Allyl complexes catalysis

Recently, Co(III)-allyl complexes have been described to be sulfonylated regiospecifi-cally by sulfonyl halides under irradiation (equation 42). Similarly, allyl methyl sulfone has been obtained from allyltrimethylsilane under copper(I) catalysis ... 

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). 

Similarly, lithium enolates of ketones add to allyl acetates via Pd° catalysis by a double inversion process,110 and potassium ketone enolates have been shown to add to preformed rr-allyl complexes with inversion (equations 156 and 157)."1... 

Although mechanistically different, a successful kinetic resolution of cyclic allyl ethers has recently been achieved by zirconium catalysis [2201. Other metals such as cobalt [221], ruthenium [222], and iron [2231 have been shown to catalyze allylic alkylation reactions via metal-allyl complexes. However, their catalytic systems have not been thoroughly investigated, and the corresponding asymmetric catalytic processes have not been forthcoming. Nevertheless, increasing interest in the use of alternative metals for asymmetric alkylation will undoubtedly promote further research in this area. 

The adoption of reaction models available for the polymerization of conjugated dienes by Ni- and Ti-catalysts to the polymerization of BD by Nd catalysis is justified by the similarities of the respective metal carbon bonds. In each of these mechanistic models the last inserted monomer is bound to the metal in a 3-allyl mode. The existence of Ni- -allyl-moieties was demonstrated by the reaction of the deuterated nickel complex [ rf- C4D6H)NiI]2 with deuter-ated BD (deuterated in the 1- and 4-position). After each monomer insertion a new 3-allyl-bond is formed [629]. As TT-allyl-complexes are known for Ti and Ni this knowledge has been adopted for Nd-based polymerization catalysts [288,289,293,308,309,630-636,638-645]. 

Allyl complexes continue to find uses in polymerization catalysis. The new isoprene-co-l,3,7-octatriene P(IP-co-OT)] copolymers with controlled... 

Initial experiments by Gibson et al. (77) indicate phase-transfer catalysis to be a promising method for the synthesis of benzyl complexes. Reaction of benzyl bromide with cyclopentadienylmolybdenum tricar-bonyl chloride, under conditions similar to those described for the preparation of o-- and 7r-allyl complexes, afforded the o--benzyl complex (64) in 89% yield. Use of benzyl chloride as substrate results in a considerably... 

Allyl complexes were amongst the earliest studied nomigid complexes, and there are a number of reviews on their rearrangements. Interest has been particularly high owing to their importance in catalysis and synthesis, with palladium and molybdenum complexes playing leading roles. 

Vinyl oxazolidinone 148 has been shown to be a good precursor to rra r-oxazolines (Scheme 38). Under palladium(O) catalysis, an intermediate 7t-allyl complex is formed with the loss of CO2. Intramolecular attack by the amide oxygen forms the thermodynamically favored /ra r-oxazoline 149. This oxazoline was then converted to the natural product balanol <19990L615>. 

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. 

The formation of allenylidene derivatives from ethynyl-hexanol and alkenyl-vinylidene mononuclear complexes (9), the formation of mononuclear ruthenium allenyl complexes from terminal alkynes (10), the intermediacy of ruthenium-allenylidene complexes in forming propargylic alcohols (II), and in the cyclization of propargyl alcohols (12), and the use of mononuclear ruthenium compounds in allylic alkylation catalysis (13) have also been reported. 

The isomerization of allylic alcohols to carbonyl compounds is usually carried out in the presence of transition-metal complexes under rather drastic conditions. The joint use of PTC and metal-complex catalysis creates a simple, mild, and efficient method for the isomerization of unsaturated alcohols [183]. Allylic alcohols are converted to ketones in a PT system with a Rh complex. An ion-pair [R4N] [RhCl4] (see Section 3.2.4.2.1) is also effective for the isomerization of allylic alcohols as well as allylarenes [184], Another example of PTC isomerization is the transformation of allylic alcohols, RCH(OH)CH=CFl2, to ketones, RCOCH2CH3 [185]. 

The great importance of group VIII complexes in homogeneous catalysis has focused more attention on their insertion chemistry, but the number of authenticated insertions of monoalkenes into M-H bonds is small. Insertions of dienes and allenes are more numerous because of the greater thermodynamic stability of the product xt-allyl complexes. Some examples of insertions of complexes of group VIII are listed in Table... 

Enantioselective catalysis with nucleophiles that attack the metal of the intermediate rr-allylpal-ladium complexes prior to allylic C-C bond formation has met with only limited success so far30-31. Phenylation of the allyl acetates 1 and 4 via re-allyl complexes with a me,sY -7r-allyl ligand using phenylzinc chloride and enantiomerically pure monophosphines 231 or 53a yield 3 and 6 of low enantiomeric excess, respectively. 

Wilke, G., et al., Angew. Chem. (Internet. Edn), 1966, S, 151 (allyl complexes and their uses in catalysis). 

You ye seen that palladium catalysis helps form carbon-carbon bonds that are difficult to make using conventional reactions. It can also help form carbon-heteroatom bonds that are difficult to make, and you have already seen some examples in the reactions of re-allyl complexes. Work starting in the 1990s by Buchwald and Hartwig has shown that Pd can be used to promote nucleophilic substitution at a vinylic or aromatic centre—a reaction which would not normally be possible. For example, aromatic amines can be prepared directly from the corresponding bromides, iodides, or triflates and the required amine in the presence of pal-ladium(0) and a strong alkoxide base. 

Tin.—Allyltin compounds, especially under rhodium-complex catalysis, react with acyl chlorides to give allyl ketones or with aryl halides to give allylarenes. Bridgehead alkyl bromides are reduced on photolysis with Bu"3SnH, whilst the system (Bu"3Sn)20-Br2 oxidizes sulphides to sulphoxides. )V-(Alkoxycar-bonyl)amino-acids are obtained upon treatment of cyclic anhydrides with Bu3SnN3 and heating in an alcohol. ... 

Compared to the externally adsorbed complex, encaging has a profound effect on the t/c ratio of the olefins formed. As suggested by the authors (36) this is firm evidence for catalysis via an anti pi-allyl complex rather than via carbanion chemistry, favoring in absence of any constraints the cis-isomer. The abundant formation of t-but-2-ene in for the encaged complex, points to the presence of steric constraints on the transition state and suggests that the less bulky syn pi-allyl intermediate is favored (36) (equation XI). 

---