Allyl aromatic Claisen rearrangement

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

The experimental KIEs were determined for the aliphatic Claisen rearrangement in p-cymene at 120°C and for the aromatic Claisen rearrangement either neat at 170°C or in diphenyl ether at 220°C. Changes in 2H, 13C or 170 composition were determined for unreacted substrates. For carbon analysis of allyl vinyl ether the C5 carbon was used as an internal standard. The C4 atom and rneta aryl protons were used as references in analysis of allyl phenyl ether. The 170 analysis was based on a new methodology. The results are summarized in Table 1, along with predicted isotope effects calculated for experimental temperatures by means of different computational methods. The absolute values of predicted isotope effects for C4 and C5 atoms varied with theoretical level and all isotope effects were rescaled to get reference effects equal to 1.000. 

The Claisen rearrangement has attracted much attention as an attractive tool for the construction of new carbon-carbon bonds. Taguchi et al. reported the enantioselective and regioselective aromatic Claisen rearrangement of catechol mono allylic ether derivatives by means of Corey s chiral boron reagent (Eq. 70) [53a,54]. The mechanism of enantioselectivity is that a rigid five-membered cyclic intermediate is formed by reaction of catechol mono allylic ethers with the chiral boron reagent and this is fol-... 

Vinyl allyl ethers undergo concerted, 6-center, non-aromatic Claisen rearrangements to unsaturated aldehydes or ketones, viz. 

Europium tris(6,6,7,7,8,8,8-heptafluoro-2 -dimethyloctane-3 -dionate). Eu(fod)3 Aromatic Claisen rearrangement Excellent chirality transfer accompanies the rearrangement of allyl aryl ethers. 

B. Allyl Aryl Ethers. Aromatic Claisen Rearrangement. 460... 

The overwhelming majority of literature devoted to isomerizations of allyl aryl ethers is connected with the aromatic Claisen rearrangement and is summarized in detail in many reviews . Although the [3,3]-sigmatropic isomerization of phenol ethers to the corresponding C-alkylated derivatives has enjoyed widespread application in organic synthesis for over seventy years, it continues to be a very important reaction for the construction of a carbon-carbon bond. This section presents only recent reports. 

For a consecutive electroeyelic ring closure Claisen rearrangement-intramolecular ami-nation process see refs 206 and 207. For studies on the sequential Claisen rearrangement of methyl-3-aryloxy-2-(aryloxymethyl)prop-2-enoates see ref 208 and for the base-catalyzed aromatic Claisen rearrangement of 3-hydroxyphenyl allyl ethers, cf. ref 209. 

In the aromatic Claisen rearrangement (65), the ether-phenol transformation occurs at about 2(X)° S-C allyl shifts are more difficult to achieve (66). As the formation of the dienone intermediates is rate-determining, the analysis of perturbance around the heteroatoms (X) leading to the intermediates should provide clues to the relative rates, for changes are the same elsewhere. Thus, the crux of the problem involves only the net change of a -X to a Qpi -X bond. Such a change is favored with X = O rather than that with X = S because trigonal carbon is harder. 

As mentioned above, the aromatic Claisen rearrangement of allyl aryl ether requires high temperature. The range of reaction temperature is normally 180-225 °C. Under such somewhat drastic conditions, undesired side reactions often occur competitively. Furthermore, in several cases, achievement of high regjo- and stereoselectivity is also very difficult To prevent the production of undesired product and to increase the selectivity, reaction conditions have been well examined about the solvent and catalyst. 

Recently, Wipf et al. reported that the aromatic Claisen rearrangement is accelerated in the presence of trimethylaluminum and water or alumosanes [36]. Although 4 molar equivalents of trimethylaluminum and the addition of one molar equivalent of water or MAO were required, the reaction smoothly proceeded under mild conditions to give oriho allyl phenol in excellent yield. They proposed that the addition of water provides a transient strong Lewis acid, which activates the substrate by complexation to the ethereal oxygen atom. 

