Alkenes enamines

Elimination unimolecular (El) mechanism (Section 5 17) Mechanism for elimination characterized by the slow for mation of a carbocation intermediate followed by rapid loss of a proton from the carbocation to form the alkene Enamine (Section 17 11) Product of the reaction of a second ary amine and an aldehyde or a ketone Enamines are char actenzed by the general structure... 

Cyclobutanes can also be formed by nonconcerted processes involving zwitter-ionic intermediates. The combination of an electron-rich alkene (enamine, enol ether) and an electrophilic one (nitro- or polycyanoalkene) is required for such processes. 

Appropriately functionalized alkenes (enamines, a-haloalkenes, silylenoi ethers, acylvinyl phosphonium salts) are frequently employed as the CC fragment in reaction with amidines, guanidines and ureas. An -chloro-A -alkylamidine will react with the enamine derived from an aldehyde to give in the first instance a 2-imidazoline (4). Such compounds deaminate readily to produce the fully aromatic imidazoles (5), especially when heated with pyridinium or triethylammonium chlorides or 50% sulfuric acid (Scheme 4.3.3). 

A variety of pyridine ring syntheses stem from Vilsmeier formylation of alkenes, enamines, vinyl ethers, ketones, and related systems followed by a separate cyclization of the derived salts with ammonium chloride or acetate.49... 

Arylcyclopropanes can be prepared from a variety of alkenes enamines react sluggishly, electron-rich alkenes are, in general, the most reactive. Thus, phenylcarbene, generated by dirhodium tetraacetate catalyzed decomposition of phenyldiazomethane, gave 2-butyl-1-phenyl-cyclopropane (2) in 6% yield from hex-l-ene, but l-butoxy-2-phenylcyclopropane (3) was obtained in 92% yield from butyl vinyl ether. ... 

Reaction of a,a -dibromoketones with enneacarbonyldiiron gives oxoallyl cations which easily react with alkenes, enamines, and amides via [3+2] and with dienes via [3+4] cycloadditions, respectively, in high yield (Scheme 10.8) [26-29],... 

First, we will consider oxygenation of chemical substrates, including alkenes, enamines, enol ethers, and ketenes. Subsequently we shall take up the oxygenation of biological substrates. 

Other Reactions Oxallyls, formed from a,a -dibromoketones and Fe2(CO)9, react with alkenes, enamines, enol ethers, amides, or dienes to give a variety of [3 + 2] and [3 + 4] cycloaddition products (Eq. 14.71). This provides a very short synthesis of the tropane skeleton from acetone and pyrrole (Eq. 14.72). As shown in Eq. 14.71, an oxallyl resembles trimeth-ylenemethane (5.22) except that one =CH2 of 5.22 is replaced by =0. 

The 3 -I- 2-cycloaddition reactions of A-alkylsufonylimines with alkenes, enamines, ynamines, diazo compounds, azirines, silyloxydienes, and azadienes have been extensively reviewed. Rhodium(III)-catalysed cycloaddition of cyclic sulfonylketimines with internal alkynes afforded multifunctional spirocyclic sultam products in high... 

Nitroalkenes 1 behave as heterodienes in [4+2] inverse electron demand cycloadditions with simple unactivated alkenes, enamines, or enol ethers (2) as dieno-philes. These reactions require the presence of a Lewis acid to enhance the reactivity of the nitroalkene and accelerate the process. The products obtained in such reactions are six-membered cyclic compounds called nitronates (3) (Scheme 22.1). These compounds can be used in turn, as 1,3-dipoles in [3+2] cycloaddition reactions. 

Typical nucleophiles known to react with coordinated alkenes are water, alcohols, carboxylic acids, ammonia, amines, enamines, and active methylene compounds 11.12]. The intramolecular version is particularly useful for syntheses of various heterocyclic compounds[l 3,14]. CO and aromatics also react with alkenes. The oxidation reactions of alkenes can be classified further based on these attacking species. Under certain conditions, especially in the presence of bases, the rr-alkene complex 4 is converted into the 7r-allylic complex 5. Various stoichiometric reactions of alkenes via 7r-allylic complex 5 are treated in Section 4. 

Again, it is noteworthy that 4-substituted 5-hydrdxythiazoles (24) react like 5-hydroxy-THISs with alkynes to give pyrroles and sometimes with alkenes to give exo-cycloadducts (Scheme 22). In the latter case other processes compete with the cycloaddition, becoming dominant when 24 is treated with azo-compounds, enamines, or heterocumulenes (31). 

