Alkenes, complexes

When thiocarbonyl derivatives are treated with an excess of electrophilic carbene complex, alkenes are usually obtained [1333-1336], The reaction is believed to proceed by the mechanism sketched in Figure 4.18, closely related to the thiocarbonyl olefination reaction developed by Eschenmoser [1337], Few examples have been reported in which stable thiiranes could be isolated [1338], The intermediate thiocarbonyl ylides can also undergo reactions similar to those of carhonyl ylides, e.g. 1,3-dipolar cycloadditions or 1,3-oxathiole formation [1338], Illustrative examples of these reactions are given in Table 4.22. 

At that period of time, this rather primitive transition structure for the peracid epoxidation of ethylene was sufficiently novel to warrant publication as a communication Today, calculating transition structures for epoxidation of a variety of complex alkenes with the actual peracid used experimentally, such as meta-chloroperoxybenzoic acid m-CPBA), is commonplace . ... 

Similar principles apply to more complex alkenes. One such case is that of isoprene, an important bio-genically produced hydrocarbon ... 

The extensive organometallic chemistry of chromium, i.e. the hexacarbonyl and its derivatives, organochromium compounds without carbonyl ligands, cyanide and isocyanide complexes, alkene, allyl, diene, cyclopentadiene and arene derivatives, and complexes of a-donor carbon ligands, has been recorded in Chapters 26.1 and 26.2 of Volume 3 of Comprehensive Organometallic Chemistry .1 In the present section, chromium complexes... 

Although some details are still debated, there is a general agreement about the basic steps of the mechanistic scheme. The first two steps are consistent with the kinetics of the reaction in that the oxidation is inhibited by Cl- and step 3 accounts for acid inhibition (Scheme 9.11). Key features of the process are the nucleophilic attack on the TT-complexed alkene, resulting in its transformation to a a-bonded intermediate (65 to 66), and p-hydride elimination (66 to 67). The disagreement is in the nature of nucleophilic attack called the hydroxypalladation step. 

In a related manner, sodium 2-bromobenzoates have been converted into 2-allylbenzoic acids. A palladium-catalyzed cyclization involving nucleophilic attack of the carboxylate on the palladium-complexed alkene yielded an isocoumarin. However, in situ catalytic hydrogenation results in the formation of an isochroman-l-one. 

An alkene complexed to platinum(II) is only slightly modified on coordination, but complexation to platinum(O) causes major changes. Platinum(O) alkene complexes show both weakening and lengthening of the carbon-carbon bond, as well as distortion of the plane of the double bond away from the platinum. In platinum(ll) alkene complexes the double bond lies approximately perpendicular to the square plane of platinum(II), but in platinum(O) complexes there is only a small dihedral angle between the platinum and alkenic planes. For platinum(II) the energy barrier to free rotation of the alkene about the platinum(D)-alkene bond is only about 40-65 kJ mol-1, whereas no rotation is observed with platinum(O) alkene complexes. Alkenes bonded to platinum(ll) exert a large trans effect but only have a small trans influence. 

Cis-trans isomerism is encountered very frequently. By one convention, the configuration of a complex alkene is taken to correspond to the configuration of the longest continuous chain as it passes through the double bond. Thus the following compound is /ra j -4-ethyl-3-methyl-3-heptene, despite the fact that two identical groups are cis with respect to each other, because... 

Most of the reactions listed in Table 6 involve prior activation of the substrate by coordination to palladium in the form of a v-, a 77-ally lie, a 77-benzylic, or an alkyl or aryl complex. Once coordinated to the metal, the substrate becomes an electron acceptor and can react with a variety of different nucleophiles. The addition of nucleophiles (Nu) to the coordinated substrate may occur in two different ways, as shown by Scheme 9 for 7r-alkene complexes 397"399 (a) external attack leading to trans addition of palladium and nucleophile across the 77-system (path A) or (b) internal addition of the coordinated nucleophile to the complexed alkene resulting in cis addition of palladium and nucleophile to the double bond. The cis and trans adducts (120) and (121) may then undergo /3-hydride elimination (/3-H), producing the vinylic oxidation product... 

Backwall and coworkers have extensively studied the stereochemistry of nucleophilic additions on 7r-alkenic and ir-allylic palladium(II) complexes. They concluded that nucleophiles which preferentially undergo a trans external attack are hard bases such as amines, water, alcohols, acetate and stabilized carbanions such as /3-diketonates. In contrast, soft bases are nonstabilized carbanions such as methyl or phenyl groups and undergo a cis internal nucleophilic attack at the coordinated substrate.398,399 The pseudocyclic alkylperoxypalladation procedure occurring in the ketonization of terminal alkenes by [RCC PdOOBu1], complexes (see Section 61.3.2.2.2)42 belongs to internal cis addition processes, as well as the oxidation of complexed alkenes by coordinated nitro ligands (vide in/ra).396,397... 

Alcohols and carboxylic acids also readily add to metal-activated alkenes2 and industrial processes for the conversion of ethylene to vinyl acetate, vinyl ethers and acetals are well established. However, very little use of intermolecular versions of this chemistry with more complex alkenes has been developed. In... 

As stated above, aliphatic amines are potent ligands for electrophilic transition metals and are efficient catalyst poisons in attempted alkene animation reactions. However, tosylation of the basic amino group greatly reduces its complexing ability, yet does not compromise its ability to nucleophilically attack complexed alkenes. Thus, a variety of alkenic tosamides efficiently cyclized under palladium(II) catalysis producing N-tosylenamines in excellent yield (equations 17 and 18).32 Again, this alkene amination proceeded through an unstable a-alkylpalladium(II) species, which could be intercepted by carbon monoxide, to result in an overall aminocarbonylation of alkenes. With ureas of 3-hydroxy-4-pentenyl-amines (Scheme 7), this palladium-catalyzed process was quite efficient but it was somewhat less so with... 

Imido-nitrido complexes, with mono-Cp Ti(IV), 4, 425-426 Imido-supporting ligands, in molybdenum complexes alkenes, 5, 535 alkynes, 5, 548... 

Mono(cyclopentadienyl) complexes alkenes and tantalum, 5, 157 alkyne and niobium, 5, 80 calcium, strontium, barium, 2, 133... 

The [Ni(alkene)(PF3)3] complexes (alkene = C1CH=CH2, CH3CH= CH2, CF3CH=CH2, and FCH=CH2) made by metal vapor synthesis are much less thermally stable than the polyene complexes (28) and decompose readily to give [Ni(PF3)J, alkene, and nickel metal. 

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