Types of Unsaturation

The principal subject of this chapter is the hydrogenation of molecules containing two carbon-carbon double bonds. These may exist in a variety of situations thus in a linear chain of carbon atoms they may be adjacent as in propadiene 

TABLE 8.1. Heats of Hydrogenation of Multiply-Unsaturated Hydrocarbons (kJ mol )  

Triplets or quartets of double bonds can also be found in linear hydrocarbons and in larger ring systems (e.g. Cio or C12) the principles outlined above also apply to them (Section 8.4.3). Compounds in which three or more double bonds exist adjacently, either in linear or cyclic systems, are referred to as cumulenes, they show some interesting features when being hydrogenated.  

One of the major themes of basic research having a practical orientation has been the use of bimetallic catalysts for improved selectivity. Palladium is the 

Our main task in this chapter is to try to understand how, when and why dienes and their relatives chemisorb to the exclusion of alkenes (the reasons are not always straightforward), and to explore the mechanisms that, additional to those revealed in the last Chapter, account for the formation of the observed products. - - -  

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Selective ozonization of compounds containing different types of unsaturated linkages is possible by choosing a proper concentration of ozone and by stopping the ozonization at the right time. It is for this reason that the calibration described above is carried out, since for certain ozonizations it is important to have the correct concentration for best results. 

In an unsaturated hydrocarbon, at least one of the carbon-carbon bonds in the molecule is a multiple bond. As a result, there are fewer hydrogen atoms in an unsaturated hydrocarbon than in a saturated one with the same number of carbons. We will consider two types of unsaturated hydrocarbons—... 

Various types of unsaturated hydrocarbons have been reported to undergo metathesis reactions by contact with appropriate catalysts. A short survey is given below. It is to be expected that in the near future still more examples will be found. 

Depending on the types of unsaturated functional units involved in the metathesis process, the reactions can be classified into three major categories diene, enyne, and diyne metathesis (Figs. 1-3). Another mode of classification... 

Before we examine the hydrogenation of each type of unsaturation, let us first take a look at the basic mechanism assumed to be operating on metal catalytic surfaces. This mechanism is variously referred to as the classic mechanism, the Horiuti-Polanyi mechanism, or the half-hydrogenated state mechanism. It certainly fits the classic definition, since it was first proposed by Horiuti and Polanyi in 193412 and is still used today. Its important surface species is a half-hydrogenated state. This mechanism was shown in Chapter 1 (Scheme 1.2) as an example of how surface reactions are sometimes written. It is shown in slightly different form in Fig. 2.1. Basically, an unsaturated molecule is pictured as adsorbing with its Tt-bond parallel to the plane of the surface atoms of the catalyst. In the original Horiuti-Polanyi formulation, the 7t-bond ruptures... 

Other metal complexes containing several types of unsaturated compounds as ligands can be prepared by similar reactions. 

C-1 Glycals, a relevant type of unsaturated branched sugar, are currently used in synthetic approaches towards C-glycosyl compounds and natural products. In this chapter we present a brief overview of the synthetic strategies employed for their preparation, which have been classified according to the type of starting material employed. Further modifications carried out on the C-1 glycals fall beyond the scope of this review and will not be discussed. 

THREE TYPES OF UNSATURATED POLYESTER RESIN PRODUCTS... 

Table 21.5 Types of unsaturated polyester resins employed in major...

Hydrogenations of all types of unsaturated halides are carried out over Raney nickel in the presence of potassium hydroxide at room temperature and atmospheric pressure with yields of 32-92%. Even perhalogenated al-kenes such as perchlorocyclopentadiene are reduced to the saturated parent compounds [63]. 

According to the type of unsaturated bond to be lithiated, carbon-carbon, carbon-oxygen and carbon-nitrogen bonds will be considered. 

Zhang has applied the cyclization of esters to the formation of a-methylene-y-butyrolac-tones, providing a novel and enantioselective entry to these substructures. The importance of this type of unsaturated lactone is evident by its ubiquitous presence in nearly one-third of all naturally occurring secondary metabolites. The Alder-ene reaction has been applied to a formal total synthesis of (+)-pilocarpine [26], a leading therapeutic reagent for the treatment of narrow and wide glaucoma. Zhang intersected Biichi s synthetic intermediate (i )-10a [27] in only two steps with 99% enantiomeric excess in 91% overall yield (Eq. 13). In comparison, Biichi synthesized (il)-lOa in five steps with 92% enantiomeric excess and 20% overall yield. 

There are two different types of P-protons, and two different unsaturated end groups are possible for isobutylene as well as some other monomers such as indene and a-methylstyr-ene. The relative amounts of the two end groups depend on the counterion, identity of the propagating center, and other reaction conditions. Only one type of unsaturated end group (internal) is possible for other monomers such as styrene, ethyl vinyl ether, and coumarone. 

A most useful haloboration agents, B-bromo-9-borabicyclo[3.3.1]nonane and BBr3, react readily with terminal alkynes via Markovnikov syn addition of the Hlg-B moiety to the carbon-carbon triple bond in an uncatalyzed process. The haloboration reaction occurs regio- and chemoselectively at terminal triple bonds, but for other types of unsaturated compounds it is nonselective 639... 

Hydroxylamine reacted with 71 to give the oxime of 7-(2,3,4,6-tetra-deoxy-/ -L-gh/cero-hex-2-enopyranosyl-4-ulose)theophylline (102), a new type of unsaturated nucleoside. Treatment of 71 with sodium boro-hydride reduced the carbonyl group, and afforded 7-(2,3,6-trideoxy-/ -L-en/thro-hex-2-enopyranosyl)theophylline (103). The n.m.r. spectrum of acetylated 103 showed H-4, 5 to be trans-diaxial and that axial addition of hydride to the carbonyl group of 71 had occurred.42... 

The best materials are produced by curing unsaturated polyester during copolymerization with different types of unsaturated monomers and oligomers, such as styrene and its derivatives, methyl methacrylate, vinylacetate, sulfone-type compounds, allyl esters, and oligo(ester acrylates).5... 

Several types of unsaturated amides have produced gitonic superelectrophiles in superacidic media. For example, reaction of the cinnamamide (60) with benzene in CF3SO3H gives the addition product in quantitative yield (eq 20).27... 

Scheme 1. Selectivity problems in different types of unsaturated substrates A-C.

Essentially, two types of unsaturated bifunctional compound display the capability of being condensed in high yield, via a transition metal-catalysed carbon-carbon coupling reaction, into their requisite generic high molecular weight polymers plus a small molecule non-conjugated acyclic dialkenes and haloaromatic derivatives. 

Figure 2. Origin of two types of unsaturated end groups in polyhexene...

In view of recent reports, it seems prospective to use the reaction of trans sily-lation for the synthesis of other types of unsaturated compounds, particularly for the synthesis of novel organosilicon starburst compounds having a silicon-bridged -conjugated structure which is expected to have potential optoelectronic properties. The effective functionalization of l,3,5-tris(dimethylvinylsi-lyl)benzene by the respective reactions with 1,4-divinylbenzene leads to the formation of a dendrimer (Fig. la) [55]. 

Vinyllithium initiates the polymerization of styrene and of similar monomers. However, the rate of initiation is fairly low, and the molecular weights exhibited by the polymers obtained are by one order of magnitude higher than the expected ones. This is not the only drawback of the method. A macromolecule obtained by anionic polymerization using vinyllithium carries an allylic double bond at the chain end, and this type of unsaturation is not adequate for the subsequent radical copolymerization because of its low reactivity towards radicals and of isomerization that can take place. 

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