Reactions of an Alkene or Alkyne

An alternate route to formation of alkyl monolayers is via Lewis acid catalyzed reactions of alkenes with the hydrogen terminated surface. In this approach, a catalyst such as ethyl aluminum dichloride is used to mediate the hydrosilylation reaction of an alkene (or alkyne), resulting in the same type of product as in the case of the photochemical or thermal reactions. This type of reaction is well known based on molecular organosilane chemistry and has also been used successfully to alkylate porous silicon [31]. Although this route has been shown to work on H/Si(lll), the resulting monolayers are found to have lower coverages than those achieved using the photochemical or thermal approach [29], Another concern with this approach is the possibility of trace metal residues from the catalyst that could adversely affect the electronic properties of these surfaces (even when present at levels below the detection limit of most common surface analysis techniques). 

The reaction of an alkene (or alkyne), CO, and H2O to directly produce a carboxylic acid is called Reppe carbony-lation chemistry or, more recently, hydrocarboxylation see Reppe Reaction). An excellent review of palladium-catalyzed Reppe carbonylation systems has been published recently by Kiss, and coverage of this important material will not be repeated here. This catalytic reaction has been known for quite some time, although the stoichiometric Ni(CO)4-based carbonylation of acetylene was the first commercial carbonylation process implemented (equation 13). The extreme toxicity of Ni(CO)4, however, has limited practical applications see Nickel Organometallic Chemistr. Co, Rh, and Pd catalysts have certainly replaced Ni(CO)4 in smaller-scale laboratory reactions, though for historical reasons a number of the fundamental mechanisms discussed in this section are based on Ni(CO)4. 

Diels-Alder reaction is the 1,4-addition of an alkene or alkyne (dienophile) across a conjugated diene. An example is the addition of pro-penal to buta-l,3-diene to give A -tetrahy-... 

In the biosynthetic schemes proposed for some halogenated natural products, positive halogen intermediates are attacked by electrons from the n bond of an alkene or alkyne in an addition reaction. 

One obvious synthetic route to isoxazoles and dihydroisoxazoles is by [3+2] cycloadditions of nitrile oxides with alkynes and alkenes, respectively. In the example elaborated by Giacomelli and coworkers shown in Scheme 6.206, nitroalkanes were converted in situ to nitrile oxides with 1.25 equivalents of the reagent 4-(4,6-di-methoxy[l,3,5]triazin-2-yl)-4-methylmorpholinium chloride (DMTMM) and 10 mol% of N,N-dimethylaminopyridine (DMAP) as catalyst [373], In the presence of an alkene or alkyne dipolarophile (5.0 equivalents), the generated nitrile oxide 1,3-dipoles undergo cycloaddition with the double or triple bond, respectively, thereby furnishing 4,5-dihydroisoxazoles or isoxazoles. For these reactions, open-vessel microwave conditions were chosen and full conversion with very high isolated yields of products was achieved within 3 min at 80 °C. The reactions could also be carried out utilizing a resin-bound alkyne [373]. For a related example, see [477]. 

Radical intermediates are also trapped by intramolecular reaction with an alkene or alkyne bond. At a mercury cathode this process competes with formation of the dialkylmercury [51], At a reticulated vitreous carbon cathode, this intramolecular cyclization of radicals generated by reduction of iodo compounds is an important process. Reduction of l-iododec-5-yne 5 at vitreous carbon gives the cyclopentane... 

In the course of reaction with an alkene or alkyne many reagents of type X—Y can be dissociated into radical pairs X- Y- as well as, or instead of, ionic species X+Y. In some cases the radical pair can add to the ir-bond(s) (see Chapter 4.1 of this volume). Furthermore, in some cases both ionic and radical behavior can be observed in the same reaction. For example, while the sulfonyl bromide bromomethanesulfo-nyl bromide, BrCFkSQzBr, adds to alkenes and alkynes by processes that most likely involve free radicals (equation 21),1S9 mechanisms are less clear cut in other related cases, (equation 22160 and Scheme 77).161... 

The following discussion deals not only with this reaction, but related reactions in which a transition metal complex achieves the addition of carbon monoxide to an alkene or alkyne to yield carboxylic acids and their derivatives. These reactions take place either by the insertion of an alkene (or alkyne) into a metal-hydride bond (equation 1) or into a metal-carboxylate bond (equation 2) as the initial key step. Subsequent steps include carbonyl insertion reactions, metal-acyl hydrogenolysis or solvolysis and metal-carbon bond protonolysis. 

