Olefins and Alkynes

Solutions of alkali metals in liquid ammonia have been developed as versatile reducing agents which effect reactions with organic compounds that are otherwise difficult or impossible/ Aromatic systems are reduced smoothly to cyclic mono- or di-olefins and alkynes are reduced stereospecifically to frani-alkenes (in contrast to Pd/H2 which gives cA-alkenes). 

The most widely employed electrophilic phosphinidene complexes are generated from M(C0)5 (M=W, Mo, Cr) complexed 7-phosphanorbornadienes 7, which were reported by Marinetti et al. in 1982 [46]. They trapped the transient phosphinidene complex 8 at 110 °C (or at 55 °C if CuCl was used as catalyst) with olefins and alkynes to obtain three-membered phosphiranes and phosphirenes, respectively. 

So far we have considered olefins and alkynes as terminating agents. Another reaction, however, was available to effect C-C coupling in the last step, the Suzuki reaction. Results were satisfactory under the conditions shown in the Table (28). 

Tetramethyl- or tetraphenyl- (cyclobutadiene)nickel dihalides undergo reductive ligand substitution with nitrogen donor ligands such as 2,2 -bipyridine or 1,4-diaza-1,3-dienes with the addition of sodium metal237. The 2,2/-bipyridyl ligand is readily displaced and reaction of this complex with a variety of olefins and alkynes leads to cycloaddition reactions with the cyclobutadiene ligand. 

Particularly interesting is the reaction of enynes with catalytic amounts of carbene complexes (Figure 3.50). If the chain-length between olefin and alkyne enables the formation of a five-membered or larger ring, then RCM can lead to the formation of vinyl-substituted cycloalkenes [866] or heterocycles. Examples of such reactions are given in Tables 3.18-3.20. It should, though, be taken into account that this reaction can also proceed by non-carbene-mediated pathways. Also Fischer-type carbene complexes and other complexes [867] can catalyze enyne cyclizations [267]. Trost [868] proposed that palladium-catalyzed enyne cyclizations proceed via metallacyclopentenes, which upon reductive elimination yield an intermediate cyclobutene. Also a Lewis acid-catalyzed, intramolecular [2 + 2] cycloaddition of, e.g., acceptor-substituted alkynes to an alkene to yield a cyclobutene can be considered as a possible mechanism of enyne cyclization. 

Rhodium( )-Catalyzed Asymmetric Hydroacylation of Olefins and Alkynes with Aldehydes I 85... 

Subsequently, direct incorporation of GO by titanocene(ii) catalyst, Gp2Ti(GO)2, under a GO atmosphere was reported.This catalytic system showed substantially higher TON and broader functional group compatibility. However, this catalyst fails to react with sterically hindered olefins and alkynes. In a recent contribution from the same group, a series of aryloxide titanium complexes 22 (figure 4) are prepared and shown to promote PKR with some sterically hindered enynes." ... 

Olefins and alkynes can also add to each other to give cyclic products in other ways (see 5-49 and 5-51). 

Allenes926 and ketenes927 react with activated olefins and alkynes. Ketenes give 1,2 addition, even with conjugated dienes.928 Ketenes also add to unactivated olefins if sufficiently long reaction times are used.929 Allenes and ketenes also add to each other.930... 

Reduction of olefins and alkynes 5-10 Reduction of aromatic rings... 

A tremendous amount of work concerning metal-induced cycloadditions of methylenecyclopropane with olefins and alkynes has been done in recent years since the first reported nickel(0) catalyzed 3+2 cycloaddition of methylenecyclopropanes with electron-poor olefins (equation 352)415 and the analogous palladium(O) codimerization (equation... 

Similar to the reforming of methane, the major product of aliphatic hydrocarbons reforming is syngas, although some dehydrogenated products such as olefins and alkynes may also be formed. Futamura et al. [49-51] reported the C02 reforming... 

It was therefore a significant breakthrough when procedures involving electron-transfer initiation began to appear in the 1960s, and today reductive initiation constitutes the most commonly used method of accomplishing the addition of Rpl to olefins and alkynes [228]. Perhaps the first example of such a process was that of Kehoe and Burton in 1966 [229,230]. 

Controlled electrolytic methodology, as discussed earlier, has also been used effectively for inducing perfluoroalkylation of olefins and alkynes [184,265]. 

Although the synthetic usefulness of such reactions has not yet been widely recognized, it should be possible to carry out hydroperfluoroalkylation of olefins and alkynes very efficiently by the strategic use of an appropriate homolytic hydrogen-transfer agent. The rates of hydrogen transfer for many such agents... 

Scheme 33 Hydrogenation of olefins and alkynes mediated by CP2T1CI/H2O...

Steric factors probably prohibit simultaneous rotation of the olefin and alkyne C2 units which would crowd all four metal-bound carbons into the same plane. Separate rotation of each unsaturated ligand was explored theoretically using the EHMO method. Rotation of the olefin destroys the one-to-one correspondence of metal-ligand tt interactions. Overlap of the filled dxz orbital with olefin n is turned off as the alkene rotates 90°, creating a large calculated barrier for olefin rotation (75 kcal/mol). Alkyne rotation quickly reveals an important point the absence of three-center bonds involving dir orbitals allows the alkyne to effectively define the linear combinations of dxy and dyz which serve as dn donor and dir acceptor orbitals for 7T and ttx, respectively. Thus there should be a small electronic barrier to alkyne rotation (the Huckel calculation with fixed metal... 

Michael addition is a facile reaction between nucleophiles and activated olefins and alkynes in which the nucleophile adds across a carbon-carbon multiple bond [25], For the preparation of hydrogels, the hydroxyl, thiol or amine functionalities have been reacted with vinyl sulfones [26-28], acrylates [29-31], acrylamides [32], and maleimides [33, 34] (Scheme 2). 

Since perfluoroalkyl-substituted olefins and alkynes possess low-lying frontier orbitals, [4 + 2] cycloaddition reactions to oxazoles and thiazoles without strongly electron-donating substituents are unfavorable. On the other hand, five-membered heteroaromatic compounds possessing an electron-rich diene substructure, like furans, thiophenes, and pyrroles, should be able to add perfluoroalkyl-substituted olefins as well as alkynes in a normal Diels-Alder process. A reaction sequence consisting of a Diels-Alder reaction with perfluoroalkyl-substituted alkynes as dienophile, and a subsequent retro-Diels-Alder process of the cycloadduct initially formed, represents a preparatively valuable method for regioselective introduction of perfluoroalkyl groups into five-membered heteroaromatic systems. 

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