Allyl vinyl ethers, Claisen

Sol 9. (c) Sodium borohydride in the presence of CeCls selectively reduces a,P-unsaturated carbonyl compounds into allylic alcohols. It is called Luche s reduction. The selective 1,2-addition is facilitated by the strongly oxophilic Ce " " ion, which coordinates with the carbonyl oxygen. The allylic alcohol I then undergoes mercury catalyzed fran -etherification to give allyl vinyl ether (Claisen substrate), which readily undergoes [3,3] shift on heating to give II. 

One of the traditional methods for applications of the aliphatic Claisen rearrangement has used Mercuric salts to prepare vinyl ether as illustrated in the use of the allylic alcohol (28) en route to aldehyde 29. The chemical development group at Boehringer-Ingelheim has developd a mild palladium acetate-phenanthroline catalyst 31 for the sequential allyl vinyl ether-Claisen process utilizing commercially available triethyleneglycol divinyl ether which avoids the use of mercuric acetate. ... 

Scheme 10.25 Allyl vinyl ether Claisen rearrangement.

Claisen Rearrangements - allyl vinyl ether to an y,5-unsaturated carbonyl... 

CLAISEN - IRELAND Rearrangment Rearrangement ol allyl phenyl ethers to o (or p-)allylphenols or of allyl vinyl ethers to y.S-unsaturated aldehydes or ketones (Claisen) Rearrangement ol allyl esters as enolale anions to y.S-unsaturated acids (Ireland)... 

Table 12.3 Claisen rearangement of allyl vinyl ether to form 5-hexenal ...

The Claisen rearrangemenC is a thermal rearrangement of allyl aryl ethers and allyl vinyl ethers respectively. It may be regarded as the oxa-version of the closely related Cope rearrangement. Claisen has discovered this reaction first on allyl vinyl ethers 1, and then extended to the rearrangement of allyl aryl ethers 2 to yield o-allylphenols 3 ... 

Two other important sigmatropic reactions are the Claisen rearrangement of an allyl aryl ether discussed in Section 18.4 and the Cope rearrangement of a 1,5-hexadiene. These two, along with the Diels-Alder reaction, are the most useful pericyclic reactions for organic synthesis many thousands of examples of all three are known. Note that the Claisen rearrangement occurs with both allylic aryl ethers and allylic vinylic ethers. 

The reactants can be made from allylic alcohols by mercuric ion-catalyzed exchange with ethyl vinyl ether.220 The allyl vinyl ether need not be isolated and is often prepared under conditions that lead to its rearrangement. The simplest of all Claisen rearrangements, the conversion of allyl vinyl ether to 4-pentenal, typifies this process. 

Scheme 6.14. Claisen Rearrangements of Allyl Vinyl Ethers and Related... 

Cu-catalyzed domino reactions have been used for the synthesis of carbocycles, as well as for heterocycles such as indoles, benzoxazoles, and quinoxalines. A very useful process is also the combination of the formation of allyl vinyl ethers, followed by a Claisen rearrangement. 

As shown earlier in many examples, the Claisen rearrangement of allyl vinyl ethers also provides a very powerful method for carbon-carbon bond formation in domino processes. Usually, the necessary ethers are formed in a separate step. However, both steps can be combined in a novel domino reaction developed by Buchwald and Nordmann [306]. This starts from an allylic alcohol 6/4-102 and a vinyl iodide 6/4-103, using copper iodide in the presence of the ligand 6/4-104 at 120 °C to give 6/4-105 (Scheme 6/4.25). The reaction even allows the stereoselective formation of two adjacent quaternary stereogenic centers in high yield. 

Nordmann and Buchwald have reported a diastereoselective Claisen rearrangement of an allyl vinyl ether to an aldehyde (Scheme 6.81) [170]. Using N,N-dimeth-ylformamide as solvent, an 80% yield with a diastereomeric ratio of 91 9 was obtained by microwave heating at 250 °C for 5 min. Conventional heating at 120 °C for 24 h provided somewhat higher yields and selectivities (90% yield, dr = 94 6). 

The Claisen rearrangement of allyl vinyl ether in aqueous solution was studied by Gao et al. [157] by using the gas-phase ab initio MEP determined previously by the Jorgensen s group [60], In this case, 69 of the 143 frames were employed along the perturbation procedure in the isothermal-isobaric ensemble at 25 °C and 1 atm. The... 

The Claisen rearrangement is an electrocyclic reaction which converts an allyl vinyl ether into a y,8-unsaturated aldehyde or ketone, via a (3.3) sigmatropic shift. The rate of this reaction can be largely increased in polar solvents. Several works have addressed the study of the reaction mechanism and the electronic structure of the transition state (TS) by examining substituent and solvent effects on the rate of this reaction. 

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