Reactions of Allyl Complexes

The reactivity of allyl complexes is included in many of the later chapters of this text. Although examples of reductive eliminations and migratory insertions of allyl complexes are known, the dominant reaction chemistry involves the attack by nucleophiles on the allyl ligand and attack of the allyl ligand on external and coordinated electrophiles. The latter reaction leads to catalytic allylic substitution reactions. The reactions of nucleophiles and electrophiles with allyl complexes are described in Chapters 10 and 11, and the catalytic allylic substitution is described in Chapter 20. 

TT-Benzyl complexes often exist in equilibrium with Ti -benzyl complexes. Consistent with the scheme for interconversions between ti - and Ti -allyl complexes, an as3Tnmetric ber zyl carbon has been shown to retain its configuration during this process (Equation 3.69).  

TT-Benzyl complexes typically form when a benzyl group is present in a coordina-tively unsaturated middle or late metal center that can participate in back-donation into the TT-benzyl urut. TT-Benzyl complexes have been prepared by the addition of benzyl Grignard reagents to unsaturated metal-halide complexes, by the addition of benzyl halides to low-valent metal complexes, or by the insertion of vinylarenes into metal hydrides or alkyls. These three types of reactions are illustrated by the examples in Equations 3.70-3.72. 

There are also complexes containing polyhapto ligands with a more extended TT-system than that in T -allyl or -benzyl groups. Pentadienyl systems are the next higher anionic homolog of an allyl system. Pentadienyl systems are often formed by the addition of protons to, or removal of hydrides from, transition metal polyene complexes. Alternatively, pentadienyl groups have been generated by the addition of 

CHAPTER 3 COVALENT (x-TYPE) LIGANDS BOUND THROUGH METAL-CARBON AND METAL-HYDROGEN BONDS 

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Scheme 24 The reaction of allyl complexes with benzylamine...

R. Baker, Chem. Ind. (London), 1980, 816-823. V Carbon-Carbon Bond Eorming Reactions of Allyl Complexes of Nickel, Palladium, and Iron. 

Other mechanistic pathways have been proposed, in particular a so-called outer-sphere mechanism (similar to a pericyclic allylic transposition) by Hugues and Powell. Although they are probably relevant to some stoichiometric reactions of -allyl complexes with dienes, they do not seem to fit the characteristic experimental features of these dienes coordination polymerizations. 

Reactions of Allylic Compounds via 7r-Allylpalladium Complexes Catalyzed by Pd(0)... 

In addition, a catalytic version of Tt-allylpalladium chemistry has been devel-oped[6,7]. Formation of the Tr-allylpalladium complexes by the oxidative addition of various allylic compounds to Pd(0) and subsequent reaction of the complex with soft carbon nucleophiles are the basis of catalytic allylation. After the reaction, Pd(0) is reformed, and undergoes oxidative addition to the allylic compounds again, making the reaction catalytic.-In addition to the soft carbon nucleophiles, hard carbon nucleophiles of organometallic compounds of main group metals are allylated with 7r-allylpalladium complexes. The reaction proceeds via transmetallation. These catalytic reactions are treated in this chapter. 

Allylic metal compounds useful for further transformations can be prepared by Pd-catalyzed reactions of allylic compounds with bimetallic reagents. By this transformation, umpolung of nucleophilic 7r-allylpalladium complexes to electrophilic allylmetal species can be accomplished. Transfer of an allyl moiety from Pd to Sn is a typical umpolung. 

Nanaomycin A 103 and deoxyfrenolicin 108 are members of a group of naphthoquinone antibiotics based on the isochroman skeleton. The therapeutic potential of these natural products has attracted considerable attention, and different approaches towards their synthesis have been reported [65,66]. The key step in the total synthesis of racemic nanaomycin A 103 is the chemo-and regioselective benzannulation reaction of carbene complex 101 and allylacety-lene 100 to give allyl-substituted naphthoquinone 102 after oxidative workup in 52% yield [65] (Scheme 47). The allyl functionality is crucial for a subsequent intramolecular alkoxycarbonylation to build up the isochroman structure. However, modest yields and the long sequence required to introduce the... 

It was suggested that this change in product distribution was due to the existence of an equilibrium between two types of complex, viz., a cr-butenyl-pentacyanocobaltate(III) and a 7r-butenyltetracyanocobaltate(III) 107, 109). However, further study of the kinetics and product distribution suggested the presence of two o-bonded complexes, viz., cr-but-l-en-3-yl and a-but-2-en-l-yl 24a). Direct evidence for the existence of a cyanide-dependent equilibrium between the a- and rr-bonded organocyanide complexes has been obtained from NMR studies of the complex prepared by the reaction of allyl halides with Co—H 109) (see also Section V,C). Both butadiene and crotyl chloride react with Co—H to give the same... 

Hoveyda and co-workers also tested optically active A-heterocyclic carbenes and their silver complexes in copper-catalyzed reactions of allylic phosphates with dialkylzincs.402 The ratios of Sn2 SN2 products were higher than 98 2 and the ee varied from 34% to 98%. 

