Less-substituted olefin

Olefins by elimination from quaternary ammonium salts (less substituted olefin preferred). 

Perfluoroolefins isomerize photochemically to yield less substituted olefins [272] Photolysis of polyfluorotriarylmtrones leads to polyfluorotriaryloxaziridines [173] (equation 43)... 

The reductive coupling of of dienes containing amine groups in the backbones allows for the production of alkaloid skeletons in relatively few steps [36,46,47]. Epilupinine 80 was formed in 51% yield after oxidation by treatment of the tertiary amine 81 with PhMeSiEh in the presence of catalytic 70 [46]. Notably, none of the trans isomer was observed in the product mixture (Eq. 11). The Cp fuMcTIIF was found to catalyze cyclization of unsubstituted allyl amine 82 to provide 83. This reaction proceeded in shorter time and with increased yield relative to the same reaction with 70 (Eq. 12) [47]. Substitution of either alkene prevented cyclization, possibly due to competitive intramolecular stabilization of the metal by nitrogen preventing coordination of the substituted olefin, and resulted in hydrosilylation of the less substituted olefin. 

The Bamford-Stevens reaction and the Shapiro reaction share a similar mechanistic pathway. The former uses a base such as Na, NaOMe, LiH, NaH, NaNHa, heat, etc., whereas the latter employs bases such as alkyllithiums and Grignard reagents. As a result, the Bamford-Stevens reaction furnishes more-substituted olefins as the thermodynamic products, while the Shapiro reaction generally affords less-substituted olefins as the kinetic products. 

These results contradict Jacobsen s earlier mechanistic theories, which would have predicted a top-on" approach for the sterically demanding tetrasubstituted olefins (Figure 1) and thus inferior results compared to the less-substituted olefins, which were assumed to approach from a skewed side-on" disposition. Furthermore, his observation that trisubstituted olefins were epoxidized in an opposite stereochemical sense compared to other olefins required invoking a stepwise mechanism, wherein the radical intermediate is steered by the pendant chiral catalyst [94JOC4378], At the current time, these results fail to coalesce into a clear unified predictive model. 

The conversion of higher substituted to less substituted olefins is not favored thermodynamically. This obstacle can be overcome when the newly formed C=C bond gains stabilization by conjugation with another double bond, e.g. C=0, or an aromatic unit, or by equilibration into a more stable product in a consecutive reaction. The E-Z geometry of the starting olefin may have a significant effect on the configuration and enantioselectivity in the product. 

Irradiation of phenyldisulfide cleaves the weak S—S bond to give a pair of thiophenyl radicals. One of these then adds to the less-substituted olefinic carbon of 25 to generate the tertiary alkyl radical 26 (Scheme 14.5). Bond rotation then ensues to give the more stable rotamer 27 in which there is minimal steric repulsion between the C(7)-methyl and the tetrahydrofuran framework. Elimination of the thiophenyl radical from 27 finalises the isomerisation to alkene 19. 

The achiral aziridination with the racemic nitrido complex 15 was successfully applied to a wide range of 1,3-dienes (Table 6.6). Cyclic conjugated dienes were smoothly aziridinated under mild conditions with no [4+1] adducts. Among these, cyclohexadiene and cycloheptadiene reacted with the complex in good yields. When the unsymmetrical dienes were employed in the reaction, the aziridi-nations of isoprene and trans-1,3-hexadiene proceeded, giving two regioisomers in 70 30 and 94 6, respectively. The major isomers in both reactions were formed by the aziridination of the less substituted olefins of the 1,3-dienes. 

Disiamylborane is a highly selective hydroborating agent with excellent steric control. Thus, it reacts selectively with less substituted olefins in the presence of more substituted 43) as ... 

The authors proposed that initial bond formation occurs from the less substituted olefin carbon to the a-carbon of the vinylogous amide to yield diradical intermediate 127 (Scheme 32, path A). This diradical can then undergo carbon-nitrogen bond homolysis to give the observed product, 126. However, the formation of 126 is also consistent with the formation of 128 by a straight cycloaddition followed by cycloreversion as outlined in Scheme 32, path B. 

These mechanistic possibilities require further investigation. Some examples of how the reaction is influenced by the structure of the olefin, the catalyst, and the solvent are given. In the presence of MX(CO)(PPh3)2 (M = Rh, Ir X = Cl, I), the oxidation of a-and /3-methylstyrenes and cis- and frans-stilbenes permitted the establishment of an activity sequence dependent on the catalyst and the stucture of the olefin. The solvent effect has been studied in the Ru complex-catalyzed oxidation of styrene and methylstyrene. In the presence of a Rh or Ir complex, the oxidation of tetramethylthylene is very selective" and takes place at a faster rate than those of the less-substituted olefins. It emerges from this that a significant role is not played by the coordinative linkage between the metal center and the olefin. Examinations have also been made on the oxidation activities of metallo-porphyrins, for example, the oxidation of cyclohexene with Co and Rh porphyrins. ... 

Selenoxide elimination occurs under relatively mild conditions in comparison to the elimination reactions described above. Selenoxides undergo spontaneous yn-elimi-nation at room temperature or below and thus have been used for the preparation of a variety of unsaturated compounds. The selenide precursors can be obtained by displacement of halides or sulfonate esters with PhSeNa. Oxidation of the selenides with hydrogen peroxide or tert-huiyX hydroperoxide, sodium periodate, or peroxycar-boxylic acids furnishes the corresponding selenoxides. Their eliminations usually favor formation of the less substituted olefin in the absence of heteroatom substituents or delocalizing groups. Since selenium compounds are toxic, they should be handled with care. 

Tosylhydrazones.—The reaction between suitable tosylhydrazones and alkyl-lithium was reported in 1967 to afford high yields of olefins, the less-substituted olefin being favoured where two possibilities exist (Hoffman-type products). In the reviewer s own experience, and that of other workers,this reaction can be unreliable, giving complex mixtures of products, some containing nitrogen. More recently, it was reported that the reaction succeeds when a limited amount of methyl-lithium in hexane is added dropwise to the steroid tosylhydrazone in tetrahydrofuran, under nitrogen e.g. 2-ene, 3-ene, 6-ene, and 1,3-diene, from... 

In these reactions the metal cleaves the allylic carbon-hydrogen bond to eliminate HCl. Since HCl is formed a base may be added to improve the 5neld of the 7t-allylpalladium complex and allow less substituted olefins to be transformed into 7r-allylpalladium complexes. Reaction of olefins with PdCl2 in acetic acid in the presence of sodium acetate , or with PdCl2(PhCN)2 in the presence of sodium carbonate gives the corresponding t-allyl complexes in good yield ... 

Allylic oxidations were also explored using PCC, Collins reagent, and selenium dioxide. While the desired enone 108 formed, the less strained, less substituted olefin 107 was invariably the dominant product. Use of the Collins reagent afforded the best yields (40%), but produced the same inseparable mixture of regioisomers (3 1,107 108). 

An indication of equilibrium positions for various types of cross-metathesis reaction is shown in Fig. 9.1. It will be seen that, in general, the equilibrium favours the less substituted olefins. However, it is sometimes possible to drive the equilibrium in the opposite direction by continuous removal of the more volatile components of the equilibrium mixture (see Ch. 5). In the rest of this chapter we... 

With a gradual increase in the complexity of the alcohol/alkoxide medium, elimination from 2-methylpent-2-yltrimethylammonium ion yields progressively more of the less substituted olefin, (127). 

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