Unsaturated molecules

1-Ethynylcyclohexene has been reduced selectively to 1-ethenylcyclohexene by the Lindlar catalyst addition of alkyl groups to give for example the arrangement C=C—CMC—C—C was without effect, but the grouping C=C—CMC—C=C 

It may be helpful to try to draw together some of the characteristics that are shared by alkyne hydrogenations in their many manifestations, and to attempt some generalisations these by definition will be statements that are more or less true most of the time. 

The outstanding quality of palladium in both these respects is well known but not well understood. Foremost among the complexities that have to be addressed are (i) the tendency to form hydride phases, and (ii) the frequent formation, especially with ethyne, of a carbonaceous overlayer, derived perhaps from oligomers, some of which escape into the fluid phase. Both these factors are invoked to account for palladium s remarkable properties, but both are responsive to reaction conditions and especially to particle size support effects may also operate. So many factors have to be kept in play when discussing mechanisms that one may safely conclude that none so far suggested is wholly satisfactory. The rich literature on alkyne hydrogenation deserves careful attention, and should be a fine source of inspiration for further research. 

Farrauto and C.H. Bartholomew, Fundamentals of Catalytic Processes, Chapman and Hall London (1997), p. 434. 

Satterfield, Heterogeneous Catalysis in Practice, 2 edn., McGraw-Hill New York (1991). 

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Kiefer J H, Mudipalli P S, Wagner A F and Harding L 1996 Importance of hindered rotations in the thermal dissociation of small unsaturated molecules classical formulation and application to hen and hcch J. Chem. Phys. 105 1-22... 

In unsaturated molecules electronic effects can be transmitted by mesomerism as well as by inductive effects. As with the latter, the mesomeric properties of a group are described by reference to hydrogen. Groups which release electrons to the unsaturated residue of the molecule are said to exert a +Af effect, whereas groups which attract electrons are said to exert a —Af effect. In aromatic structures the important feature of an M-substituent is that it influences the 0- and p-positions selectively. 

Ca.ta.lysis, Iridium compounds do not have industrial appHcations as catalysts. However, these compounds have been studied to model fundamental catalytic steps (174), such as substrate binding of unsaturated molecules and dioxygen oxidative addition of hydrogen, alkyl haHdes, and the carbon—hydrogen bond reductive elimination and important metal-centered transformations such as carbonylation, -elimination, CO reduction, and... 

Kadical-addition reactions to unsaturated molecules-. Chain-termination reactions-. 

Hydrides. Zirconium hydrides react easily with unsaturated molecules. This process, termed hydrozirconation, replaces the hydrogen with the unsaturated group ... 

The accessibility of the +4 and +6 oxidation states for sulfur and, to a lesser extent, selenium gives rise to both acyclic and cyclic molecules that have no parallels in N-O chemistry. Thus there is an extensive chemistry of chalcogen diimides RN=E=NR (E = S, Se, Te) (Section 10.4). In the case of Te these unsaturated molecules form dimeric structures reflecting the increasing reluctance for the heavier chalcogens to form multiple bonds to nitrogen. The acyclic molecule N=Sp3,... 

Cycloaddition reactions also have important applications for acyclic chalcogen-nitrogen species. Extensive studies have been carried out on the cycloaddition chemistry of [NSa]" which, unlike [NOa]", undergoes quantitative, cycloaddition reactions with unsaturated molecules such as alkenes, alkynes and nitriles (Section 5.3.2). ° The frontier orbital interactions involved in the cycloaddition of [NSa]" and alkynes are illustrated in Fig. 4.13. The HOMO ( Tn) and LUMO ( r ) of the sulfur-nitrogen species are of the correct symmetry to interact with the LUMO (tt ) and HOMO (tt) of a typical alkyne, respectively. Although both... 

A Semi-Empirieal Theory of the Electronic Spectra and Electronic Structure of Complex Unsaturated Molecules R. Pariser and R. G. Parr... 

A cycloaddition reaction is one in which two unsaturated molecules add to one another, yielding a cyclic product. As with electrocyclic reactions, cycloadditions are controlled by the orbital symmetry of the reactants. Symmetry-allowed... 

