Bonding deinsertion

Aside from two-center (Patterns 1 and 2) and three-center (Patterns 3, 4, 11, and 12) processes, most of the processes shown in Scheme 1.3 are four-center processes involving either addition (Patterns 5—10) or 0-bond metathesis (Pattern 13). In this context, it should be noted that addition is simply a four-center metathesis in which one molecule happens to be multiply-bonded. In addition to these metathetical processes, there is yet another fundamentally important four-center metathetical process termed migratory insertion and deinsertion (Patterns 14 and 15). It should be clear from Patterns 14 and 15 shown in Scheme 1.3 that distinction between insertion and deinsertion is only a relative and semantic issue. In the current discussion, a process involving cleavage of the C—Zr bond is termed migratory insertion, while the reverse process is termed migratory deinsertion. 

These reactions resemble those described in section 2.2.1 because both types involve insertion of the transition metal into the Si-H or Ge-H bond. However, here the molecule eliminated is not a neutral ligand but is formed by deinsertion of two ligands which are sigma bonded to the transition metal after the addition of R3MH. 

Cleavage of SUicon (or Germaniniii)-TransitiMetal Bonds by Deinsertion... 

These results (bond lenght, deinsertion, NMR) can be explained as an equilibrium which is established slowly between a three-center complex and two independent compounds (cf. Graham Ref. ). 

Electrophiles such as halogens may cleave the transition metal-silicon (or germanium) bonds. We have already seen that in the case of the hydrido complexes they induce deinsertion reactions with retention of configuration at silicon (cf. Sect. 3.1). 

Organometallics such as Grignard and lithium reagents give rise to deinsertion in hydrido complexes (cf Sect. 3.1). However, their reactivity is quite different toward normal sigma bonded transition metal-group IVg metal complexes. 

In fact, the C-H bond activation by the zirconium or tantalum hydride on 2,2-dimethylbutane can occur in three different positions (Scheme 3.5) from which only isobutane and isopentane can be obtained via a P-alkyl transfer process the formation of neopentane from these various metal-alkyl structures necessarily requires a one-carbon-atom transfer process like an a-alkyl transfer or carbene deinsertion. This one-carbon-atom process does not preclude the formation of isopentane but neopentane is largely preferred in the case of tantalum hydride. 

The product of this reaction appears to have formed by insertion of a CO group into an Mn—CHj bond. The reverse of this reaction is called decarbonylation but may also be called deinsertion or, more broadly, elimination. Infrared studies with CO have revealed that the reaction actually proceeds by migration of the methyl ligand rather than by CO insertion. 

Insertion into element-hydrogen bonds tend to be less favored thermodynamically than insertions into other bonds (e.g., element-carbon). This is often attributed to the high element-hydride bond strength, which is lost upon insertion. Since the insertion reaction is also entropically disfavored, the reverse deinsertion of the unsaturated moiety to generate an element-hydride bond can be thermodynamically favored. When the hydride exists in the P position of the inserted product, this process is commonly referred to as /S-hydride elimination. Nevertheless, there are many examples of insertions into element-hydride bonds that generate stable compounds, and when this insertion reaction is an uphill process, chelation to the element or subsequent chemistry (i.e., catalytic cycles) can be employed to facilitate the initial insertion step. 

As shown in equation 7.59, OA of the aldehyde yields the cis-hydridoacylrhodium complex, 39. Heating results in ligand dissociation to give 40, which undergoes migratory deinsertion (to be discussed in Chapter 8) to produce 41. Isomerization of 41 to a d.v-hydridoalkyl complex presumably occurs before final the final RE step to yield the C-H elimination product. These reactions have applications in catalytic industrial process known as olefin hydroformylation (Section 9-2). Equation 7.60 illustrates straightforward RE to form a new C-H bond. 

Equations 8.1 and 8.2 describe the general process of insertion of a ligand into an M-Y bond. The former shows a process known as 1,1-insertion and the latter its 1,2 counterpart. The reverse reactions are known interchangeably as deinsertion, extrusion, or elimination. 

Hydrozirconation occurs with yyn-addition of the Zr-H bond across a C=C or C=C bond (equation 8.16). Due to lower steric hindrance, the addition also tends to be regiospecific, with the zirconium attached to the less substituted position (just as in hydroboration). Internal alkenes and alkynes isomerize to 1-alkyl and 1-alkenyl complexes, respectively—presumably by alternating reactions of insertion and deinsertion—until the complex with the least steric hindrance is formed. 

Mechanistic studies on the intramolecular hydroacylation by using deuterium-labeling experiments have been reported by several groups [95-100]. The results of their studies showed that the addition of the Rh-H bond to a carbon-carbon double bond takes place in syn fashion [95,96]. They also demonstrated that C-H bond cleavage, hydrid transfer to the double bond, and carbonyl deinsertion are all fast and reversible steps (Scheme 3) [99]. 

Aldehyde CH bonds are reactive in oxidative addition, so it is not unexpected to find that aldehydes readily undergo catalytic reactions involving this oxidative addition. Several catalysts decarbonylate aldehydes as a result of the acyl hydride formed after the C-H addition undergoing deinsertion of CO, followed by reductive elimination of the alkane product (Eq. 2.49). The hard step in the process is the thermally induced dissociation of the resulting tightly bound CO. One such catalyst is [Rh(triphos)Cl] (triphos = PhP(CH2CH2PPh2)2) [134]. 

For clarifying the factors influencing the ease of CO insertion and its reverse process, it is desirable to know the metal-carbon bond energies in the initial metal alkyl and the product metal acyl species. However, the presently available thermochemical data for the bond dissociation energies in acyl-transition metal complexes are not sufficient to allow us to advance a reasonable argument for the thermodynamic feasibilities of insertion and deinsertion processes [22-24],... 

The nature of the ligand into which the CO is to be inserted strongly influences the ease of CO insertion. The metal to hydride bond is known to be resistant to CO insertion, whereas deinsertion from the formyl to the hydride proceeds readily [26]. However, in certain cases the unfavored insertion into metal hydrido complexes takes place when a -formyl bond is formed, giving an extra stability to the product [27]. 

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