Complexes of alkenes

A complex with a CH group bridging a main group and a transition element has been prepared from Cp2TiCl2 (p. 290) and AlMe3. 

It reacts with ketones to produce methylene derivatives in higher yields than are obtained by the Wittig reaction. It adds to alkynes and to alkenes forming metallocycles. This is a pivotal step in the accepted mechanism for olefin metathesis (p. 373). 

The four membered titanacyclobutane ring is nearly planar. Studies of Tebbe s compound are also of interest in connection with Ziegler-Natta polymerization of alkenes. Commercial catalysts incorporate TiCl and aluminium alkyls (p. 371). 

The complexes formed by mono-olefins are often rather unstable. Thus tetracarbonyl(ethene)iron, a yellow oil, b.p. 34 C/12 mm, decomposes slowly at room temperature with loss of ethene to give Fe3(C0)i2. Photolysis can be useful in effecting the displacement of CO, for example when the thermal reaction has a high activation energy (p. 169). Often complexes of alkenes which carry electron attracting groups form more readily and are more robust than those of ethene itself. 

Bidentate diolefins such as 1, 5-cyclooctadiene or norbornadiene can chelate to a metal centre forming much more stable products than those obtained from monoolefins. 

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Reaction with additional alkyne releases an organic anthraquinone (128). Numerous TT-complexes are known. Examples include complexes of alkenes,... 

Complexes of Alkenes, Alkynes, and Arenes with Gold Compounds 299... 

The bonding in n complexes of alkenes is the result of two major contributions. The filled n orbital acts as an electron donor to empty d orbitals of the metal ion. There is also a contribution to bonding, called back-bonding, from a filled metal orbital interacting with the alkene n orbital. These two types of bonding are represented in Fig. 8.1. 

To some degree we have oversimplified electrophilic substitution by neglecting the possible role of the l l charge-transfer complexes that most electrophiles form with arenes (see Section 10-30 for discussion of analogous complexes of alkenes) ... 

Many complexes of alkenes, cycloalkenes, alkynes, and cycloalkynes with transition metals are now known. Some examples are ... 

Simple protonation is not expected in the case of formally d° metal complexes, as the metal has no lone pairs, but the proton may attack an M—H bonding electron pair and lead to products (equation 11). The protonation of many complexes of alkenes and other unsaturated hydrocarbons often occurs at carbon, not at the metal (e.g. equation 2852). 

There are also several situations where the metal can act as both a homolytic and heterolytic catalyst. For example, vanadium complexes catalyze the epoxidation of allylic alcohols by alkyl hydroperoxides stereoselectively,57 and they involve vanadium(V) alkyl peroxides as reactive intermediates. However, vanadium(V)-alkyl peroxide complexes such as (dipic)VO(OOR)L, having no available coordination site for the complexation of alkenes to occur, react homolyti-cally.46 On the other hand, Group VIII dioxygen complexes generally oxidize alkenes homolytically under forced conditions, while some rhodium-dioxygen complexes oxidize terminal alkenes to methyl ketones at room temperature. 

Epoxidation of alkenes by MoO(02)2(HMPA)(H20) (15) has been shown to proceed via two steps reversible complexation of alkene to the metal, displacing the equatorial HMPA ligand, followed by irreversible oxygen transfer to the alkene.11 118 This reaction has the following... 

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. 

Despite the complexities of alkene ozonolysis47, the reaction between alkenes and ozone may be summarized by Scheme 7. The reaction involves several steps48 with the formation of a variety of intermediates, such as a primary ozonide (1,2,3-trioxolane) (12), its isomer of rearrangement 13 and a carbonyl oxide (14). 

In organometallic complexes of alkenes, where a transition metal uses an empty d orbital to coordinate to the n bond of the alkene, the vinyl carbons move upfield to the range of 8 7-110, varying widely and depending on the exact structure of the complex.5 The reasons for this upfield shift include both shielding by the electron-rich electropositive metal, and a change in hybridization of the vinyl carbons toward less s character (i.e., more sp3 like). 

The various synthetic routes to complexes of alkenes and alkynes... 

Bromine jr-complexes of alkenes can be detected spectroscopically with bulky alkenes these intermediates can be stopped at the jr-complex stage,12 but normally the sequence is ... 

Mechanism The Wilkinson catalyst (6.9), a 6-electron complex, loses one or two triph-enylphosphine ligands and converts into a 14- or 12-electrons complex. The activation of hydrogen occurs by uptake on the metal complex catalyst via an oxidative addition. This is followed by ir-complexation of alkene to metal. Intramolecular hydride transfer and subsequent reductive elimination release the alkane and complete the cycle (Scheme 6.2). 

Gold(l) complexes of alkenes and alkynes appear to play an important role as intermediates of the gold(l)-catalyzed addition of water, alcohols, carboxylic acids, or amines to these substrates. The elfect of this 7r-complexation (22) is superior to the performance of mercury(ll) in this type of reactions. " In a similar way, gold(III) salts have found applications in homogeneous catalysis. ... 

Rntheninm complexes of alkenes, dienes, or cyclodienes in oxidation states 0 and II are known. From a preparative view, an important class of componnds is the oligomeric or polymeric complexes of composition [(dialkene)RnX2] , which often serve as reagents for further transformations. Their synthesis is similar to the procedure that has been detailed for phosphine complexes that is, when RuCl3(aq) is refluxed in alcohol in the presence of the diene, an insoluble brown to orange oligomeric complex is deposited. However, the use of... 

While the dependence on phosphine concentration and carbon monoxide strongly depends on the reaction parameters, it is important to note most catalyst systems will show a first-order dependence in alkene and rhodium concentration. A recent study involving fluoroalkylphosphine ligands illustrates this once again [37]. Thus, with very few exceptions, complexation of alkene to the metal centre is the rate-determining step. One exception was discussed above. 

Scheme 11.18 shows a reasonable mechanism for polymerization that consists of first complexation of alkene, followed by 1,2-insertion. Ideally, the two steps will continue until synthesis of high-molecular-weight polymer occurs. Chain growth will cease if chain transfer occurs. Scheme 11.19 indicates two possible mechanisms for chain transfer, both of which are reasonable and indistinguishable by experiment. In both cases, the steric bulk of the diimine ligand seems to hinder the transfer process.102... 

Whereas transition metal complexes of alkenes and their chemistry have been well explored, comparatively little is known about the structure and reactivity of n complexes obtained from strained olefins. The stability of transition metal complexes of alkenes in general is preferably discussed in terms of the Dewar-Chatt-Duncanson model (171). A mutual er-type donor-acceptor interaction accounts for the bonding overlap of the bonding 71-MO of the olefin with vacant orbitals of the metal together with interaction of filled d orbitals with the 7r -MO of the double bond (back bonding) leads to a partial transfer of. electron density in both directions (172). The major contribution to the stabilizing interaction is due to back-bonding. 

Vibrational spectra of r-bonded complexes of alkenes and alkynes with transition metals have been reviewed by Davidson. In contrast to c-bonded complexes (Sec. lV-2), the C=C and C=C stretching bands of jr-bonded complexes show marked shifts to lower frequencies relative to those of free ligands. 

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