Cycloaddition reactions are those in which two unsaturated molecules add together to yield a cyclic product. For example, Diels-AJder reaction between a diene (four tt electrons) and a dienophile (two tt electrons) yields a cyclohexene. Cycloadditions can take place either by suprafacial or antarafacial pathways. Suprafacial cycloaddition involves interaction between lobes on the same face of one component and on the same face of the second component. Antarafacial cycloaddition involves interaction between lobes on the same face of one component ancl on opposite faces of the other component. The reaction course in a specific case can be found by looking at the symmetry of the HOMO of one component and the lowest unoccupied molecular orbital (LUMO) of the other component. 

Pariser, R., and Parr, R. G., 1. J. Chem. Phys. 21, 466 II. J. Chew. Phys. 21, 767, "A semi-empirical theory of the electronic spectra and electronic structure of complex unsaturated molecules."... 

Fluorinated compounds are frequently referred lo by code, such as F-l 15. To translate this code into a molecular formula, add 90 to 115. The firsl digit of the sum is the number of carbons the second, the number of hydrogens the third, the number ol fluorines chlorines complete the valences (i.e., C2F5CI is F-l 15 = 90 + 115 = 205. or C H Fj). A four-digit number is used for unsaturated molecules. See Tables 14.1-14.3 for the numbering system for chlorofluorocarbons. 

Osmate esters are important intermediates in the reactions of 0s04 in the stereospecific m-hydroxylation of alkenes and other unsaturated molecules [182],... 

Few CIDNP studies on free radical reactions with olefins and related unsaturated molecules have been reported, and relatively little chemically useful information seems to have been derived, despite the potential relevance in polymerizing systems. Thus CIDNP has been reported in the decomposition of benzoyl peroxide in the presence of styrene and... 

Observed geometrical features or stabilities of small ring molecules containing an endocyclic double bond have been explained in terms of the Ji-a interactions [137, 138]. The n relaxation is predicted to stabilize such small ring unsaturated molecules of heavy atoms recently prepared [139-142],... 

Some transition metal systems M(CO)R react with a wide range of L, including phosphites, phosphines, arsines, stibines, organic amines, iodide, and CO, to mention a few, yielding the corresponding acyls. Other systems, e.g., CpFe(CO)2R (2S), display a marked selectivity toward various L. Certain unsaturated molecules L [SOj (239), CF2=Cp2 (238), inter alia] insert themselves into the M—R bond instead of effecting the reaction shown in Eq. (8). 

Quite independently, of these fragmentary remarks, a distinctive role of HO (and later LU and SO, too) in unsaturated molecules was pointed out 43> in a general form and with substantiality (cf. Chap. 2). With respect to the molecular complex formation, the theory of charge-transfer force was proposed 47>. A clue tograsp the importance of HO—LU interaction was thus brought to light simultaneously both from the side of ionic reaction and from the side of molecular complex formation. 

The insertion of unsaturated molecules into metal-carbon bonds is a critically important step in many transition-metal catalyzed organic transformations. The difference in insertion propensity of carbon-carbon and carbon-nitrogen multiple bonds can be attributed to the coordination characteristics of the respective molecules. The difficulty in achieving a to it isomerization may be the reason for the paucity of imine insertions. The synthesis of amides by the insertion of imines into palladium(II)-acyl bonds is the first direct observation of the insertion of imines into bonds between transition metals and carbon (see Scheme 7). The alternating copolymerization of imines with carbon monoxide (in which the insertion of the imine into palladium-acyl bonds would be the key step in the chain growth sequence), if successful, should constitute a new procedure for the synthesis of polypeptides (see Scheme 7).348... 

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... 

Figure 10.1 Selected examples of naturally available, biologically active unsaturated molecules and the molecular structures.

These different classes of molecules reflect the variety of reaction conditions which are present in ISM. Thus ion-molecule reactions dominate in the dark clouds these reactions are driven by ionisation processes and lead to unsaturated molecules, ions and radicals. 

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