Aromatic Claisen

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

Allyl aryl ethers undergo accelerated Claisen and [1,3] rearrangements in the presence of a mixture of trialkylalanes and water or aluminoxanes. The addition of stoichiometric quantities of water accelerates both the trimethylaluminum-mediated aromatic Claisen reaction and the chiral zirconocene-catalyzed asymmetric carboalumination of terminal alkenes. These two reactions occur in tandem and, after oxidative quenching of the intermediate trialkylalane, result in the selective formation of two new C-C bonds and one C-0 bond (Eq. 12.70).153 Antibodies have also been developed to catalyze Claisen154 and oxy-Cope155 rearrangements. 

This reaction also is concerted and proceeds via a six-membered transition state, but here the species (59), corresponding to the ene-one intermediate (53a) in the aromatic Claisen rearrangement, is in fact the end-product. This is so because there is in (59) no energetic driving force, comparable to re-aromatisation in (53a— 52a), to promote its enolisation. 

Gamer AL, Koide K (2008) Oxidation state-specific fluorescent method for palladium(II) and platinum(IV) based on the catalyzed aromatic Claisen rearrangement. J Am Chem Soc 130 16472-16473... 

Concerning the aromatic Claisen rearrangement certain salient observations are summarized in the following equations [233]. 

An aromatic Claisen rearrangement has been used as a key step in a total synthesis of racemic heliannuols C and E.18 A formal synthesis of (-)-perhydrohistrionicotoxin has used Claisen rearrangement of an amino acid ester enolate as the key step, in which almost total chirality transfer was observed from (S, )-oct-3-en-2-ol in the sense predicted by a chair-shaped transition state with chelation control of enolate geometry.19 Treatment of 1-(cyclohex-l-enyl)-6-methoxy-2-propargylindanol derivatives with base... 

The disconnection for the aromatic Claisen is to reverse the rearrangement. This is a little simpler than those we have seen so far as one C-C bond is broken 59 and one C-0 bond made. But you must remember to turn the allylic system back to front. This is easily seen if the starting material is drawn as 59a with the dotted line representing a reconnection. The rest is a normal ether disconnection. 

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 dienone (8 equation 2), prepared independently by allylation of sodium 2,6-dimethylphenolate, has been shown to undergo irreversible rearrangement to the p-allylphenol (9) and aryl allyl ether (7) = -2.5 7S-KX) C). The rate constant ki for the forw reaction is negligible. In the aromatic Claisen-Cope sequence, the Claisen rearrangement is the slower step because the arorttaticity of the ring must be overcome. 

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. 

Three main reaction types are considered below, bond fissions, reverse ene reactions, and non-aromatic Claisen-type rearrangements. 

In the non-aromatic Claisen-type rearrangements, a sigma bond breaks, a sigma bond is formed, and two t-bonds migrate concertedly. The basic reaction is illustrated below... 

Aromatic Claisen rearrangements in 2,3-unsaturated sugar systems are useful for the stereo-controlled synthesis of aryl-branched sugars [87] (O Scheme 20). The a-anomer 97 is much less reactive in comparison to the /3-anomer 99. This thermal rearrangement is carried out by refluxing in iV,iV-diethylaniline. The efficiency of the reaction is almost independent of the nature of the p-substituent in the phenyl group. 

Rearrangements. By applying Corey s diazaborolidine catalyst to aromatic Claisen rearrangement, a chiral side chain is introduced into the ortho position of catechols. Highly enantioselective rearrangement of 0-allylimidates 111 containing a methoxybinaphthyl substituent has been witnessed. ... 

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... 

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