Metal-assisted

Metal-assisted cycloaddition with participation of heterocycles 97CRV523. 

Metal-assisted cycloaddition in formation of heterocycles 97CRV523. 

Compared with the related reactions of tutrones, there have only appeared a few pnbhcacions of metal-assisted or metal-catalyzed 1,3-dipolar cycle additions of nltnle oxides This Is due to... 

Fruhauf H-W Metal-Assisted Cycloaddition reactions in Organotransition Metal Chemistry Chem. Rev. 1997 97 523-596... 

Enzyme-mediated oxidation reactions offer highly diverse options for the modification of existing functional groups as well as for the introduction of novel function in chiral catalysis. Biooxidations often enable us to obtain complementary solutions to metal-assisted transformations and organocatalysis and are considered one of the important strategies of green chemistry . 

Biooxidative deracemization of racemic sec-alcohols to single enantiomers [47,48] is complementary to combined metal-assisted lipase-mediated strategies [49,50]. In general, deracemization can be realized by either an enantioconvergent, a dynamic kinetic resolution, or a stereoinversion process. The latter concept is particularly appealing, as only half of the substrate needs to be converted, as the remaining half already represents the product with correct stereochemistry. 

Metal-assisted enantioselective catalytic reactions are one of the most important areas in organic chemistry [1-3]. They require the appropriate design and the preparation of chiral transition metal complexes, a field also of major importance in modern synthetic chemistry. These complexes are selected on both their ability to catalyze a given reaction and their potential as asymmetric inducers. To fulfill the first function, it is absolutely required that the catalysts display accessible metal coordination sites where reactants can bind since activation would result from a direct interaction between the metal ion... 

Quantum calculations on metal-assisted tautomerization indicate a substantial stabilization to protonation of the endocyclic nitrogen atoms (99). In the case of HgCHg, adducts are formed by binding at either A-N6 or C-N4, which increases the stability of the respective N1H+ or N3H+ protonated species by 10-14 kcal/mol. For Pt11 binding at C-N4 the effect is much greater at 30-34 kcal/mol. 

Two-dimensional planar interpenetrating networks have been formed using the spacer ligand 2,2 -bis-l,6-naphthyridine with a zinc salt.274 Helicate structures have been synthesized which rely heavily on non-covalent interactions in the metal-assisted self-assembly process in solution.275... 

Weber, Lothar, Transition-Metal Assisted Syntheses of Rings and Cages from... 

An interesting case of metal-assisted ring opening of (109) has been described by Wolczanski using the low-valent (silox)3Nb fragment that binds (109) in the //2(N,C) mode (Scheme 50). Derivative (113) undergoes C—N insertion by thermolysis in benzene at 70 °C only. The reaction gives 0.5 equiv. of (109) and 0.5 equiv. of (114) as a thermodynamic mixture of cis,cis-, trans,cis-, trans,trans- and cis,trans-isomers.255-257... 

The general form of metal-assisted cycloaddition as shown in Scheme 31 can be readily elaborated using monohaptopropargyl complexes (Scheme 32)61 specific examples illustrating the synthesis of A3-pyrrolinone derivatives are shown in Scheme 33.59... 

The diazines pyridazine, pyrimidine, pyrazine, and their benzo derivatives cinnoline, phthalazine, quinazoline, quinoxaline, and phenazine once again played a central role in many investigations. Progress was made on the syntheses and reactions of these heterocycles, and their use as intermediates toward broader goals. Some studies relied on solid-phase, microwave irradiation, or metal-assisted synthetic approaches, while others focused attention more on the X-ray, computational, spectroscopic, and natural product and other biological aspects of these heterocycles. Reports with a common flavor have been grouped together whenever possible. 

The polymerization of olefins and di-olefins is one of the most important targets in polymer science. This review article describes recent progress in this field and deals with organo-transition metal complexes as polymerization catalysts. Recent developments in organometallic chemistry have prompted us to find a precise description of the mechanism of propagation, chain transfer, and termination steps in the homogeneously metal-assisted polymerization of olefins and diolefins. Thus, this development provides an idea for designing any catalyst systems that are of interest in industry. 

