Alkyl ligands migration

Migration of a hydride ligand from Pd to a coordinated alkene (insertion of alkene) to form an alkyl ligand (alkylpalladium complex) (12) is a typical example of the a, /(-insertion of alkenes. In addition, many other un.saturated bonds such as in conjugated dienes, alkynes, CO2, and carbonyl groups, undergo the q, /(-insertion to Pd-X cr-bonds. The insertion of an internal alkyne to the Pd—C bond to form 13 can be understood as the c -carbopa-lladation of the alkyne. The insertion of butadiene into a Ph—Pd bond leads to the rr-allylpalladium complex 14. The insertion is usually highly stereospecific. 

In the case of terminal C=C (1,2 addition units), i.e. when R=R =H and R" (or R111) = polymer chain, two types of hydride migration are possible, namely (i) The Markownikoff s addition which would lead to the formation of B type repeating units and (ii) The anti Markownikoff s addition which would result in the formation of the observed repeating units C. In the case of Markownikoff s type addition the hydride transfer occurs to Ca and results in the formation of branched alkyl-rhodium intermediate complex shown by Structure 2. Whereas when anti Markownikoff s addition occurs, the resulting intermediate alkyl-rhodium complex has linear alkyl ligand as shown by Structure 3. 

Pichler and Schulz were the first to suggest that the initial step in metal-catalyzed CO/H2 reactions might be the migration of catalyst-bound hydride to CO to yield a formyl intermediate [Eq. (1)] (3). The alkyl ligand in many isolable metal carbonyl alkyls can similarly be induced to migrate to coordinated CO (4. ). Thus, Eq. (1) has some precedent in homogeneous chemistry. Other suggestions for the involvement of formyl intermediates have come from Wender (6) and Henrici-Olive and Olive (7). 

We mentioned in section 4.1 that (H2)TiH+ in a triplet state was also much more stable than the singlet TiH3+ with C3V symmetry the proton binds to the H ligand to remove it. On a positively charged titanium centre, the alkyl ligands do not remain bound to the titanium they migrate to bind to the... 

When E is carbon, the produced phosphenium complex is too reactive to be detected. The alkyl ligand immediately migrates to the phosphenium phosphorus to give 37. The 16 electron species can be converted into an isolable complex 38 by the addition of PPh3.22,23... 

The migration of an alkyl ligand to a coordinated CO generates a coordinatively unsaturated species (21-XXIXa) which can be stabilized either by the solvent (S) or as a r -bound acyl (21-XXIXb) by coordination of the oxygen to the metal. In general, several mechanistic pathways are therefore possible ... 

In the mechanism proposed by Biloen et al. (2d) it is assumed that a CH3 fragment "jumps" on top of a CH2 jg fragment to form a CH3 - CH2(ads) species. This could be formally interpreted as a cis-migration of an alkyl ligand,... 

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