Jr-allylic complexes

Ethenylcyclopropyl tosylates 131 and 2-cyclopropylideneethyl acetates 133, readily available from the cyclopropanone hemiacetals 130, undergo the re-gioselective Pd(0)-catalyzed nucleophilic substitution via the unsymmetrical 1,1-dimethylene-jr-allyl complexes. For example, reduction with sodium formate affords a useful route from 131 to the strained methylenecyclopropane derivatives 132. The regioselective attack of the hydride is caused by the sterically... 

Also, the trimerization reaction was observed by using jr-allyl complexes of palladium stabilized by chelating ligands, such as 28 and 29 (38). The reaction was carried out at 70°C in dimethylformamide (DMF) or dimethyl sulfoxide (DMSO), and 1,3,6,10-dodecatetraene (27) was obtained in 60% yield at 30% conversion. 

The stereochemistry of the dialkoxylation arises from two external attacks by the alcohol, one on the rr-diene complex and the second on the intermediate jr-allyl complex. This is in accordance with the other palladium-catalyzed 1,4-syn additions discussed above. 

In a stoichiometric reaction the 6jr-allyl)palladium complex 66 was isolated and characterized5815. In a subsequent reaction the jr-allyl complex was reacted with benzoquinone in acetic acid to give an allylic acetate, which was hydrolyzed and oxidized to theaspirone. Interestingly, a quite high diastereoselectivity for the turns methyl isomer was obtained in the palladium-mediated spirocyclization (equation 28). 

In the catalytic reaction this diastereoselectivity drops, and it was demonstrated that the jr-allyl complex with the cis tetrahydrofuran is kinetically favored and is trapped by the... 

It is postulated that the mechanism of the silane-mediated reaction involves silane oxidative addition to nickel(O) followed by diene hydrometallation to afford the nickel -jr-allyl complex A-16. Insertion of the appendant aldehyde provides the nickel alkoxide B-12, which upon oxygen-silicon reductive elimination affords the silyl protected product 71c along with nickel(O). Silane oxidative addition to nickel(O) closes the catalytic cycle. In contrast, the Bu 2Al(acac)-mediated reaction is believed to involve a pathway initiated by oxidative coupling of the diene and... 

Liebeskind and coworkers have examined the reactivity of (2//-pyran)Mo(CO)2Cp+ cations 210, which may be prepared in optically active form from carbohydrate precursors. Nucleophilic attack on cation 210 occurs at the diene terminus bonded to the ring oxygen to give jr-allyl complexes 51 (Scheme 53)85. Hydride abstraction from 51 gives the cation 54 addition of a second nucleophilie occurs regioselectively to give... 

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). 

Fig. 25. Geometrical relationships for the formation and reaction of a jr-allyl complex at a single atomic center.

Both hydride cind jr-allyl complexes of these metals are well known in the -f-2 oxidation state (60), but neither hydride nor r-allyl complexes of platinum(IV) are known. 

Under certain conditions, platinum alkene complexes can be converted to jr-allyl complexes by hydrogen loss (equation 281).860 Allyl alcohol insertion into a platinum(II) carbonyl bond gives an allyloxycarbonylplatinum(II) intermediate which can be decarboxylated to form the jj3-allyl complex (equation 282).861,863... 

When the cyclopropyl platinum(II) complex dA-P(.(CHCH2CH2)2(PMe2Ph)2 is treated with HC1 then AgN03, the jr-allyl complex is formed (equation 285).866 -Allyl complexes can also be formed by insertion of alkenes into platinum-hydride bonds, and this reaction is discussed in Section 52.2.3.3. 

The two types of addition were proposed to occur through different surface absorbed forms240,243 (Scheme 6). The 24 -adsorbed intermediate is suggested to result in 1,2 addition (Type A metals). The 25 and 26 jr-allyl species, in contrast, ensure 1,4 addition. The selectivity of the formation of stereoisomeric 2-butenes, in turn, depends on the interconversion of the possible half hydrogenated syn and anti surface jr-allyl complexes... 

Scheme 30 shows the proposed reaction mechanism, which involves the formation of an acylpalladium species as the key intermediate, in tautomeric equilibrium with a cyclic jr-allyl complex (in this and in the following Schemes, unreactive ligands are omitted for clarity). The main reason for the high activity of the Pdl42"-based catalyst in this process lies in the very efficient mechanism of reoxidation of Pd(0), which involves oxidation of HI by 02 to I2, followed by oxidative addition of the latter to Pd(0). It is worth nothing that under these conditions Pd(0) reoxidation occurs readily without need for Cu(II) or organic oxidants. 

In the presence of alkenes, palladium(II) salts form Pd(II)-olefin complexes. For olefins with allylic hydrogen atoms, these complexes undergo a rapid rearrangement to jr-allyl complexes by a process called allylic C-H activation [34], Nucleo-... 

Lithium lanthanum jr-allyl complexes, LiLn(All)4 dioxane, where Ln = Ce, Nd, Sm, Gd, Dy have been synthesized and used as catalysts in the polymerization of butadiene. The data show the predominance of 1,4-trans product. The catalytic activity of the lanthanides was nearly the same as evidenced by the percent yield in the range 78-90. 

