Thiophene, organometallic chemistry

Volume 78 of Advances in Heterocyclic Chemistry contains four contributions. A. P. Sadimenko of Fort Hare University of South Africa has covered organometal-lic compounds of furan, thiophene, and their benzannulated derivatives. This constitutes the first installment of a projected series on the organometallic chemistry of heteroaromatic ligands, a subject of great fundamental and technical importance that has exploded in the 1990s. 

This chapter is a continuation of the one on organometallic chemistry of furans and thiophenes [01AHC(78)1]. It starts with the trends of coordination of... 

The thiophene ring can be elaborated using standard electrophilic, nucleophilic, and organometallic chemistry. A variety of methods have been developed to exploit the tendency for the thiophene ring (analogous to that of furan and pyrrole) to favor electrophilic substitution and metallation at its a-carbons. Substitution at the p-carbons is more challenging, but this problem can also be solved by utilizing relative reactivity differences. 

The organometallic chemistry of thiophenes and benzothiophenes has been reviewed extensively up to the year 2001 <2001AHC7, 20010M1259>. Therefore, only some contributions of the last 5 years are presented in this section. 

Many other metals have been shown to be active in HDS catalysis, and a number of papers have been published on the study of periodic trends in activities for transition metal sulfides [15, 37-43]. Both pure metal sulfides and supported metal sulfides have been considered and experimental studies indicate that the HDS activities for the desulfurization of dibenzothiophene [37] or of thiophene [38, 39] are related to the position of the metal in the periodic table, as exemplified in Fig. 1.2 (a), 1.2 (b), and 1.2 (c). Although minor differences can be observed from one study to another, all of them agree in that second and third row metals display a characteristic volcano-type dependence of the activity on the periodic position, and they are considerably more active than their first row counterparts. Maximum activities were invariably found around Ru, Os, Rh, Ir, and this will be important when considering organometallic chemistry related to HDS, since a good proportion of that work has been concerned with Ru, Rh, and Ir complexes, which are therefore reasonable models in this sense however, Pt and Ni complexes have also been recently shown to promote the very mild stoichiometric activation and desulfurization of substituted dibenzothiophenes (See Chapter 4). 

In conclusion, an extensive literature is available on the reaction networks that are thought to operate in HD,S of various types of thiophenic molecules besides the great advances that have been made in direct studies on molybdenum sulfides and related catalysts, this is another area in which organometallic chemistry has made an impressive contribution to HD,S catalysis, as a number of reaction pathways and mechanisms for the hydrogenation and hydrogenolysis of thiophenes on metal complexes in solution has been well established with the aid of a variety of physical techniques. 

In conclusion, this first Chapter attempts to provide the non specialist with and a general basis for understanding the importance and the major advances and drawbacks of heterogeneous HDS and HDN reactions, as well as the principal challenges that need to be addressed in the future. It is also meant to provide the context in which the organometallic chemistry related to these processes will be discussed in the rest of the book. Chapter 2 will be devoted to the description of metal complexes of the thiophenes, their syntheses, structures, bonding characteristics and reactivity patterns. 

A lot of synthetic effort has been devoted to the synthesis of polymerizable thiophene derivatives, but the principal aim of this review is to focus on the polymerization chemistry. The basic synthesis procedures for polythiophene can be considered in parallel with poly(p-phenylene). Indeed, a similar classification can be drawn direct oxidation of thiophene, organometallic coupling, and electrochemical syntheses. Frecursor methods for polythiophene are rather exceptional. 

Organometallic chemistry is particularly important in thiophene chemistry. Thiophenes can be directly metallated at an a-position using a strong base at low temperature, and the resulting nucleophile will react with electrophiles to effect... 

Many of these studies utilized noble metals such as Ir, Os, Rh, Ru, or Re, whereas others used more conventional metals such as Mn, Fe, Mo, or Co. The particular metal on which the observations were made is not important at this point. What is important is that all of the important steps required for direct sulfur removal and hydrogenation of thiophene and more condensed derivatives have been shown to occur with soluble metal complexes. Thus, organometallic complex chemistry can be of great value in elucidating the mechanisms involved in conventional HDS processes and perhaps can point the way to improved catalyst formulations. 

Prior to about 1985, less than a handful of metal complexes containing coordinated thiophenes were known, and virtually no mention of them could be found in the HDS literature of that time. Today, not only the number and the variety of such compounds have grown to be very large, but also they have become of obliged reference for workers in the heterogeneous HDS field. In turn, organometallic chemists have become aware of the wealth of information available on the chemistry of thiophenes on solid catalyst surfaces, and of the main standing issues that need to be understood and solved in HDS catalysis. 

Some early proposals for the modes of adsorption of thiophenes on metal sulfides have been probed by comparisons with the structures of well-characterized metal complexes this has allowed the identification of the most reasonable alternatives and of new possibilities not previously considered. Tlieoretical studies on such complexes at increasing levels of sophistication have also contributed in an Important manner to provide a clear and consistent picture of the different possible bonding modes of thiophenes to metal centers. When these theoretical and experimental results from molecular chemistry are combined with the information available from surface techniques and heterogeneous catalysis, the chemisorption of this type of organosulfur compounds on metal sulfides arises as a very well understood phenomenon. This is no doubt one of the most important achievements of the organometallic modeling approach to HDS chemistry. 

---