Conjugated diene complexes oxidation

Unconjugated dienes form the 1,3-diene complexes after isomerization to conjugated dienes. Formation of the stable conjugated diene complexe is the driving force of the isomerization. For example, the 1,4-diene in the synthetic intermediate 29 of prostaglandin A can be protected as the diene complex 30 after isomerization to the conjugated diene when it is treated with Fe2(CO)9. This method was applied to the synthesis of prostaglandin C (13). The diene complex 30 is stable for the oxidation of the lactol and introduction of the a-chain [7]. 

Conjugated dienes are oxidized to epoxides (or diols, if water is present) with the MTO/H2O2 system [11]. Urea/H202 avoids the subsequent epoxide ring opening. Electron-rich and conjugated dienes are more easily oxidized than electron-poor dienes and dienes with isolated double bonds. According to kinetic measurements complex 3 plays no important role as catalyst in this case. Compound 2 is an active species [11a]. 

In Grignard reactions, Mg(0) metal reacts with organic halides of. sp carbons (alkyl halides) more easily than halides of sp carbons (aryl and alkenyl halides). On the other hand. Pd(0) complexes react more easily with halides of carbons. In other words, alkenyl and aryl halides undergo facile oxidative additions to Pd(0) to form complexes 1 which have a Pd—C tr-bond as an initial step. Then mainly two transformations of these intermediate complexes are possible insertion and transmetallation. Unsaturated compounds such as alkenes. conjugated dienes, alkynes, and CO insert into the Pd—C bond. The final step of the reactions is reductive elimination or elimination of /J-hydro-gen. At the same time, the Pd(0) catalytic species is regenerated to start a new catalytic cycle. The transmetallation takes place with organometallic compounds of Li, Mg, Zn, B, Al, Sn, Si, Hg, etc., and the reaction terminates by reductive elimination. 

Several types of Pd-catalyzed or -promoted reactions of conjugated dienes via TT-allylpalladium complexes are known. The Pd(II)-promoted oxidative difunctionalization reactions of conjugated dienes with various nucleophiles is treated in Chapter 3, Section 4, and Pd(0)-catalyzed addition reactions of conjugated dienes to aryl and alkenyl halides in this chapter. Section 1.1.1. Other Pd(0)-catalyzed reactions of conjugated dienes are treated in this section. 

Pd-cataly2ed reactions of butadiene are different from those catalyzed by other transition metal complexes. Unlike Ni(0) catalysts, neither the well known cyclodimerization nor cyclotrimerization to form COD or CDT[1,2] takes place with Pd(0) catalysts. Pd(0) complexes catalyze two important reactions of conjugated dienes[3,4]. The first type is linear dimerization. The most characteristic and useful reaction of butadiene catalyzed by Pd(0) is dimerization with incorporation of nucleophiles. The bis-rr-allylpalladium complex 3 is believed to be an intermediate of 1,3,7-octatriene (7j and telomers 5 and 6[5,6]. The complex 3 is the resonance form of 2,5-divinylpalladacyclopentane (1) and pallada-3,7-cyclononadiene (2) formed by the oxidative cyclization of butadiene. The second reaction characteristic of Pd is the co-cyclization of butadiene with C = 0 bonds of aldehydes[7-9] and CO jlO] and C = N bonds of Schiff bases[ll] and isocyanate[12] to form the six-membered heterocyclic compounds 9 with two vinyl groups. The cyclization is explained by the insertion of these unsaturated bonds into the complex 1 to generate 8 and its reductive elimination to give 9. 

Additions of suifinic acids to polyenes ( hydrosulfonylation ), however, proceed with very strong acids or under catalysis of Pd complexes (equation 17). With copper(Il) arenesulfinates, azulene has been oxidatively sulfonylated in the 1- and 2-positions of the five-membered ring (equation 18). The sulfonylmercuration has also been applied with success to conjugated dienes (equation 19). 

Nickel(O) complexes are extremely effective for the dimerization and oligomerization of conjugated dienes [8,9]. Two molecules of 1,3-butadiene readily undergo oxidative cyclization with a Ni(0) metal to form bis-allylnickel species. Palladium(O) complexes also form bis-allylpalladium species of structural similarity (Scheme 2). The bis-allylpalladium complexes show amphiphilic reactivity and serve as an allyl cation equivalent in the presence of appropriate nucleophiles, and also serve as an allyl anion equivalent in the presence of appropriate electrophiles. Characteristically, the bis-allylnickel species is known to date only as a nucleophile toward carbonyl compounds (Eq. 1) [10,11],... 

Some derivatization methods mentioned in other sections of this review include chemical ionization by nitric oxide (MS) or epoxidation (MS), formation of jr-complexes for NMR (shift agents) etc. Also, the Diels-Alder reaction, which was mentioned several times as a tool for derivatization of conjugated dienes and polyenes, was extensively described and reviewed in the literature. 

