Allyl complexes from olefins

A completely different way of obtaining t-allyl complexes from olefins is by hydride abstraction, e.g. ... 

An allylhydrido-metal complex is initially formed when the allylic C—H bond is cleaved. For Rh such allylhydrido-metal complexes are not isolated during the formation of n-allyl complexes from olefins, but the allylhydridorhodium complex, XIX, prepared by oxidative addition of HCl to the allylRh(I) complex, XVIII ... 

Often a base such as NaOAc is necessary to make the reaction go smoothly (281), Basic solvents such as A, A -dimethylformamide (DMF) also aid in the formation of 77-allyl compounds from 1-olefins (200) in this case [(DMF)2H][Pd2Cle] is also formed. In a recent study of the formation of 77-allylic complexes from 1-olefins in DMF under mild conditions, it was suggested that bases may not only shift the equilibrium to the right by neutralizing the acid but may also aid in removal of the proton from the allylic position (40). One possible mode of promotion might be the stabilization of intermediate Pd(IV) hydrido species. However, more work is required before the role of basic agents in Pd(II) catalysis is finally understood. 

Several new types of allylic complexes have recently been prepared. Atkinson and Smith prepared Tr-allylic complexes from 2,2,4-trimethy-pent-3-en-l-ol of the type [(CHg—CHMeCH)CMe2CHaOH]PdCl 2, in which the alcohol function is not coordinated to the Pd(II) (10). In contrast, the Rh(III) and Pt(II) complexes are olefin complexes, with the alcohol function complexed to the metal. 

S.8.2.3.8. TT-Allyl Complexes from Allylic C—H Bond Cleavage in Olefins by Metal Complexes... 

This reaction is a preparative method for the synthesis of 7c-allylpalladium complexes from olefins. Thus the Tt-allyl complex III can be prepared in 90% yield from 2,3-dimethyl-1-butene ... 

By using a basic solvent, e.g., DMF, 7t-allyl complexes can be prepared under mild conditions, but yields are moderate. The promoting effect of the base may favor a mechanism in which the base abstracts a hydrogen in the allylic position (cf. Scheme 1). The facile formation of Tt-allylpalladium complexes from olefins with slightly acidic allylic hydrogens ... 

Equations 3.64-3.66 illustrate routes to allyl complexes from dienes, diene complexes, and olefins. Allyl complexes have been prepared by the insertion of a conjugated diene into a metal hydride, alkyl, or acyl linkage, as illustrated for the cobalt complexes in Equation 3.64. ° Alternatively, allyl complexes have been prepared by nucleophilic or electrophilic attack on a coordinated diene. Equation 3.65 shows the formation of allyl complexes by the addition of carbanions to a cationic diene complex, and Equation 3.66 shows the formation of a cationic diene complex by the protonation of a neutral 1,3-diene complex. Allyl complexes have also been formed by the abstraction of an allylic proton from a metal-olefin complex, either by a base or by the metal itself. This reaction has been proposed as a step in the isomerization of olefins (Equation 3.67) and in the allylic oxidation of olefins (Equation 3.68). - ... 

In the case of isomerization which proceeds according to the 7c-allyl mechanism, 1,3-hydrogen transfer takes place. These reactions are catalyzed by palladium(II) complexes which easily form 7r-allyl complexes from 7r-olefin compounds. Also, compounds of nickel, rhodium, iron, etc., are utilized as catalysts. Effective isomerization is possible if the hydrogen addition to both terminal carbon atoms of the 7r-allyl asymmetric grouping takes place. 

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). 

In 1981, it was demonstrated (70) that anions of nitro compounds can be involved in C,C-coupling with allyl acetates at the allylic carbon atom with the use of metal complex catalysis. For many years, this observation did not come to the attention of chemists interested in the synthesis of cyclic nitronates. However, Trost demonstrated (71) that this process can be used in the synthesis of five-membered cyclic nitronates from olefins (18) containing two acyl groups in the different allylic positions (Scheme 3.21). 

The fact that organosamarium allyl complexes of the type Cp 2Sm(CH2CH=CHR) can arise from the treatment of Cp 2Sm or [Cp 2Sm(/r-H)]2 with a variety of olefin and diene substrates makes samarium chemistry more intriguing. The reaction modes are illustrated in Scheme 18. These allylsamarium complexes 55 react with C02 to afford the carboxylate products 56, which participate in monometallic/bimetallic interconversions (Equation (10)). Carbon disulfide and 0=C=S also insert into carbon-samarium bonds, which form only monometallic species.29... 

Abstract Aldehydes obtained from olefins under hydroformylation conditions can be converted to more complex reaction products in one-pot reaction sequences. These involve heterofunctionalization of aldehydes to form acetals, aminals, imines and enamines, including reduction products of the latter in an overall hydroaminomethylation. Furthermore, numerous conversions of oxo aldehydes with additional C.C-bond formation are conceivable such as aldol reactions, allylations, carbonyl olefinations, ene reactions and electrophilic aromatic substitutions, including Fischer indole syntheses. 

