Intramolecular migratory insertion

There is a substantial body of organometallic reactions which are called insertion reactions. This is a superficial misleading designation the real process is composed of the intramolecular migration of a ligand onto an adjacent one followed by ligation of a solvent molecule or another ligand into the vacated coordination site  

Hydrogen migration onto the adjacent olefin ligand is a key step in hydrogenation reactions and will be treated in the succeeding section. 

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Tamao and Ito proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of 1,7-diynes initiated by oxidative addition of the silane to an Ni(0) species to form an Ni(ii) silyl hydride complex. Gomplexation of the diyne could then form the nickel(ii) diyne complex la (Scheme 1). Silylmetallation of the less-substituted G=C bond of la, followed by intramolecular / -migratory insertion of the coordinated G=G bond into the Ni-G bond of alkenyl alkyne intermediate Ila, could form dienylnickel hydride intermediate Ilia. Sequential G-H reductive elimination and Si-H oxidative addition would release the silylated dialkylidene cyclohexane and regenerate the silylnickel hydride catalyst (Scheme 1). 

Equation (11) illustrates an intramolecular migratory insertion of an acyl with an alkyne to generate a lactone ring system. 

Although linear nitrosyl complexes are formally isoelectronic with carbonyl complexes, few examples of migratory insertions of nitrosyl ligands have been reported. One example of such an insertion is shown in Scheme 9.6. In the presence of phosphines, intramolecular migratory insertion occurs to form the stable nitrosoalkane complexes at the right of the scheme. A mechanism similar to that for carbonyl insertion in Scheme 9.1 appears to be followed, except that JCj is the rate-determining step (i.e., k [L] >> k ). 

The insertions of imines into late transition metal-carbon bonds are even less common. In one case, the insertion of an imine into Ni- and Pd-acyl bonds occurs with 2,1-regiochemistry to form an aminoalkyl product (Equation 9.80). Tlus reaction is likely to occur through a polar transition state formed by attack of a nucleophilic nitrogen at the electrophilic acyl carbon. One set of examples of 1,2-insertions of imines into late metal-carbon bonds have been reported. This example involves insertion of N-aryl aldimines into rhodium-aryl complexes containing a labile pyridine ligand (Equation 9.81). The rates of these reactions were inverse order in added pyridine, suggesting that the reaction occurs by an intramolecular migratory insertion mechanism after replacement of the coordinated pyridine by the imine. 

Nolan and co-workers also showed that alkenes could react with NHCs in what appeared to be a net migratory insertion, although in this case an inter-molecular attack was proposed. Reaction of diene adduct 17 with an excess of IPr led to 18 (Equation (2.3)). Calculations (R = H) showed that 18 formed via initial addition of an NHC ligand to the Pt centre followed by intermolecular attack of a second NHC moiety. The intermolecular attack occurred with an activation barrier of +7.6kcalmol whereas intramolecular migratory insertion involving the Pt-bound NHC would entail a barrier of -f 40kcalmoH ... 

The reaction is postulated to proceed via an intramolecular migratory insertion of amine, followed by ortho-C-H bond activation and cyclization (Scheme 7.7). Notably, a catalytlcally active cationic ruthenium-acetyhde complex has been isolated and characterized in this case. 

In a synthetic route to the E-azaebumane series, an intramolecular heteroaryl Heck reaction was the major cyclization strategy [140], Under Jeffery s ligand-free conditions, E-azaebumane skeleton 182 was prepared from bromopyridine 181. The migratory insertion occurred at C(2) of the indole ring. 

Yttrium-catalyzed enyne cyclization/hydrosilylation was proposed to occur via cr-bond metathesis of the Y-G bond of pre-catalyst Cp 2YMe(THF) with the Si-H bond of the silane to form the yttrium hydride complex Ig (Scheme 8). Hydrometallation of the C=G bond of the enyne coupled with complexation of the pendant G=G bond could form the alkenylyttrium alkyl complex Ilg. Subsequent / -migratory insertion of the alkene moiety into the Y-C bond of Ilg could form cyclopentylmethyl complex Illg. Silylation of the resulting Y-C bond via cr-bond metathesis could release the silylated cycloalkane and regenerate the active yttrium hydride catalyst. Predominant formation of the /ra //j--cyclopentane presumably results from preferential orientation of the allylic substituent in a pseudo-equatorial position in a chairlike transition state for intramolecular carbometallation (Ilg —IHg). 

Acylpalladium intermediates can be involved in intramolecular processes leading to the formation of carbo- or heterocycles. In this chapter we discuss the cyclizations via the attack of acylpalladium intemediates at carbon centers and formation of new G-G bonds. The basic scheme (Scheme 7) of such processes includes the oxidative addition of Pd(0) to G(j )-X bonds (X = halogen or triflate), migratory insertion of GO, and subsequent intramolecular addition of acylpalladium intermediate to double or triple bonds to yield cyclic ketones. 

All these protocols allow us to form a new carbocycle in a bimolecular process. The cyclization involving two different molecules besides CO has been realized, involving carbapalladation of norbornene, migratory insertion of CO, and subsequent intramolecular Heck-like attack at thiophene residue. Thallium acetate is required as electrophilic co-catalyst (Scheme 18). ... 

The pentacyclic framework of natural product maxonine was prepared in an intramolecular 1-exo Heck cyclization. The migratory insertion of the pendant olefin into the arylpalladium complex could have led either to the formation of an eight or a seven membered ring, of which only the latter was observed (5.3.), 3... 

The results implicate the mechanism shown in Scheme 6. Thus initial migratory insertion of CO into the Zr-C bond is followed by intramolecular insertion of the oxycarbene into the Zr-H bond to produce what could be considered an aldehyde adduct of bis(pentamethylcyclopentadienyl)zirconium(II). The final step involves simple /J-hydride abstraction to give the enolate hydride, 24. 

As discussed earlier, the generally accepted mechanism for the Heck reaction involves the steps of oxidative addition, coordination of the alkene, migratory insertion, and P-hydride elimination [2,3], With the intramolecular Heck reaction emerging as an important synthetic reaction over the past decade, the individual steps of this mechanism have come under closer scrutiny, and attention is beginning to be directed at determining the identity of the enantioselective step [41],... 

Small unsaturated molecules often insert into the M-G bonds and cause expansion of metallacycles. The mechanism of the process is often described as migratory insertion when the reagent is first coordinated by the metal center and then enters further intramolecular reactions <1994JA12117, 1995RTC73, 1996CCR(155)209>. 

Danoponlos et al. reported the crystal structure of a palladium pincer carbene complex that is the product of intramolecular 1,2-methyl migration from palladium to the carbene carbon atom, a process also referred to as a migratory insertion of the carbene into the Pd-methyl bond [434] (see Figure 3.150). The importance of this compound stems from the fact that it was the first unambiguous experimental evidence for this process actually to take place after it had been suspected for several years with the suspicion being backed by several theoretical calculations [447-449]. 

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