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Organolanthanide-Catalyzed Hydroamination/Cyclization Reactions of Aminoalkynes. Computational Investigation of Mechanism, Lanthanide Identity, and Substituent Effects for a Very Exothermic C−N Bond-Forming Process
journal contribution
posted on 2006-11-06, 00:00 authored by Alessandro Motta, Ignazio L. Fragalà, Tobin J. MarksThis contribution focuses on organolanthanide-mediated catalytic hydroamination processes and analyzes
the exothermic hydroamination/cyclization of a prototypical aminoalkyne, H2N(CH2)3C⋮CR, mediated
by Cp2Sm− complexes, using density functional theory. The reaction is found to proceed in two discrete
steps, namely, cyclization with concerted Ln−C and C−N bond formation and subsequent Ln−C
protonolysis. Dissociation of the cyclized amine then follows to regenerate the active catalyst. Analysis
is carried out for (i) insertion of the triple bond moiety into the Sm−N bond via a four-center transition
state, (ii) subsequent Sm−C protonolysis by a second substrate molecule, (iii) the effects of other Ln+3
ions and aminoalkyne R substituents on the reaction energetics, and (iv) comparison to the analogous,
essentially thermoneutral process for aminoalkenes. DFT energetic profiles are computed for the turnover-limiting aminoalkyne C⋮C triple bond insertion into the Ln−NH− linkage, and the geometries and
stabilities of reactants, intermediates, and products are analyzed. The picture that emerges involves
concerted, rate-limiting, exothermic insertion of the alkyne fragment into the Ln−N(amido) bond via a
highly organized, seven-membered chairlike cyclic transition state (ΔH⧧calcd = 4.6 kcal/mol, ΔS⧧calcd =
−11.9 eu). The resulting cyclized complex then undergoes exergonic protonolysis to yield an amine−amido complex, the likely resting state of the catalyst. Large rate accelerations effected by smaller
lanthanide ions and certain alkyne substituents can be understood in terms of approach distances and
charge buildup in the cyclization transition state.
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Keywords
exergonic protonolysisCp 2Sm complexeshydroamination processesSubstituent Effectsalkyne substituentsbond moietylanthanide ionsLarge rate accelerationsLncyclization transition statecharge buildupH 2NDFTthermoneutral processcatalystalkyne fragmentbond insertionsubstrate moleculereaction energeticsComputational InvestigationLanthanide Identityaminoalkyne R substituentscyclized amineapproach distances
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