The protonation of the a-allylic cyanocobaltate complexes has been reported by Kwiatek and Seyler 50) to proceed with the liberation of the corresponding olefin. Thus the complex prepared from butadiene [Eq. (35)] on treatment with aqueous HCl liberates 1-butene. The carbonium ion which probably forms first can cleave directly to 1-butene or it may first rearrange to a Tr-olefin complex, from which the olefin is then displaced with either HgO or chloride ... 

A final example concerns the question of 7r-allyl and related complexes with hydrogen. Again the evidence cited in the preceding section suggests that the principal reaction of jr-allyl complexes with hydrogen is to yield olefins, desorbed from the surface, although the possibility that a TT-bonded olefin is formed first is a geometrically feasible process (Fig. 25). 

One may inquire whether the evidence that 77-allyl complexes yield desorbed olefins when formed from dienes and hydrogen, or from alkenes, is pertinent to the question concerning the course of the exchange of such complexes formed by the adsorption of saturated hydrocarbons. The composition of the surface must be different under the two circumstances in one there must be few sites not occupied by olefin or half-hydrogenated intermediates, while in the other (the exchange of saturated hydrocarbons) many sites must be vacant. Consequently, in the absence of an excess of any unsaturated hydrocarbon, there is no driving force for the desorption (or displacement) of the unsaturated intermediates which are formed on the surface and intermediates of any degree of unsaturation remain bonded to the surface and leave it only as saturated hydrocarbon. Yet the evidence obtained from the reactions of the unsaturated hydrocarbons must indicate the paths which may be traversed under either circumstance. 

This review deals with metal-hydrocarbon complexes under the following headings (1) the nature of the metal-olefin and -acetylene bond (2) olefin complexes (3) acetylene complexes (4) rr-allylic complexes and (5) complexes in which the ligand is not the original olefin or acetylene, but a molecule produced from it during complex formation. ir-Cyclopentadienyl complexes, formed by reaction of cyclopentadiene or its derivatives with metal salts or carbonyls (78, 217), are not discussed in this review, neither are complexes derived from aromatic systems, e.g., benzene, the cyclo-pentadienyl anion, and the cycloheptatrienyl cation (74, 78, 217), and from acetylides (169, 170), which have been reviewed elsewhere. 

With the addition of 1,3-butadiene, the initially yellow hydride solutions turn red with the formation of relatively stable l-Me-it-allyl-nickel complexes, and olefin isomerization activity stops. By measuring the rate of formation of the rc-allyl complexes in the presence of added P(OEt)3, it was possible to measure the rate constant for dissociation of L from HNiL4 and show that this is the rate-determining step (42). 

The cationic Jt-allyl complex is often isolable and has been the subject of considerable study. X-ray structures of the 7t-allyl complexes with chiral ligands have been a primary source of structural information from which the design and predictive model of chiral catalysts derive. Chiral-metal-olefin complexes, which constitute another important class of intermediates have also been isolated, albeit few in number [31]. These static studies have been complemented by a growing number of NMR studies taking advantage of modem heteronuclear correlation and NOE techniques, which offer opportunities to monitor solution structures of the catalytic species [32-34],... 

Although metal-olefin complexation can be a source of enandoselection, reactions exploiting this mechanistic motif have not been developed much. Due to the facile enantioface interconversion process, the origin of the enantioselection often reverts back to Type C alkylation (Figure 8E, 1). To transfer chiral recognition of the coordination process to the ee of the product, kinetic trapping of the incipient 7t-allyl complex is required prior to any isomerization process. For this reason, few successful examples have come from the use of more reactive heteroatom nucleophiles (N, O and S) and/or intramolecular reactions. 

Krylov (62) studied the adsorption of oxygen and propylene on vanadium oxide/MgO and molybdenum oxide/MgO catalysts by ESR and IR at 25°C. He observed the formation of Qr radicals and ir-allyl complexes during the simultaneous adsorption of 02 and C3H . The data indicated that an electron transfer took place from the olefin to the oxygen through the transition metal ion forming the following complex ... 

Abstract The synthesis and X-ray structure of various octahedral zirconium complexes and their catalytic properties in the polymerization of a-olefins are described. Benzamidinate, amido, allylic, and phosphinoamide moieties comprise the study ligations. For the benzamidinate complexes, a comparison study between homogeneous and heterogeneous complexes is presented. For the phosphinoamide complex, we show that the dynamic symmetry change of the complex from C2 to C2v allows the formation of elastomeric polymers. By controlling the reaction conditions of the polymerization process, highly stereoregular, elastomeric, or atactic polypropylenes can be produced. The formation of the elastomeric polymers was found to be the result of the epimerization of the last inserted monomer to the polymer chain. 

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