Activated alkene or alkyne

Cycloaddition reactions between phosphaalkenes or -alkynes and activated alkenes or alkynes are also an available synthetic route to the four-membered rings. These reactions afforded not only phosphorus-containing rings, such as phosphetanes, phosphetenes, and phosphetes, but also arsine-containing rings. 

The carbonyl ylides are trapped in a [2-1-3]-cycloaddition with activated alkenes or alkynes. For example, the reaction of the a-chloro ether 3.56 with cesium fluoride (CsF) in the presence of dimethyl fumarate gives only the trans cycloaddition product 3.57, while the reaction with dimethyl maleate gives exclusively the cis-isomer 3.58 (Scheme 3.24). 

DIELS - ALDER Cyclohexene synthesis 4+2 Thermal cydoaddition between a diene and an activated alkene or alkyne, sometimes catalyzed by Lewis acids. 

Harvey, R.G., Reactions of triethyl phosphite with activated olefins, Tetrahedron, 22, 2561, 1966. Pande, K.C., Hydrophosphinylation of activated alkenes or alkynes, Dow Chemical, U.S. Patent Appl. US 3622654, 1971 Chem. Abstr., 16, 46303, 1972. 

Pyran 2-ones undergo Diels-Alder reactions with activated alkenes or alkynes. For instance, from 27/-pyran-2-one and maleic anhydride, the e Jo-adduct 15 is formed which undergoes thermal decarboxylation to the 1,2-dihydrophthalic anhydride 16 ... 

Pyridones can act as diene components in [4+2] cycloadditions. The Diels-Alder reaction with activated alkenes or alkynes lead to derivatives of 2-azabicyclo[2.2.2]octa-5,7-dien-3-one 12 by the addition of acetylenedicarboxylate to l,4,6-trimethyl-2-pyridone 11, as shown below [84] ... 

Competitive or preferential nucleophilic (trans) addition is specially important for late transition metals since, in general, they are good a-acceptors and efficiently activate alkenes or alkynes upon coordination. Coordinated X groups that have lone pairs (X = amido, alkoxide, etc.) may give a 1,2-cis addition product by nucleophilic intramolecular attack, depending on the availability of this lone... 

In recent years, much attention has been paid on the Baylis-Hillman reaction for construction of C-C bond between an aldehyde and an activated alkene or alkyne [248]. The major drawbacks of the Baylis-Hillman reaction are its slow reaction rate and limited range of substrates. To overcome these shortcomings, many efforts have been made, such as using Lewis acids or various other additives to activate carbonyl electrophiles [249]. The use of TiCU to mediate Baylis-Hillman reaction represents a commonly applied approach to enhance the reactivity. This part of work has been well documented in a recent comprehensive review [248d]. 

The behavior of strained,/Zuorimiret/ methylenecyelopropanes depends upon the position and level of fluorination [34], l-(Difluoromethylene)cyclopropane is much like tetrafluoroethylene in its preference for [2+2] cycloaddition (equation 37), but Its 2,2-difluoro isomer favors [4+2] cycloadditions (equation 38). Perfluoromethylenecyclopropane is an exceptionally reactive dienophile but does not undergo [2+2] cycloadditions, possibly because of stenc reasons [34, 45] Cycloadditions involving most possible combinations of simple fluoroalkenes and alkenes or alkynes have been tried [85], but kinetic activation enthalpies (A/f j for only the dimerizations of tetrafluoroethylene (22 6-23 5 kcal/mol), chlorotri-fluoroethylene (23 6 kcal/mol), and perfluoropropene (31.6 kcal/mol) and the cycloaddition between chlorotnfluoroethylene and perfluoropropene (25.5 kcal/mol) have been determined accurately [97, 98] Some cycloadditions involving more functionalized alkenes are listed in Table 5 [99. 100, 101, 102, 103]... 

Catalytic hydration and alcoholation of unsaturated compounds such as alkenes or alkynes would be a high value-adding step in the synthesis of compounds of complicated structure as well as in the large-scale production of industrially useful simple compounds. The activation of the O-H bond of water, alcohol, or carboxylic acid by transihon metals is relevant to a variety of such catalytic processes. 

In combination with the use of tetrasulfur tetranitride, trithiazyl trichloride, or any equivalent source of N-S-N , the technique of functionalizing a two-carbon source such as active methylene, alkene, or alkyne into thiadiazole (see Section 5.09.9.1.4) followed by reduction (see Section 5.09.5.6) provides a rapid route to 1,2-diamines. 

Cationic zirconocene species efficiently activate alkenes toward carbon—carbon bond formation via carbometalation, as has been demonstrated in studies of alkene polymerization. Today, some zirconocene catalysts are available that allow single additions of metal-alkyls (mainly aluminum-alkyls) to alkenes or alkynes, thereby forming stable alkyl- or alkenyl-metals that do not undergo any further oligomerization. On the other hand, carbozirconation with Cp2ZrRCl in the presence of stoichiometric or catalytic amounts of activators has also been realized. 

The reaction of (TPP)Rh with terminal alkenes or alkynes is of special interest due to the cleavage of the carbon-carbon bond adjacent to either the alkene or the alkyne functionality and results in the ultimate formation of (TPP)Rh(R). This overall reaction implies activation of a relatively inert carbon-carbon bond, especially for the case of the terminal alkene. However, the ultimate formation of (P)Rh(R) is not surprising if one considers the relative stability of the rhodium carbon bond in this species(17). 

A development of the last two decades is the use of Wacker activation for intramolecular attack of nucleophiles to alkenes in the synthesis of organic molecules [9], In most examples, the nucleophilic attack is intramolecular, as the rates of intermolecular reactions are very low. The reaction has been applied in a large variety of organic syntheses and is usually referred to as Wacker (type) activation of alkene (or alkynes). If oxygen is the nucleophile, it is called oxypalladation [10], Figure 15.4 shows an example. During these reactions the palladium catalyst is often also a good isomerisation catalyst, which leads to the formation of several isomers. 

The amide [Sm(45a) N(SiMe3)2 ] was a catalyst for an aUene-based hydroamination/ cyclisation. As an illustration, one such product upon hydrogenation yielded a naturally occurring alkaloid. Scheme 4.8. " " The same samarium(lll) amide was also active for the intramolecular hydrophosphination/cyclisation of phosphino-alkenes or -alkynes e.g., H2P(CH2)3C=CPh was transformed into 76. " ... 

Ito and co-workers have also used the Pd(OAc)2/t-alkyl isocyanide catalyst to affect the double silylation of carbon-carbon multiple bonds in an intramolecular system to yield silacarbocycles.59 Alkenes or alkynes that are tethered to a disilanyl group through a carbon chain, an ether linkage, or an amine functionality undergo intramolecular addition of the disilane moiety to the multiple bond. Activation of the disilane by the presence of electron-withdrawing groups on silicon is not necessary for the reaction to... 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

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