Alkenes production maximization

The observed stereoehemistry of [2jc -I- 2jc] eyeloadditions is in accord with the stereochemieal predietions that ean be made on the basis of this model. For example, E-and Z-2-butene give stereoisomerie products with ethoxyketene. For monosubstituted alkenes, the substituent is vieinal and cis to the ethoxy group in the eyelobutanone product. This is exactly the stereoehemistry predicted by the model in Fig. 11.16, sinee it maximizes the separation of the alkyl and ethoxy substituents in the transition state. 

The presence of ether among the decomposition products depends on its stability (maximal for Me20 and minimal for Bu 20). All Al(OR)3 except the methoxide form alcohols, alkenes, traces of water, and hydrogen. 

In the SN1 mechansim, a competition between elimination and substitution also results from the ability of the nucleophile to act as a base. However, in this case the competition occurs at the carbocation stage of the reaction. Figure 8.12 shows an example. Elimination reactions are discussed in more detail in Chapter 9. Chapter 10 presents methods to minimize elimination when the substitution product is desired and methods to maximize elimination when the alkene is the desired product. For now it is important only to recognize that eliminations may decrease the yields in substitution reactions. 

As we saw in Chapter 8, elimination reactions often compete with nucleophilic substitution reactions. Both reactions can be useful in synthesis if this competition can be controlled. This chapter discusses the two common mechanisms by which elimination reactions occur, the stereochemistry of the reactions, the direction of the elimination, and the factors that control the competition between elimination and substitution. Based on these factors, procedures are presented that can be used to minimize elimination if the substitution product is the desired one or to maximize elimination if the alkene is the desired product. 

Structural effects on the C-basicity of enamines are, however, more complex. Because of the paucity of values for pX H+, we shall anticipate our discussion of the kinetics of C-protonation (see Section III) so that some of the information presented there can be incorporated into the present section. The justification for doing so is that many of the effects that influence the stability of the iminium ion are expected to be operational in the transition state. In particular, the coplanarity of the atoms about the C=N bond in the iminium ion (already preferred for some enamines, but only when geometrically possible24) should be maintained or improved in the transition state in order to maximize p-n overlap (equation 4)25. This means that, besides the ability of the amino nitrogen to bear a positive charge, other factors such as formation of the C=N double bond (with attendant rehybridization at nitrogen), and steric interactions between groups attached to the alkene and amine moieties will be important both in the transition state and in the iminium ion product. 

Cracking catalysts using combinations of medium and large-pore zeolites in order to maximize the production of products to be used in reformulated gasoline has been r orted [23]. In a study of the craclmg of n-heptane over MCM-22, ZSM-5 and Beta it was shown that the yield of propene was greatest in the case of MCM-22 and the overall alkane/alkene ratio of the products lay between ZSM-5 (0.94) and Beta (1.17). 

Typically, nonstabilized ylides are utilized for the synthesis of (Z)-alkenes. In 1986, Schlosser published a paper summarizing the factors that enhance (Z)-selectivity. Salt effects have historically been defined as the response to the presence of soluble lithium salts. Any soluble salt will compromise the (Z)-selectivity of the reaction, and typically this issue has been resolved by the use of sodium amide or sodium or potassium hexamethyldisilazane (NaHMDS or KHMDS) as the base. Solvent effects are also vital to the stereoselectivity. In general, ethereal solvents such as THF, diethyl ether, DME and t-butyl methyl ether are the solvents of choice." In cases where competitive enolate fomnation is problematic, toluene may be utilized. Protic solvents, such as alcohols, as well as DMSO, should be avoided in attempts to maximize (Z)-selectivity. Finally, the dropwise addition of the carbonyl to the ylide should be carried out at low temperature (-78 C). Recent applications of phosphonium ylides in natural product synthesis have been extensively reviewed by Maryanoff and Reitz. 

For monosubstituted alkenes, the substituent is vicinal and cis to the ethoxy group in the cyclobutanone product, a structure that maximizes the separation of the alkyl and ethoxy substiments in the TS. 

If you want to synthesize an alkene, you should choose the most hindered alkyl halide in order to maximize the elimination product and minimize the substitution product. For example, 2-bromopropane would be a better starting material than... 

