Olefin conjugation complex

The first metal-olefin complex was reported in 1827 by Zeise, but, until a few years ago, only palladium(II), platinum(Il), copper(I), silver(I), and mercury(II) were known to form such complexes (67, 188) and the nature of the bonding was not satisfactorily explained until 1951. However, recent work has shown that complexes of unsaturated hydrocarbons with metals of the vanadium, chromium, manganese, iron, and cobalt subgroups can be prepared when the metals are stabilized in a low-valent state by ligands such as carbon monoxide and the cyclopentadienyl anion. The wide variety of hydrocarbons which form complexes includes olefins, conjugated and nonconjugated polyolefins, cyclic polyolefins, and acetylenes. 

Both conjugated and nonconjugated olefins form complexes with the transition-metal carbonyls. Despite the fact that the first known complex, Zeises salt K(PtC2H4Cl3), discovered in 1827, was that of a simple olefin, complexes of monoolefins are rather limited in number. However, nonconjugated diolefins (L) react with group-VI carbonyls to form complexes of the type LM(CO)4 an example is provided by tetracarbonyl-bicyclo-(2,2, l)hepta-2,5-diene chromium (2) (Fig. 1). In contrast, the iron carbonyls... 

Isomeric (s-cis- and (i-fra/w-V-conjugated diene)zirconocene and -haf-nocene complexes exhibit pronounced differences in their characteristic structural data as well as their spectroscopic features. These differences exceed by far the consequences expected to arise simply from the presence of conformational isomers of the 1,3-diene unit. While (f-rra/u-butadiene)-zirconocene (3a) shows a behavior similar to a transition metal olefin TT-complex, the (.r-cu-diene)ZrCp2 isomer 5a exhibits a pronounced alkylmetal character (23, 45). Typical features are best represented by a tr, 7T-type structure for 5 (55). However, the distinctly different bonding situation of the butadiene Tr-system/bent-metallocene linkage is not only reflected in differences in physical data between the dienemetallocene isomers 3 and 5, but also gives rise to markedly different chemical behavior. Three examples of this are discussed in this section the reactions of the 3/5 isomeric mbcture with carbon monoxide, ethylene, and organic carbonyl compounds. 

Keywords Living polymerization, Living copolymerization, Rare earth metal complexes, Alkyl methacrylate, Alkyl acrylates, Lactones, Ethylene, 1-Olefins, Conjugated dienes, Acetylene... 

Equations 3.64-3.66 illustrate routes to allyl complexes from dienes, diene complexes, and olefins. Allyl complexes have been prepared by the insertion of a conjugated diene into a metal hydride, alkyl, or acyl linkage, as illustrated for the cobalt complexes in Equation 3.64. ° Alternatively, allyl complexes have been prepared by nucleophilic or electrophilic attack on a coordinated diene. Equation 3.65 shows the formation of allyl complexes by the addition of carbanions to a cationic diene complex, and Equation 3.66 shows the formation of a cationic diene complex by the protonation of a neutral 1,3-diene complex. Allyl complexes have also been formed by the abstraction of an allylic proton from a metal-olefin complex, either by a base or by the metal itself. This reaction has been proposed as a step in the isomerization of olefins (Equation 3.67) and in the allylic oxidation of olefins (Equation 3.68). - ... 

A dinuclear ruthenium(II)-NHC complex (structure 80, Fig. 28) has been reported to efficiently catalyze the oxidation of stilbenes and other disubstituted olefins conjugated to an aromatic system to the corresponding... 

Thallium(III) acetate reacts with alkenes to give 1,2-diol derivatives (see p. 128) while thallium(III) nitrate leads mostly to rearranged carbonyl compounds via organothallium compounds (E.C. Taylor, 1970, 1976 R.J. Ouelette, 1973 W. Rotermund, 1975 R. Criegee, 1979). Very useful reactions in complex syntheses have been those with olefins and ketones (see p. 136) containing conjugated aromatic substituents, e.g. porphyrins (G. W. Kenner, 1973 K.M. Smith, 1975). 

