Catalytic 1,3‐H Atom Shift of a Terminal Benzylic Alkyne by Iron and Alkali Metal Silylamides – Switching between Allene and Internal Alkyne

Abstract

Herein the examination of the transformation of alkynes by low‐coordinate iron silylamides is presented. An anionic linear iron(I) silylamides ([Fe(N{Dipp}SiR3)2]−; Dipp=2,6‐di‐iso‐propylphenyl, R=Me) acts as precatalyst for the cyclotrimerization of diphenyl acetylene but is unable to transform internal alkynes with aliphatic substituents or terminal alkynes accordingly. For a benzylic, terminal alkyne, however, the 1,3‐H‐shift to the internal alkyne is observed and proceeds via phenyl allene. Using 10 mol% of the iron complex, the terminal alkene is selectively transformed to phenyl allene within minutes, and further fully transformed to the internal alkyne within 24 h. The transformation is retraced on a stoichiometric level and leads to the isolation of a π‐alkyne complex with a shifted triple bond. Further, the anionic, trigonal iron(II) silylamide [Fe(NR2)3]− also mediates the catalytic conversion of the terminal alkyne but is restricted to allene formation. Overall, a deprotonation/reprotonation mechanism is assumed for these transformations. This was ultimately proven by using potassium hexamethyldisilazanide KNR2, which is an even more active catalyst for the complete triple bond shift.

Intricate Road to Linear Anionic Nickel(I) Hexamethyldisilazanide [Ni(N(SiMe3)2)2]−

“…They are highly reactive, as expected from thefor these metalslow oxidation state in conjunction with the low-coordinate environment. As such, they have been exploited for a number of different reactivities such as C–F bond cleavage, , C–C multiple bond transformation, − reduction, , and a platform for either low-coordinate π-acceptor adducts (CO or alkynes), ,, imido, or chalcogenido complexes . Given the multifaceted use of these linear compounds, we were naturally interested in the respective nickel­(I) complex, which constitutes the heaviest open-shell linear metal­(I) hexamethyldisilazanide.…”

Synthesis, Properties, and Reactivity of a Linear NHC-Based Chromium(I) Silylamide

“…Complexes with two-coordinate, open-shell 3d-metal­(I) ions, known for chromium to nickel, are a particular class of compounds in coordination chemistry. − They show interesting single molecule magnetic properties, − and can be used for bond activation, which has focused so far on the metals iron, − cobalt, ,,− and especially nickel. − In contrast, the chemistry of two-coordinate chromium­(I) is considerably less developed. This is remarkable as the linear chromium­(I) complex [TerCr­(PMe 3 )] (Ter = 2,6-Tripp 2 -3,5- i Pr 2 -C 6 H, Tripp = 2,4,6- i Pr 2 -C 6 H 2 ) was the very first example within this class of molecules (Figure A) .…”

Homotropic Cooperativity in Iron-Catalyzed Alkyne Cyclotrimerizations

“…Herein, we showcase an unprecedented case of homotropic cooperativity in the regioselective [2+2+2]-cycloaddition of acetylenes to 1,2,4-(aryl)benzenes catalyzed by a two-coordinate iron complex ( 1 , Figure 1 ). The potential of such complexes in this field is poorly explored, with complexes [Fe(IPr){N(SiMe 3 )(dipp)}], 8 [Fe{N(SiMe 3 )(R)} 2 ] − , 9 and [Fe{N(SiMe 3 ) 2 } 2 ], 10 as the sole previously reported examples ( Figure 1 ).…”