Double bonds, overview

K. Tani and Y. Kataoka, begin their discussion with an overview about the synthesis and isolation of such species. Many of them contain Ru, Os, Rh, Ir, Pd, or Pt and complexes with these metals appear also to be the most active catalysts. Their stoichiometric reactions, as well as the progress made in catalytic hydrations, hydroal-coxylations, and hydrocarboxylations of triple bond systems, i.e. nitriles and alkynes, is reviewed. However, as in catalytic hydroaminations the holy grail", the addition of O-H bonds across non-activated C=C double bonds under mild conditions has not been achieved yet. 

A variety of double bonds give reactions corresponding to the pattern of the ene reaction. Those that have been studied from a mechanistic and synthetic perspective include alkenes, aldehydes and ketones, imines and iminium ions, triazoline-2,5-diones, nitroso compounds, and singlet oxygen, 10=0. After a mechanistic overview of the reaction, we concentrate on the carbon-carbon bond-forming reactions. The important and well-studied reaction with 10=0 is discussed in Section 12.3.2. 

The insertion of double bonds into Al—C and Al—H bonds has been known for a very long time. The importance of direct -hydrogen transfer as a general reaction category has not been recognized as clearly we hope that the present overview illustrates how pervasive this reaction is in aluminium chemistry. 

RINGS CONTAINING TWO ENDOCYCLIC DOUBLE BONDS 4.2.3.1 Overview... 

Metal-nitrogen bonds, in Zr(IV) bis-Cp compounds, 4, 910 Metal-nitrogen double bonds, synthetic reaction overview, 11, 151-178... 

Although several enzymes can enantioselectively catalyze the hydrocyanation of R2C=0 and R2C=NR bonds [7], (asymmetric) hydrocyanation of C=C double bonds has no precedents in biology. In homogeneous catalysis asymmetric hydrocyanation is still underdeveloped, as is apparent from the relatively few reports in the literature. In the following paragraphs a short overview will be given divided into the two major substrate classes investigated, cyclic (di)enes and vinylarenes. 

Of the three possible substrates, thiophene, benzo[ ]thiophene, and dibenzo[, thiophene, benzo[ ]thiophene is the most easily hydrogenated to the dihydro derivative this is ascribable to the more pronounced olefinic character of the C(2)-C(3) double bond in benzo[, ]thiophene as compared to that in thiophene. There is no example in the literature of the hydrogenation of dibenzothiophene either to the tetrahydro or the hexahydro stage. The hydrogenation of benzol ]thiophene is catalyzed by transition metals such as Ru, Os, Rh, and Ir. An excellent overview of homogeneous catalytic hydrogenation of thiophenic substrates has been presented recently <2004JOM (689)4277>. 

Figure 12.1. Overview of eicosanoid metabolism, a Stmetures of eicosanoid precursor fatty acids, and their occurrence in membrane phospholipids (PC is shown as an example). Araehidonie acid is the prototypic precursor eicosatrienoic and eicosapen-tanoic acid differ from it by the a lackingor an additional double bond, respectively, b Conversion of preemsor fatty acids occurs by various enzymes, notably cyclooxygenases and lipoxygenases. Isoprostanes are non-enzymahe derivatives that may form in vivo at appreciable rates one characterishc featme is that they occur as racemic mixtures. Their physiological significance is not entirely clear.

Figure 1 gives an overview of some calculational results which clearly shows that silylboranes with a B=B double bond are less stable than those with a B=Si double bond. The most stable species is a nido-pentaborane. Consequently, the quest for a silyleneborane needs a strategy in which electronic as well as a steric effects must play a decisive factor. 

The inability of certain nucleophiles to add to the carbon-nitrogen double bond of iminesfimine derivatives, coupled with the propensity of basic reagents to preferentially abstract protons a to the imine double bond, has limited the utility of the group in synthetic organic chemistry. While unique solutions exist for individual reactions, they are at best only applicable to a particular structural class of azomethines. Since structurally diverse imine derivatives have been utilized for the preparation of highly functionalized amines, an overview of some of the structural features of azometiiines and nonstabilized... 

In this account, an overview of the methods employed for the synthesis of conjugated dienes and polyenes is presented. Dienes and polyenes with isolated double bonds are excluded, as they are accessed through methods usually employed for alkene synthesis. Oligomerizations and polymerization reactions leading to polyenes are also not covered. Synthesis of 1,2-dienes, i.e. allenes, is excluded from the purview as there is a volume in the present series devoted to this functional group. Synthesis of heterodienes, conjugated enol ethers, [n]-annulenes and related compounds are also not covered here. However, enynes, dienynes and enediynes syntheses have been included in a few cases in view of their emerging importance. 

Discrimination between the symmetrical [2n- -2a] and the [(27t-l-2(r)-f 27t] cycloaddition is difficult, because both form the same product (except if the exocyclic double bond in the methylene-cyclopropane is substituted). The unsymmetrical [2n + 2[Pg.2207]

Cholesterol is synthesized mainly in the liver by a three-stage process. All 27 carbon atoms in the cholesterol molecule are derived from acetyl-CoA. The first stage is the synthesis of the activated five-carbon isoprene unit, isopentenyl pyrophosphate. Six molecules of isopentenyl pyrophosphate then condense to form squalene in a sequence of reactions that also synthesize isoprenoid intermediates that are important in protein isoprenylation modifications. The characteristic four-ring structure of cholesterol is then formed by cycUzing of the linear squalene molecule. Several demethylations, the reduction of a double bond, and the migration of another double bond result in the formation of cholesterol. Figure 34-1 provides an overview of cholesterol biosynthesis. 

---