REACTIONS OF THIOPHENES ON MONONUCLEAR COMPLEXES

A good number of mononuclear metal complexes are known to activate C-S bonds, and it is clear that a high electron density around the metal atom generally favors the oxidative addition reaction. One commonly encountered case is the interaction of electron rich, coordinatively unsaturated (16-electron) metal fragments with thiophenes which results in the rupture of one of the C-S bonds with the consequent formation of the corresponding saturated 18-electron thiametallacycles. 

The high selectivity observed for the ring-opened product in this case was explained by Jones in terms of a slight preference (of the order of 0.5 Kcal mo ) for Ti S-coordinalion of thiophene to Rh -which was claimed to lead to C-S bond scission-over Ti (C=C) bonding which was thought in turn to be responsible for C-H bond activation, as represented in Fig. 4.2. 

Other interesting Cp systems which have been used in HDS-related chemistry include Angelici s Cp Ir( n -T), which is formed by reduction of the corresponding t C coordinated precursor this ri -T derivative isomerizes to the ring opened isomer by the action of basic alumina or triethylamine, or alternatively, by UV irradiation. Also, attack on cationic [CpRu(T VT)] by H or other nucleophiles promotes ring opening [8] (see Section 2.5). 

Boncella has very recently reported [81] the use of the -non Cp- complex [W(NPh) o-(Me3SiN)2C,HJ (CjHjN) ], to open the rings of thiophene, 2-methylthiophene, and benzothiophene at 65° C in toluene solution. This is another particularly important case representing the only example of C-S activation induced by W(IV), and the only stable complexes containing ring-opened thiophenes on W, one of the most frequently used metals in heterogeneous HDS  

In particular, it is lower in energy than the thennal processes which are necessary to generate the 16e fragments from their I Sc precursors . 

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