Claisen rearrangement of allyl phenyl ethers

Wipf and coworkers used a Claisen rearrangement of allyl phenyl ethers 4-309 followed by an enantioselective carboalumination using the chiral Zr-complex 4-310 and trimethyl aluminum (Scheme 4.67) [104]. After an oxidative work-up of the intermediate trialkylalane, the corresponding alcohols 4-311 were obtained with up to 80% ee and 78% yield. One can also transfer an ethyl group using triethyl aluminum with even better ee-values (up to 92%), but the yields were rather low (42%) due to a more sluggish oxidative cleavage of the Al-C bond. 

Claisen rearrangement of allyl phenyl ethers. These were thoroughly studied before the rearrangement of allyl vinyl ethers... 

A unified theoretical explanation using molecular orbital theory has been proposed. Grimme [65] investigated the PFR of phenyl acetate as well as the photo-Claisen rearrangement of allyl phenyl ether and the 3-cleavage of para-substituted phenoxyacetones. A unified description of the three reactions has been invoked according to MNDOC-CI and AMl/AMl-HE calculations. No matter what ex-... 

Ramamurthy and coworkers studied the photo-Fries rearrangement of phenyl acetate and phenyl benzoate and photo-Claisen rearrangement of allyl phenyl ether (Fig. 34) included in two types of zeolite (faujasites X and Y and pen-tasils ZSM-5 andZSM-11) [192], The photolysis was performed with the zeolite slurry in either hexane or iso-octane. One of the most remarkable observations is that the product distribution is altered within zeolites from that in isotropic solvent. Furthermore, while in solution, nearly a 1 1 mixture of ortho and para isomers 40 and 41 (Fig. 34) was obtained, within zeolites one is able to direct the photoreaction selectively toward either the ortho or the para products by conducting the reaction either within faujasites or pentasils, respectively (Fig. 34). 

Rapid Claisen rearrangement of allyl phenyl ether and meta-substituted derivatives has been reported to occur with poor regioselectivity at 250 °C in dicationic ionic liquids, especially (24).23 Claisen rearrangement of resorcinol allyl ethers (25) was found to have poor regioselectivity under thermal or microwave conditions, but it was further found that selectivity for the 6-substituted product (26) over (27) could be improved to 13 1 using boron trichloride and an appropriately protected ether (25 ... 

Volumes of activation for the Claisen rearrangement of allyl phenyl ether in various solvents are given in Table 35. This parameter was calculated according to... 

Figure 1. Mechanism of the ortho-Claisen rearrangement of allyl phenyl ether to 2-allylphenol. A typical reaction condition is heating at 200 °C for 6 h, yield 73% [2].

In the present study, the Claisen rearrangement of allyl phenyl ether, the simplest allyl aryl ether, has been investigated in the presence of various zeolites. This paper presents the preliminary results of the extensive investigation of the ability of zeolites to catalyse the Claisen rearrangement. 

Figure 3. Reaction scheme of the zeolite-catalysed Claisen rearrangement of allyl phenyl ether, and cyclisation and isomerisation of the initial rearrangement product.

H-mordenite and HB are able to catalyse the Claisen rearrangement of allyl phenyl ether. The reaction is accompanied by the cyclisation of the initial rearrangement product and other acid catalysed side reactions. A drawback is the formation of dimers and oligomers, but improvement with respect to these undesired side reactions can be achieved by deactivation of the outer surface of the zeolite with triphenylphosphine in the case of H13. 

Problem 4.17. A small amount of 4-allylphenol is often obtained from the Claisen rearrangement of allyl phenyl ether. Draw a concerted mechanism for this reaction, name the mechanism, and determine whether the suprafacial-suprafacial rearrangement is thermally allowed or disallowed by the Woodward-Hoffmann rules. If it is not allowed, draw a multistep mechanism for the reaction. 

