Electrophiles heterocyclic chelates

The pentasulfido complex reacts with tertiary phosphines to give dimers [Ti(MeCp)2]2Sj, (x = 4 and 6). The pentasulfido complex also gives heterocycles of the type Ti(MeCp)2S4CR2 when treated with ammonium sulfide in the presence of certain alkylating agents. In fact, the Me2CS4 chelate is a minor side product in the synthesis of Ti(MeCp)2Sj. The compound Ti(MeCp)2S5 reacts with electrophilic acetylenes to give dithiolenes... 

Complex 15 illustrates the importance of incorporating an N, 0-chelate of sufficient steric bulk on an electrophilic metal center to achieve overall improved reactivity. While these room temperature transformations are promising, this complex is not thermally robust and thus displays limited substrate scope. In particular, heating to promote reactivity with unstrained internal alkenes or challenging N-heterocyclic substrates results in no appreciable product formation and catalyst decomposition, as noted by NMR spectroscopy. 

In seeking to produce stable iron(O) complexes for the activation of arenes, a variety of interesting arsenic-sulfur heterocycles has been synthesized. Thus, with use of bis(3-chlofopropyl)phenylarsine (66) as the electrophile, the potentially chelating macrocycles (73)-(80) were prepared under high-dilution conditions in ethanol at 70°C from sulfide or various thiolates (Scheme 12) <81TL5035, 85CB2703>. The products were purified on alumina columns with use of benzene-diethyl ether as eluant. Except for crystalline (75), (77), and (78), the products were oils. 

Chelate-assisted C-H bond trifluoromethylations are rare, and so far, only two reagents have been used successfully in these reactions. The first reagent 19 is an electrophilic source of CF, which reacts under Pd catalysis with substrates bearing directing groups (Scheme 23.27) [92, 93]. The reaction also requires the presence of Cu(OAc)j and trifluoroacetic acid as promoters both amide-based and heterocyclic directing groups can be employed. 

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