Functionalization of alkenes

Three oxidative reactions of benzene with Pd(OAc)2 via reactive rr-aryl-Pd complexes are known. The insertion of alkenes and elimination afford arylalk-enes. The oxidative functionalization of alkenes with aromatics is treated in Section 2.8. Two other reactions, oxidative homocoupling[324,325] and the acetoxylation[326], are treated in this section. The palladation of aromatic compounds is possible only with Pd(OAc)2. No reaction takes place with PdCl2. 

The carbopalladation is extended to homoallylic amines and sulfides[466. Treatment of 4-dimethylamino-l-butene (518) with diethyl malonate and Li2PdCl4 in THF at room temperature leads to the oily carbopalladated complex 519, hydrogenation of which affords diethyl 4-(dimethylamino) butylmalonate (520) in an overall yield of 91%. Similarly, isopropyl 3-butenyl sulfide (521) is carbopalladated with methyl cyclopentanonecarboxylate and Li2PdCl4. Reduction of the complex affords the alkylated keto ester 522 in 96% yield. Thus functionalization of alkenes is possible by this method. 

When the 7r-allylpalladium complexes are formed by the reaction of aikenes with PdCU and react with nucleophiles, the whole reaction constitutes the stoichiometric functionalization of alkenes[4,5]. 

Table 12.8. Relative Rates of Radical Addition as a Function of Alkene Substitution"...

The reaction of carboethoxynitrene with cis- and trani-4-methyl-2-pentene was studied as a function of alkene concentration. At large alkene concentrations, azir-idination is stereospecific. Upon dilution of the alkene, the stereospecificity is lost. These results are completely analogous to studies of carbenes in which a... 

Solvomercuration,3111 484 187 or the addition of mercury(II) salts, is a convenient route to organomercurials. On the other hand, replacement of mercury with hydrogen allows Markovnikov functionalization of alkenes.488 A method called mercuration-demercuration, for instance, has been developed for the Markovnikov hydration of alkenes under mild conditions489 [Eq. (6.82)490] ... 

A kinetic scheme was proposed [122] with the fluorescent exciplex as precursor of the photoproducts (ortho as well as meta adducts). Quantum yields of adduct formation, exciplex emission, and benzene fluorescence were measured as a function of alkene concentration. The kinetic data fit the proposed reaction scheme. The authors have also attempted to prove the intermediacy of the exciplex in the photoaddition by adding a quencher to the system benzene + 2,2-dimethyl-... 

Thus activation and functionalization of alkenes, enones and conjugated dienes are possible based on the 7i-allylpalladium complex formation from these unsaturated compounds. 

Figure 4.12 Variation in FO energies as a function of alkene and alkyne deformation.

Beller, M., Seayad, J., Tillack, A., and Jiao, H. (2004) Catalytic Markovnikov and anti-Markovnikov functionalization of alkenes and alkynes recent developments and trends. Angewandte Chemie, International Edition, 43, 3368-3398. 

Nucleophilic amination of alkenes functionalized by an allylic C-X (x = heteroatoms, halides) as outlined in Eq. (1) is a simple and direct procedure for the synthesis of allyl amines, since very efficient methods for the selective allylic functionalization of alkenes are available. 

Replacement of the phenyl ring of 27 with an alkene leads to different products as a function of alkene substitution (Scheme 7). The cyclization of 29 (R = Me) gives... 

Ever since the initial discovery of the Wacker process [1], i.e. the Pd/Cu-catalyzed oxidation of ethylene to acetaldehyde (1) in water, methods for the palladium (II) - mediated oxidative functionalization of alkenes have found widespread application in the synthesis of complex molecules [2J. 

Allylic alcohols represent a small fraction of the total population of alkenes found in organic molecules. Asymmetric epoxidation of allylic alcohols therefore taps only a small portion of the synthetic potential inherent in a completely general asymmetric epoxidation of isolated (nonfunctionalized) alkenes. A partial solution to this problem now exists. The recent development of a catalytic asymmetric process for the dihydroxylation of aUcenes provides an indirect route to epoxides or epoxide-like functionalization of alkenes. The stereochemistry of the process, the scope of enantioselectivity and chemical yield and a summary of key chemical transformations are presented in this section. Since this roach to alkene functionalization is at an early stage of development, the results sununarized here are certain to benefit from extensions and improvements as research in this area progresses. 

Electrooxidation of halide salts is quite useful for the generation of reactive species of halogen atoms under mild conditions. Functionalization of alkenes involving the formation of halohydrins, 1,2-halides, a-halo ketones, epoxides, allylic halides and others has been achieved by electrochemical reactions and is well documented in the literature. On the other hand, electrogenerated carbenium ions can be captured by nucleophilic halide anions, providing a new route to halogenated compounds... 

The recommendations for ligand choice as a function of alkene substitution can be summarized as follows (The structures of the ligands are given in Table 5). 

In (a), the optical experiments, the reaction of 804 with the alkenes was studied by monitoring the rate of decay of 804 as a function of the concentration of alkene. It was foimd that the bimolecular rate constant for reaction of 804 (obtained from the plot of kobs(erved) as a function of [alkene]) increases strongly with increasing water content of the solvent, as shown in Figure 1, where allyl alcohol is used as an example. This indicates that the transition state for the reaction (which consists in addition to the C=C double bond, Eq. 10 see later and Refs [46] and [47]) is highly ionic [48]. 

Chlorosulfides are often used for vicinal functionalization of alkenes. Silyl enol ethers can react with a-chloromethyl phenyl sulfides (5) in the presence of ZnBr2 or TiCU to give the corresponding -keto sulfide, which could be easily transfonned into the a-methylene ketone via sulfide oxidation to sulfoxide, followed by pyrolytic elimination (Scheme 23). 

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