Following the success with the titanium-mediated asymmetric epoxidation reactions of allylic alcohols, work was intensified to seek a similar general method that does not rely on allylic alcohols for substrate recognition. A particularly interesting challenge was the development of catalysts for enantioselective oxidation of unfunctionalized olefins. These alkenes cannot form conformationally restricted chelate complexes, and consequently the differentiation of the enan-tiotropic sides of the substrate is considerably more difficult. 

The isomerisation of allyl and propargylic alcohols involves the 1,3-shift of an oxygen atom rather than a hydrogen atom. Isomerisation of allyl alcohols can be catalysed by a variety of metal oxo complexes and in this instance the reaction does not involve metal carbon bonds as we will see. One could imagine that allylic metal species can participate in isomerisation of allylic compounds, but for the alcohols themselves this is not an easy reaction. In chapter 13 reactions of allyl acetates and the like, which are more prone to... 

The Pd(0)-catalyzed displacement of allylic acetates (297) with various nucleophiles via the allylic Pd(II) complex (298) is a well-established procedure (Scheme 114). Through attack of electrons (+2e ) in place of nucleophiles, (298) is expected to undergo a reductive cleavage providing allylic carbanions (299) and the acetate anion along with Pd(0) complexes. The latter can then be captured by various electrophiles (polarity inversion. Scheme 114) leading to (300) [434]. This procedure is useful for the deprotection of allyl esters under neutral conditions. Recently, a mechanistic study of the Pd-catalyzed reaction of allylic acetate (297), using carbonyl compounds as an electrophile, has been reported [435]. 

The mechanism of the Zn chloride-assisted, palladium-catalyzed reaction of allyl acetate (456) with carbonyl compounds (457) has been proposed [434]. The reaction involves electroreduction of a Pd(II) complex to a Pd(0) complex, oxidative addition of the allyl acetate to the Pd(0) complex, and Zn(II)/Pd(II) transmetallation leading to an allylzinc reagent, which would react with (457) to give homoallyl alcohols (458) and (459) (Scheme 157). Substituted -lactones are electrosynthesized by the Reformatsky reaction of ketones and ethyl a-bromobutyrate, using a sacrificial Zn anode in 35 92% yield [542]. The effect of cathode materials involving Zn, C, Pt, Ni, and so on, has been investigated for the electrochemical allylation of acetone [543]. 

Mazet et al. have reported an efficient asymmetric isomerization reaction of allylic alcohols [60, 61]. In a preliminary report they utilized the BArp analog of Crabtree s complex to efficiently catalyze a hydride transfer from the a position of the allylic alcohol to the p position of the olefin with a concomitant formation of a formyl group. A subsequent report detailed a remarkable enantioselective variant of this process catalyzed with Ir(12g) and (12h) (Scheme 12). 

Reactions of allylic electrophiles with stabilized carbon nucleophiles were shown by Helmchen and coworkers to occur in the presence of iridium-phosphoramidite catalysts containing LI (Scheme 10) [66,69], but alkylations of linear allylic acetates with salts of dimethylmalonate occurred with variable yield, branched-to-linear selectivity, and enantioselectivity. Although selectivities were improved by the addition of lithium chloride, enantioselectivities still ranged from 82-94%, and branched selectivities from 55-91%. Reactions catalyzed by complexes of phosphoramidite ligands derived from primary amines resulted in the formation of alkylation products with higher branched-to-linear ratios but lower enantioselectivities. These selectivities were improved by the development of metalacyclic iridium catalysts discussed in the next section and salt-free reaction conditions described later in this chapter. 

Carreira et al. reported the kinetic resolution of branched allylic carbonates catalyzed by an iridium complex derived from a chiral [2.2.2]-bicyclooctadiene [48]. Reactions of allylic carbonates with phenol were run to 50% conversion of the carbonate, leaving unreacted allylic carbonate in high enantiomeric excess (Scheme 32). The phenyl ether products were also isolated in mid-to-high enantiomeric excess. 

The preparation of (jr-allyl)(PHOX)Ir complexes was possible by using standard methods. A typical example is depicted by a crystal structure in Figure 9.1. Here, the configuration around the Ir center is as to be expected on the basis of the trans influences of the ligands. The reaction of this complex with dimethyl sodiomalonate proceeds by ligand exchange and addition of the nucleophile at the... 

Addition reactions of allylic boron compounds have proven to be quite general and useful. Several methods for synthesis of allylic boranes and boronate esters have been developed.36 37 The reaction has found some application in the stereoselective synthesis of complex structures. 

Jiang S, Agoston GE, Chen T, Cabal M-P, Turns E (1995) BF3 Et20-promoted allylation reactions of allyl(cyclopentadienyl)iron(II) dicarbonyl complexes with carbonyl compounds. Organometallics 14 4697 -709... 

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