Allyl species

With two competing allylic species, a secondary center -CH2- is brominated preferentially over a primary center -CH3. 

The next step is the insertion of a lattice oxygen into the allylic species. This creates oxide-deficient sites on the catalyst surface accompanied hy a reduction of the metal. The reduced catalyst is then reoxidized hy adsorbing molecular oxygen, which migrates to fill the oxide-deficient sites. Thus, the catalyst serves as a redox system. ... 

It has been reported that an allylic C-Si bond can be cleaved by tetrabutylammonium fluoride to give an anionic allylic species, which chemoselectively adds to carbonyl compounds (nitriles, esters, and epoxides failed) to form homoallylsilyloxy compounds13. 

Complexes with unsaturated ligands (a-vinyl. a-allyl, and alkynyl) have been reported, each prepared from Fe(TPP)CI with the appropriate Grignard (vinyl, 2- methylvinyl.2,2-dimethylvinyl,allyl,or2-methylallyl)orlithiumreagent(LiC= C-n-Pr or LiC CPh) and observed by NMR spectroscopy (Scheme 4). The vinyl and alkynyl complexes are stable in solution at 25 C, whereas the allyl species decompose quickly if allowed to warm to room temperature. All were too reactive to be purihed by chromatography. The vinyl and allyl complexes show characteristic low spin behavior, although the temperature dependence of the vinyl... 

Although this type of reaction is symmetry forbidden in an unadsorbed molecule, theoretical calculations showed that in a molecule adsorbed on transition metals, such a shift is allowed [3-5], Later, other theoretical calculations suggested another type of 1,3-hydrogen shift, one in which the allylic cxo-hydrogen is abstracted by the surface fi-om an adsorbed alkene (either 1,2-diadsorbed or n-complexed) and the resulting 7i-allyl species moves over the abstracted hydrogen in such a way that it adds to the former vinylic position and causes, in effect, a stepwise intramolecular 1,3-hydrogen shift (bottom shift) [6],... 

Four kinds of adsorbed species are postulated to result from the adsorption of alkenes on noble metals, Jt-com plcxcd double bonds, Jt-complexed jt-allyl species, mono-O-bonded alkanes, and di-o-bonded alkanes. Distinguishing among these is difficult and has been the subject of much speculation. For example, alkenes may adsorb either by Jt-complexing or by di-C-adsorption, and 7t-allyl species are thought to occur readily on Pd but not so readily on Pt.55... 

In discussing the reaction pathways, we believe that the general evidence leads to the conclusion that hydrogenolysis proceeds via adsorbed hydrocarbon species formed by the loss of more than one hydrogen atom from from the parent molecule, and that in these adsorbed species more than one carbon atom is, in some way, involved in bonding to the catalyst surface. In the case of ethane, this adsorption criterion is met via a 1-2 mode or a v-olefin mode. Mechanistically it is difficult to see how the latter could be involved in C—C bond rupture in ethane. With molecules larger than ethane, other reaction paths are possible One is via adsorption into the 1-3 mode, and another involves adsorption as a ir-allylic species. 

These features do not preclude (1) and (2) (or their near equivalent) as a pathway for hydrogenation provided (1) is irreversible, but they do preclude alkyl reversal as an isomerization pathway. Over chromia, Burwell et al. (3) suggest that allyl species may furnish an isomerization pathway, viz ... 

When propylene chemisorbs to form this symmetric allylic species, the double-bond frequency occurs at 1545 cm-1, a value 107 cm-1 lower than that found for gaseous propylene hence, by the usual criteria, the propylene is 7r-bonded to the surface. For such a surface ir-allyl there should be gross similarities to known ir-allyl complexes of transition metals. Data for allyl complexes of manganese carbonyls (SI) show that for the cr-allyl species the double-bond frequency occurs at about 1620 cm-1 formation of the x-allyl species causes a much larger double-bond frequency shift to 1505 cm-1. The shift observed for adsorbed propylene is far too large to involve a simple o--complex, but is somewhat less than that observed for transition metal r-allyls. Since simple -complexes show a correlation of bond strength to double-bond frequency shift, it seems reasonable to suppose that the smaller shift observed for surface x-allyls implies a weaker bonding than that found for transition metal complexes. 

If the x-allyl species is the reactive species in hydrogenation, one would expect behavior dramatically different from that for ethylene, which ad-... 

The reactions described above are those expected if a 7r-allyl species is functioning as an intermediate. Thus, it appears that not only is the ir-allyl formed on zinc oxide but it is an important intermediate in the reactions of propylene. 

Isomerization of butene via a 7r-allyl species introduces an added dimension to the stereochemistry. The 7r-allyl species from propylene is presumed to be planar with its plane approximately parallel to the surface. Since it is attached to the electropositive zinc, it may have considerable carbanion character. A corresponding structure for adsorbed butene would lead to two isomeric forms, viz ... 

