Aromatic allyl ethers

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... 

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 %). 

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]. 

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. ... 

Allyl ethers of perfluoroaromatic phenols have been observed to alkylate the aromatic nucleus [106] or to undergo Claisen rearrangement [1071 (equation 54). 

Ether groups in the benzene ring of quinazoline behave as in ethers of homocyclic aromatic compounds, e.g., they can be demethylated with anhydrous aluminum chloride. Allyl ethers also undergo a Claisen rearrangement/ ... 

Scheme 10.35 Aromatic Claisen rearrangements of catechol mono allylic ethers with sulfur-containing boron catalyst.

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

A simple synthesis of 3-substituted and 23-disubstituted 4-chloiofuians was accomplished. It involves a CuCl/bipy-catalyzed regioselective cyclization of l-acetoxy-2.22-trichloroethyl allyl ether followed successively by dechloroacetoxylation with Zn dust and tandem dehydro-halogenation-aromatization with tBuOK/18-crown-6 <99CC2267>. 

Allylation of arenes and alcohols. Allyltrimethylsilane and some related al-lyltrialkylmetal reagents of Sn and Ge in combination with iodosylbenzene (1 equiv.) activated by BF3 etherate (0.25-1 equiv.) allylate aromatics in CH2C12 at -30° to 25°. Under these conditions alcohols are converted into allyl ethers, even though iodosylbenzene is a known oxidant for alcohols.4... 

Solutions of these metals in liquid ammonia effect (i) the reduction of a range of functional groups such as carbonyl and acetylenic and also conjugated and aromatic systems, and (ii) cleavage of benzyl and allyl ethers and thioethers. These reactions are usually carried out by the general procedure of adding the metal to a solution of the substrate in liquid ammonia to which dry methanol or ethanol or t-butanol has been added to provide a ready proton source (alcohols are more acidic than ammonia).34... 

Although the Claisen rearrangement was first observed in the enol allyl ethers,1-2 the reaction is much more useful and important in the aromatic series. Some interesting observations have been made, however, with the open-chain systems. The original reports concerned the rearrangement of ethyl O-allylacetoacetate, O-allylacetylacetone (XIVo), and O-allyloxymethylenecamphor (XV). 

When steric hindrance in substrates is increased, and when the leaving anion group in substrates is iodide, SET reaction is much induced (Cl < Br < I). This reason comes from the fact that steric hindrance retards the direct nucleophilic reduction of substrates by a hydride species, and the a energy level of C-I bond in substrates is lower than that of C-Br or C-Cl bond. Therefore, metal hydride reduction of alkyl chlorides, bromides, and tosylates generally proceeds mainly via a polar pathway, i.e. SN2. Since LUMO energy level in aromatic halides is lower than that of aliphatic halides, SET reaction in aromatic halides is induced not only in aromatic iodides but also in aromatic bromides. Eq. 9.2 shows reductive cyclization of o-bromophenyl allyl ether (4) via an sp2 carbon-centered radical with LiAlH4. 

In terms of functional group compatibility, ethers, alcohols, tertiary amines, acetals, esters, amides and heterocycles are compatible with the Pauson-Khand reaction. In the intramolecular version, relatively few carbon skeletons undergo the cyclization. Most intramolecular PKRs use systems derived from hept-l-en-6-yne (6) or propargyl allyl ethers (7) or amines (8). Other interesting and more recent substrates are enynes connected through aromatic rings like 9-11, which have allowed us and other groups to obtain aromatic polycycles (Fig. 1) [28-31]. 

Allyl phenyl ethers undergo an intramolecular [3,3]-sigmatropic rearrangement (the Claisen rearrangement) to form the C-alkyl derivative (Scheme 4.18). A consequence of the electrocyclic mechanisms is that the y-carbon atom of the allyl ether becomes attached to the aromatic ring. 

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