In 1959 Carboni and Lindsay first reported the cycloaddition reaction between 1,2,4,5-tetrazines and alkynes or alkenes (59JA4342) and this reaction type has become a useful synthetic approach to pyridazines. In general, the reaction proceeds between 1,2,4,5-tetrazines with strongly electrophilic substituents at positions 3 and 6 (alkoxycarbonyl, carboxamido, trifluoromethyl, aryl, heteroaryl, etc.) and a variety of alkenes and alkynes, enol ethers, ketene acetals, enol esters, enamines (78HC(33)1073) or even with aldehydes and ketones (79JOC629). With alkenes 1,4-dihydropyridazines (172) are first formed, which in most cases are not isolated but are oxidized further to pyridazines (173). These are obtained directly from alkynes which are, however, less reactive in these cycloaddition reactions. In general, the overall reaction which is presented in Scheme 96 is strongly... 

Other isocyanates undergo [2 + 2] cycloaddition, but only with very electron rich alkenes. Thus phenyl isocyanate gives /3-lactams with ketene acetals and tetramethoxyethylene. With enamines, unstable /3-lactams are formed if the enamine has a /3-H atom, ring opened amides are produced 2 1 adducts are also found. Photochemical addition of cis- and traH5-stilbene to phenyl isocyanate has also been reported (72CC362). 

The chemical reactivity of these two substituted ethylenes is in agreement with the ideas encompassed by both the MO and resonance descriptions. Enamines, as amino-substituted alkenes are called, are vety reactive toward electrophilic species, and it is the p carbon that is the site of attack. For example, enamines are protonated on the carbon. Acrolein is an electrophilic alkene, as predicted, and the nucleophile attacks the P carbon. 

Systems usually fluonnated by electropositive fluorine reagents include acti-vated alkenes (enol ethers, enol acetates, silyl enol ethers, and enamines), activated aromatic systems, certain slightly activated carbon-hydrogen bonds, and selected organometallics. 

The types of cycloadditions discovered for enamines range through a regular sequence starting with divalent addition to form a cyclopropane ring, followed by 1,2 addition (i) of an alkene or an alkyne to form a cyclo-cyclobutane or a cyclobutene, then 1,3-dipolar addition with the enamine the dipolarophile 4), and finally a Diels-Alder type of reaction (5) with the enamine the dienophile. 

The reactions of electrophilic alkenes (alkenes attached to electron-withdrawing groups) with enamines produce one or more of the following products simple alkylation (2), 1,2 cycloaddition (3), and 1,4 cycloaddition (4). Competition with C alkylation by N alkylation is inconsequential and therefore will be largely ignored (5,7). A stepwise ionic mechanism leading to these products necessarily involves the formation of a zwitterion intermediate (1) as the first step, which is then followed either by one of the... 

Olefins conjugated with electron-withdrawing groups other than a carbonyl group undergo reactions with enamines in a manner similar to the carbonyl-conjugated electrophilic alkenes described above. Namely, they condense with an enamine to form a zwitterion intermediate from which either 1,2 cycloaddition to form a cyclobutane ring or simple alkylation can take place. 

The similarity between the reactions of alkenes and cyclopropanes is further demonstrated by the reactions of electrophilic cyclopropanes and cyclopropenes with enamines. Cyclopropylcyanoester74, when treated with the pyrrolidine enamine of cyclohexanone, undergoes what would be a 1,2 cycloaddition in the analogous alkene case, but is actually a 1,3 cycloaddition here, to form adduct 75 (90). A similar reaction between the... 

A pseudo 1,2 cycloaddition (actually a 1,3 cycloaddition, but may be considered a 1,2 type if a three-membered ring is considered analogous to an alkene) is observed when the pyrrolidine enamine of cyclohexanone is allowed to react with N-carbethoxyaziridine (129) to produce octahydro-indole 130 91). Octahydroindoles and pyrrolidines can also be produced through the intramolecular alkylation of the enamines of certain halo-ketourethanes 176a). 

Examine the eleetrostatic potential map of eaeh nueleophile (enamine, silyl enol ether, lithium enolate and enol) with emphasis on the face of the nucleophilic alkene carbon. Rank the nucleophiles from most electron rich to least electron rich. What factors are responsible for this order (Hint For each molecule, consider an alternative Lewis structure to that given above that places a negative charge on the nucleophilic carbon.)... 

Dipole Acrylic ester 1-Alkene Vinyl ether Enamine... 

Enamines 1 are useful intermediates in organic synthesis. Their use for the synthesis of a-substituted aldehydes or ketones 3 by reaction with an electrophilic reactant—e.g. an alkyl halide 2, an acyl halide or an acceptor-substituted alkene—is named after Gilbert Stork. 

Enamines react with acceptor-substituted alkenes (Michael acceptors) in a conjugate addition reaction for example with o ,/3-unsaturated carbonyl compounds or nitriles such as acrylonitrile 8. With respect to the acceptor-substituted alkene the reaction is similar to a Michael addition ... 

The double bond of an enamine (alkene + amine) is much more nucleophilic than a typical alkene double bond. Assuming that the nitrogen atom in an enamine is. -hybridized, draw an orbital picture of an enamine, and explain why the double bond is electron-rich. 

Reaction of a,/MJnsaturated Fischer Carbene Complexes with Alkenes, Butadienes, Enamines, and Imines... 

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