Ti═NR vs Ti−R′ Functional Group Selectivity in Titanium Imido Alkyl Cations from an Experimental Perspective

Abstract

A study of the reactions of the titanium imido methyl cation [Ti(NtBu)(Me3[9]aneN3)X]+ (5+, X = Me) and key analogues (6+, X = CH2SiMe3; 7+, X = Cl) with a range of unsaturated substrates provides a comprehensive evaluation of reaction site selectivity in transition-metal cations which contain both Ti═NR and Ti−R′ functional groups as potential sites for 2π + 2π cycloaddition and migratory insertion, respectively. Cations 5+ and 6+ reacted with MeCN to form Ti−R′ insertion products. Insertion reactions were also exclusively observed in their reactions with N,N′-disubstituted carbodiimides or tert-butyl isocyanate. In contrast, reaction of the chloride cation 7+ with diisopropylcarbodiimide or tert-butyl isocyanate afforded the unusual σ adducts [Ti(NtBu)(Me3[9]aneN3)(L)Cl]+ (L = iPrNCNiPr, tBuNCO), which slowly underwent 2π + 2π cycloaddition. Reaction of 5+ with the internal alkynes PhCCR (R = Ph, SiMe3) also showed exclusive preference for insertion into the Ti−Me bond. In contrast, reaction with terminal alkynes RCCH (R = Ph, SiMe3) yielded only the azametallacyclic products via 2π + 2π cycloaddition reactions. The chloride cation 7+ also underwent a 2π + 2π cycloaddition with PhCCH but was unreactive toward PhCCPh, ethylene, styrene, or 1-hexene.