47 Alkenes
de Meijere, A., Science of Synthesis, (2010) 47, 1.
Alkenes are endowed with a C=C bond, and this constitutes the simplest, yet one of the most versatile functional groups in organic molecules. In terms of worldwide annual production by the petrochemical industry, the simple alkenes, namely ethene, propene, and the isomeric butenes, play the dominant role, and are building blocks for a vast number of chemical intermediates and final consumer products. In this volume of Science of Synthesis (consisting of two subvolumes, 47a and 47b), the various methods for the preparation of alkenes are discussed and evaluated. The focus is on purely hydrocarbon alkenes and cycloalkenes without any functional groups directly attached to the C=C bond; such functionalized compounds constitute other product classes that are covered in other volumes of Science of Synthesis, according to the organizational system employed in the series. However, some of the established methods that need to be covered here, at least briefly for systematic reasons, have mostly or even solely been used to prepare such functionally substituted examples, while alkenes with remote functional groups are also included here, with cross-references to other volumes of Science of Synthesis whenever necessary. Previously published reviews, book chapters, and books on any of the presented methods are referred to wherever applicable.
There is a stunningly great variety of methods to access alkenes from appropriately functionalized alkanes (see Table 1 for a schematic listing). The first sections of this volume are devoted to the various carbonyl alkenation (olefination) reactions such as the Wittig reaction and related phosphorus-based alkenations (Section 47.1.1.1), the Peterson alkenation (Section 47.1.1.2), and the Julia and Julia–Kocienski alkenations as well as further related sulfur-based alkenations (Section 47.1.1.3), which all have some mechanistic similarities. The more recently developed alkenations of carbonyl compounds with metal carbenes such as the Tebbe and Petasis reagents, as well as with gem-dimetallic species, are also in the same group of transformations (Section 47.1.1.4), and so is the so-called McMurry coupling of (preferably) two identical carbonyl compounds (Section 47.1.1.5). All of these methods have found a wide range of applications in the synthetic laboratory, and some have even been employed in industrial scale production. Alkene metathesis (Section 47.1.1.6), on the other hand, had been used in the petrochemical industry for the conversion of simple alkenes into higher alkenes long before this methodology became applicable to more complex organic molecules with the advent of new classes of catalysts with wide functional-group tolerances. This development was initiated mainly by Grubbs and by Schrock and their coworkers, for which these two scientists shared the Nobel Prize in 2005. Other groups have joined in and made important contributions from the 1990s onwards.
The various transition-metal-catalyzed cross couplings (Section 47.1.2) including the Mizoroki–Heck reaction, the SN′ allylations of organometallic compounds, and π-allyl substitutions, all of which were discovered in the 1960s and 1970s, some initially as stoichiometric reactions, now find a wide range of applications in research laboratories, and some have made their way into the production processes for fine chemicals in industry. The metal-catalyzed oligomerization of alkenes to higher alkenes, discussed in Section 47.1.2.4, has its importance only on the industrial scale.
Table 1 Schematic View of the Synthetic Routes to Alkenes Including Cycloalkenes, and the Corresponding Section in Volume 47
| Method | Representative Reaction(s)a | Section |
|---|---|---|
| Wittig and related phosphorus-based alkenations |  |
47.1.1.1 |
| Peterson alkenation |  |
47.1.1.2 |
| Julia, Julia–Kocienski, and related sulfur-based alkenations |  |
47.1.1.3 |
| alkenation with metal carbenes and related reactions |  |
47.1.1.4 |
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| McMurry coupling and related reductive dimerization reactions |  |
47.1.1.5 |
| alkene metathesis |  |
47.1.1.6 |
| cross coupling and Heck reactions |  |
47.1.2.1 |
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| SN′ allylation of organometallic compounds |  |
47.1.2.2 |
| π-allyl substitution |  |
47.1.2.3 |
| oligomerization of alkenes to higher alkenes |  |
47.1.2.4 |
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| Diels–Alder reactions ([4 + 2] cycloadditions) |  |
47.1.3.1 |
| ene reactions |  |
47.1.3.2 |
| 4π-electrocyclic reactions |  |
47.1.3.3.1 |
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| Cope rearrangement |  |
47.1.3.3.3 |
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| oxidative decarboxylation and decarbonylative elimination |  |
47.1.4.1 |
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| oxidative decarboxylation of dicarboxylic acid derivatives |  |
47.1.4.2 |
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| base- and otherwise catalyzed elimination from alkyl halides, methanesulfonates, toluenesulfonates, ethers, sulfides, ammonium salts, and sulfonium salts |  |
47.1.4.3 |
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| acid-catalyzed dehydration of alcohols |  |
47.1.4.4 |
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| pyrolytic elimination from esters, xanthates, phosphates, thiophosphates, sulfamates, amine N-oxides, ammonium hydroxides, phosphonium salts, and others |  |
47.1.4.5 |
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| reductive elimination from β-halohydrins and their esters or ethers |  |
47.1.4.6 |
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| reductive elimination from gem-dihalides |  |
47.1.4.7 |
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| reductive extrusion from three- to six-membered heterocycles |  |
47.1.4.8 |
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| reactions of (arylsulfonyl)hydrazones with strong bases (Bamford–Stevens and Shapiro reactions) |  |
47.1.4.9 |
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| dehydrogenation of CH2—CH2 fragments |  |
47.1.4.10 |
| [2 + 2]-cycloaddition reactions |  |
47.1.5.1 |
| hydrogenation reactions (catalytic hydrogenation and chemical reduction) |  |
47.1.5.2 |
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| elementometalation (including hydrometalation) and subsequent cross-coupling reactions |  |
47.1.5.3 |
| carbometalation and subsequent cross-coupling reactions |  |
47.1.5.4 |
| dissolving-metal (Birch-type) reduction of arenes |  |
47.1.6.1 |
| catalytic hydrogenation and chemical reduction of allenes |  |
47.1.6.2 |
| catalytic hydrogenation and chemical reduction (e.g., dissolving-metal reduction, hydrocarbonation by organometallic reagents, diimide reduction) of 1,3- and higher dienes |  |
47.1.6.3, 47.1.6.4 |
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| isomerization of alkenes |  |
47.1.7 |
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| synthesis from other alkenes without isomerization (electrophilic and nucleophilic substitution) |  |
47.1.8 |
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| syntheses of cyclopropenes |  |
47.2.1 |
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| syntheses of nonconjugated di-, tri-, and oligoenes |  |
47.3.1 |
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a Possible stereoisomers and/or regioisomers in products are not shown.