The reactions shown in Scheme 1 require activation of the aromatic C-H bond by a metal and subsequent insertion of an alkene or alkyne in the aryl-carbon palladium bond (Chapter III.1.3.2.5). C-H activation has been the topic of many studies since the 1960s and several metal complex systems are known to induce... 

To the best of our knowledge, X-ray structural data of complexes with simple dihapto interactions between a lithium atom and the n system of an alkene or alkyne ligand are unknown, but there is some spectroscopic evidence for weak it interactions in solutions of 3-alkenyllithium compounds from 7Li-and H-NMR data (4). Interactions of this sort are presumably important in addition (polymerization) reactions between organolithium compounds and alkenes or alkynes. 

For thermally induced [2 + 2] cycloadditions, the concerted mechanism is operative only in particular cases, such as in the reactions between an alkene or alkyne and a ketene. The ketene can be generated directly in the reaction mixture from the appropriate acid chloride with triethylamine. The cycloaddition reaction is stereospecific and occurs exclusively in a cis fashion. Although the intermolecular cycloaddition with ketene itself proceeds in poor yields due to the propensity of the unsubstituted ketene to undergo dimerization, it is quite an efficient reaction with ketenes containing electron-withdrawing substituents. Usually, a-chloro ketenes are employed as reagents formed in situ from the corresponding a-chloro acid chlorides. Typical examples are represented in the preparation of cycloadducts such as 378 and 379 (Scheme 2.127). The latter cycloadduct, prepared in modest yield (ca. 20%),... 

Metal-mediated cyclizations that rely on the initial complexation of an alkene or alkyne around a low oxidation state metal center are often sensitive to the presence of additional substituents (particularly electron-donating substituents), and relatively more stringent reaction conditions are often required for successful cychzation. This effect was noted in the Ni-catalyzed formal [4 -I- 4] cycloaddition reactions developed by Wender and Tebbe and is apparent when one compares the reported facility of Pd-catalyzed linear dimerization of 1,3-butadiene versus that of substituted 1,3-dienes. Similarly, the initial attempts at Pd-catalyzed cyclization of bisdiene 70a (Scheme 22) were rather disappointing. Using 0.05 equiv of [Pd(OAc)2/3 PhjP] (THF, 65 °C, 12 h), only a small... 

The hydroamination reaction is the addition of an N-H bond across the G=G or C=C bonds of an alkene or alkyne. This is a highly atom-economical method of preparing substituted amines that are attractive targets for organic synthesis and the pharmaceutical industry. Different homogeneous catalysts have been utilized for hydroamination reactions. However, the heterogeneous catalysts for this kind of reactions have received less attention. 

In this section, only examples of Mizoroki-Heck reactions where a proper addition of the cr -aryl- or a -alkeny Ipalladium(II) complex to a double bond of an alkene or alkyne occurs are considered. As a consequence, an often-met deviation from the classic Mizoroki-Heck mechanism, the so-called cyclopalladation, will not be treated in further detail [12, 18]. However, as it is of some importance, especially in heterocycle formation and mainly because it will be encountered later during polycyclization cases, it shall be mentioned briefly below. Palladacycles are assumed to be intermediates in intramolecular Mizoroki-Heck reactions when j3-elimination of the formed intermediate cannot occur. These are frequently postulated as intermediates during intramolecular aryl-aryl Mizoroki-Heck reactions under dehydrohalogenation (Scheme 6.1). The reactivity of these palladacycles is strongly correlated to their size. Six-membered and larger palladacycles quickly undergo reductive elimination, whereas the five-membered species can, for example, lead to Mizoroki-Heck-type domino or cascade processes [18,19]. 

The biosynthesis of certain halogenated marine natural products is intriguing. Some of their halogens appear to have been introduced as electrophiles rather than as Lewis bases or nucleophiles, which is their character when they are solutes in seawater. But how do marine organisms transform nucleophilic halide anions into electrophilic species for incorporation into their metabolites It happens that many marine organisms have enzymes called haloperoxidases that convert nucleophilic iodide, bromide, or chloride anions into electrophilic species that react like F, Br, or CF. In the biosynthetic schemes proposed for some halogenated natural products, positive halogen intermediates are attacked by electrons from the 77 bond of an alkene or alkyne in an addition reaction. 

