Deinsertion

Evidence was shown for migration of an alkyl group in carbonyl insertion, and deinsertion steps between the methyl carbonyl rhodium complex [ r/Vi/ -Indenyl-l -(CH2)3PPh2 Rh(CO)-Me](BF4) and the acetyl rhodium complex [ r/5 r/l-(Indenyl-l -(Cl I2)3PPh2) RhI(COMe)] by crystallography as well as by 1H NMR spectroscopy.28... 

Figure 1 provides several electrochemical windows of important, relevant processes, including the reduction of alkyl carbonates, ethers, Li insertion into graphite, and Li metal deposition. Recent studies revealed two major failure mechanisms of graphite electrodes in repeated Li insertion/ deinsertion processes 21... 

Figure 3. An illustration of morphology, surface processes and changes during a Li insertion-deinsertion cycle of an electrode comprising synthetic graphite flakes in EC-DMC solutions, in which the electrodes behave reversibly.

Figure 5. 7Li NMR spectra recorded during 3 cycles of reversible lithium insertion-deinsertion in the composite carbon electrode. The peak at 0ppm is due to ionic lithium (Li+PF6 and passivation layer). The peak of lithium at 263 ppm is not shown.

NMR studies clearly indicated a rapid and reversible ethylene insertion-deinsertion process, as illustrated in Eq. (18). 

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. 

O Connor proposed a mechanism involving deinsertion of carbon monoxide from the vinylketene complex 106 to form the new cobaltacyclobutene 109. The cobalt may then undergo a 1,3-shift to the carbonyl of the ester group to create the oxycobaltacycle 110, before deinsertion of the cobalt moiety forms the furan 108. Alternatively, 109 may rearrange to the vinyl-carbene 111, which then undergoes ester-carbonyl attack on the carbene carbon to form the zwitterionic species 112, which finally aromatizes to yield the furan 108. Notice that this latter postulate is identical to the final steps of the mechanism formulated by Wulff (see Section V,B) for the reaction between a cobalt carbene and an alkyne, in which a cobaltacyclobutene is a key intermediate.51... 

These results led to the proposal of the following mechanism. Decomplex-ation of the central C2 fragment allows coordination of the alkyne (intermediate 119), which then inserts to form the metallacycle 120. Deinsertion (reductive eliminate of the cobalt moiety allows ring closure to give the cyclohexadienone complex 121, which upon decomplexation yields the desired phenol. The regiochemistry of the alkyne insertion determines the ratio of 116 117 (for simplicity, only the sequence leading to 116 has been shown). 

A promising EW may be obtained by combining tungsten trioxide, WO3, a well-known primary electrochromic electrode, which is coloured by the following lithium insertion-deinsertion process ... 

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... 

This structure may be related to the ease of deinsertion of these compounds in the presence of phosphines. The reaction takes place at room temperature without irradiation ... 

Deinsertion is not limited to hydrido complexes since alkyl and halogeno derivatives undergo the same type of reaction ... 

However, deinsertion involving two electron ligands is not a general reaction for all the hydride complexes. Indeed, the iron complex (CO)4Fe(H)SiPh3 undergoes CO replacement rather than 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. ). 

In this case, where cleavage may proceed both by nucleophilic attack and deinsertion, it has been proved that the former possibility occurs ... 

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. 

Lithium reagents display a more complicated reactivity as basic agents. Nonfunctional hydrides undergo deinsertion (cf. Sect. 3.1) while functional ones undergo hydrogen abstraction followed by substitution of the corresponding function. 

However, anions containing Cr, Mo and W undergo deinsertion of germane with retention of configuration at germanium -... 

The reaction with acetyl chloride affords the diagonal methyl derivative after CO deinsertion, as in the case of Mel (Scheme 17) ... 

---