Investigations Directed at Catalytic Carbon−Carbon and Carbon−Oxygen Bond Formation via C−H Bond Activation

Abstract

A potential catalytic cycle for the formation of new C-C and C-O linkages from hydrocarbon feedstocks and readily available olefin and ketone substrates mediated by Cp′M(NO)(L) (M ) Mo, W; Cp′ ) Cp*, Cp; L ) Lewis base) fragments has been investigated. The cycle is based on three steps: (1) oxidative addition of the hydrocarbon substrate to the metal center, (2) subsequent hydrometalation of the olefin or the ketone, and (3) final reductive elimination of the coupled product. Of the various Cp′M-(NO)(L) groups examined, the Cp*W(NO)(PPh 3 ) fragment has been found to be the best candidate for mediating these catalytic steps since it is not prone to form unreactive Cp*W(NO)(PPh 3 ) 2 as are some of the other fragments that readily decompose to 18e Cp′M(NO)L 2 complexes. Hence, Cp*W(NO)(PPh 3 ) has been utilized to determine if the oxidative addition and hydrometalation steps can occur sequentially under one-pot experimental conditions. However, olefins are too π-acidic and readily form stable 18e Cp*W(NO)(PPh 3 )(η 2 -olefin) adducts, which prevent oxidative addition of the hydrocarbon substrate to the tungsten center. Similarly, benzophenone, Ph 2 CO, and diisopropyl ketone, i Pr 2 CO, also form 1:1 η 2 -CdO adducts with the π-basic tungsten center in the 16e fragment. Nevertheless, oxidative addition and hydrometalation do occur sequentially to form the desired aryl alkoxide complex, Cp*W(NO)(OCH i -Pr 2 )(Ph), in addition to the Cp*W(NO)(η 2 -OC i Pr 2 )(PPh 3 ) adduct, when benzene and diisopropyl ketone are employed as the two substrates. The solid-state molecular structures of cis-Cp*W(NO)[η 2 -(CH 2 -NMe)P(NMe 2 ) 2 ](H), Cp*W(NO)(OCH i Pr 2 )(Ph), and Cp*W(NO)(η 2 -OC i Pr 2 )(PPh 3 ) have been established by single-crystal X-ray crystallographic analyses.

Agostic NSiH⋅⋅⋅Mo Complexes: From Curiosity to Catalysis

“…Similarly, reaction of 3 with styrene gives the hydrido vinylsilyl complex 5 (Scheme 3), which probably emerges from styrene coupling with the silanimine intermediate D (Scheme 2), followed by C À H activation in the CH 2 CHPh fragment. Similar alkene insertion into a Zr À Si bond has been observed previously by Surprisingly, NMR experiments indicated that reactions of 3 with acetone and acetophenone lead to the selective formation of the ketone complexes 6 [17] and 7, [18] respectively, accompanied by the evolution of one equivalent of PhSiH 3 and the formation of a difficult-to-characterize mixture of volatile silicon-containing compounds (Scheme 3). All attempts to trace the fate of the extruded silylene fragment…”

“…[18] As for ketones, the reaction of (ArN) 2 Mo(PMe 3 ) 3 with benzaldehyde gives an h 2 complex (8), characterized by an up-field C À H signal at d = 5.69 ppm in its 1 H NMR spectrum. [17] These observations suggest that in the hydrosylation of carbonyl compounds the active catalyst is probably formed from 3 after silane elimination, but before silylene extrusion from D.…”

Unusual Structure, Fluxionality, and Reaction Mechanism of Carbonyl Hydrosilylation by Silyl Hydride Complex [(ArN)Mo(H)(SiH2Ph)(PMe3)3]

“…The formulation of complex 24 as an η 2 adduct is further supported by the appearance of red‐shifted CO stretches in the IR spectrum (1596 and 1487 cm −1 versus 1720 cm −1 for the free PhHCO) 11a–c,e. 50, 51 Complex 24 is unstable under vacuum; however, cooling the reaction mixture down to −30 °C allowed us to crystallize complex 24 in its analytically pure form. Alternatively, a mixture of complexes 23 and 24 can be obtained through a transfer hydrogenation reaction of benzaldehyde with isopropoxy derivative 18 (Scheme ).…”

“…However, the 31 P{ 1 H} NMR spectrum at −60 °C shows two doublets ( 2 J (P,P)=218.7 Hz) at δ =8.0 and 6.4 ppm for two nonequivalent trans‐ PMe 3 ligands. In the 1 H NMR spectrum, the carbonyl protons of two η 2 ‐benzaldehyde ligands appear as broad, upfield‐shifted signals at δ =5.42 (br s) and 5.18 ppm (br d, J (H,P)=9.6 Hz),51 that are coupled in the 1 H 13 C HSQC spectrum to the carbonyl carbon atoms at δ =85.2 and 86.1 ppm, respectively.…”

“…However, the 31 P{ 1 H} NMR spectrum at À60 8C shows two doublets ( 2 JA C H T U N G T R E N N U N G (P,P) = 218.7 Hz) at d = 8.0 and 6.4 ppm for two nonequivalent trans-PMe 3 ligands. In the 1 H NMR spectrum, the carbonyl protons of two h 2 -benzaldehyde ligands appear as broad, upfield-shifted signals at d = 5.42 (br s) and 5.18 ppm (br d, JA C H T U N G T R E N N U N G (H,P) = 9.6 Hz), [51] that are coupled in the 1 H À 13 C HSQC spectrum to the carbonyl carbon atoms at d = 85.2 and 86.1 ppm, respectively. Increasing the temperature to À40 8C resulted in the dissociation of one PMe 3 group and the formation of the Scheme 10), which was characterized by IR and multinuclear NMR spectroscopy and by X-ray diffraction ( Figure 5).…”