Ethyl ligands

We were concerned that synthesis of a,6-dialkoxyethyl complexes 16 and 17 would be limited by overreduction of 13 and 14. Both appended a-alkoxyethyl (30,63) and 6-alkoxyethyl (16,64) groups separately undergo facile reductive cleavage to ethyl ligands. Model studies were accordingly carried out in order to first establish conditions favoring generation of 6-alkoxyethyl and a-alkoxyethyl compounds via monohydridic reduction of suitable substrates. 

Fig. 8 Proposed mechanisms for scrambling the two carbons of the ethyl ligand (a) for making equivalent the two P atoms (b). Adapted from [115]...

A chiral metal center, as is found in a pseudotetrahedral iron complex with cyclopentudienyl. carbonyl, triphenylphosphine, and ethyl ligands, hus also beer used to address the question of alkyl migration versus carbonyl insertion. Inversion of... 

In the phenyl series (Figure 4.10b), when R is methyl or ethyl, conformers E and F are dominant, with E favored. Note that the selectivity in the phenyl series for methyl and ethyl ligands is greater than in the methoxy series (Table 4.2). This is because the phenyl group is bulky and has a low energy a orbital, so that the electronic and steric effects act in concert. For the isopropyl and tert-h Ay ligands, the importance of the G/H conformers increases, and when R is fert-butyl they predominate. 

It has been proposed that (i) and (ii) could be rationalised if the mechanism involved the transient formation of an ethyl intermediate [95], It was argued that the three electrons and protons added to the coordinated acetylene would result in a coordinated ethyl species. If the active site is coordinatively-unsaturated, p-elimination of the ethyl ligand can occur to produce ethylene and a metal hydride. Such behaviour is well-established in organometallic chemistry and specifically has been demonstrated [96] at the M(R2PCH2CH2PR2)2 site (M = Mo or W R = Ph or Et) as shown in Figure 31. 

In the reaction using ZnEt2, exchange of the allyl and ethyl ligands occurs between Pd and Zn, affording jt-allylzinc and ethylpalladium intermediates. The former compound reacts with aldehyde to induce its allylation, while the latter is converted into a Pd(0) complex via -hydrogen elimination of the ethyl ligand (Eq. 5.39) [145,146]. 

An irreversible j6-H elimination is observed in the decomposition of PCP-based rhodium(III)-alkyl complexes (Eq. 6.15) [72], Isotopic labeling with C proves that there is no incorporation of C in the methyl end of the ethyl ligand, discounting fast reversible /1-H elimination. This occurs in an irreversible fashion. Deuterium labeling reveals a kinetic isotope effect of kn/ko = 1.4 for ethyl, supporting that /1-H elimination is rate determining. The activation parameters in toluene were A// = 21.2 kcal/mol, A5 = —21.1 eu, and AG gg = 27.5 kcal/mol. 

Similar to the polymerization of ethylene on Ziegler catalysts, the first reaction step is the coordination of an ethylene molecule at a Cr center. The initiator of the polymerization reaction is thought to be a Cr-H group, into which ethylene inserts to form an ethyl ligand (Eq. 8-12). It was shown that only isolated Cr centers on the surface are catal5h ically active. 

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