N-Allyl complexes

The application of these catalysts in the initial state (without any special treatment of the surface organometallic complexes of such cata-lysts) for ethylene polymerization has been described above. The catalysts formed by the reaction of 7r-allyl compounds with Si02 and AUOj were found to be active in the polymerization of butadiene as well (8, 142). The stereospecificity of the supported catalyst differed from that of the initial ir-allyl compounds. n-Allyl complexes of Mo and W supported on silica were found to be active in olefin disproportionation (142a). 

The other mechanism, called the n-allyl complex mechanism, does not require external hydrogen ... 

In order to explain the competitive formation of the 1 1 and 1 2 adducts and the formation of the 2,6-octadienyl rather than the 1,6-oc-tadienyl chain, a mechanism was proposed (62, 69) in which the insertion of one mole of butadiene to the Pd—H bond gives the 77-methallyl complex (68) at first, from which 1-silylated 2-butene is formed. At moderate temperature and in the presence of a stabilizing ligand, further insertion of another molecule of butadiene takes place to give C5-substituted n-allyl complex 69. The reductive elimination of this complex gives the 1 2 adduct having 2,6-octadienyl chain. In the usual telomerization of the nucleophiles, the reaction of butadiene is not stepwise and the bis-n--allylic complex 20 is formed, from which the l, 6-octadienyl chain is liberated. 

The alkylidyne tricobalt nonacarbonyl complexes (2) are produced from the reaction of the cobalt tetracarbonyl anion with 1,1,1-trihaloalkanes [4], under conditions analogous to those used for the synthesis of the n-allyl complexes. Although the yields for (2) appear to be low (Table 8.3), they are better than, or comparable with, those obtained by the traditional procedures [8] and are obtained under more amenable conditions. 

The following two-step operation was chosen as an example of the former case. The first step involves an allylation to generate the 1,6-enyne intermediate 1, via the reaction between a metal n -allyl complex derived from 52 and propargyl anion 51, followed by the PK-type reaction to furnish the bicyclopentenone 2 (Scheme 11.13). Since these two specific reactions have opposing electronic requirements (the first prefers Lewis basic character, while the second can be facilitated by Lewis acidic catalysts), finding the right combination of catalysts was the key to success. 

The reaction of 02 with propene adsorbed on ZnO has been studied by EPR and IR spectroscopy (395,396). The n allyl intermediate is thought to react with 02 to give a hydroperoxide (not detected) which forms glycidalde-hyde, which is postulated as an intermediate in the formation of acrolein on the surface. Krylov and others (397,398) have also studied the reaction of n allyl complexes with 02. 

Substitution of Hydroxyl Group. The substitution activity of the hydroxyl group of allyl alcohol is lower than that of the chloride group of allyl chloride and the acetate group of allyl acetate. However, allyl alcohol undergoes substitution reactions under conditions in which saturated, alcohols do not react. Reactions proceed in catalytic systems in which a n-allyl complex is considered as an intermediate. 

An additional issue of regiocontrol arises in asymmetric induction when the n-allyl complex possesses a primary terminus. Although steric factors favor the formation of the achiral linear product, alkylations with reactive nucleophiles often benefit from electronic effects leading to the branched product [ 147,148]. Of particular interest is the reaction of the crotyl system because... 

Metal-n-allyl complexes are important in a number of stereoselective catalytic reactions and are therefore attractive for computational chemists (see also Chapter 13, Section 13.2). An empirical force field study based on the MM2 parameterization scheme aimed at predicting stereoselective nickel(0)-catalyzed cycloadditions was recently conducted 56. As in a similar study 57, where a force field for the structure optimization of palladium-allyl systems was developed, dummy atoms were needed to define the structural model. Based on the assumptions required to model a catalytic process, the results obtained have to be interpreted with caution. 

The coordination of the Pd(0)-catalyst to fhe double bond forms an r 2 n-allyl complex. An oxidative addition, during which the leaving group is expelled, gives an i)3 7t-allyl complex. This step is also called ionization ... 

In general, 2-alkenes will form the more-substituted n-allylic complex, and 1-methylcycloalkanes react to give the r-allyl complex from abstraction of one of the -CH3 hydrogens. If a given aUcene is umeactive, use of the very electrophilic Pd(02CCp3)2 may result in reaction. When the alkene contains a particularly acidic C-H bond, such as one 0 to a carbonyl group, that C-H bond is normally the one abstracted to give the allylic complex (equation 54). 

Reactions of n-Allyl Complexes Derived From 1,3-Dienes... 

This method for n-allyl complexes is of wider application than most others, and examples are found in the V, Cr, Mn, Fe, Co and Ni triads because allylic halides, esters, etc., are reactive electrophiles. Mild conditions can be used, and this is often required owing to the limited thermal stability of most organotransition-metal complexes. A second reason is that metal complexes of the above triads easily adopt low-valent. 

The 86% yield of the key complex XXVIII is the maximum achieved and varies . The synthesis of ( j -RC3H4)FeX(CO)3 (X = Cl, Br, I, NOj and R = H, 1-Me, 2-Me, 1-Ph, 2-Ph, 2-Br can start from Fe2(CO)9 . The diallyl complex XXIX decomposes at 57-58°C is unstable does not give tractable products with PPhj, P(OPh)j or pyridine but can be stored under an inert atmosphere at —76°C . The complex XXIX as well as XXX cf. [Eq. (n)] can also be obtained in fair yield via the phase-transfer process previously mentioned for Mo and Mn complexes Under UV irradiation n-allyl complexes emanating from FefCO), and allylic halides may be prepared in decent yield . 

The synthesis and properties of 7r-allyl transition metal complexes have been discussed elsewhere (127-129). We shall draw attention to the most interesting properties of n-allyl complexes such as structure, coordinative unsaturation of the metal, and the ability of the 77-allyl ligand to enter into intermolecular rearrangements. 

The other mechanism, called the n-allyl complex mechanism, does not require external hydrogen and proceeds by hydrogen abstraction to form the ri. Tj.allyl complex 15 (see p. 117 and 10-60). 

Cyclopropenes react with palladium(II) compounds to give n allyl complexes 1,2,3-triphenylcyclopropene forms the ring-opened product 310 (equation 93), whilst its 3-methyl and tetraphenyl counterparts give " indenyl complexes 309. 

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