Allyl complexes from allenes

Palladium(II) is one of the most important transition metals in catalytic oxidations of allenes [1], Scheme 17.1 shows the most common reactions. Transformations involving oxidative addition of palladium(O) to aryl and vinyl halides do not afford an oxidized product and are discussed in previous chapters. The mechanistically very similar reactions, initiated by nucleophilic attack by bromide ion on a (jt-allene)pal-ladium(II) complex, do afford products with higher oxidation state and are discussed below. These reactions proceed via a fairly stable (jt-allyl)palladium intermediate. Mechanistically, the reaction involves three discrete steps (1) generation of the jt-allyl complex from allene, halide ion and palladium(II) [2] (2) occasional isomeriza-... 

Scheme 3-26 Fonnation of Tr-allyl complexes from 1,3-dienes and allenes.

PtMeL2] proceed in a Markownikov manner by electrophilic attack of Pf thus [Pt(A -2-methallyl)L2] is formed from allene and [PtMe-(acetone)L2], whereas the analogous 1,3-butadiene cation does not lead to a 7r-allylic derivative by Pt—Me insertion. The hydro cation, however, can react by either a Markownikov or an anti-Markownikov mechanism with either Pt+ or attack on the unsaturated ligand. This apparent versatility leads to the formation of Tr-allylic complexes from both allenes and 1,3-dienes with [PtHLg]. ... 

Because of the exceptional C-F bond strength, the successful preparation of a-halocyclopropyl c-complexes is realized by substitution of 1-bromo-l-fluoro-trans-2,3-dimethylcyclopropane 179 with Fp [90], Silica gel column chromatography of the thus obtained cr-complex 180 results in ring opening to the alcohol 181 as a single stereoisomer. The allene complex 182 is produced by treatment with BF3OEt2, indicating that 181 is derived from 182 and water. The 7i-allyl complex 183 is formed by photolysis via a disrotatory process. 

When, furthermore, phenols (368) are coupled with 1 in the presence of a Pd° catalyst, the phenoxy-methyl-1,3-dienes 369 are produced [158]. As aryl allyl ethers, these can be made to undergo a Claisen rearrangement (205 °C, DMF) and the ensuing 2-(l,3-dienylmethyl)phenols 370 finally cydize in the presence of a trace of acid to a mixture of exo-methylene chromans 371 (major product) and dihydrobenzofur-ans 372 - a remarkable generation of functional and structural complexity from simple starting materials with 100% atom economy and underlining impressively the synthetic versatility of modern allene chemistry ... 

In Scheme 17.2 palladium is coordinated from below, but it is also possible that it coordinates from above and forms the other enantiomer of the chiral syn,syn Jt-allyl complex. If palladium has another chiral ligand then these Jt-allyl complexes become diastereomers. Thus, from an unsymmetrically substituted allene (R R ), eight diastereomeric Jt-allyl complexes can be formed. If one of the diastereomers is preferred then further reaction of the Jt-allyl moiety leads to an enantiomerically enriched product. 

Acetylation occurs at the 2-position of allene systems (Scheme 8.14). The intermediate 7t-allyl complex breaks down via the nucleophilic displacement of the cobalt carbonyl group by the hydroxide ion to produce the hydroxyketone (7) [ 11 ]. An alternative oxygen-initiated radical decomposition of the complex cannot, however, be totally precluded. The formation of a second major product, the divinyl ketone (8), probably arises from direct interaction of the dicobalt octacarbonyl with the allene and does not require the basic conditions. 

Evidence derived from a study of the stereochemistry of hydrogenation of 1,2-cyclononadiene and 1,2-cyclodecadiene led Moore (108) to conclude that allyl complexes like those postulated above must be intermediates. He noted that, of the four ways in which either allene could be adsorbed on a surface, two, a and b, would yield via cis addition of hydrogen the cis-cycloalkene and two, c and d, the tram isomer. Examination of... 

An asymmetric preparation of alkenylcyclopropanes has also been realized by the use of palladium(O) complexes carrying chiral ferrocenylphosphine ligands (equation 22)38. The requisite rt-allyl palladium intermediates can also be generated from allene and meth-ylenecyclopropane derivatives, 1839 and 1940, in the presence of palladium(O) complex and alkenyl or aryl halide (equations 23 and 24). The cobalt complexes, 20, similarly afford the corresponding alkenylcyclopropanes upon exposure to LDA (equation 25)41. 

These -propargyl rhenium complexes undergo kinetic addition of nucleophiles at the central C atom to produce rhenacyclobutenes. The nucleophiles range from PR3 to malonate, acetylides, pyridines, and water. The derivatives of the addition of pyridines, however, are unstable and undergo further rearrangements to allene or acetylene complexes. Protonation of the metaUacyclobutenes produces u -allyl complexes. 

In this section, Pd(0)-catalyzed reactions of allenes with nucleophiles are treated, which are clearly different mechanistically from the reactions explained in the above. Attack of nucleophiles may occur at C-1, C-2, and C-3 carbons of the allenes 63. Among them, attack at C-3 to give 64 is predominant. Most importantly, reactions of allenes with pronucleophiles start by the oxidative addition of pronucleophiles to Pd(0) to generate H-Pd-Nu 65. The formation of 64 by hydro-carbonation can be explained in two ways in the case where Nu-H is the carbon pronucleophile. As one possibility, hydropalladation of one of the two double bonds occurs to afford the terminal palladium intermediate 66, which is stabilized by the formation of 7r-allyl complex 67, and reductive elimination provides the C-3 adduct 68. Another possibility is carbopalladation to generate 69, and subsequent reductive elimination provides 68. Of these two possibilities, the hydropalladation mechanism is preferable. 

Oxidative Methods.—Oxidation of Alcohols. Benzoyl peroxide catalysed by nickel(ii) bromide gives high yields of aldehydes and ketones from the corresponding alcohols. Similar yields are obtained with t-butyl hydroperoxide catalysed by diaryl diselenides, a method particularly recommended for benzylic or allylic alcohols. Ketones are obtained from secondary alcohols using hydrogen peroxide catalysed by molybdenum or tungsten peroxo-complexes/ and nickel peroxide has been employed to prepare a-allenic aldehydes and ketones from allenic alcohols. ... 

Me3SiPdSnBu3 is formed primarily from 6/1-237, which then adds to the allene moiety in 6/1-236 to give a a- or Jt-allyl palladium complex. This undergoes an intramolecular carbonyl allyl addition to afford the cis-cycloalkariols 6/1-238 (Scheme 6/1.61). 

Closely related to both allyl carbenoids and the allenyl carbenoids discussed above, propargyl carbenoids 101 are readily generated in situ and insert into zirconacycles to afford species 102 (Scheme 3.27), which are closely related to species 84 derived from allenyl carbenoids [65], Protonation affords a mixture of allene and alkyne products, but the Lewis acid assisted addition of aldehydes is regioselective and affords the homopropargylic alcohol products 103 in high yield. Bicydic zirconacyclopentenes react similarly, but there is little diastereocontrol from the ring junction to the newly formed stereocenters. The r 3-propargyl complexes derived from saturated zirconacycles are inert towards aldehyde addition. 

The mechanism of this reaction was considered on the basis of hydropalladation (Scheme 14). To minimize steric repulsions, the palladium hydride complex approaches the C=CH2 moiety of the allene in the anti-Markovnikov mode from the opposite side of the substituent. This addition gives a 7t—allyl palladium complex with the (Z)-configuration,18 which is converted to the (Z)-product by C-P bond formation, with regeneration of the Pd(0) catalyst. 

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