Dimers four-bridge

The catalytic activity of rhodium diacetate compounds in the decomposition of diazo compounds was discovered by Teyssie in 1973 [12] for a reaction of ethyl diazoacetate with water, alcohols, and weak acids to give the carbene inserted alcohol, ether, or ester product. This was soon followed by cyclopropanation. Rhodium(II) acetates form stable dimeric complexes containing four bridging carboxylates and a rhodium-rhodium bond (Figure 17.8). 

Rhodium(II) forms a dimeric complex with a lantern structure composed of four bridging hgands and two axial binding sites. Traditionally rhodium catalysts faU into three main categories the carboxylates, the perfluorinated carboxylates, and the carboxamides. Of these, the two main bridging frameworks are the carboxylate 10 and carboxamide 11 structures. Despite the similarity in the bridging moiety, the reactivity of the perfluorinated carboxylates is demonstrably different from that of the alkyl or even aryl carboxylates. Sohd-phase crystal structures usually have the axial positions of the catalyst occupied by an electron donor, such as an alcohol, ether, amine, or sulfoxide. By far the most widely used rhodium] 11) catalyst is rhodium(II) acetate [Rh2(OAc)4], but almost every variety of rhodium] 11) catalyst is commercially available. 

Schlenk intermediates and the addition path have been made so far. However, our results seem to provide simple and realistic pathways. The Grignard addition occurs in a dimeric dichlorine-bridged form. A vicinal-magnesium bonding alkyl and C=0 interaction causes C—C bond formation via a four-center interaction as shown in Scheme 20 (polar mechanism). When the interaction is improbable owing to the steric effect, the Mg—0=C bond formation precedes the C—C bond formation and the Mg—C bond is ruptured (SET). 

Dinuclear Rh(II) compounds are another class of effective catalysts (227). Electrophilic carbenes formed from diazo ketones and dimeric Rh(II) carboxylates undergo olefin cyclopropanation. Chiral Rh(II) carboxamides also serve as catalysts for enantioselective cyclopropanation (Scheme 95) (228). The catalysts have four bridging amide ligands, and... 

Figure 24 Schematic structure of copper(II) dimeric species held together by four bridging purine ligands (after ref. 28).

The dimeric Rh11 complex [Rh2(MeCOS)4(MeCOSH)2] contains four bridging thioacetate groups the two thioacetic acid ligands are in the thione form and are each bound via the sulfur atom to one of the rhodium atoms.133 The Ru" complex [Ru(MeCOS)2(CO)(Ph3 P)2] is known in two isomeric forms.134... 

The iron protein has been found in animals, viruses and some prokaryotic organisms. The best studied example is from Escherichia coli.S19 The enzyme consists of two subunits, Bi and B2. Subunit B, has a molecular weight of 160 000 and is made up of two polypeptide chains. It contains redox-active SH groups, which play a role in the reaction, and has two binding sites for substrates and sites for effector molecules. It can bind any of the four ribonucleotide substrates. Subunit B2 has a molecular weight of 78 000 and also has two polypeptide chains. It contains a dimeric oxo-bridged iron site associated with a remarkably stable tyrosine radical. The formation of the active enzyme from B, and B2 requires the presence of magnesium ions. 

It is of considerable interest that some 14 years after Calvin postulated these structures, van Niekerk and Schooling (8) showed by X-ray analysis that cupric acetate monohydrate, Cu(OAc)i.HjO, exists in the solid as dimeric molecules, the two copper ions being joined by four bridging acetate groups. The copper-copper distance in the dimer is only 2.64 A., as compared with 2.56 A. in the metal. 

A few four-coordinate complexes of the formulas CuL3C104 (L = Ph3P, Ph2EtAs, Ph2-n-PrAs L3 = tox) and Cu(Ph3P)2C104 have been reported (94, 96, 97) the last one is a dimer with bridging bidentate... 

Recently, Rh" dimers with more than one type of bridging ligand in the same complex have been characterized these will be considered along with a few complexes with less than four bridging... 

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