Firm assignments for these C=C bands require more detailed experiments but a tentative assignment can be made. The bands at 1550-1570 cm-1 are probably due to a ir-allyl species the shift from the double-bond region for butenes is about 100 cm-1 compared to the shift of 107 cm-1 observed for the 7r-allyl formed from propylene, but the butene is less firmly held. With propylene we observed a x-complex in which the shift in C=C stretch was about 30 cm-1. We believe the band at 1610 cm 1... 

Results with butene are not as extensive as those with propylene. Nevertheless, on the basis of the ground work laid by the more extensive propylene studies, we are able to apply similar criteria to the more limited data for butene and conclude that a x-allyl species forms. Some preliminary studies suggest that two x-allyl species form from 1-butene (65), corresponding to the syn and anti forms. The results for propylene, the fact that x-allyl species form from butene, and the fact that zinc oxide is an effective catalyst for butene isomerization strongly suggest that these x-allyls are intermediates in the isomerization reaction. 

The deprotonation of alkenes by organometallic reagents affords allyl species. As the simplest example of delocalized organometallic systems, the alkali metal allyl system has been studied in solution and the solid state in quite some detail this work has been further supported by theoretical studies. Allyl species are usually very reactive undergoing complex rearrangement reactions, and often, the reaction products cannot be directly characterized. Instead, they are often identified by their reaction products. 

The study of solvated alkali metal allyl species remains a complex topic due to a variety of reorganization processes. Structural data on alkali metal allyl derivatives include [G3H5Li(TMEDA)] 133,139 where solvated lithium ions act as... 

The arrangement of the molecular orbitals of the allyl species will be useful when discussing the bonding of this ligand in metal complexes (Chapters 16 and 21). 

I FIGURE 5.17 Molecular orbital diagram for the allyl species. 

Depending on the nature of surface chain growth species, on the other hand, one is confronted mainly with the alkyl mechanism,6 based on the insertion of a methylene species C CHj) into the metal-alkyl bond, or with the alkenyl mechanism,2 wherein a surface vinyl species ( CH=CH2) reacts with a surface methylene ( CII2) to form an allyl species ( CH2CH=CH2). 

The second pathway is represented by Eqs. (8)—(11). These reactions involve reduction of the Nin halide to a Ni° complex in a manner similar to the generation of Wilke s bare nickel (37, 38) which can form a C8 bis-77-alkyl nickel (17) in the presence of butadiene [Eq. (9)]. It is reasonable to assume that in the presence of excess alkyaluminum chloride, an exchange reaction [Eq. (10)] can take place between the Cl" on the aluminum and one of the chelating 7r-allyls to form a mono-77-allylic species 18. Complex 18 is functionally the same as 16 under the catalytic reaction condition and should be able to undergo additional reaction with a coordinated ethylene to begin a catalytic cycle similar to Scheme 4 of the Rh system. The result is the formation of a 1,4-diene derivative similar to 13 and the generation of a nickel hydride which then interacts with a butadiene to form the ever-important 7r-crotyl complex [Eq. (11)]. 

Osborn and Green s elegant results are instructive, but their relevance to metathesis must be qualified. Until actual catalytic activity with the respective complexes is demonstrated, it remains uncertain whether this chemistry indeed relates to olefin metathesis. With this qualification in mind, their work in concert is pioneering as it provides the initial experimental backing for a basic reaction wherein an olefin and a metal exclusively may produce the initiating carbene-metal complex by a simple sequence of 7r-complexation followed by a hydride shift, thus forming a 77-allyl-metal hydride entity which then rearranges into a metallocyclobutane via a nucleophilic attack of the hydride on the central atom of the 7r-allyl species ... 

An alternative means of activating an activated r/>3-C-H bond is by the addition of a nucleophile to an allylic species as in aminations using catalytic amounts of Pd(n) (Equation (34)).43... 

This is thought to proceed via an initial [2 + 2]-addition, followed by protonation of an allyl species. Phosphine decootdination from the metal is essential when dppe is used instead of triphenylphosphine, no reaction takes place (Scheme 16).74... 

Michael additions of 7r-allyl species to alkynes were employed for the synthesis of elaborated carbocycles as in the ruthenium-catalyzed cycloisomerization of 1,6-enynes (Equation (188)).1... 

An electron-rich metal can deprotonate the dicarbonyl derivative, affording the hydridopalladium intermediate 23, which can undergo a Tr-allyl 24 formation through diene insertion (which can be assimilated to a hydridopalladation of olefin) (Scheme 7). The attack of the enolate to the -jr-allyl species occurs with good enantioselectivity in the presence of the chiral ligand. The final product 21 is released and the palladium(O) complex 22 is regenerated. 

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