Synthesis of alkenes

Saytzeff rule states that the major product of elimination will be the most substituted alkene. 

Notice that in elimination, the base abstracts a hydrogen. This is a different behavior than that of a nucleophile in a substitution reaction. In a substitution reaction, the nucleophile attacks the carbon- 

Hydrogenation is an example of an addition reaction, In order for hydrogenation to occur at nr appreciable rate, a firterogt m ous catalyst is employed. A heterogeneous catalyst is a catalyst that exists in a different phase (i.e. gas, liquid, solid, aqueous, etc,) than the reactants or products. Normally tiny shavings of metal act as the catalyst to promote syn-addition (same side addition). 

Hydrogenation is an exothermic reaction with a high energy of activation. Heats of hydrogenation can be used to measure relative stabilities of alkenes. The lower the heat of hydrogenation, the more stable the alkene. 

We can synthesize alkenes by processes such as dehydrogenation of alkanes, dehydrohalogenation of alkyl halides, and dehydration of alcohols. 

When monohydric alcohols undergo dehydration, isomeric alkenes can be formed by the loss of water by f (or 1,2-) elimination. Selective synthesis of certain alkenes can, however, be accomplished-when primary alcohols are treated with appropriate solid catalysts terminal alkenes are formed. In contrast, either 1- or 2-alkenes can be produced by dehydration of secondary 2-alkanols. The reactivity of alcohols follows the sequence tertiary secondary primary. 

Phosphates are also active dehydration catalysts [6,8,9]. Although a mixed (El and E2) mechanism has been reported [24], most of these materials and particularly BPO4 have typical El behavior [25-27]. The involvement of the carbocatio-nic intermediate results in a complex mixture of isomeric alkenes [27,28] and intramolecular and intermolecular dehydration (alkene and ether formation, respectively) are often parallel processes. For example, aluminum phosphates with P/Al 1 yield a mixture of alkenes and ethers whereas those with P/Al 1 give alkenes selectively [6,29]. Dehydration activity usually correlated with surface acidity [29-31]. The strongly acidic sites of AIPO4 were found to promote alkene formation [26]. 

Numerous studies, including mechanistic and kinetic investigations mostly with simple alcohols, have been performed with molecular sieves as dehydration catalysts [8,32-34]. Although highly active these are rarely used for converting alcohols with complicated structures to alkenes. The reason is that these catalysts are not selective-a prevalent El mechanism, i. e. the involvement of carbocationic intermediates, and parallel inter- and intramolecular processes result in the formation of isomeric alkenes and ethers. Alcohols with specific structure, however, can be selectively transformed to alkenes. For example, 1-phenyl-1-ethanol is transformed to styrene in 95 % yield over HZSM-5 zeolite at 493 K [34]. Ether formation, however, was shown to be significant when a-(p-tolyl)ethanol was reacted over zeolite HY [35]. A low concentration of the reactant alcohol inside the zeolite is required to prevent such dimerization-type reaction a suitable competing solvent should be selected. 

Zeolites are also excellent catalysts in the selective dehydration of tertiary alcohols. Shape selectivity, i. e. selective dehydration of 1-butanol to 1-butene over CaA zeolite in the presence of 2-butanol has also been reported [36]. The bulky 2-methylcyclohexanol, in turn, undergoes dehydration over zeolite Y, although resulting in a mixture of alkenes [37]. 

Acidic clays are widely applied in the dehydration of alcohols [38]. Although similar to zeolites in their capacity to induce the formation of both alkenes and ethers, selective alkene synthesis is possible. Various layered materials (clays, ion-exchanged montmorillonite, pillared layered clays) are very active and, in general, selective in transforming primary, secondary, and tertiary aliphatic alcohols to 1-alkenes [39-43]. Al -exchanged montmorillonite, however, induces ether formation from primary alcohols and 2-propanol [41]. Substituted 1-phenyl-1-ethanols yield the corresponding styrene derivatives at high temperature (653-673 K) [44]. 

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Wittig reaction (Section 17 12) Method for the synthesis of alkenes by the reaction of an aldehyde or a ketone with a phosphorus yhde... 

Most important for the synthetic chemist is an index to the synthesis of functional groups, e.g. synthesis of alkenes from ketones, as well as conversion of ketones to alkenes. 

Except for terpene chemistry, the Wagner-Meerwein rearrangement is of limited synthetic importance. It is rather found as an undesired side-reaction with other reactions, for example in the synthesis of alkenes by elimination reactions. 

We had a brief introduction to radical reactions in Section 5.3 and said at that time that radicals can add to alkene double bonds, taking one electron from the double bond and leaving one behind to yield a new radical. Let s now look at the process in more detail, focusing on the industrial synthesis of alkene polymers. 

