets out the principal modes of coordination of fluorinated molecules to transition metal centers and the principal pathways for C-F bond activation with one or two examples for each type. Section 1.26.2 provides a moderately comprehensive set of examples of intermolecular G-F bond activation organized by aromatic, alkene, and aliphatic C-F bonds and for the period 1997-2005. Section 1.26.3 follows the same pattern but for intramolecular G-F activation. 

The reaction of alkenes with alkenes or alkynes does not always produce an aromatic ring. An important variation of this reaction reacts dienes, diynes, or en-ynes with transition metals to form organometallic coordination complexes. In the presence of carbon monoxide, cyclopentenone derivatives are formed in what is known as the Pauson-Khand reaction The reaction involves (1) formation of a hexacarbonyldicobalt-alkyne complex and (2) decomposition of the complex in the presence of an alkene. A typical example Rhodium and tungsten ... 

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]. 

The mechanism for the reaction catalyzed by cationic palladium complexes (Scheme 24) differs from that proposed for early transition metal complexes, as well as from that suggested for the reaction shown in Eq. 17. For this catalyst system, the alkene substrate inserts into a Pd - Si bond a rather than a Pd-H bond [63]. Hydrosilylation of methylpalladium complex 100 then provides methane and palladium silyl species 112 (Scheme 24). Complex 112 coordinates to and inserts into the least substituted olefin regioselectively and irreversibly to provide 113 after coordination of the second alkene. Insertion into the second alkene through a boat-like transition state leads to trans cyclopentane 114, and o-bond metathesis (or oxidative addition/reductive elimination) leads to the observed trans stereochemistry of product 101a with regeneration of 112 [69]. 

In order to rationalize the catalyst-dependent selectivity of cyclopropanation reaction with respect to the alkene, the ability of a transition metal for olefin coordination has been considered to be a key factor (see Sect. 2.2.1 and 2.2.2). It was proposed that palladium and certain copper catalysts promote cyclopropanation through intramolecular carbene transfer from a metal carbene to an alkene molecule coordinated to the same metal atom25,64. The preferential cyclopropanation of terminal olefins and the less hindered double bond in dienes spoke in favor of metal-olefin coordination. Furthermore, stable and metastable metal-carbene-olefin complexes are known, some of which undergo intramolecular cyclopropane formation, e.g. 426 - 427 415). 

Other transition-metal oxidants can convert alkenes to epoxides. The most useful procedures involve /-butyl hydroperoxide as the stoichiometric oxidant in combination with vanadium, molybdenum, or titanium compounds. The most reliable substrates for oxidation are allylic alcohols. The hydroxyl group of the alcohol plays both an activating and a stereodirecting role in these reactions. /-Butyl hydroperoxide and a catalytic amount of VO(acac)2 convert allylic alcohols to the corresponding epoxides in good yields.44 The reaction proceeds through a complex in which the allylic alcohol is coordinated to... 

The mechanism for the stereoselective polymerization of a-olefins and other nonpolar alkenes is a Ti-complexation of monomer and transition metal (utilizing the latter s if-orbitals) followed by a four-center anionic coordination insertion process in which monomer is inserted into a metal-carbon bond as described in Fig. 8-10. Support for the initial Tt-com-plexation has come from ESR, NMR, and IR studies [Burfield, 1984], The insertion reaction has both cationic and anionic features. There is a concerted nucleophilic attack by the incipient carbanion polymer chain end on the a-carbon of the double bond together with an electrophilic attack by the cationic counterion on the alkene Ti-electrons. 

Species (A) and (B) constitute the main class of unsaturated carbenes and play important roles as reactive intermediates due to the very electron-deficient carbon Cl [1]. Once they are coordinated with an electron-rich transition metal, metal vinylidene (C) and allenylidene (D) complexes are formed (Scheme 4.1). Since the first example of mononuclear vinylidene complexes was reported by King and Saran in 1972 [2] and isolated and structurally characterized by Ibers and Kirchner in 1974 [3], transition metal vinylidene and allenylidene complexes have attracted considerable interest because of their role in carbon-heteroatom and carbon-carbon bond-forming reactions as well as alkene and enyne metathesis [4]. Over the last three decades, many reviews [4—18] have been contributed on various aspects of the chemistry of metal vinylidene and allenylidene complexes. A number of theoretical studies have also been carried out [19-43]. However, a review of the theoretical aspects of the metal vinylidene and allenylidene complexes is very limited [44]. This chapter will cover theoretical aspects of metal vinylidene and allenylidene complexes. The following aspects vdll be reviewed ... 

As noted in the introduction, in contrast to attack by nucleophiles, attack of electrophiles on saturated alkene-, polyene- or polyenyl-metal complexes creates special problems in that normally unstable 16-electron, unsaturated species are formed. To be isolated, these species must be stabilized by intramolecular coordination or via intermolecular addition of a ligand. Nevertheless, as illustrated in this chapter, reactions of significant synthetic utility can be developed with attention to these points. It is likely that this area will see considerable development in the future. In addition to refinement of electrophilic reactions of metal-diene complexes, synthetic applications may evolve from the coupling of carbon electrophiles with electron-rich transition metal complexes of alkenes, alkynes and polyenes, as well as allyl- and dienyl-metal complexes. Sequential addition of electrophiles followed by nucleophiles is also viable to rapidly assemble complex structures. 

Among the transition-metal catalysts that have been used, only those of Pd(II) are productive with diazomethane, which may be the result in cyclopropanation reactions [7,9,21] of a mechanism whereby the Pd-coordinated alkene undergoes electrophilic addition to diazomethane rather than by a metal carbene transformation in any case, asymmetric induction does not occur by using Pd(II) complexes of chiral bis-oxazolines [22],... 

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