Electron-rich alkenes, 2 + 4 addition

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). 

Early work established that S4N4 forms di-adducts with alkenes such as norbornene or norbomadiene. Subsequently, structural and spectroscopic studies established that cycloaddition occurs in a 1,3-S,S"-fashion. The regiochemistry of addition can be rationalized in frontier orbital terms the interaction of the alkene HOMO with the low-lying LUMO of S4N4 exerts kinetic control. Consistently, only electron-rich alkenes add to S4N4. 

The synthesis of a-substituted phosphonates 89, via the electrophilic addition of phosphorylated C-radicals 88 (generated by reaction of BujSnH to the readily accessible a-phosphoryl sulfides (or selenides)) and electrophilic addition to electron rich alkenes, has been described [57] (Scheme 26). A large excess of alkene is necessary to minimize the competitive formation of the undesired compound 90 resulting from direct reduction of the initial radical 88. The ratio 89/90 has been measured for each example. The synthesis of the a-mono- or a,a-di-substituted (R or phosphonates 89 shows that the free radical approach... 

Iodine was found to be an efficient catalyst for the aziridination of alkenes (Scheme 6) utilizing chloramine-T (A-chloro-A-sodio-p-toluenesulfonamide) as the nitrogen source. For example, when 2 equiv. of styrene (45a) were added to chloramine-T in the presence of a catalytic amount of iodine (10mol%) in a 1 1 solvent mixture of acetonitrile and neutral buffer, the corresponding aziridine (46) was obtained in 91% yield. The reaction proved to work with other acyclic and cyclic alkenes, such as oct-l-ene and cyclohexene. The aziridination of para-substituted styrene derivatives (45b-e) demonstrated that, as expected for an electrophilic addition, electron-rich alkenes reacted faster than electron-poor alkenes. However, with 1 equiv. of I2, mainly iodohydrin (47) was formed. A catalytic cycle has been proposed to account for the fact that only a catalytic amount of iodine is required (Scheme 1) ... 

At least two different Pd(0) species can be involved in both the oxidative addition and n-coordination steps, depending on the anions and ligands present. High halide concentration promotes formation of the anionic species [PdL2X] by addition of a halide ligand. Use of trifhioromethanesulfonate anions promotes dissociation of the anion from the Pd(II) adduct and accelerates complexation with electron-rich alkenes. 

Only very few examples of alkene-]2-i-2] cycloadditions are known ]345, 347, 348]. By using a large excess of the moderate electron-rich alkene p-propenyl-anisol ]348] or even less electron-rich alkyl-subshtuted 1,3-butadienes [347] no thermal [2-1-2] cycloaddition occurs, but a photochemical cycloaddition can be enforced. The mechanism is proven to be stepwise via a biradical or dipolar intermediate ]347-351], comparable to the addition of the alkynes. During the addihon of cis- and trons-alkenes the existence of this relahvely long lived intermediate leads to a loss of stereochemical integrity. Addihon of ds-4-propenylanisol or trans-4-propenylanisol results in both cases exclusively in the trans-adduct (Scheme 4.61). 

Alkyl-5-alkyliminothiatriazolines (50) decompose slowly around 40-60 "C and rapidly at 125°C with formation of sulfur, nitrogen, and carbodiimide (51) (Equation (4)). However, the carbo-diimides (51) formed react with undecomposed thiatriazoline (50), which in part explains the low yields of isolable carbodiimide. It has not been possible by trapping experiments to decide whether the decomposition involves an intermediate as addition of electron-rich alkenes or heterocumulenes induces immediate nitrogen evolution in a bimolecular reaction (see Section 4.19.5.2) <78JCS(P1)1440>. 

The Diels-Alder addition between 1,2,4-triazines and alkenes gives dihydropyridines or pyridines, provided that the alkene is electron-rich (72LA(758)120). Since most of the electron-rich alkenes are unsymmetrical some points of ambiguity arise, summarized in Scheme 11. 

The scope and utility of cation radical induced cyclobutanation1 is greatly enhanced by the option of cross additions, the first of which was the formation of a 3 2 mixture of diastereomeric cyclobutanes in the irradiation of an equimolar mixture of phenyl vinyl ether and 1,1-dimethylindene in the presence of tetraphenylpyrylium tetrafluoroboratc in acetonitrile.2 The scope of PET cyclobutanations was further extended in a synthetic sense by the observation of cross additions of electron-rich alkenes to conjugated dienes.3 4 Examples of such reactions are shown below in the formation of compounds 1, 2, 3 and 4. 

A second major mode of photocydoaddition involves 1.2-addition to the aromatic ring, and this predominates if there is a large difference in electron-donor/acceptor capacity between the aromatic compound and the alkene. It is therefore the major reaction pathway when benzene reacts with an electron-rich alkene such as 1,1-dimethoxyethylene (3.43) or with an electron-deficient alkene such as acrylonitrile (3.441. When substituted benzenes are involved, such as anisole with acrylonitrile (3.45), or benzonitrile with vinyl acetate (3.46), reaction can be quite efficient and regioselective to give products in which the two substituents are on adjacent carbon atoms. 

When an alkyl or aryl ketone, or an aryl aldehyde, reacts with an alkyl-substituted ethylene, or with an electron-rich alkene such as a vinyl ether, the mechanism involves attack by the (n,n triplet state of the ketone on ground-state alkene to generate a 1,4-biradical that subsequently cyclizes. The orientation of addition is in keeping with this proposal, since the major product is formed by way of the more stable of the possible biradicals, as seen for benzophenone and 2-melhylpropene (4.64). As would be expected for a triplet-state reaction, the stereoselectivity is low, and benzophenone gives the same mixture of stereoisomers when it reacts with either trans or... 

Dihydropyridazines (117) result from Diels-Alder addition of. v-tetrazines (115) with electron-rich alkenes (e.g. 116). Frequently the products aromatize, as in (117) — (118) (see also Section 3.2.1.10.2.iv). 

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