Other Multiple Bonds

Other multiple bonds, eg,, —C=N, —N=N—, also undergo the addition. However, those reactions not involving boron—carbon bond... 

The insertion of alkynes into a chromium-carbon double bond is not restricted to Fischer alkenylcarbene complexes. Numerous transformations of this kind have been performed with simple alkylcarbene complexes, from which unstable a,/J-unsaturated carbene complexes were formed in situ, and in turn underwent further reactions in several different ways. For example, reaction of the 1-me-thoxyethylidene complex 6a with the conjugated enyne-ketimines and -ketones 131 afforded pyrrole [92] and furan 134 derivatives [93], respectively. The alkyne-inserted intermediate 132 apparently undergoes 671-electrocyclization and reductive elimination to afford enol ether 133, which yields the cycloaddition product 134 via a subsequent hydrolysis (Scheme 28). This transformation also demonstrates that Fischer carbene complexes are highly selective in their reactivity toward alkynes in the presence of other multiple bonds (Table 6). 

In Part 2 of this book, we shall be directly concerned with organic reactions and their mechanisms. The reactions have been classified into 10 chapters, based primarily on reaction type substitutions, additions to multiple bonds, eliminations, rearrangements, and oxidation-reduction reactions. Five chapters are devoted to substitutions these are classified on the basis of mechanism as well as substrate. Chapters 10 and 13 include nucleophilic substitutions at aliphatic and aromatic substrates, respectively, Chapters 12 and 11 deal with electrophilic substitutions at aliphatic and aromatic substrates, respectively. All free-radical substitutions are discussed in Chapter 14. Additions to multiple bonds are classified not according to mechanism, but according to the type of multiple bond. Additions to carbon-carbon multiple bonds are dealt with in Chapter 15 additions to other multiple bonds in Chapter 16. One chapter is devoted to each of the three remaining reaction types Chapter 17, eliminations Chapter 18, rearrangements Chapter 19, oxidation-reduction reactions. This last chapter covers only those oxidation-reduction reactions that could not be conveniently treated in any of the other categories (except for oxidative eliminations). 

Besides the weak bonds listed in the previous table, there are other multiple bonds that endow the molecules in which they are situated with a positive enthalpy of formation. Such compounds are termed endothermic compounds. The danger they represent does not necessarily come from the fact that they are unstable, but is related to the exothermicity of their decomposition reaction. The most convincing examples are the acetylenic compounds, and in particular, acetylene. It is also the case for ethylene, aromatic compounds, imines and nitriles. 

The ordinary olefines are not reduced under these conditions. But sodium in liquid ammonia reduces double bonds if they are conjugated to aromatic systems or other multiple bonds. 

Intramolecular Cycloadditions Involving Other Multiple Bonds. Ketocarbene Dipoles as Building Blocks in Heterocyclic Synthesis. ... 

Intramolecular Cycloadditions Involving Other Multiple Bonds... 

ADDITIONS OF CARBON-CENTERED RADICALS TO OTHER MULTIPLE BONDS AND 765... 

One of the mildest general techniques to extend a carbon chain entails the addition of a carbon-centered radical to an alkene or alkyne. The method for conducting these addition reactions often determines the types of precursors and acceptors that can be used and the types of products that are formed. In the following section, synthetically useful radical additions are grouped into chain and non-chain reactions and then further subdivided by the method of reaction. Short, independent sections that follow treat the addition of carbon-centered radicals to other multiple bonds and aromatic rings and the additions of hete-roatom-centered radicals. 

Besides alkenes and alkynes, other multiple bonds can be used as acceptors in addition reactions of carbon radicals provided the usual requirements of reactivity and selectivity are met. Other types of carbon-carbon multiple bonds that have been used as acceptors include dienes,162 allenes,61 enolates (and ni-tronates, see below) and quinones.223 Even highly strained cr-bonds have served as acceptors on occasion.224... 

The CGMT model was intended to describe (non-cyclic) homonuclear and heteronuclear double-bonded systems. However, in the meantime other multiple-bonded species have also been synthesized, as will be seen in the following. They also often exhibit structural features which are not familiar from the analogous carbon derivatives, and are not yet fully understood. Some of them are, however, related to the phenomena discussed above and may be understood on a similar base. I will not go into details in these cases, but will refer to recent literature when available. 

