Cycloaddition and Ene Reactions

Chiral C2-symmetric bisoxazoline-copper(II) complexes [30, 31] were introduced as catalysts for cycloaddition and ene reactions of glyoxylates with dienes [9] leading to intense activity in the use of these catalyst for different cycloaddition reactions. 

Desimoni et al. have found that chiral BOX-manganese(II) complexes can catalyze the intramolecular cycloaddition and ene reactions, with the latter as the major product [37]. 

Singlet molecular oxygen ( A is an electron acceptor powerful enough to react with olefins in the pseudoexcitation band. The [2h-2] cycloaddition and ene reactions and the stereoselectivities are reviewed in this subsection. 

Benzyne gives both [2 + 2] cycloaddition and ene reaction products with simple alkenes.136... 

Cycloaddition and ene reactions. Dienes >C=C—C=C< such as buta-1,3-diene, isoprene, 2,3-dimethylbuta-l,3-diene, fraws-piperylene, cyclopentadiene or anthracene react with 92 in Diels-Alder fashion to give [2 + 4] cycloadducts 410 (equation 128)62. Ene products 411 are formed additionally when the relative reaction rates for the [2 + 4] cycloaddition reaction and the ene reaction are comparable (e.g. for isoprene and 2,3-dimethyl-l,3-butadiene) Alkenes with allylic hydrogen (propene, 2-butene, isobutene) give ene products see equation 129. 

Since [4 + 2]cycloaddition and ene reactions are generally assumed to proceed in a concerted manner via isopolar activated complexes, they should exhibit virtually the same small, often negligible, response to changes in solvent polarity. This is what, in fact, has been found cf. for example [138, 682, 683]. However, two-step [2 + 2]-cycloaddition reactions of singlet oxygen to suitably substituted electron-rich alkenes proceed via dipolar activated complexes to zwitterionic intermediates (1,4-dipoles or perepoxides). In this case, the relative amounts of 1,2-dioxetane and allylic hydroperoxides or e do-peroxides should vary markedly with solvent polarity if two or even all three of the reaction pathways shown in Eq. (5-145) are operative [681, 683, 684]. 

TABLE 4. Relative rates of cycloadditions and ene reactions of Me2Si=C(SiMe3)2 at 100 °C (after References 246 and 303)... 

Cycloaddition and ene reactions. Japanese chemists have considered theoretical aspects of the mechanism of ene, [2 + 2], and [6 + 2] cycloaddition reactions of singlet oxygen and believe that perepoxides are involved as quasiintermediates. 

Like the esters of acetylenedicarboxylic acid (6), dicyanoacetylene (2) has often been employed in cycloaddition reactions it participates in Diels-Alder and 1,3-dipolar cycloadditions, and ene reactions as well as homo additions are also known [9,11], Although kinetic measurements with 2 seem not to have been performed so far, from working knowledge it appears that 2 is a more reactive dienophile than any other activated acetylene including hexa-... 

In some cases, products of reactions of TADs are best explained by the existence of 1,4-dipolar intermediates, and sometimes such intermediates have been observed. Some examples of [2 + 2] cycloaddition and ene reactions have been treated with the respective Sections IV,B and IV,C. There are many other types of reactions where such intermediates have been suggested and/or proved. For example, vinylethers 304 give with PTAD mixtures of 1,2-addition and polymeric products, the formation of which can be easily explained by 1,4-dipolar intermediates 305 (72JOC1454). In the presence of alkyl ketones, e.g., acetone, PTAD reacts spontaneously with vinyl ethers via these 1,4-dipoles 305, which are able to add to weakly dipolarophilic acetone to form 306, together with a polymeric material (71JOC2838 83JOC822). A similar reaction takes place when 1-phenyl-4-vinylpyrazole (307) is treated with PTAD at -60°C in acetone (85TL6357). 

These data, together with observations on the reaction of 1,2-didehydro-o-carborane (an analog of o-benzyne) with enamines, tend to support the two independent mechanisms proposal. Competition between [2+2] cycloadditions and ene reactions in the case of... 

On the other hand, carboryne, l,2-dehydro-t -carborane, is a three-dimensional relative of benzyne (Fig. 7.1) [6]. It can react with alkenes, dienes, and alkynes in [2-1-2], [4-1-2] cycloaddition and ene-reaction patterns [7], similar to those of benzyne [8], Although these reactions show the potential for the preparation of functionalized carboranes in a single operation, they are complex and do not proceed in a controlled manner. In view of the spectacular role of transition metals in synthetic chemistry, we envisage that the aforementioned reactions may work efficiently and in a controlled way with the help of transition metals. In this connection, we initiated a research program to develop transition-metal-mediated/catalyzed synthetic methodologies for the functionalization of carboranes. This chapter summarizes the recent progress in this research area. 

W. Oppolzer, Regio-and Stero-selective Synthesis of Cyclic Natural Products by Intramolecular Cycloaddition- and Ene-reactions , Pure Appl. Chem., 1981, 53, 1181. 

The Jt system of ruthenium allenylidenes can also participate in pericyclic reactions such as cycloadditions and ene reactions to afford functionalized polycyclic products. 

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