Dienes compounds, inter-intramolecular

Summary The synthesis and structural features of inter- and intramolecularly stabilized l-metalla-2-sila-l,3-diene compounds will be discussed. 

Anodic C, C-coupling is a very powerful tool to synthesize cyclic compounds with high regio- and stereoselectivity. It involves inter- and intramolecular coupling of arylolefins, dienes, enolethers, phenol ethers, and aromatic amines and often opens a quick entry into complex natural products in a few steps. Although the mechanism is fully established in only a few cases, it does appear to involve the coupling of two radical cations at the site of their highest radical density and is further controlled by steric constraints. This important type of reaction is reviewed in Chap. 5 and in Refs. [89, 90]. 

As discussed in Section 6.9 1, 3-dienes and dienophiles in which multiple bonds are not activated by electron-withdrawing or electron-releasing substituents fail to undergo cycloaddition except under the most severe conditions. Particular difficulty is encountered in the cycloaddition of two unactivated species since homodimerization can be a competitive and dominant reaction pathway. The use of transition-metal catalysts, however, has proved to be a valuable solution. Complexation of unactivated substrates to such catalysts promotes both inter- and intramolecular cycloadditions. Consequently, the cycloaddition of such unactivated compounds, that is, simple unsubstituted dienes and alkenes, catalyzed by transition metals is a major, important area of study.655 In addition, theoretical problems of the transformation have frequently been addressed in the more recent literature. 

These 107i-systems are isoelectronic with the pentalene dianion and have been of some theoretical interest. 1,5-Diheteropentalene systems are very popular substrates for the investigation of inter- and intramolecular cycloaddition reactions due to their diene character. Such cycloaddition processes allow for a rapid entry into complex polyheterocyclic rings and makes these compounds potentially useful for natural product synthesis. 

A number of new Ni-catalyzed protocols have been developed recently for the coupling of carbonyl compounds and unsaturated substrates such as alkynes or 1,3-dienes. For instance, a three-component intramolecular coupling (alkyne/aldehyde/hydrosUane) has led to the formation of bicyclic N-heterocycles. A number of similar inter- and intramolecular coupling reactions have been developed into useful synthetic routes (Scheme 27). 

This chapter will mainly describe new developments in the area of theoretical methods and experimental structural methods, and in the rapidly growing field of inter- and intramolecular Diels-Alder reactions of these electron-poor azadienes. Quantitative data for the reactivity of dienes and dienophiles in these (4 -1- 2) cycloadditions with inverse electron demand will be discussed. The synthesis and reactions of dihydro, tetrahydro, and hexahydro tetrazines cannot be discussed broadly beyond the scope of <84CHEC-I(3)53l>. Verdazyls, a well-known class of compound, cannot be treated in detail within the frame of this contribution (see Section 6.21.5.9). 

Since its discovery over sixty years ago [1] the Diels-Alder reaction has lost none of its attraction. [2, 3] It enables, in a one-step inter- or intramolecular reaction, the rapid preparation of cyclic compounds having a six-membered ring. During the course of the [4 -I- 2] cycloaddition four new stereocenters can be introduced directly, and their stereo-control is a topic of major interest in modem synthetic chemistry. [4-6] In addition, in intermolecular reactions, the relative positions of the reaction partners (regiochemistry) must be taken into account. If a concerted reaction is assumed, both a cis addition (suprafacial mode) and a preferred endo orientation (Alder rules) can be expected. But how can the absolute configuration of the desired product be controlled There are three basic possibilities the use of a chirally modified diene, a chirally modified dienophile, or a chiral catalyst. Although the first successes resulted from the attractive, hut difficult, catalytic route, [4b, 7] the majority of the investigators are concerned with the stoichiometric... 

C.xi. Doubly Activated Methylene and Methines (C—H Acidic Compounds) as the H—Y Trapping Reagent Inter- and Intramolecular Diene Coupling... 

Isomerisation studies of cobalt(iii) complexes cover a variety of compounds. The rate law for isomerisation of [Co(dien)(OH2)a] + indicates some isomerisation via the hydroxo-cation [Co(OH)dien(OHa)2] +. Rates and activation parameters for direct isomerisation of the tris-aquo-complex were determined. Further kinetic results for isomerisation of cis- and rra 5 -[Co(OH)2 en2]+ in strongly basic solution indicate an intramolecular mechanism. Isomerisation of cw-[CoCl2(diars)2] in methanol is also intramolecular, but isomerisation of cw-[Co(02C CH3)2en2]+ in acetic acid occupies an intermediate position between intra- and inter-molecu-larity, for the essential step is solvent-assisted acetate exchange within ion-pairs. This assignment of mechanism results from consideration of kinetics of acetate exchange as well as of isomerisation. ... 

The Diels-Alder (DA) reaction is one of the most useful processes in preparative organic chemistry. Its potential in heterocyclic chemistry and natural products synthesis is very well known. It provides the chemist with one of his best tool for the preparation of cyclic compounds having a six-membered ring. The process is in one step inter or intramolecular from a diene and dienophile bearing an almost unlimited number of variants, ft is worth noting that these variants exist not only in the substitution of the reaction component but also in the electronic nature of the dienes and dienophiles. 

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