Terminal Alkyne Coupling Reactions through a Ring: Mechanistic Insights and Regiochemical Switching

Abstract

The mechanism and selectivity of terminal alkyne coupling reactions promoted by rhodium(I) complexes of NHC‐based CNC pincer ligands have been investigated. Synthetic and kinetic experiments support E‐ and gem‐enyne formation through a common reaction sequence involving hydrometallation and rate‐determining C−C bond reductive elimination. The latter is significantly affected by the ligand topology: Employment of a macrocyclic variant enforced exclusive head‐to‐head coupling, contrasting the high selectivity for head‐to‐tail coupling observed for the corresponding acyclic pincer ligand.

Terminal Alkyne Coupling Reactions through a Ring: Mechanistic Insights and Regiochemical Switching

“…Selected data for 7:Rh1-C2 1.940 (5) ,C2-C3 1.204 (8) ;Rh1-C2-C3 176.6(5)8 8;R h1-C4 1.983 (5) ,C4-C5 1.310 (8) ;Rh1-C4-C5 140.1(4)8 8, C2-Rh1-C4 96.6(2)8 8;R h1-C6 2.435 (5) ,Rh1-N101 2.228 (5) ,Rh1-C109 2.066 (5) ,Rh1-C115 2.075(6) ;for 8:R h1-Cnt(C2,C3) 1.981 (3) ,C2-C3 1.255(4) ,C2-C4 1.425(4) ,C4-C5 1.338 (4) ; C2-C4-C5 124.7(3)8 8;Rh1-N101 2.105 (2) ,Rh1-C109 2.052 (3) ,Rh1-C115 2.042 (3) ;Py-Rh-CCtwist 38.7(2)8 8. [13] Scheme 3. Preparation of PhCCC(CH 2 )Ph and E-PhCCCH=CHPh.…”

“…Presumably as aconsequence of the confined metal coordination sphere evident in the solid state, 7 is appreciably dynamic in solution on the NMR timescale (500 MHz;see the Supporting Information). [13] Scheme 2. [13] Scheme 2.…”

“…By comparison to the acyclicc ongener CNC-Me,t he steric constraints imposed by the flexible ring raise the barrier for C À Cb ond reductive elimination from aRh III gem-alkenyl alkynyl species relative to the alternative Rh III E-alkenyl alkynyl intermediate by at least DG°(298 K) = 13 AE 6kJmol À1 ,t riggering as witch in product selectivity.T hese observations provide new insight into how terminal alkyne coupling reactions can be controlled and, more generally,s howcase an unconventional approach for tuning the reactivity of pincer ligands. [13] Scheme 3. Our future work is focused on exploring the application of these lutidine-based CNC ligands in catalysis,supramolecular,a nd organometallic chemistry.…”

“…IV in Scheme 1). Notable spectroscopic features of 6 include C 1 symmetry,t wo 18H tBu resonances,g eminal alkene 1 H resonances at d 3.21 and 4.79, and 13 Cr esonances at d 101.9 (RhC C, 1 J RhC = 55 Hz) and 153.8 (RhC(CH 2 )Ar', 1 J RhC = 38 Hz) that display large 103 Rh coupling. [15] Isolation of 6 from solution was encumbered by facile reductive elimination of Ar'CCC(CH 2 )Ar'.…”

“…Selected data for 1:Rh1-Cnt(C2,C3) 2.033 (3) ,C2-C3 1.373(4) , Rh1-N101 2.116(2) ,Rh1-C109 2.037(2) ,Rh1-C115 2.042 (2) ;Py-Rh-C=Ctwist 16.0(2)8 8;f or 2:R h1-Cnt(C2,C3) 1.996 (3) ,C2-C3 1.363 (5) ,Rh1-N101 2.116(2) ,Rh1-C109 2.042 (3) ,Rh1-C115 2.061 (3) ;Py-Rh-C=Ct wist 31.7(2)8 8. [13] Scheme 2. Reactionsof1 and 2 with HCCAr'.…”