Reductive coupling and related reactions with Mo and Ti tris- anilides

Abstract

Chapter 1: The capability of Mo(N[t-BulAr)₃ to act as a powerful one, two, and three electron reductant have been previously demonstrated. In this work, we show that Mo(NIt-BulAr)₃ forms a metastable adduct with the moderate [pi] acid PhCN; coordination of PhCN activates the nitrile carbon towards reaction with a variety of one-electron reagents such as dichalcogenides, nitric oxide, hydrogen atom donors, cobaltocene, and elemental phosphorus. Evidence is presented for the existence of an inner-sphere electron transfer mechanism for these reactions. Chapter 2: With the facile cleavage of N₂ by Mo(Njt-Bu]Ar)₃ already established, a Mo(NIt-Bu]Ar)₃-mediated process for the incorporation of N₂ into organic molecules is an exciting prospect; its realization depends critically on the development of methods for cleavage of the Mo-N bond formed in the early stages of the process. In this chapter, we demonstrate that appropriately-substituted Mo(IV) ketiminates (synthesized using the methods of Chapter 1) undergo [beta]-elimination to cleave the Mo-N bond and liberate PhCN. We present the kinetics of the reaction, substituent effects, and-in three cases - activation parameters. Chapter 3: Deprotonation of the titanium formate complex (ArIt-BuJN)₃TiOC(O)H with LiN(i-Pr)₂ resulted in the release of free CO and the formation of a titanium(IV) oxoanion complex, isolated as its lithium salt. Chapter 4: Previous work from these labs has shown that the unique combination of well-defined composition, steric bulk, and strong reducing ability embodied in Ti(N[t- Bu]Ar)₃ lends itself particularly well to mechanistic studies of the classical Pinacol coupling.

(cont.) As shown in Chapter 1, a similar relationship can be drawn between Mo(NIt- BulAr)₃ and reductive nitrile coupling. In this chapter we draw on this mechanistic understanding to develop three new classes of reductive cross-couplings: nitrile is coupled with pyridine to form dihydropyridines, with benzophenone to form substituted 1,4-cyclohexadienes, and with carbon dioxide to form a-iminocarboxylates.