Vinyl ethers and enamines

Rotation around the C(sp )-N bond of enamines is slower (4-6 kcal/mol barrier) than the corresponding process in saturated amines. On the other hand, cis-trans-isomerization of the C=C bond is faster than it is in alkenes without a heteroatom. Both of these effects illustrate the importance of resonance for this 

For example, pyrrolidine enamines are more nucleophilic than piperidine and morpholine-based enamines. This difference in reactivity of cyclic amines originates from variations in pyramidalization at nitrogen. The nitrogen atom in a five-membered cycle is much more flat than the nitrogen atom in a six-membered cycle. 

Planarization in the pyrrolidine enamines increases p-character in the lone pair and renders nitrogen a stronger donor. Enamines derived from imidazolidinones are 10 -1(P times less reactive than those based on the proline-derived Hayashi-Jprgensen catalyst.  

Selected organic donors with their first reductive potentials relative to SCE 

Rotation around C-X bonds in heteroatom-substituted alkenes The preferred orientations of substituent X in the R2C=CR -X systems relative to the vinyl moiety are also interesting. Whereas the flat conformation is preferred for X=0, S to allow the perfect alignment of the p-type lone pairs atX with the it-systan, the varying degrees of pyramidalization can be observed for X=N as already discussed. When the second lone pair is present (X=0, S), rotation around the 0-R and S-R bonds is controlled by stereoelectronics in a well-defined manner that arranges this lone pair antiperiplanar to the orbital. As with aldehydes, esters etc., this situation again illustrates the interplay of different effects that manifest themselves in different observed properties 7i-effects (n -) n ) are important for reactivity whereas sigma effects (n o ) define shape of the molecules. 

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There are also substituents that can act as electron-releasing groups through resonance. Among familiar examples are alkoxy and amino groups in vinyl ethers and enamines, respectively. 

The reaction of vinyl ethers and enamines with nitroalkenes is highly regiose-lective, with only the head-to-head adduct observed. The endo approach of the dienophile is preferred in the thermal cycloaddition, however, the mode of approach can be controlled by the choice of the Lewis acid promoter (214). Facial discrimination has been obtained by the use of chiral groups on the both the nitroalkene (215,216) and the enamine (217) or vinyl ether (218), as well as with chiral Lewis acids (46,66,94,219,220). 

Scheme 13 In situ trapping of vinyl ether and enamine structures in the mixture of uncontrolled thermal decomposition of NMMO...

Origin and formation pathways of the different vinyl ether and enamine structures in mixtures from the uncontrolled NMMO degradation were difficult to assess, since the high temperatures during such processes also allowed disfavored decomposition processes to proceed, which are thermodynamically forbidden or disfavored under the usual Lyocell process conditions. 

Many common reactions of aliphatic amines, ethers, and sulfides 1 involve initial addition of an electrophilic reagent utilizing the lone pair of electrons on the heteroatom salts, quaternary salts, coordination compounds, amine oxides, sulfoxides, and sulfones are formed in this way. Corresponding reactions are very rare (cf. Section 3.3.1.3) with pyrroles, furans, and thiophenes. These heterocycles react with electrophilic reagents at the carbon atoms 2, 3 rather than at the heteroatom. Vinyl ethers and enamines 4 show intermediate behavior, reacting frequently at the -carbon but sometimes at the heteroatom. 

Recent work176 shows that route A is more probable. Acetoacetic ester, vinyl ethers, and enamines react analogously with acetyl-benzoquinone. The reaction of acetoacetic ester probably goes via the enol form, as the reaction rate is solvent-dependent, i.e., it depends on the enol content176 ... 

Vinyl Ethers and Enamines from Wittig-Style Reactions... 

It is significant that if an electron-poor diene is utilized, the preference is reversed and electron-rich alkenes, such as vinyl ethers and enamines, are the best dienophiles. Such reactions are called inverse electron demand Diels-Alder reactions, and the reactivity relationships are readily understood in terms of frontier orbital theory. Electron-rich dienes have high-energy HOMOs that interact strongly with the LUMOs of electron-poor dienophiles. When the substituent pattern is reversed and the diene is electron poor, the strongest interaction is between the dienophile HOMO and the diene LUMO. The FMO approach correctly predicts both the relative reactivity and regioselectivity of the D-A reaction for a wide range of diene-dienophile combinations. 

The oxidative functionalization of olefins through ir-olefin complexes of palladium also has a long history, including the industrial production of acetaldehyde and vinyl acetate. Related reactions, including the conversion of olefins to vinyl ethers and enamines, have been studied in more recent times for fine chemical synthesis. These oxidative C-0 and C-N bond formations have been conducted with a variety of oxidants, including Oj, and have been studied as both intermolecular and intramolecular processes. 

The reactivity of isocyanates in [2+2] cycloaddition reactions is as follows alkyl < aryl < nitroaryl << arenesulfonyl < halosulfonyl. Also, the reactivity of the substrate is determined by the substituents. For example, vinyl ethers and enamines are more reactive than olefins. Often the formation of the [2+2] cycloadducts involves polar linear intermediates, which can be intercepted by the isocyanate or the substrate to form six-membered ring [2+2+2] cycloadducts (see Section 3.3.1.4). Also, diynes react with isocyanates to give six-membered ring [2+2+2] cycloadducts. In the latter reactions catalysts play an important role. From Q, ty-diynes macrocyclic adducts are obtained. 

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