Rhodium porphyrins synthesis

The stabilizing effect of an axial ligand has been previously observed in the synthesis of cobalt corrolates. Such an effect has been used to synthesize the complex where no peripheral p substituents are present on the macrocycle, which decomposes if attempts are made to isolate it in the absence of triphenyl-phosphine [10]. The behavior of rhodium closely resembled that of cobalt and it seems to be even more sensitive to the presence of axial ligands. [Rh(CO)2Cl]2 has also used as a metal carrier with such a starting material a hexacoordinated derivative has been isolated. The reaction follows a pathway similar to that observed for rhodium porphyrinates the first product is a Rh+ complex which is then oxidized to a Rh3+ derivative [29]. 

The electrosynthesis of metalloporphyrins which contain a metal-carbon a-bond is reviewed in this paper. The electron transfer mechanisms of a-bonded rhodium, cobalt, germanium, and silicon porphyrin complexes were also determined on the basis of voltammetric measurements and controlled-potential electrooxidation/reduction. The four described electrochemical systems demonstrate the versatility and selectivity of electrochemical methods for the synthesis and characterization of metal-carbon o-bonded metalloporphyrins. The reactions between rhodium and cobalt metalloporphyrins and the commonly used CH2CI2 is also discussed. 

The synthesis of metalloporphyrins which contain a metal-carbon a-bond can be accomplished by a number of different methods(l,2). One common synthetic method involves reaction of a Grignardreagent or alkyl(aryl) lithium with (P)MX or (PMX)2 where P is the dianion of a porphyrin macrocycle and X is a halide or pseudohalide. Another common synthetic technique involves reaction of a chemically or electrochemically generated low valent metalloporphyrin with an alkyl or aryl halide. This latter technique is similar to methods described in this paper for electrosynthesis of cobalt and rhodium a-bonded complexes. However, the prevailing mechanisms and the chemical reactions... 

Two aspects of porphyrin electrosynthesis will be discussed in this paper. The first is the use of controlled potential electroreduction to produce metal-carbon a-bonded porphyrins of rhodium and cobalt. This electrosynthetic method is more selective than conventional chemical synthetic methods for rhodium and cobalt metal-carbon complexes and, when coupled with cyclic voltammetry, can be used to determine the various reaction pathways involved in the synthesis. The electrosynthetic method can also lead to a simultaneous or stepwise formation of different products and several examples of this will be presented. 

Oxidative amination of carbamates, sulfamates, and sulfonamides has broad utility for the preparation of value-added heterocyclic structures. Both dimeric rhodium complexes and ruthenium porphyrins are effective catalysts for saturated C-H bond functionalization, affording products in high yields and with excellent chemo-, regio-, and diastereocontrol. Initial efforts to develop these methods into practical asymmetric processes give promise that such achievements will someday be realized. Alkene aziridina-tion using sulfamates and sulfonamides has witnessed dramatic improvement with the advent of protocols that obviate use of capricious iminoiodinanes. Complexes of rhodium, ruthenium, and copper all enjoy application in this context and will continue to evolve as both achiral and chiral catalysts for aziridine synthesis. The invention of new methods for the selective and efficient intermolecular amination of saturated C-H bonds still stands, however, as one of the great challenges. 

A complementary, but less pronounced, increase in cis syri) selectivity is provided when diazoacetates with rather small ester groups and rhodium catalysts with bulky ligands, such as iodorhodium(IIl) porphyrins and mcso-tetrakis(2,4,6-triarylbenzoato)di-rhodium(ll) complexes, are employed. For ethyl 2-alkylcyclopropane-l-carboxylates so obtained, a cisfran.s ratio of 2-4 was typical. Notable cis selectivities have also been achieved in the synthesis of ethyl 2-phenylcyclopropane-l-carboxylate with the catalyst [(r/ -C5H5)Fe(CO)2(THF)][BFJ (cis/trans 5.25, see Section 1.2.1.2.4.2.6.3.1.) and copper catalysts prepared in situ from a copper salt [copper(I) iodide, copper(II) acetate or triflate] and sodium tetrakis(7,8,8-trimethyl-4,5,6,7-tetrahydro-2Ff-4,7-methanoindazolyl)borate (3) cis/ trans 2.1-3.2). °... 

The above method has been used for the synthesis of metal alkyl (or aryl) o-bonded porphyrins of iron , cobalt rhodium titanium iridium gallium indium thallium, silicon germanium " " and tin ... 

Diastereoselective synthesis of 2-substituted 2E/-chromenes result from metalloradical coupling-cyclization reactions of Al-tosylhydrazones with terminal alkynes mediated by cobalt(II) complexes of porphyrins (14JA1090). Cyclopropenes readily rmdergo a rhodium(III)-catalyzed C-H aaivation with N-phenoxyacetamides to produce 2,2-disubstituted 2H-chromenes in moderate-to-excellent yields (Scheme 16) (14AGE13234). 

A general investigation of the Vlllth group metals led to the discovery of much improved and often unusual efficiences (up to 100%) and selectivities with palladium(II) and rhodium(II) carboxylates. Some metal carbonyls (Rh6(C0)16 and Ru3(C0)12, VKC0)6, Mo(C0)6) [6] catalyze typical carbene reactions. More recently, new catalytic systems,(Rh(III) (porphyrin) VII [7], Co(III) (oxime) VIII [8]. ..) have been described. Their application in synthesis is however limited for different reasons such as the cost, the difficult synthesis of the complex or their lack of versatility. 

While major advances in the area of C-H functionalization have been made with catalysts based on rare and expensive transition metals such as rhodium, palladium, ruthenium, and iridium [7], increasing interest in the sustainability aspect of catalysis has stimulated researchers toward the development of alternative catalysts based on naturally abundant first-row transition metals including cobalt [8]. As such, a growing number of cobalt-catalyzed C-H functionalization reactions, including those for heterocycle synthesis, have been reported over the last several years to date (early 2015) [9]. The purpose of this chapter is to provide an overview of such recent advancements with classification according to the nature of the catalytically active cobalt species involved in the C-H activation event. Besides inner-sphere C-H activation reactions catalyzed by low-valent and high-valent cobalt complexes, nitrene and carbene C-H insertion reactions promoted by cobalt(II)-porphyrin metalloradical catalysts are also discussed. 

C-H alkylation and amination reactions involving metal-carbenoid and metal-nitrenoid species have been developed for many years, most extensively with (chiral) dirhodium(ll) carboxylate and carboxamidate complexes as catalysts [45]. When performed in intramolecular settings, such reactions offer versatile methods for the (enantioselective) synthesis of hetero- and carbocy-cles. In the past decade, Zhang and coworkers had explored the catalysis of cobalt(II)-porphyrin complexes for carbene- and nitrene-transfer reactions [46] and revealed a radical nature of such processes as a distinct mechanistic feature compared with typical metal (e.g., rhodium)-catalyzed carbenoid and nitrenoid reactions [47]. Described below are examples of heterocycle synthesis via cobalt(II)-porphyrin-catalyzed intramolecular C-H amination or C-H alkylation. 

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