1,6‐Enyne Cyclizations Catalyzed by N‐Heterocyclic Carbene Supported Gold Complexes: Deconvoluting Sterics and Electronics

Abstract

A series of N‐heterocyclic carbene (NHC) supported gold(I) complexes were synthesized, characterized in solution as well as in the solid state, and examined as precatalysts for 1,6‐enyne cycloisomerization reactions. The product mixtures obtained for a variety of 1,6‐enyne isomerizations were governed primarily by the electronic properties of the aforementioned precatalysts. In particular, precatalysts containing electronic‐rich NHC ligands showed increased selectivity for bicyclic products, whereas analogous precatalysts supported by electron‐deficient NHCs preferentially afforded olefinic products.

A Comparison of the Stability and Reactivity of Diamido‐ and Diaminocarbene Copper Alkoxide and Hydride Complexes

“…Aw hole equivalent of Et 3 SiH was now consumed by reactionw ith 1 in the presenceo fp hosphine. The proton NMR spectrum of 9 was consistentw itht he C s symmetric 6-MesDAC-Hl igand, the NCHNr esonance at d = 5.96 ppm now correlating to a 13 Cr esonance at d = 68.7 ppm, split into ad oublet with a 2 J CÀP coupling of 62 Hz. The IR spectrum showed carbonyl bandsf or 9 at 1662 and 1632 cm À1 ,a pproximately 80 cm À1 lower in frequency than those of 1 and 3 due to the loss of conjugationb etween the amide Nl one pairs and diamidomethine carbon centre.…”

“…[20] The orange solution turned instantly dark red and then, within seconds, afforded ay ellow suspension. Thes olid component was identified by powder X-ray diffraction as nanoparticulate Cu 2 O( see the Supporting Information), whereas the 1 HNMR spectrum of the solution showedt he presence of as ingle copper-containing species( present alongside tBuOH), which was characterised by X-ray diffraction as ad ifferent ringopenedc arbene product, [(6-MesDAC)Cu(OC(O)CMe 2 C(O)N(H)- (1)ÀC (1) 1.874(2), Cu(1)ÀO (1) 1.8016 (15), O(1)ÀC (23)1 .407(3), C(1)ÀCu (1)-O(1) 175.31(8), Cu(1)-O(1)-C(23) 124.75 (13).…”

“…[11] Derivatives of the six-and seven-membered ring carbenes, which contain carbonyl groups a-t ot he nitrogen atoms, are also known. These diamidocarbenes, or DACs, exhibit ambiphilic properties; [12] they are nucleophilic enought oc oordinate to metal centres, [12,13] but can also behave as electrophilesi nb ringinga bout small molecule activation. [13a, 14] In previousw ork, [15] we described what provedt ob e ultimately fruitless attempts to prepare the diamidocarbene alkoxide complex [(6-MesDAC)CuOtBu] (1)t hrough the addition of MOtBu (M = Li, Na, K) to Cu(m-Cl)] 2 .I n contrast to Na and K tert-butoxide, which gave intractable mixtures, LiOtBu reactedc leanly,b ut only to bring about partial salt metathesis to give the ate complex [(6-MesDAC)CuCl· LiOtBu·2THF].W en ow report alternative strategies that have allowed the successful synthesis of 1,t ogether with the diaminocarbene analogue[ (6-Mes)CuOtBu] (2).…”

The chemistry of the carbon-transition metal double and triple bond: Annual survey covering the year 2014

“…1241), followed by ring expansion, oxonium ion hydrolysis, and demetallation. The effect of ancillary 6-membered ring NHC ligands on the formation of fused cyclopropanes vs. ring opened compounds noted that the more electron-rich ligands afforded more of the cyclopropane compound [1183]. A computational study of enyne cycloisomerization followed by ring opening with ethanol was reported, emphasizing regioselectivity (5-vs 6-membered ring M a n u s c r i p t 151 formation) issues [1184].…”

Physical Organic Chemistry by Any Other Name Would Smell as Sweet

“…Bielawski and co‐workers explored the electronic effects of N ‐heterocyclic carbenes (NHCs) in gold complexes to tune the ratio of bicyclic versus olefin products for 1,6‐enyne cyclizations (Figure 12). 13 By designing NHCs that were sterically similar, they were able to separate steric and electronic effects on the reaction outcome. They found that 13 C NMR resonances of the carbenoid nuclei correlated with the Tolman electronic parameters, which are a measure of the electron‐donating properties determined by the frequency of the IR CO stretch.…”