A mild aerobic palladium-catalyzed 1,4-diacetoxylation of conjugated dienes has been developed and is based on a multistep electron transfer46. The hydroquinone produced in each cycle of the palladium-catalyzed oxidation is reoxidized by air or molecular oxygen. The latter reoxidation requires a metal macrocycle as catalyst. In the aerobic process there are no side products formed except water, and the stoichiometry of the reaction is given in equation 19. Thus 1,3-cyclohexadiene is oxidized by molecular oxygen to diacetate 39 with the aid of the triple catalytic system Pd(II)—BQ—MLm where MLm is a metal macrocyclic complex such as cobalt tetraphenylporphyrin (Co(TPP)), cobalt salophen (Co(Salophen) or iron phthalocyanine (Fe(Pc)). The principle of this biomimetic aerobic oxidation is outlined in Scheme 8. 

The products of electrochemical oxidation of conjugated dienes are considerably affected by the reaction conditions such as the material of the electrode, the supporting electrolyte and the solvent. The oxidation of butadiene with a graphite or carbon-cloth anode in 0.5 M methanolic solution of NaClCU mainly yields dimerized products along with small amounts of monomeric and trimeric compounds (equation 5)1. The use of platinum or glassy carbon mainly gives monomeric products. Other dienes such as isoprene, 1,3-cyclohexadiene, 2,4-hexadiene, 1,3-pentadiene and 2,3-dimethyl-l,3-butadiene yield complex mixtures of isomers of monomeric, dimeric and trimeric compounds, in which the dimeric products are the main products. 

In acyclic systems the 1,4-relative stereoselectivity was controlled by the stereochemistry of the diene. Thus, oxidation of (E,E)- and (E,Z)-2,4-hexadienes to their corresponding diacetates affords dl (>88% dl) and mesa (>95% me so) 2,5-diacetoxy-3-hexene, respectively. A mechanism involving a t vans-accto xy pal I adation of the conjugated diene to give an intermediate (rr-allyljpalladium complex, followed by either a cis or trans attack by acetate on the allyl group, has been suggested. The cis attack is explained by a cis migration from a (cr-allyl)palladium intermediate. The diacetoxylation reaction was applied to the preparation of a key intermediate for the synthesis of d/-shikimic acid, 3,... 

Isolated double bonds can be oxidatively cleaved in systems containing a conjugated diene moiety if it is protected as a tricarbonyl(diene)iron complex44. Dienal 39 was acquired in 49% yield by a two-step osmylation-periodate cleavage sequence (equation 27). In contrast, ozonolysis of the polyene complexes is reported to lead to destruction of the complex. 

The complete shift of a conjugated diene to a new conjugated diene system has also been observed in the -ionone system (33). Such diene rearrangements require a stoichiometric quantity of Fe(C0)5. The rearranged diene ligand is conveniently liberated from the Fe by oxidation of the complex with FeCls-... 

Catalyst conditions very similar to those employed in Eq. 42 and Scheme 12 were used recently by Lloyd-Jones, Booker-Milburn, and coworkers to achieve Pd-catalyzed diamination of conjugated dienes with urea nucleophiles (Eq. 44) [181], Both dioxygen and BQ were evaluated as oxidants, and BQ proved to be significantly more effective (Eq. 44). The beneficial effect of BQ probably arises from its ability to promote nucleophihc attack on the intermediate Tt-allyl palladiiun complex (see below. Sect. 4.4). This hypothesis is supported by the observation that the oxidative amination of styrene with urea, which does not undergo the second nucleophilic attack, proceeds equally effectively with both O2 and BQ as the oxidant (Eq. 45). 

A recently described method for insertion of a carbon monoxide molecule into the monoepoxide of a conjugated diene gives /3-lactones in high yield. This is achieved by reaction of iron pentacarbonyl with the starting vinyloxirane to give the 7r-allyl iron complex (66), which on oxidation with cerium(IV) ammonium nitrate gives the /3-lactone. In some cases, y-Iactone products can also be obtained from this reaction (8lJCS(Pi)270). 

Haase and Dunkley (1969B) reported that although high concentrations of ascorbic acid in model systems of potassium linoleate were prooxidant, a decrease in the rate of oxidation was observed. Haase and Dunkley (1969C) further noted that certain concentrations of ascorbic acid and copper inhibited the formation of conjugated dienes, but not the oxidation of ascorbic acid, and caused a rapid loss of part of the conjugated dienes already present in the system. They theorized that certain combination concentrations of ascorbic acid and copper inhibit oxidation by the formation of free radical inhibitors which terminate free- radical chain reactions, and that the inhibitors are complexes that include the free radicals. 

---