As described in Section 2.3.2, vinylaziridines are versatile intermediates for the stereoselective synthesis of (E)-alkene dipeptide isosteres. One of the simplest methods for the synthesis of alkene isosteres such as 242 and 243 via aziridine derivatives of type 240 and 241 (Scheme 2.59) involves the use of chiral anti- and syn-amino alcohols 238 and 239, synthesizable in turn from various chiral amino aldehydes 237. However, when a chiral N-protected amino aldehyde derived from a natural ot-amino acid is treated with an organometallic reagent such as vinylmag-nesium bromide, a mixture of anti- and syn-amino alcohols 238 and 239 is always obtained. Highly stereoselective syntheses of either anti- or syn-amino alcohols 238 or 239, and hence 2,3-trans- or 2,3-as-3-alkyl-2-vinylaziridines 240 or 241, from readily available amino aldehydes 237 had thus hitherto been difficult. Ibuka and coworkers overcame this difficulty by developing an extremely useful epimerization of vinylaziridines. Palladium(0)-catalyzed reactions of 2,3-trons-2-vinylaziri-dines 240 afforded the thermodynamically more stable 2,3-cis isomers 241 predominantly over 240 (241 240 >94 6) through 7i-allylpalladium intermediates, in accordance with ab initio calculations [29]. This epimerization allowed a highly stereoselective synthesis of (E) -alkene dipeptide isosteres 243 with the desired L,L-... 

Scheme 2.60 Epimerization of y-aziridinyl-a,(i-enoates and synthesis of alkene isosteres.

This method provides a convenient synthesis of alkenes with the double bond in a relatively unstable position. Thus reduction of the p-toluenesulfonylhydrazones of a,(3-unsaturated aryl ketones and conjugated dienones gives rise to nonconjugated olefins. Unsaturated ketones with endocyclic double bonds produce olefins with double bonds in the exocyclic position. The reduction of p-toluenesulfonylhydrazones of conjugated alkynones furnishes a simple synthesis of 1,3-disubstituted allenes. ... 

Suggest a synthesis of alkene (1>, needed for a morphine synthesis. ... 

Trialkylsilyl groups have a modest stabilizing effect on adjacent carbanions (see Part A, Section 3.4.2). Reaction of the carbanions with carbonyl compounds gives (3-hydroxyalkylsilanes. (3-Hydroxyalkylsilanes are converted to alkenes by either acid or base.270 These eliminations provide the basis for a synthesis of alkenes. The reaction is sometimes called the Peterson reaction.211 For example, the Grignard reagent derived from chloromethyltrimethylsilane adds to an aldehyde or ketone and the intermediate can be converted to a terminal alkene by acid or base.272... 

A number of procedures for stereoselective synthesis of alkenes involving alkenylboranes have been developed. For each of the reactions given below, show the structure of the intermediates and outline the mechanism in sufficient detail to account for the observed stereoselectivity. 

This is an extremely useful reaction for the synthesis of alkenes. It involves the addition of a phosphonium ylid, e.g. (136), also known as a phosphorane, to the carbonyl group of an aldehyde or ketone the ylid is indeed a carbanion having an adjacent hetero atom. Such species are generated by the reaction of an alkyl halide, RR CHX (137), on a trialkyl- or triaryl-phosphine (138)—very often Ph3P—to yield a phosphonium salt (139), followed by abstraction of a proton from it by a very strong base, e.g. PhLi ... 

Crozet and co-workers have used S l reactions for synthesis of new heterocycles, which are expected to be biologically active (see also Section 7.3, which discusses synthesis of alkenes). For example, 2-chloromethyl-5-nitroimidazole reacts with the anion of 2-nitropropane to give 2-isopropylidene-5-nitroimidazole. It is formed via C-alkylation of the nitronate ion followed by elimination of HN02 (Eq. 5.33).51a Other derivatives of nitroimidazoles are also good substrates for SrnI reactions.5113 0... 

A. Sen (ed.) Catalytic Synthesis of Alkene-Carbon Monoxide Copolymers and... 

Scheme 1.13. Synthesis of alkene- and alkyne-ZrCp2 complexes by P-H abstraction of dialkylzirconocenes in the presence of 7t-compounds (Negishi-Takahashi protocol).

During August, the host queen typically abandons her nest, but the parasite stays on. By the end of August, a new generation of parasites emerges and the parasitic queen has no further need for her disguise. Her alkenes reappear, and with them she resumes her own unique odor. How she turns the synthesis of alkenes off and on is still a mystery. 

Consiglio G, Milani B (2003) In Sen A (ed) Catalytic Synthesis of Alkene-Carbon Monoxide Copolymers and Cooligomers, chap. 6. Kluwer Academic, Dordrecht, p 189... 

A synthesis of alkenes from 1,2-diols depends upon the initial formation of the 1,3,2-dioxaphospholans (128), followed by rupture of the heterocyclic ring and removal of phosphorus with Li-NH3 or Na-Ci 0H 8. The reaction is reasonably stereo-... 

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