Agostic complexes via nucleophile addition/protonation reactions of .eta.6-benzene tris(trifluorophosphine)chromium

Reactivity of (η6-Arene)tricarbonylchromium Complexes toward Additions of Anions, Cations, and Radicals

“…These latter steps are in direct analogy to the known arene complex chemistry of nucleophilic addition followed by protonation as first reported by Semmelhack 65 and mechanistically elucidated by Ku ¨ndig. 66 While ketyl-olefin coupling reactions represent one of the most widely studied SmI 2 -promoted radical processes, 39c,67 ketylarene coupling reactions are rare. 68 Use of arene chromium complexes can now dramatically broaden the scope of these coupling reactions.…”

Transition-Metal-Mediated Dearomatization Reactions

“…Extensions to other nucleophiles have been reported, and with different aromatic systems, deviations of this pattern have become apparent. Hydride (LiBHEt 3 ) has been shown to add to (benzene)Cr(CO) 3 and also to the closely related (benzene)Cr(PF 3 ) 3 complex. , When an imine, oxazoline, or hydrazone substituent is attached to the aromatic ring, hard organolithium reagents (e.g. n-BuLi, MeLi) will act as nucleophiles rather than as bases as is the case in reactions with (benzene)Cr(CO) 3 . , Limitations to the scope of nucleophilic additions to (arene)Cr(CO) 3 complexes result from the moderate electrophilicity imparted by the Cr(CO) 3 group (comparable to an arene nitro substituent), the activation of the arene hydrogens toward deprotonation, and the potential attack by the nucleophile at CO.…”

“…Hydride addition followed by protonation yielded complex 7 . Kinetic investigations show 1,5-hydride migration (which involves C−H bond breaking) and 1,2-hydride (which involves M−H bond breaking) exchange processes in the agostic complex to be so rapid that only cooling to −130 °C stopped the processes which interconvert the complexed isomeric dienes (Scheme ) 8 Addition of Hydride Followed by Protonation Leads to a Highly Fluxional Agostic System: (iii) 1,2-H-Exchange; (iv) 1,5-H-Migration …”

Reduction of Borazines Mediated by Low‐Valent Chromium Species

“…Because hydride equivalents can be generated using H 2 and/or protonation of reduced metal fragments, these reactions demonstrate the feasibility of a reduction strategy that could in principle proceed through low-energy pathways, which remains a challenge for the regeneration of B-N hydrogen storage materials.The low-valent chromium fragment {Cr(CO) 3 } was targeted as a platform capable of binding borazine and subsequently mediating the stepwise reduction of the ring. Prior studies on analogous arene-{Cr(CO) 3 } adducts showed that the sequential addition of hydride and proton equivalents afforded cyclohexadiene adducts; [8] we hypothesized that similar reactivity would be possible when arenes were replaced with borazines. Alkyl-substituted borazines were selected as suitable model substrates for spent AB fuels because the thermodynamic requirements for protio and alkyl B=N bond reduction are similar.…”