A Rigid Cyclic (Alkyl)(amino)carbene Ligand Leads to Isolation of Low‐Coordinate Transition‐Metal Complexes

Abstract

Keywords agostic interactions; carbene ligands; low-coordinate complexes; palladium; rhodiumIn the last decade the use of diamino N-heterocyclic carbenes (NHCs) A as ancillary ligands for transition-metal catalysts and as organic catalysts on their own has proven very fruitful.[1] The efficiency of NHCs is attributed to their strong σ-donor properties and sterically demanding structure.[1,2] Not only have NHCs yielded improved transition metal catalysts, but they have also lead to the isolation of unusual low-coordinate metal complexes,[2] which often play a key role in catalytic processes. Recently, we reported the synthesis of stable cyclic (alkyl)(amino)carbenes (CAACs) B[3] and demonstrated that CAACs can be even stronger σ donors than NHCs, and lead to highly efficient palladium catalysts for the α-arylation of ketones and aldehydes. Here we report that the peculiar steric and electronic properties of rigid CAAC ligands allow the preparation of low-coordinate metal complexes, hitherto not isolable with any other ligands.To further evaluate the electronic properties of rigid CAAC B1, we attempted to prepare the [RhCl(CO) 2 (B1)] complex, since the CO stretching frequencies of this type of rhodium complexes are recognized as an excellent measure of the σ-donor ability of the ligand L, and a large amount of data is available for comparison.[4] Reaction of carbene B1 with half an equivalent of [{RhCl(cod) 2 } 2 ] (cod = 1,5-cyclooctadiene) afforded complex 1, which after purification by column chromatography on silica gel was obtained as orange crystals in 79% yield (Scheme 1). The structure of 1 was unambiguously determined by single-crystal X-ray diffraction.[5] Following the classical procedure, a solution of 1 in chloroform was treated with an excess of CO. Surprisingly, the 13 C NMR spectrum of the product (95% yield) showed only one CO signal, with a large coupling constant (J Rh,C = 134 Hz), and not two signals with smaller J values (J Rh,C = 33-82 Hz), as observed for [RhCl(CO) 2 (L)] complexes. [4] It is known that when bulky ligands L are present [RhCl(CO) 2 L] complexes can readily lose one CO ligand ** We are grateful to the NIH (R01 GM 68825) and RHODIA for financial support of this work.

Allene formation by gold catalyzed cross-coupling of masked carbenes and vinylidenes

“…The most common systems involve LAuCl complexes (L ϭ monodentate ligand), which, through in situ salt metathesis reactions, generate the active species often postulated to be LAu ϩ . Because CAACs have been shown to stabilize cationic species (22), where other ligands were ineffective, they seemed particularly well suited for the preparation of robust LAu ϩ catalysts.…”

“…Considering the lack of efficient and versatile catalytic processes to assemble directly the skeletal carbons of the allene -system from two different fragments, a general coupling protocol is highly desirable. Here, we report the synthesis of an isolable cationic Au(I) 2 -toluene complex featuring a cyclic (alkyl)(amino)carbene (CAAC) ligand (21)(22)(23)(24). We demonstrate its ability to mediate the efficient catalytic coupling of alkynes and enamines to yield a wide range of nonterminal, unsymmetrically substituted, allenes.…”

Beyond ConventionalN-Heterocyclic Carbenes: Abnormal, Remote, and Other Classes of NHC Ligands with Reduced Heteroatom Stabilization

“…131 Subsequent metal coordination gave the complexes 185 and 187 (see Scheme 63). 132 The Rh center spontaneously lost one CO ligand upon carbene bonding to afford 185 complex as a rare 14e species. The 14e palladium carbene complex 187 was accessible by coordination to [Pd(allyl)Cl] 2 and subsequent Ag + -mediated halide abstraction.…”

Synthesis of an extremely sterically shielding N-heterocyclic carbene ligand

“…8 Ligands of the CAAC type have been introduced by the Bertrand group. 9,10 These ligands stand out due to their "flexible steric bulk", which enables them to influence transition metal catalysts' activities. 11,12 Berthon-Gelloz and Markó enhanced the IPr system 13 by formally replacing four isopropyl groups by four benzydryl substituents, and adding two methyl substituents, thereby generating the even bulkier IPr* ligand.…”