Concatenating Structural Constraint Effects at Tin for the Sequential Generation, Stabilization, and Transfer of Acyclic Aminocarbenes

Abstract

Structural constraint approaches have been employed toward different ends in recent years, from augmenting the nucleophilicity in pyramidalized low-valent p-block compounds to enhancing the Lewis acidities at planarized tetravalent p-block elements. While previous studies exploited these effects separately, this work introduces a strategy to concatenate structural constraint approaches at individual stages of a reaction sequence in a row to unlock a synthetic path unattainable by conventional methodologies. The boosted nucleophilicity resulting from the constrained tetracoordinated calix[4]pyrrolato stannate(II) dianion enables the reductive formation of sterically unprotected acyclic aminocarbenes. These amino carbenes are stabilized at the concomitantly formed square-planar stannane(IV) as air-stable adducts. Transfer of the carbenes onto copper(I) by cooperativity of the calix[4]pyrrole ligand finalizes this protocol to hitherto unreported yet prototypical carbene complexes. Detailed spectroscopic and quantum theoretical analyses establish the synergy of structural constraints and element−ligand cooperation as the linchpin to this reaction path and its selectivity.

Conformational and Substitution Effects on the Donor and Reducing Strength of Tin(II) Porphyrinogens

“…[2,[9][10][11] We recently reported the tetracoordinated, structurally constrained meso-octaethylcalix [4]-pyrrolato stannate(II) dianion [2 Et ] 2À and explored its chemistry as a strong tin-centered σdonor and nucleophile as well as reducing agent (Figure 1A). [12,13] The calix [4]pyrrolato ligand acts as a macrocyclic, tetradentate scaffold which imparts a structurally constrained coordination environment around the tin(II) center, leveraging its reducing and nucleophilic properties. Beneficially, the straightforward synthesis of the calix [4]pyrrole ligand system H 4 [1 R ] via the acid-catalyzed condensation of pyrrole with ketones allows its fast modification simply by employing different ketones or pyrroles as starting material (Figure 1 B).…”