Abstract
The prevailing reductive chemistry of Sm(II) has been accessed and explored mostly by the use of samarocene precursors. The highly reducing character of these congeners, along with their Lewis acidity and predominantly ionic bonding, allows for the relatively facile activation of C–H bonds, as well as peculiar transformations of unsaturated substrates (e.g., C–C couplings). Among other important C–C coupling reactions, the reaction of phenylacetylene with different mono- or bimetallic samarocene complexes affords trienediyl complexes of the type {[(C5Me5)2Sm]2(µ-η2:η2-PhC4Ph)}. In contrast, when t-butylacetylene is used, uncoupled monomers of the type (C5Me5)2Sm(C≡C–tBu) were obtained. Although this type of reactivity may appear to be simple, the mechanism underlying these transformations is complex. This conclusion is drawn from the density functional theory (DFT) mechanistic studies presented herein. The operating mechanistic paths consist of: (i) the oxidation of each samarium center and the concomitant double reduction of the alkyne to afford a binuclear intermediate; (ii) the C–H scission of the acetylinic bond that lies in between the two metals; (iii) a dual metal σ-bond metathesis (DM|σ-SBM) process that releases H2; and eventually (iv) the C–C coupling of the two bridged μ-alkynides to give the final bimetallic trienediyl complexes. For the latter mechanistic route, the experimentally used phenylacetylene was considered first as well as the aliphatic hex-1-yne. More interestingly, we shed light into the formation of the mono(alkynide) complex, being the final experimental product of the reaction with t-butylacetylene.
Highlights
The versatility of coupling reactions mediated by single-electron transfer (SET) from Sm(II)complexes is clearly illustrated by the chemistry of SmI2 [1,2]
All these sequences share a common initiation step, the double SET one, in which a doubly reduced phenylacetylene unit contained in a dinuclear samarium(III) complex is formed
From this “key intermediate” the reaction can proceed either through: (A) a direct insertion of PhC≡CH that leads to an alkynyl bis-Sm(III) complex; (B) a peculiar direct H–H coupling type of Transition State (TS); (C) a C–H activation leading to a μ-H:μ-C motif
Summary
The versatility of coupling reactions mediated by single-electron transfer (SET) from Sm(II). Lewis acidity and the ionic bonding of these Sm(II) complexes offer ligation of substrates that can undergo facile activation of C–H bonds, or partial reduction of unsaturated compounds [8] These two elementary steps can combine to achieve dehydrocoupling of alkynes, as was initially reported by. With HC≡CR (R = tBu and Ph) to form the corresponding trienediyl dinuclear complexes [13] The latter group concluded that the uncoupled dimer [(Cp*)2La(C≡C–R)]2 is the direct kinetic precursor of this reaction, and that neither a redox-active lanthanide ion nor a phenyl substituent on the alkyne is required for its completion. The formation of the uncoupled complexes, instead of the trienediyl one where tBuC≡CH was used, is discussed Based on these studies, novel transition states which lead to unique reaction pathways in the domain of f-block chemistry are revealed
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
