Cis-Propenyl ethers

In presence of the basic catalyst potassium t-butoxide, the solvent DMSO accelerates some rearrangement reactions. For example, allyl ethers are rearranged to cis-propenyl ethers. 

The 2+2 cycloadditions of benzyne to cis- and trani-propenyl ether gave cis- and fran -benzocyclobntanes as the main products, respectively [ 117,118], Stereospecific [2+2] cycloaddition reactions were observed between the benzyne species generated by the halogen-Uthium exchange reaction of ort/io-haloaryl triflates and the ketene silyl acetals (Scheme 23) [119],... 

The characteristics of the 1,3-dipolar cycloaddition mechanism of azides and other 1,3-dipoles (such as diazoalkanes, azo-methine imines, nitrones, nitrile imines, nitrile oxides) have been described in detail by Huisgen.191 19 According to the author, the addition of a 1,3-dipole (a b c) to a dipolarophile (d e) occurs by a concerted mechanism in which the two new a bonds are formed simultaneously although not necessarily at equal rates (32). As a consequence, a stereoselective cis addition is observed. Thus, the addition of p-methoxyphenyl azide to dimethyl fiynarate (33) yields l-(p-methoxyphenyl)-4,5-froiw-dicarbomethoxy-AMriazoline (34),194 and 4-nitrophenyl azide gives exclusively the respective cis-addition products 35 and 36 on addition to irons- and cis-propenyl propyl ether.196... 

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... 

Enantiomer selection is also found in vinyl ether polymerization [226,227], The polymerization of cis- and trans- 1-methylpropyl propenyl ethers using (-)-menthoxyaluminum dichloride [227] and the copolymerization of rac- 1-methylpropyl vinyl ether with optically active monomers [226] are enantiomer selective. 

Stereochemical integrity may also be lost when the reaction of a stereoisomer occurs via an intermediate which retains a stereogenic element, but whose bonding permits interconversion of stereoisomers faster than its conversion of stereoisomeric products. [2 + 21-Cycloaddition of TCNE and cis-propenyl methyl ether [30] yields cis- and trans-adducts, 22 in Scheme 9.13, in ratios which depend on the solvent (84 16 in favour of the cis-adduct in acetonitrile). The dipolar 23 was proposed as an intermediate. The initial bonding destroys the double bond character between Cl and C2 of the enol ether reactant, and the much... 

Each product exhibits different properties of the crystalline phase. Another example is the enantioelective cationic polymerization of racemic cis-1 --methylpropyl-l-propenyl ether initiated by (— )-menthoxyaluminium dichloride in toluene at 351 K. The polymer is optically active ( + ) and the remaining monomer can be transformed to a polymer with negative optical activity by BF3OEt2 [100]. 

Propenyl Ethers and Unsaturated Cyclic Ethers Propenyl ethers (CH3—CH=CH—OR R = ethyl, isobutyl, etc. cis- and trans-isomers) and 3,4-dihydrofuran are linear and cyclic a,/3-unsaturated ethers, that can be regarded as / -substituted vinyl ether derivatives. For these monomers a few controlled/living cationic polymerizations have been reported. The HI/I2 system is generally effective for both linear and cyclic monomers [181,182,183], whereas a recent study by Nuyken indicates that the IBVE-HI adduct coupled with nBu4NC104 is suited for 3,4-dihydrofuran (see Section V.A.4) [184]. A variety of mono- and bifunctional propenyl ethers can readily be prepared by the ruthenium complex-catalyzed isomerization of corresponding allyl ethers [185]. 

A thorough stereochemical study of a very similar reaction with cis- and trunj-propenyl methyl ether as electron-rich alkenes provides comparable findings. The trans isomer of 1,2-bis(trifluoromethyl)-l,2-dicyanoethylene and cis-propenyl methyl ether (112) gave adducts (113), (114) and (115). In some cases, adducts revealing some loss of stereochemistry in the enol ether component were detected and identified. 

Huisgen, R., Mayr, H. Steric course, kinetics, and mechanism of the [2 + 2] cycloadditions of alkylphenylketenes to ethyl cis and trans-propenyl ether. Tetrahedron Lett. 1975, 2969-2972. 

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-... 

Similar studies were performed for the radical polymers of cis-propenyl vinyl ether and 2-methylpropenyl vinyl ether, with the conclusion that the polymer structures were basically the same as that of poly (divinyl ether) (5 ). 

Vinylboronic acids undergo analogous reactions to give the mono- and divinyl-thallium derivatives in high yields " with retention of configuration, e.g., heating a 1 1 cis-propenylboronic acid and Tl(III) chloride in aq ether for 8.5 h produces 30% cis-propenylthallium dichloride and 30% bis(cis-propenyl)thallium chloride. The same reaction with trans-propenylboronic acid produces 28% trans-propenylthallium dichloride and 67 % bis(trans-propenyl)thallium chloride. 

Enol ethers are more reactive toward formaldehyde and MesAl than simple alkenes. Reaction with dihydropyran gives a 75% yield of a 92 8 mixture of 33 and 34 (See Figure 10). The major product is again formed by cis addition of hydroxymethyl and methyl groups. Quite different results are obtained with acyclic enol ethers. 20 Reaction of ethyl propenyl ether, as a 78 22 cis-trans mixture, with 2 equivalents of paraformaldehyde and 2 equivalents of MesAl at 0 in CH2CI2 gives a 65% yield of an 18 1 mixture of threo- and c yr/ir< -3-ethoxy-2-meAyl-l-butanol (37 and 38). Identical results are obtained from either pure stereoisomer of ethyl propenyl ether. 

Potassium tert-butoxide Stereospecific migration of carbon-carbon double bonds cis-Propenyl from allyl ethers... 

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. 

Cis- and trans-isomevs of methyl propenyl ether, structurally similar to 2-phenylvinyl methyl ether, also undergo equimolar copolymerization with Under the conditions of the study, i.e., benzene solvent, free-radical initiator, and 60°C polymerization temperature, the cw-isomer was shown to be the most reactive. The orders of the two monomers toward the MA radical (1/ri) were cis = 0.431 and trans = 0.347. The specific viscosities of the copolymers, measured in acetone at 30°C, varied from 0.11 to 0.42 dl/g. Alternating copolymers were obtained over a wide variation of monomer feed ratios. 

For the isomerization of 2-propenyl ether the catalytic activity is in the order Ca0>La203, SrO, MgO>ZnO, AI2O3, SiOz —The initial isomerized product was exclusively cis form, indicating the intermediacy of an anionic allyl species as in the case of butene isomerization over solid bases. In the case of isomerization of 2-propenylamine, MgO and CaO were very active, while Z1O2 and ZnO were inac-tive. By the isomerization of., . -dimethyl-2-propenylamine, 100% cis-N,N- - TO-penylamine was initially formed, so that an anionic allyl intermediate was proposed. 

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