The ultraviolet spectra 27,28 exhibit, in agreement with the general postulate of Braude et al.2%-30 a bathoehromic shift to 225-235 m/j. caused by the auxochromic action of the nitrogen-free electron pair. This shift is approximately the same as that caused by introduction of a conjugated double bond, and is increased by further conjugation with other multiple bonds, e.g. in diene-amines prepared from A -3-oxosteroids.31,32 Spectral maxima (at 280-285 mfi, e 19,000-26,000) point to the conjugation of three mobile electron pairs but cannot decide the position of the double bonds the molecular extinction coefficient indicates a transoid (5) rather than cisoid arrangement (e.g. 6).33... 

Many fewer photocatalytic organic reductions have been reported. Reductions of organic substrates are less thoroughly studied, largely because of the early emphasis on the use of organic compounds as oxidizable source for the production of hydrogen gas. Nonetheless, some examples do exist, such as the hydrogenation of olefins, vinyl ethers, and a, S-unsaturated enones and alkynes [166, 167]. Similarly, other multiple bonds can be reduced, e.g., the N=N double bond of diaryl azo compounds [168] or carbonyl C=0 bonds [169, 170]. 

The chemistry of 2 and 3 has been investigated over the past ten years. Thus, for example, reactions of these species with nitriles, isonitriles, ketones, 1,4-dihetero-1,3-dienes, and many other multiple bond systems have been realized [2], However, the important groups of the alkenes and 1,3-dienes were missing from this series. The objective of the present work was to rectify this omission. 

The Chalk-Harrod mechanism has been widely accepted with various modifications to account for the hydrosilylation of other multiple bonds (C=C), C=0, C=N), homogeneously catalyzed by various metal complexes. 

Partial Wiener indices are other multiple bond descriptors derived by a splitting of the Wiener index into different multiple bond contributions. 

Carbon—carbon double bonds which are conjugated to other multiple bonds can also be reduced selectively by dissolving metals. For example, in the selective reduction of ergosterol (7) the distribution of the regioisomers depends on the hydroxy substituent, as well as on the solvent and the metal employed 1 °. Excellent yields of the (ran. -fused product 8 are obtained when sodium in /erf-butyl alcohol/tetrahydrofuran is used. Only a trace of the epimeric civ-product, formed as a result of / -face protonation, is found. The isolated double bond is not affected under the reaction conditions. 

The methyl disproportionation reaction appeared to occur at the higher temperatures. Hydrogen-rich species—methane and hydrogen— were observed in the gaseous products. In the absence of carbon monoxide or other multiple-bonded gaseous molecules, it may be assumed that an amine or nitrogenous species was the hydride donor, and that hydrogen-deficient moieties (nitriles, polyunsaturates, etc.) remained as adsorbed coke (0.2 wt % N, 2.2 wt % C) on the zeolite surface. 

Contrary to some reports, electrophilic addition reactions may occur in other multiple-bond systems. In many of the reactions of aldehydes and ketones the first stage involves the addition of some entity across the carbon-oxygen bond, e.g., the formation of oximes, semicarbazones, hydrazones, hydrates (1,1-diols) and their ethers, and the aldol condensation. Most of these reactions entail a subsequent loss (elimination) of a small molecule e.g. water, ammonia, ethanol) and, while one must be careful to determine whether the rate-determining stage involves attack on the carbonyl compound or elimination from the adduct , there are some systems in which it is evident that electrophilic attack is involved in the slow stage of the reaction sequence. Examples of such reactions are the acid-catalysed formation of oximes of aliphatic - and aromatic carbonyl compounds, of furfural semi-carbazone , and of 1,1-diols from aldehydes or ketones . 

As well as such additions to aldehydes and ketones, there are a number of addition reactions involving other multiple bonds some of these show definite electrophilic character-. ... 

Substances that have terminal double or triple bonds, if unconjugated to other multiple bonds, are oxidized by P-450 in the e.r. to toxic substances which attack this porphyrin and deactivate it. Examples are ethylene, acetylene, vinyl chloride and the hypnotic ethchlorvynol which is l-chloro-3-ethylpent-l-en-4-yn-3-ol. Thus, ethylene leads to the A-2-hydroxyethy 1-derivative of P-450. A further adverse effect is that other drugs, if given at the same time, escape the usual metabolic transformation and so build up in the patient (Ortiz de Montellano, Beilan and Matthews, 1982). 

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