Historically, the first multivalent glycoconjugate obtained by metal-assisted self-assembly was described in 1994 by Sakai and Sasaki (Fig. 35).267 The methodology was based on the trimerization of unsymmetrical Bipy-GalNAc (307) induced by the presence of Fe(II) salts, to give the tridentate Fen(bipy-GalNAc)3 cluster 308 as a mixture of four diastereomeric isomers (A-fac, A-fac, A-mer, and A-mer). 

For (XX), L py, it is likely that the major reaction path involves initial skeletal isomerization to give (XXI) followed by rapid solvolysis of this isomer. The solvolysis of this isomer is strongly metal-assisted since the intermediate carbonium ion is stabilised by the metal-alkene resonance form as shown in the Scheme. The product is the 1-D2 isomer. Now, the skeletal isomerization of (XX) is expected to be retarded by free pyridine and cannot occur when L2 = 2,2 -bipyridyl C7). Hence under these conditions the reaction must occur by solvolysis of (XX) giving largely the 3-D2 isomer. However, the product formed under these conditions is still about 30% of the 1-D2 isomer (Table I). 

In the case of DTPA derivative complexes, both proton- and metal-assisted (Zn2+ or Cu2+) pathways are important for the dissociation (143). For the DTPA-amide derivatives, the proton-assisted dissociation is much faster, while the metal-assisted pathway is slower (147,148), with a dependency of the... 

Decarbonylation of the acyl is likely to be metal-assisted (Ru11) giving rise to a Ru carbonyl, which is subsequently decarbony-lated by nucleophilic attack by nBu3P. This phosphine can displace coordinated carbonyl, as exemplified by reaction 3 ... 

However, formation of an R species, either free or within a radical-pair cage with the metal (14), is strongly favored in view of the methylcyclopentane noted in the cyclohexen-4-al decarbonylation, since the rearrangement shown in eq. 5, metal-assisted if necessary, seems plausible (15) ... 

With the transition-metal-assisted ring-opening and isomerization of small rings in mind, it is astonishing that only one such study for a [4]radialene is known. The cobalt(I)... 

The earliest developments in transition metal-assisted formation of aromatic C-P linkages were the result of the efforts of Tavs28 that were concerned with the use of Ni(II) halide salts for the reaction of aryl halides with trialkyl phosphites. These reactions involved conditions reminiscent of the Michaelis-Arbuzov reaction (heating at an elevated temperature) and produced arylphosphonate products in reasonable yield (Figure 6.8). 

Enantiomerically pure 84 has been obtained in only seven synthetic operations in 40% overall yield starting from simple and commercially available precursors (Scheme 25). All C-C bond formations are metal-assisted or metal-catalyzed, with the RCM-based cyclization of the 12-membered ring (82->83) being particularly efficient [33]. 

The use of a lipophilic zinc(II) macrocycle complex, 1-hexadecyl-1,4,7,10-tetraazacyclododecane, to catalyze hydrolysis of lipophilic esters, both phosphate and carboxy (425), links this Section to the previous Section. Here, and in studies of the catalysis of hydrolysis of 4-nitrophenyl acetate by the Zn2+ and Co2+ complexes of tris(4,5-di-n-propyl-2 -imidazolyl)phosphine (426) and of a phosphate triester, a phos-phonate diester, and O-isopropyl methylfluorophosphonate (Sarin) by [Cu(A(A(A/,-trimethyl-A/,-tetradecylethylenediamine)l (427), various micellar effects have been brought into play. Catalysis of carboxylic ester hydrolysis is more effectively catalyzed by A"-methylimidazole-functionalized gold nanoparticles than by micellar catalysis (428). Other reports on mechanisms of metal-assisted carboxy ester hydrolyses deal with copper(II) (429), zinc(II) (430,431), and palladium(II) (432). 

---