Aromatic allylation

Rearrangements, especially those only involving heat or a small amount of catalyst to activate the reaction, display total atom economy. A classic example of this is the Claisen rearrangement, which involves the rearrangement of aromatic allyl ethers as shown in Scheme 1.2. Although... 

We will describe representative procedures for the epoxidation of a disub-stituted aromatic allylic alcohol (A), a trisubstituted aromatic allylic alcohol (B) and a disubstituted aliphatic allylic alcohol (C). 

Hydroxylation Aromatic, allylic, benzylic CYP4S0 or flavin Phenols... 

Kitazume and Zulfiqar have investigated the Claisen rearrangement of several aromatic allyl ethers in ionic Hquids, catalyzed by scandium(III) trifluoromethane-sulfonate [72]. The reaction initially gave the 2-aUylphenol but this reacted further to give 2-methyl-2,3-dihydrobenzo[b]furan (Scheme 5.1-41). The yields in this reaction were highly dependant on the ionic liquid chosen, with [EDBU][OTf giving the best yields (e.g., 91 % for R = 6-CH3). Reactions in [BMIMjlBFJ and [BMIM][PF j gave low yields (9-12 %). 

Cleavage of allylic alcohols. The system selectively cleaves the double bond and the adjacent bond bearing the hydroxyl group of acyclic and aromatic allylic alcohols. Cyclic allylic alcohols are oxidized in low yield to dicarboxylic acids. 

Allylstannanes rapidly undergo the Kocheshkov comproportionation reaction with SnCU, even at -50 °C. The initial product is apparently formed with allylic inversion, but further transmetallation may give other allylic isomers.35 The SnCl3 group which is formed enhances the electrophilic power of the allyl group. This has been exploited in aromatic allylation (equation 9-13),36 and transmetallations of this type provide the basis for some, but not all, of the examples of the catalysis of the allylation of carbonyl compounds by allylstannanes in the presence of Lewis acids37,38 (Section 9.1.3.4). 

Allyl Triflone as an Allylating Agent. Frejd reported a low yield method using allyl triflone for aromatic allylation through a dlazotlzatlon/allylatlon process (eq 7). ... 

The Claisen rearrangement is intramolecular in nature. It was confirmed by a crossover experiment in which two aromatic allyl ethers 95 and 96 were heated together and found to yield same products 97 and 98 as when they were heated separately. No crossover products 99 and 100 were found [72]. 

Several other stabilized carbocations were utilized for asymmetric organocata-lyzed transformations. A comparable and similar benzylic situation is given by the utilization ofallylic alcohols (Scheme 4.16). These substrates are completely unre-active under organocatalyzed reactions conditions. But, by a combination of metaland organocatalysis, the same authors were able to realize an enantioselective a-alkylation of enolizable aldehydes with allylic alcohols [47]. The corresponding allylic cations were formed by the use of catalytic amounts of indium salts. Thus, various several different aromatic allylic alcohols were converted into the corresponding aldehydes 52a-c with high enantioselectivity. 

The structure of a vinylsilane Pt(0) complex, a likely hydrosilylation catalyst, has been determined (11). The compound is obtained by reacting vinylsilanes with [Pt(COD)2], as well as from H2PtCl6 or K2PtCl4. On the other hand, the kinetics of the hydrosilylation of aromatic allyl ethers appear to suggest the involvement of Cl-bridged Pt(II) dimers, although no direct evidence for the Pt oxidation state or the presence of Cl-bridges was presented. ... 

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