Ethers, ethyl propenyl 2 + 2 cycloaddition reactions

On addition the stereochemistry at the C=C bond of the vinyl ether remains constant. This was established by labeling studies and by the use of cis and trans ethyl propenyl ether. From the similarity of the activation parameters, the reaction characteristics, and the stereoselectivity of the cycloaddition step to those of the reactions of the same complexes with 1,3-dienes or ynamines an associative concerted nonsynchronous process was deduced.224... 

There have been a number of unsuccessful attempts to trap 1,4-zwitterionic intermediates, - but there have also been some reported successes. At best, then, such trapping experiments are not uniformly diagnostic for zwitterionic intermediates. truns-Propenyl ethyl ether reacts with TONE in ethanol solvent at 25 C to give product (140). " The normal [2 + 2] adduct (139) gives this same product when allowed to react in ethanol, but at a slower rate. Just how fast this solvolysis might take place under the conditions of the cycloaddition reaction, with enol ether and TCNE present, is not clear. Acetone does not interfere with the addition of ethyl vinyl ether to TCNE, but the adduct (141) is converted over one week at room temperature to the six-membered ring product (142), which may be postulated as being derived from ci ture of a 1,4-zwitterionic intermediate in equilibrium with the cyclobutane structure. Alternatively, since acetone like ethanol is known to react with TCNE, it may be a case of acid-catalyzed solvent-assisted solvolysis. 

The [2 + 2] cycloaddidon of the propynyl tungsten complex (12b) has been found to be stereospecific with the cis and trans isomers of ethyl propenyl ether." TTie reaction with cis-propenyl ethyl ether gave rise to the cyclobutenyl complex (18), in addition to a minor amount of the ring-opened dienyl carbene complex (19). The stereochemistry about the double bonds in the dienyl complex (19) revealed that it was derived from a thermal electrocyclic conrotatory ring opening of the cis-cyclobutenyl carbene complex (18) and thus the stereochemistry is completely retained in this cycloaddition. In support of this analysis it was found that cyclobutenyl complex (18) could be thermally opened to the dienyl complex by heating in an inert solvent at 50 C. In the cycloaddition of the propynyl complex (12) with trans-... 

Huisgen has also studied the effects of substitution in the keten in the reactions of a series of alkylphenyl ketens to ethyl cis- and trans-propenyl ethers. With the cis-enol ethers the thermodynamically less stable cyclobutanone is always produced. This is the same result as that found in the addition of ketens to cyclopentadiene and other cis-olefins, and the mechanistic implications are the same. With the trans-enol ether, the thermodynamically more stable product is formed, and this observation can be rationalized in terms of a [tc2 + k2 J cycloaddition if the preferred orientation complex has the substituent on the keten between the alkoxy-group and a hydrogen rather than between a methyl group and a hydrogen on the enol ether. In all the cases studied, the cis-enol ether reacted more rapidly than its trans-isomer. This cis trans reactivity ratio is not found in [2 + 2] additions proceeding via zwitterionic intermediates. For example, the rate ratio for the reaction of TCNE with cis- and trans-1-alkenyl ethers is very close to unity. 

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