The classical Diels–Alder reaction, including its modern catalyzed versions, along with the so-called ene reactions and electrocyclic reactions make up Sections 47.1.3.1–47.1.3.3. Among these, the Diels–Alder reaction has by far the widest application, as it is the simplest and most atom-economical way to prepare cyclohexene derivatives of any sort. Ene reactions have only a rather limited range of applicability, but the modern “metallo-ene” reactions {see, for example, Science of Synthesis, Vol. 36 [Alcohols (Section 36.2.3.1.4.536.2.3.1.4.5)]} are among the most versatile methods for the elegant construction of cyclic and oligocyclic skeletons.
Of course, elimination reactions constitute the largest arsenal of methods for the preparation of alkenes (Section 47.1.4). They range from eliminations of carbonyl or carboxy groups, the latter either without or along with the elimination of a second carboxy or a hydroxy group, via base- or acid-catalyzed or pyrolytic eliminations of HX (in which X can be any leaving group), to the reductive elimination of two vicinal or geminal leaving groups. Such eliminations can proceed regioselectively and some of them are even stereoselective. In particular, the reductive extrusions of oxygen, sulfur, and sulfur dioxide from oxiranes, thiiranes, and thiirane 1,1-dioxides (including in situ formed thiirane 1,1-dioxides in the so-called Ramberg–Bäcklund reaction), respectively, occur stereoselectively. This also holds for the reductive extrusion of sulfur and carbon dioxide or sulfur and carbon disulfide from dioxolane- and dithiolane-2-thiones, respectively. The mechanistically interesting Bamford–Stevens and Shapiro reactions, which occur upon treatment of ketone (arylsulfonyl)hydrazones with 1 and 2 equivalents, respectively, of an organometallic reagent (usually butyl- or methyllithium), complement the vast range of elimination reactions. The latter have most frequently been employed for the preparation of cyclic and oligocyclic alkenes.
Alkynes in general are more precious than alkenes, and they are frequently prepared from the latter. However, they also serve as valuable starting materials for alkenes by various addition reactions (Section 47.1.5). While [2+2]-cycloaddition reactions to furnish cyclobutenes have a rather limited range of applications, reductions by stereospecific catalytic hydrogenation and by chemical reduction are frequently employed to transform oligofunctional alkynes, which are more easily assembled than the correspondingly substituted alkenes, into the latter target compounds. Modern transition-metal-catalyzed elementometalations [including hydrometalation (Section 47.1.5.3) and carbometalation (Section 47.1.5.4)] especially, with or without subsequent cross coupling, have gained enormous importance since the 1980s for the access to a wide range of functionalized and nonfunctionalized alkenes, as well as conjugated and nonconjugated di-, tri-, and oligoenes.
Partial reductive removal of double bonds from cumulated and conjugated dienes, trienes, and arenes can also serve as an access to simpler alkenes and cycloalkenes (Section 47.1.6). Thus, the so-called Birch reduction, i.e. the treatment of arenes with lithium metal in liquid ammonia (or the Benkeser version using lithium in a primary amine), provides convenient access to substituted cyclohexa-1,4-dienes and cyclohexenes. Catalytic hydrogenation as well as Birch-type reduction of allenes (1,2-dienes) occurs selectively at the least-substituted double bond, and 1,3-dienes formally undergo 1,4-addition of hydrogen under Birch reduction conditions; this can be performed in the presence of an additional nonconjugated double bond in the same molecule.
Isomerizations of alkenes (Section 47.1.7) are only of industrial importance, with the exception of the isomerization of vinylcyclopropanes to cyclopentenes, which has gained ever increasing attention since its discovery around 1960 as a method to access five-membered carbocycles. Certain alkenes are most easily accessible by electrophilic or nucleophilic substitutions of appropriately activated (i.e., usually functionally substituted in the case of nucleophilic substitutions) alkenes without isomerizations (Section 47.1.8).
Methods for the syntheses of cyclopropenes (Section 47.2) and of nonconjugated di-, tri-, and oligoenes (Section 47.3) are discussed in two separate sections, although most of the employed methods are basically the same as those discussed in various preceding sections.
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