Transition-metal-catalyzed processes often work nicely for the aromatic Claisen rearrangement. Mechanistic aspects might be different from those under usual thermal conditions. Platinum complexes catalyzed the reaction of allyl 2-naphthyl ether 41 to afford l-allyl-2-naphthol 42 regioselectively in excellent yield [40]. Molybdenum hexacarbonyl also catalyzed the one-pot conversion of allyl aryl ethers to coumaran derivatives such as 43 from 17 [41]. 

Enantioselective aromatic Claisen rearrangement was reported from our group in 1997 [65]. The Claisen rearrangement of catechol mono allyl ether derivative 80 was catalyzed by chiral bis-sulfonamide-boron reagent 81. Although a stoichiometric amount of chiral reagent is required for this reaction, this is only one successful example of the enantioselective aromatic Claisen rearrangement. 

The )5-methyl substituent on the allyl moiety in the substrate of the aromatic Claisen rearrangement accelerates the formation of the coumaran derivative, often observed as a byproduct arising from the initially formed ortho allyl phenol. 

Followed by the aromatic Claisen rearrangement of allyl ortho vinylaryl ether 92, the Cope rearrangement of 1,5-hexadiene unit constructed at the ortho position proceeded to give ortho-1,4-pentadienylphenol derivatives 93, prior to the para rearrangement [73]. 

Danishefsky et al. developed a stereospedfic route for the synthesis of deoxy analog of mitomycin via an aromatic Claisen rearrangement [84]. The aUyl aryl ether 112 was prepared from allylic alcohols and phenol derivative via the Mitsu-nobu reaction. The aromatic Claisen rearrangement of 1,3-disubstituted aUyhc ether 112 proceeded under thermal conditions (N,N-dimethylaniline reflux) to afford the ortho rearrangement product 113 in 80% yield. 

The comparison of experimental and theoretically predicted kinetic isotope effects (KIEs) can be used to probe the accuracy of the computationally predicted transition structure provided that the experimental KIEs have been determined accurately. A comparison of predicted and experimental KIEs by Meyer et al. [55] led to the conclusion that, There is a firm disagreement in about half the cases between predicted and literature experimental heavy atom KIEs for both the aliphatic and aromatic Claisen rearrangements . Therefore, they reinvestigated the experimental KIEs for the Claisen rearrangement of aUyl phenyl and allyl vinyl ether and compared the determined values (solution) with the calculated data (gas phase). Eor the Claisen rearrangement of allyl vinyl ether, the transition structure was calculated on the MP4(SDQ)/6-31G level of theory and the predicted KIEs were in excellent agreement with the new experimental KIEs. The authors collected and compared previously reported data for the calculated distance between C- l/C-6 and O/C-4 and added their own predicted data (Scheme 11.40). 

General. Chiral 2-bromo-l,3-bis(4-methylphenyl sulfonyl)-4,5-diphenyl- 1,3,2-diazaborolidine (1) is used to control the stereochemistry of enantioselective aromatic Claisen rearrangements, allylations of aldehydes, aldol reactions, and formation of chiral propa-l,2-dienyl and propargyl alcohols. Included is the discussion of both the R,R) and the (5,5) chiral controllers. 

The thermal rearrangement of allyl phenyl ethers to o-allyl phenol and its mechanism is very well known to organic chemists. Both the aliphatic and aromatic Claisen rearrangements involve a 3,3-sigmatropic shift. There are reviews providing usefulness of this rearrangement reaction. ... 

Theoretical and mechanistic studies using QM/MM simulations have also looked at the solvent effects and on-water reactivity of the aromatic Claisen rearrangements of allyl p-R-phenyl ethers (R = CH3, Br, and OCHj) and allyl naphthyl ethers and showed that such aqueous systems can provide increased rate accelerations, yields, and specificity for several types of organic reaction classes compared to organic solvents [58]. Biologically relevant aromatic Claisen rearrangements have also been explored, which utilize the on-water effect [59]. 

A straightforward synthetic approach for a series of novel phosphorus containing heterocycles (410) and (411) has been developed by applying the sequential aromatic Claisen rearrangement, coupling of allyl or vinyl phosphonates (408) or (409), respectively, and a ring closing methathesis protocol (Scheme 124). ... 

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/ ... 

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