Reactions Involving Olefinic Centers

Solution-phase synthetic procedures leading to (NHC)2Pd complexes revolve around ligand displacement reactions involving palladium centers bearing weakly coordinating ligands such as phosphines or olefins (see Scheme 1). For instance, a 14-electron species such as Pd(P(o-tolyl)3)2 reacts with free NHC... 

However, the reactivity of such compounds is still low e.g., when R, = R2 = R3 = CH and X = Cl, the silanes produced from CF3C2H4Li and C6F13C2H4Li were obtained in 40 and 20% yields, respectively. As a result of this poor reactivity, this method was abandoned and efforts now center around hydrosilylation reactions involving fluorinated olefins. 

This observation may well explain the considerable difference between metal-olefin and metal-acetylene chemistry observed for the trinuclear metal carbonyl compounds of this group. As with iron, ruthenium and osmium have an extensive and rich chemistry, with acetylenic complexes involving in many instances polymerization reactions, and, as noted above for both ruthenium and osmium trinuclear carbonyl derivatives, olefin addition normally occurs with interaction at one olefin center. The main metal-ligand framework is often the same for both acetylene and olefin adducts, and differs in that, for the olefin complexes, two metal-hydrogen bonds are formed by transfer of hydrogen from the olefin. The steric requirements of these two edgebridging hydrogen atoms appear to be considerable and may reduce the tendency for the addition of the second olefin molecule to the metal cluster unit and hence restrict the equivalent chemistry to that observed for the acetylene derivatives. 

The olefin oxygenations carried out with dioxygen seem to be metal-centered processes, which thus require the coordination of both substrates to the metal. Consequently, complexes containing the framework M (peroxo)(olefin) represent key intermediates able to promote the desired C-0 bond formation, which is supposed to give 3-metalla -l,2-dioxolane compounds (Scheme 6) from a 1,3-dipolar cycloinsertion. This situation is quite different from that observed in similar reactions involving middle transition metals for which the direct interaction of the olefin and the oxygen coordinated to the metal, which is the concerted oxygen transfer mechanism proposed by Sharpless, seems to be a more reasonable pathway [64] without the need for prior olefin coordination. In principle, there are two ways to produce the M (peroxo)(olefin) species, shown in Scheme 6, both based on the easy switch between the M and M oxidation states for... 

The intrinsic stability of the aromatic n system has two major consequences for the course of reactions involving it directly. First, the aromatic ring is less susceptible to electrophilic, nucleophilic, and free-radical attack compared to molecules containing acyclic conjugated n systems. Thus, reaction conditions are usually more severe than would normally be required for parallel reactions of simple olefins. Second, there is a propensity to eject a substituent from the tetrahedral center of the intermediate in such a way as to reestablish the neutral (An + 2)-electron system. Thus, the reaction is two step, an endothermic first step resulting in a four-coordinate carbon atom and an exothermic second step, mechanistically the reverse of the first, in which a group is ejected. The dominant course is therefore a substitution reaction rather than an addition. 

The Kharasch addition reactions promoted by [RuCl2(PPh3)3] are believed to proceed through a redox chain mechanism (Eqs. 1-3) [ 16]. Their kinetics show a first-order dependence both on the ruthenium complex and on CC14. Whereas no clear-cut evidence for alkene coordination to the metal was found with catalyst precursor 1 (which readily loses one phosphine ligand), olefin coordination cannot be excluded because there is a saturation kinetic rate dependence on the alkene. This observation led to the proposal of a reversible step involving olefin coordination to the metal center [ 16,19,20]. Recent work with other ruthenium-based catalysts further supports olefin coordination (see later). 

A2 4 A-i accounts for the possible interrelation between the frequency factors of two monomolecular reactions the formation of a complex four-centere intermediate during the insertion reaction compared with simple olefin desorption The better agreement between the calculated A and experimental Ap values corresponds to the case for which the rate-determining step involves olefin coordination. 

The generalized concept of carbocations and electrophilic reactions indicates that the initial interactions of electrophiles with 7r-donor systems (olefins, acetylenes, aromatics) involves three-centered bond carbonium ion formation. The 7r-bond provides the bonding electron pair which interacts with the empty orbital of the electrophile. Thus in principle there is no difference between the electrophilic reactivity of... 

A related transformation to the previous carbene transfer reaction involves a nitrene ligand bonded to the metal center, in a metallonitrene intermediate in situ generated upon the appropriate selection of the catalyst and the nitrene precursor. As shown in Scheme 17, some transition metal complexes react with such a precursor to generate an unsaturated intermediate, generally electrophilic in nature, which might react with olefins or C—H bonds affording aziridines or amines in a catalytic manner. The most employed nitrene sources are hypervalent I(III) compounds such as PhI=NTs, chloramine-T or organic azides. 

Structural features of olefins with distorted double bonds have been discussed within the deformation space defined by the eight bond angle deformations. The out-of-plane bond angle distortions are of particular interest because they are involved in addition reactions of the double bond. The symmetrical Blg-type deformation is related to concerted anti-additions, whereas the J3lu-type distortion (cf. Table 1) is appropriate for concerted syn-addition and those reactions that involve three-center intermediates and the formation of transition metal complexes. Twist or torsion is due to the Alu-type oop distortion and may be related to addition reactions, which in principle would lead—in the extreme case of a 90° twist angle—to an eclipsed rather than a staggered arrangement. 

In the previous section one of the two major aspects of silicon syntheses was described. A major problem for the organosilicon chemist is producing a silicon-carbon bond. A second synthetic area involves functional-group transformations. Most useful reactions at silicon centers are substitution processes. This is an important difference between the chemistry of silicon compounds and that of carbon compounds. The synthetic or reaction processes in organic compounds are increased enormously by the availability of double bonds present in olefins and carbonyl derivatives. In fact, substitution at sp3-carbon centers may represent the least interesting of organic reactions. 

Klyueva et al. have investigated the acidic properties of erionite modified by isomorphous substitution of B , Ga , and Fe " " by Si and Al . The incorporation of these elements in the aluminosilicate framework led to the generation of new acid centers. These acid centers have a lower concentration of aluminum cations than aluminosilicates, leading to s unples with lower acidity. Consequently, the rate of reactions involving hydrogen transfer, like olefin conversion into paraffins, was lower on isomorphous-substituted erionite samples. Table 5 shows that this enhanced the selectivity toward light olefins. The production of aromatics may... 

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