Development and Investigation of NW(OR)3, NMo(OR)3, and Mo2(OR)6 Complexes for Triple-Bond Metathesis.

Abstract

The application of alkyne metathesis in the development of polymers, pharmaceuticals, natural products, and many other products has surged. In response, an expansion of current catalysts and methods to synthesize the catalysts is desired. This work focuses on the systematic design and development of tungsten and molybdenum-based alkylidyne and nitride complexes for triple-bond metathesis. Nitrile-alkyne cross metathesis (NACM), a new type of metathesis involving the exchange of alkyne and nitrile moieties was developed. NACM activity was found with three tungsten nitride complexes, [N≡W(OCMe2CF3)3]3 (1), N≡W(OCMe¬(CF3)2)3(DME) (2), and [Li(DME)2][N≡W(OCMe(CF3)2)4] (3). Optimization of the NACM conditions including reaction medium, temperature, concentration, and catalyst loading were completed. NACM resulted in a mixture of symmetrical and unsymmetrical alkyne products with 2 favoring the formation of symmetrical alkynes relative to 1. These product ratio differences were accounted for through variances in catalyst resting state during NACM. The selective formation of symmetrical or unsymmetrical alkynes in high yield can be achieved by altering the reaction conditions. Although 2 exhibits greater NACM activity than 1, broader substrate compatibility is found with 2. Catalyst deactivation was substrate-dependent; conversion to alkylidyne complexes prior to deactivation occurring with some substrates. The utility of NACM was illustrated through the synthesis of large arylene-ethynylene macrocycles. Methods to readily synthesize alkylidyne species from N≡Mo(OR)3 and Mo2(OR)6 were developed. The irreversible formation of benzylidyne complexes from N≡Mo(OCMe(CF3)2)3 and N≡Mo(OC(CF3)3)3(MeCN) was achieved. The first evidence for the formation of RC≡Mo(OR)3 complexes (OR=OCMe3, OCMe2CF3, and OCMe(CF3)2) through the interaction of Mo2(OR)6 complexes with internal alkynes was discovered. The first example of the reversible formation of alkylidyne and Mo2(OR)6 species (OR=OCMe3, OCMe2CF3) was also achieved. Extension of the formation of molybdenum alkylidyne complexes from nitride complexes that contain OCMe¬3 and OCMe2CF3 ligands was achieved in situ in the presence of Lewis acids. Attempted isolation of alkylidyne complexes formed in situ was unsuccessful due to decomposition of the formed alkylidyne species in the presence of the Lewis acid. Several simple Lewis acids were found to increase the rate of alkyne metathesis with N≡Mo(OR)3 and Mo2(OR)6 complexes (OR= OCMe3, OCMe2CF3, and OCMe(CF3)2).