The Claisen rearrangement of allyl phenyl ethers proceeds in the presence of the Nafion-H and silica/Nafion-H nanocomposites. The 2-methyldihydrobenzofuran is formed as a major product (75%) in the presence of the Nafion-H beads the minor product is the orf/zo-allylphenol (25%) (equation 10). However, the ortto-allylphenol is formed as the major product in the presence of the Nafion/H-silica nanocomposites. 

The stereochemistry of the Claisen rearrangement of allyl phenyl ethers has been examined. The new double bond is primarily trans whether it is formed from a CIS or a trans precursor. These results are accommodated by a chair-like cyclic transition state. For the two isomeric reactants, the preferred geometry for the transition state would be A and B, respectively. Both lead to a trans double bond in... 

Except for the construction of isoprenyl aromatic natural products, the Claisen rearrangement of allyl phenyl ethers has been relatively little exploited in natural product synthesis. The potential value of the reaction, however, has now been demonstrated in a synthesis of (+)-latifine (128). Thus, the allyl phenol ether (125) was found to re-arrange smoothly in refluxing N,N-dimethylaniline to give (125) in 75% yield, which was then elaborated to the natural product via the amine (12 ) in a further seven steps. 

As mentioned in the solvent effect section, protic solvents accelerate the Claisen rearrangement. In particular the reaction rate enhances by increasing the acidity of the protic solvent used and trifluoroacetic acid (pKj=0.2) is one of the most effective catalysts. It was reported that in trifluoroacetic acid the Claisen rearrangement of allyl phenyl ether proceeds at room temperature to afford the ortho-rearrangement product 3 and 2-methylcoumaran 25 along with a small amount of phenol and unreacted starting ether [28, 29],... 

The Claisen rearrangement of allyl phenyl ether in NCW was first reported by An et al. (Fig. 9.61). At 200 and 240°C for a period of 10 min, the conversion to 2-allylphenol increased significantly, 10 and 84% respectively. At higher temperatures (245 and 250°C) and longer reaction time (60 min), an array of products appeared. These products included phenol, 2-(2-hydroxyprop- l-yl)-phenol, and 2-methyl-2,3-dihydrobenzofuran. The results are summarized in Table 9.7. 2-Methyl-2,3-dihydrobenzofuran was identified as the thermodynamic product. Figure 9.62 summarizes the proposed mechanistic pathways to each of the products. ... 

Claisen rearrangements of allyl phenyl ethers to orr/io-allylphenols were thoroughly studied before the analogous rearrangements of allyl vinyl ethers. Entry 13 in Scheme 7.10 illustrates this type of Claisen rearrangement. The rearrangement step is a concerted one and leads to a cyclohexadienone, which then enolizes to an orr/io-allyIphenol. 

Table 11.1 Noncatalyzed Claisen rearrangement of allyl phenyl ethers comparison of batch and microflow — a literature overview.

Reaction Claisen rearrangement of allyl phenyl ether. 

Claisen rearrangements of allyl phenyl ethers to ortho-a y phenols were the first [3,3]-sigmatropic rearrangements to be thoroughly studied. The reaction... 

SCHEME 6 Claisen rearrangement of allyl phenyl ether and subsequent intramolecular hydroxyalkylation giving 2-methyldihydrobenzofuran (11). 

Nevin et al. also reported the Claisen rearrangement of allyl phenyl ether catalyzed by mesoporous (A1)MCM-41 [57], In this case, conversion to products was higher when increasing the Al content in the mesoporous silica suggesting, therefore, that the reaction occurs inside the pores. The Si/Al ratio in the catalysts has an effect in the reaction selectivity (A1)MCM-41 with lower Si/Al ratios led preferentially to compoxmd 10 at lower temperatures. 

S.G. Waghofikar, S. Mayadevi, N.E. Jacob, S. Sivasanker, Claisen rearrangement of allyl phenyl ether over zeolites beta, mordenite and Y, Microporous Mesoporous Mater. 95 (2006) 8-16. 

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