Cyclic dienes preorganize the geometry of the two donble bonds, favoring chelation over jr-allyl formation. Acyclic dienes tend to give jr-allyl complexes. However, the reaction of allyl chloride with PdCl2(PhCN)2 at low temperatnres in the absence of solvent gives the 1,5-hexadiene complex (eqnation 40). 

Najera and coworkers introduced a new class of cyclic alanine templates (227, equation 59), the structure of which was anchored on Schollkopf s bislactim ether . Palladium-catalyzed allylations of the chiral pyrazinone derivative 227 with allylic carbonates (228) as substrates led to the formation of y,i5-unsaturated amino acids (229a-c) under very mild and neutral reaction conditions, whereas the required base for enolate preparation has been generated in situ from the allylic carbonate during jr-allyl complex formation. With this protocol in hand, the alkylated pyrazinones 229 were obtained with excellent regio- and diastereoselectivities (>98% ds). Finally, hydrolysis with 6 N aqueous HCl under relatively drastic conditions (150 °C) led to the free amino acids. 

This reaction constitutes a special type of process in which a hydrogen atom and a nucleophile are added across the diene with fonnation of a carbon-hydrogen bond in the 1-position and a carbon-Nu bond in the 4-position. Some examples of such reactions are hydrosilylation [12-18], hydrostannation [19,20] amination [21,22], and addition of active methylene compounds [21 a,23,24], These reactions are initiated by an oxidative addition of H-Nu to the palladium(0) catalyst, which produces a palladium hydride species 1 where the nucleophile is coordinated to the metal (Scheme 8-1). The mechanism commonly accepted for these reactions involves insertion of the double bond into the palladium-hydride bond (hydride addition to the diene), which gives a (jr-allyl)palladium intermediate. Now depending on the nature of the nucleophile (Nu) the attack on the jr-allyl complex may occur either by external trans-aVtBck (path A) or via a cw-migration from palladium to carbon (path B). 

The use of dienecarboxylic acids (cf. Section 8.3.1.1, under Intramolecular 1,4-diacyloxylation ) under the conditions for haloacyloxylation in acetone resulted in a highly stereoselective chlorolactonization [Eqs.(44) and (45)] [67]. The reaction proceeds >98% as a 1,4-c/5 addition and involves the same intermediate lactone jr-allyl complex as in the intramolecular diacyloxylation. 

Intramolecular reactions of allylic acetates with conjugated dienes catalyzed by Pd(0) lead to a 1,4-addition of a carbon and an oxygen nucleophile to the diene. The reaction, which is formally an isomerization, involves tw different yr-allyl complexes (Scheme 8-4) [44]. Reaction of 22 in the presence of the Pd(0) catalyst Pd2(dba)3-CHCl3 (dba = dibenzyl-ideneacetone) and LiOAc/HOAc in acetonitrile at reflux produces the cyclized isomer 25 in 62% yield. The double bond was exclusively of E stereochemistry, while the ring stereochemistry was a mixture of cis and tram isomers. Oxidative addition of the Pd(0) to the allylic acetate gives the intermediate jr-allyl complex 23. Subsequent insertion of a diene double bond into the allyl-palladium bond produces another jr-allyl intermediate (24), which is subsequendy attacked by acetate to give the product 25. 

L. Mond and his co-workers discovered it in 1888. The platinum methyls, prepared in 1907 by W. J. Pope, were amongst the first-known transition metal alkyls, and the discovery by W. Reppe in 1940 that Ni complexes catalyse the cyclic oligomerization of acetylenes produced a surge of interest which was reinforced by the discovery in I960 of the jr-allylic complexes of which those of Pd arc by far the most numerous. 

Ionization of substrates 1 and 2 leading to the symmetrically 1,1-disubstituted diastereomeric Ti-allyl complexes 3 and 4 also allows complete conversion to one product enantiomer, provided that nucleophilic attack occurs at the carbon bearing substituent R2. High enantiomeric excess may be achieved if a rapid equilibration between the two intermediate re-allyl species is established and the soft carbanion preferentially attacks one of them. Interconversion of the reactive complexes is possible via epimerization by nucleophilic attack of free palladium(O) anti to the jr-allyl complex or by n-a-n rearrangement involving the formation of a Pd-C c-bond at the symmetrically substituted allyl terminus. This process is only fast for R1 = H, due to a low degree of steric congestion or for R1 = aryl because of rc-benzyl participation. 

In the dimerization reaction of butadiene catalyzed by palladium complexes, nucleophiles (YH), such as amines, alcohols, phenols, carboxylic acids 41 4S>, and active methylene compounds 46) are introduced. This reaction can be explained by the attack of these nucleophiles on the jr-allylic complexes formed as intermediates-in the reactions. Takahashi, Shibano, and Hagihara confirmed by using deuterium that the hydrogen of YH migrates to C6 of the dimeric product, probably via the oxidative addition reactions of YH to the palladium species 42). 

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