Catalytic cyclopropanation of alkenes has been reported by the use of diazoalkanes and electron-rich olefins in the presence of catalytic amounts of pentacarbonyl(rj2-ris-cyclooctene)chromium [23a,b] (Scheme 6) and by treatment of conjugated ene-yne ketone derivatives with different alkyl- and donor-substituted alkenes in the presence of a catalytic amount of pentacarbon-ylchromium tetrahydrofuran complex [23c]. These [2S+1C] cycloaddition reactions catalysed by a Cr(0) complex proceed at room temperature and involve the formation of a non-heteroatom-stabilised carbene complex as intermediate. 

Ferrocen-l,l -diylbismetallacycles are conceptually attractive for the development of bimetal-catalyzed processes for one particular reason the distance between the reactive centers in a coordinated electrophile and a coordinated nucleophile is self-adjustable for specific tasks, because the activation energy for Cp ligand rotation is very low. In 2008, Peters and Jautze reported the application of the bis-palladacycle complex 56a to the enantioselective conjugate addition of a-cyanoacetates to enones (Fig. 31) [74—76] based on the idea that a soft bimetallic complex capable of simultaneously activating both Michael donor and acceptor would not only lead to superior catalytic activity, but also to an enhanced level of stereocontrol due to a highly organized transition state [77]. An a-cyanoacetate should be activated by enolization promoted by coordination of the nitrile moiety to one Pd(II)-center, while the enone should be activated as an electrophile by coordination of the olefinic double bond to the carbophilic Lewis acid [78],... 

In the case of r)2-coordination of the exocyclic C=C bond, it becomes substantially elongated compared with the double bond of free alkenes, as a result of back donation from the metal to the 7t orbitals of the double bond. For instance, in complex 17b the coordinated bond length is 1.437 A (see Fig. 3.2).18 This is also reflected in the loss of planarity around the quaternary exocyclic carbon, the methylenic carbon being bent out of the ring plane by 10.78°.18 Similar structural features were also observed with other P2Pd conjugated olefin complexes.39... 

On the basis of the above result, the class 4 of chiral porphyrin complex (18) possessing a chiral strap, and facial chirality caused by it, has been introduced.66,67 Epoxidation with the complex (18) in the presence of 1,5-dicycohexylimidazole, which blocks the nonbridged side of the complex, shows good to high enantioselectivity when the substrates are conjugated mono- and m-di-substituted olefins (Scheme 11). 

Besides ruthenium porphyrins (vide supra), several other ruthenium complexes were used as catalysts for asymmetric epoxidation and showed unique features 114,115 though enantioselectivity is moderate, some reactions are stereospecific and treats-olefins are better substrates for the epoxidation than are m-olcfins (Scheme 20).115 Epoxidation of conjugated olefins with the Ru (salen) (37) as catalyst was also found to proceed stereospecifically, with high enantioselectivity under photo-irradiation, irrespective of the olefmic substitution pattern (Scheme 21).116-118 Complex (37) itself is coordinatively saturated and catalytically inactive, but photo-irradiation promotes the dissociation of the apical nitrosyl ligand and makes the complex catalytically active. The wide scope of this epoxidation has been attributed to the unique structure of (37). Its salen ligand adopts a deeply folded and distorted conformation that allows the approach of an olefin of any substitution pattern to the intermediary oxo-Ru species.118 2,6-Dichloropyridine IV-oxide (DCPO) and tetramethylpyrazine /V. V -dioxide68 (TMPO) are oxidants of choice for this epoxidation. 

Kim et al. have reported that the copper complex bearing the unique dinitrogen ligand (51) catalyzes the aziridination of conjugated as well as nonconjugated olefins with good enantio-selectivity (Scheme 37).159... 

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