Oxidative cleavage (Sections 8.16 and 8.19) A reaction in which the carbon-carbon double bond of an alkene or alkyne is both cleaved and oxidized, yielding compounds with carbon-oxygen double bonds. 

In academic labs, hydrogenation experiments are usually done on small quantities of reagents, usually less than 1 gram of an alkene or alkyne. In these situations, it is not necessary to worry about the inherent exothermicity of the reaction. However, on larger scale reactions, it is wise to consider how much heat will be liberated and if there are circumstances that might result in a reaction that could go out of control. Some of the catalysts used in hydrogenations are air-sensitive and should be added to a flask first under a blanket of nitrogen gas, before any flammable solvent is added. It is best to... 

Simultaneous formations of C C and C—N bonds at both terminals of an alkene or alkyne are called carboamination reactions. Most cases of these types of reactions take place in the presence of a metal catalyst, especially a transition metal catalyst. 

The types of cycloadditions discovered for enamines range through a regular sequence starting with divalent addition to form a cyclopropane ring, followed by 1,2 addition (i) of an alkene or an alkyne to form a cyclo-cyclobutane or a cyclobutene, then 1,3-dipolar addition with the enamine the dipolarophile 4), and finally a Diels-Alder type of reaction (5) with the enamine the dienophile. 

The 2n component 2, the so-called dipolarophile (analogously to the dieno-phile of the Diels-Alder reaction) can be an alkene or alkyne or a heteroatom derivative thereof. Generally those substrates will be reactive as dipolarophiles, that also are good dienophiles. 

Non-heteroatom-stabilised Fischer carbene complexes also react with alkenes to give mixtures of olefin metathesis products and cyclopropane derivatives which are frequently the minor reaction products [19]. Furthermore, non-heteroatom-stabilised vinylcarbene complexes, generated in situ by reaction of an alkoxy- or aminocarbene complex with an alkyne, are able to react with different types of alkenes in an intramolecular or intermolecular process to produce bicyclic compounds containing a cyclopropane ring [20]. 

A simple test to distinguish between 2-pentene and cyclopentane is to add a few drops of a red Br2 solution to the unknown liquid. The reddish color will disappear if the liquid is an alkene or alkyne, e.g., 2-pentene, due to the addition of Br2 to the multiple bond. No such addition reaction occurs between Br2 and cyclopentane. 

The reactive (TPP)Rh in Scheme II is electrogenerated from l(TPP)Rh(L)d ci in the presence of an alkene or an alkyne. The formation of an intermediate is observed. This intermediate is not detected by chemical reaction methods and was tentatively assigned as a it complex(15). Similar it complexes have been reported for ruthenium porphyrin species(17 18). 

Several routes to the pyrrolo[l,2-f][l,2,3]triazole skeleton have been described. Intramolecular dipolar cycloaddition of azido-alkenes or alkynes seems to be the most convenient process, although the cyclization efficiency seems to be highly substrate dependent (Scheme 16) <2002JA2134, 2003T1477, 2005SL2187, 2005TL8639>. The formation of this bicyclic system by an intramolecular Heck reaction is an attractive alternative. The recent syntheses of sulfamides by intramolecular cyclization of alkenes or allenes offer a complementary route to the classical... 

Carbo-oxylation (carbohydroxylation and carboalkoxylation), whereby an organic group and an oxy (hydroxyl or alkoxy) group add across a carbon-carbon double bond or a triple bond, is an important transformation in organic synthesis. We found that the reaction of a cation pool with an alkene or alkyne followed by the trapping of the resulting carbocation by water led to the... 

Addition reactions form the basis for tests that distinguish alkenes and alkynes from alkanes. Bromine, Br2, has a deep reddish-brown colour. When bromine is added to an alkene or alkyne, an addition reaction takes place. As the bromine is used up, the brown colour of the bromine disappears. Since alkanes cannot undergo addition reactions, no reaction takes place when bromine is added to an alkane. 

Metallacyclobutanes or other four-membered metallacycles can serve as precursors of certain types of carbene complex. [2 + 2] Cycloreversion can be induced thermally, chemically, or photochemically [49,591-595]. The most important application of this process is carbene-complex-catalyzed olefin metathesis. This reaction consists in reversible [2 + 2] cycloadditions of an alkene or an alkyne to a carbene complex, forming an intermediate metallacyclobutane. This process is discussed more thoroughly in Section 3.2.5. 

---