Alkene also migratory insertion

Dissociation of Me3P from the 18-electron complex gives a 16-electron complex. Association of the carbonyl group gives a Ti(II) n complex that can also be described as a Ti(IV) metallaoxirane. Dissociation of the second Me3P, association of the alkene, and migratory insertion into the C2-Ti bond gives a five-membered metallacycle. 

In the last decade an enormous revival of late transition catalysts for the polymerisation of alkenes has taken place [45] (remember that the first discovery of Ziegler for ethene polymerisation also concerned nickel and not titanium). The development of these catalysts is due to Brookhart in collaboration with DuPont (Figure 10.28) [46], Detailed low-temperature NMR studies have revealed the mechanism of the reaction [47], Interestingly, the resting state of the catalyst is the ethene-metal-alkyl complex and not the metal-alkyl complex as is the case for the ETM catalysts. For ETM catalysts the alkene complex intermediates are never observed. Thus, the migratory insertion is the rate-determining step (the turnover limiting step , in Brookhart s words) and the reaction rate is independent of the ethene concentration. 

The general mechanistic features of the ethene/CO copolymerisation cycle (Scheme 7.2) are substantially valid also for styrene. In particular, the propagation steps are similar for both alkenes and consist of subsequent alternated migratory insertions of alkyl to CO and of acyl to olefin, with P-chelate and y-chelate resting states. The structures of the first intermediates in the syndiotactic copolymerisation of styrene derivatives with CO have been determined by an in situ NMR study using [(Pr DAB)Pd(Me)(NCMe)]BAr4 as precursor (Scheme 7.21) [38]. 

In the process of olefin insertion, also known as carbometalation, the 1,2 migratory insertion of the coordinated carbon-carbon multiple bond into the metal-carbon bond results in the formation of a metal-alkyl or metal-alkenyl complex. The reaction, in which the bond order of the inserted C-C bond is decreased by one unit, proceeds stereoselectively ( -addition) and usually also regioselectively (the more bulky metal is preferentially attached to the less substituted carbon atom. The willingness of alkenes and alkynes to undergo carbometalation is usually in correlation with the ease of their coordination to the metal centre. In the process of insertion a vacant coordination site is also produced on the metal, where further reagents might be attached. Of the metals covered in this book palladium is by far the most frequently utilized in such transformations. 

Chalk and Harrod provided the first mechanistic explanation for the transition metal catalyzed hydrosilation as early as in 1965. Their mechanism was derived from studies with Speier s catalyst and provided a general scheme, which could be used also for other transition metals. The catalytic cycle consists of an initial oxidative addition (see Oxidative Addition) of the Si-H bond, followed by coordination of the unsaturated molecule, a subsequent migratory insertion (see Insertion) into the metal-hydride bond and eventually a reductive elimination (see Reductive Elimination) (Scheme 3 lower cycle). The scheme provides an explanation for the observed Z-geometry in the hydrosilation of alkynes, which is a consequence of the syn-addition mechanism. The observation of silated alkenes as by-products in the hydrosilation of alkenes along with the lack of well-established stoichiometric examples of reductive elimination of aUcylsilanes from alkyl silyl metal complexes... 

Alkyl iridium compounds are also accessible via insertion (see Migratory Insertion) of alkenes into Ir H bonds. Analogously, alkenyl iridium compounds may be formed via insertion of alkynes into Ir-H bonds. These types of reactions have been studied to shed tight on the mechanism of alkene and alkyne hydrogenation processes. For example, HIr(CO)(PPh3)2 (65) will react with ethylene and higher olefins to produce the alkyl iridium compounds (equation 17). 

Migratory insertions (see Migratory Insertion) of alkenes or alkynenes into M-H bonds are also a common method... 

Our laboratory has also taken advantage of multiple insertion processes to access the scopadulan diterpenes (Scheme 6-24) [52]. An early example was the economical conversion of aryl iodide 136 to tetracycle 137, an intermediate which was subsequently converted to ( )-scopadulcic acid B (138). In this conversion, migratory insertion of the neighboring exocyclic alkene provided a tricyclic intermediate having no /5-hydrogens, which subsequently underwent a transannular 5-exo Heck reaction with the trisubstituted... 

Insertions and (3-eliminations are also the microscopic reverse of each other. In an insertion, an A=B 77 bond inserts into an M-X bond (M-X + A=B —> M-A-B-X). The M-X and A=B bonds are broken, and M-A and B-X bonds are formed. Insertion is usually preceded by coordination of the A=B 77 bond to the metal, so it is sometimes called migratory insertion. In an insertion, an M-X bond is replaced with an M-A bond, so there is no change in oxidation state, d electron count, or total electron count. However, a new a bond is formed at the expense of a 77 bond. The nature of the reaction requires that the new C-M and C-H bonds form to the same face of the A=B 77 bond, resulting in syn addition. The reaction of a borane (R2BH) with an alkene to give an alkylborane is a typical insertion reaction that you have probably seen before. 

In hydroboration, a boron hydride (R2BH) adds across an alkene (R CH=CH2) to give R CH2CH2BR2. The 16-electron, d° Zr(IV) complex Cp2Zr(H)Cl, popularly known as Schwartz reagent, undergoes a closely related reaction. The mechanism involves coordination of an alkene to the electrophilic Zr center followed by migratory insertion of the alkene into the Zr-H bond. The reaction proceeds for alkynes also, by exactly the same mechanism. 

We can also think of 1,2-insertion as migration of the hydride to the (3-position on the alkene or alkyne, much the same as we saw for 1,1-CO migratory insertion. 

The insertion of alkenes into M-R bonds (whether M-H or M-C) is now considered as a migratory insertion process and requires a mutually cis arrangement of the alkene and the M-H or M-C bond. Since in many practical processes the alkene is an external ligand which needs to coordinate immediately prior to insertion, a few theoretical studies consider also the coordination step as part of the insertion process. 

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