Allyl ligand, analogy

Polyenes are counted so that all bonding and nonbonding TV orbitals associated with the polyene are occupied. The most commonly encountered examples are illustrated in (20). Below each structure is the number of Jt electrons (which interact with the metal in a a fashion) used for polyene-M a bonding provided that the metal is bonded to all carbon atoms in the polyene. Thus Cp donates six electrons only if it is an r] ligand. For an -Cp situation, it donates four electrons analogous to a r-allyl ligand, etc. 

Formation of the u-propenyliron derivative is rather unusual, and has no analogy in the chemistry of 77-allyl complexes. Probably in such complexes the facility to partially or completely replace the 7r-allyl ligand by CO and other groups can be explained by electron deficiency at the metal atom. 

An analogous scheme has been proposed 161) for the subsequent transformations of complex (VII) in the 7r,<7-transfer of allyl ligand from palladium to mercury. 

Propene.—Dent and Kokes demonstrated most elegantly, using i.r. spectroscopy and 6 selectively labelled propenes, that on adsorption propene forms both a ff-complex (vc=c shifted 30 cm from the value in gas phase) and a TT-allyl species ( CH2—CH—CH2, i c = c shifted 100 cm" ). The latter had a saturation coverage of 30% and was analogous to the 7r-allyl ligands of transition metal complexes. They showed that it displaced type (/) hydrogen to C3H5 H... 

The stereochemistry of addition in equation 8.36 is interesting. If LiEt3BD is used instead of its protium analog, the deuterium ends up at the C-2 of the allyl ligand syn to the Cp. As we shall see in the examples to follow, nucleophilic addition onto it ligands almost always occurs anti to the metal fragment. This seems reasonable because anti addition involves a pathway with less steric hindrance than syn. 

These results indicate that the n-allylic ligand bonded to Pd of the catalyst is incorporated in the initiating polymer end. Analogous insertion of 1,3-di-ene into the Pd-(l-methyl- -allyl) bond was reported to form a C-C bond between the diene and the methyl-substituted carbon of the n-allylic ligand, although it is sterically less favorable than the alternative insertion of the C=C bond into the Pd-CH2 bond (Eq. 43) [161]. 

AUylation of a camphorsultam-modified glycine derivative 26 proceeds on deprotonation and addition of a Pd catalyst and an allyl carbonate. A three-component coupling involving a-allylation of A-acryloyloxazolidin-2-one by reactions with free radicals is rendered enantioselective in the presence of a bis(4-phenyloxazoline)methane ligand analogous to 12. Similarly, a stepwise Baylis-Hillman reaction to furnish chiral a-methylene-P-hydroxy ketones is effective if catalyzed by a tartaric acid derivative 27. 

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