What is required from a preparative viewpoint is a means of obtaining 100% elimination when an alkene is required, and 100% substitution when the substituted product is required. In practice this ideal situation can rarely be obtained, but there are guidelines that can be applied to maximize the yield of the desired product. A detailed discussion of the factors affecting the ratio of elimination to substitution is beyond the scope of this book. What we will do is discuss some of the more important points, and leave you with the set of rules that practising chemists apply when planning a reaction. 

The combined data in Tables 7-9 for the additions of styrene radical cations to their neutral precursors (dimerizations) and to other alkenes lead to a potentially important conclusion with respect to the design of cross-addition reactions. These data indicate that dimerization rate constants are frequently several orders of magnitude greater than the rate constants for cross addition. The absolute rate constants for the two reactions can be used to adjust the concentrations of the neutral styrene that leads to the radical cation and the alkene in order to maximize the yield of the cross-addition product. The kinetic and mechanistic data obtained for these reactions thus provides the basis for the development of synthetic strategies that utilize radical cation chemistry. 

In 2,9-diaryl-l,10-phenanthrolines (see Figures 7.10 and 7.20), bridgeheads and 1,10-phenanthroline unit are connected directly, leading to a rather stiff entity. The active centre, the copper ion bound to the nitrogen atoms of the 1,10-phenanthroline can only be reached from below through the bimacrocycle. In the cyclopropanation, the active species is a copper-carbenoid. When the alkene approaches it, the substituents of the alkene and the carboxylate of the carbenoid try to avoid sterical compression within the cavity and take up maximal distance to one another. Thus, the exo-product is formed with high selectivity. 

Hydroformylation is a metal-catalyzed reaction in which an olefin, CO, and H react to produce an aldehyde. The reaction was discovered at BASF by Otto Roelen, and was called hydroformylation by Adkins. This transformation is also sometimes referred to as the "oxo" process. In a formal sense, the elements of formaldehyde are added across a C=C bond. Common side reactions include aUcene hydrogenation, aldehyde hydrogenation, and alkene isomerization. Hydroformylation is one of the largest volume reactions conducted with homogeneous catalysts in the chemical industry. It is used to produce over 14 billion pounds of aldehydes per year (two pounds per year for every person on Earth ). The aldehydes are converted to alcohols, acids, and other materials as useful end products. One large-volume use of hydroformylation is the conversion of propene to a mixture of -butyraldehyde and /-butyraldehyde (Equation 17.2). Since the desired product is n-butyraldehyde, a great deal of effort has been expended to maximize the n i (noimal to iso, or often also called l/b for linear to branched) ratio of aldehydes and to understand the factors that control it. 

FIGURE 11.5 A schematic description of the Shell higher olefins process (SHOP). Keim s nickel catalyst gives 1-alkenes of various chain lengths. The subsequent steps allow the chain lengths to be manipulated to maximize the yield of C10-C14 products. Finally, SHOP alkenes are often hydroformylated, in which case the internal alkenes largely give the linear product, as discussed in Chapter 9. 

The mechanism of the Alder-Ene reaction is therefore considered to be duplicitous in that both concerted and stepwise processes may be operating in both the thermal and Lewis acid-catalyzed cases depending on the enophile and particular conditions. A concerted process should maximize allylic resonance by turning the axis of the breaking C-H bond parallel to the 7c-orbitals of the neighbouring alkene. The stepwise process may occur provided the optimum geometry of the TS is not accessible or if an intermediate radical, biradical or cation is formed and properly stabilized. The transition metal-catalyzed reaction is mechanistically different, while formally providing the Alder-Ene product. 

VI. ENEAMiNE-AssisTED ALKYLATION OF KETONES. As shown in Figure 9.13 (vide supra) the kinetic enolate of 2-methylcyclohexanone (formed at low temperature) is the enolate anion of the less substituted alkene and, reasonably, alkylation with, for example methyl iodide will yield 2,6-dimethylcyclohexanone. Regrettably, however, the yield of alkylated product from the kinetic enolate is difficult to maximize since even the smallest perturbation (e.g., rise in temperature and presence of excess ketone.) tends to drive the system toward equilibrium, resulting in the more highly substituted enolate anion. Enamines can be employed to overcome this tendency to produce alkylation on the more substituted side. 

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