Chiral-at-Ruthenium-SEGPHOS Catalysts Display Diastereomer-Dependent Regioselectivity: Enantioselective Isoprene-Mediated Carbonyl tert-Prenylation via Halide Counterion Effects

Abstract

The first correlation between metal-centered stereogenicity and regioselectivity in a catalytic process is described. Alternate pseudo-diastereomeric chiral-at-ruthenium complexes of the type RuX(CO)[η 3 -prenyl][(S)-SEGPHOS] form in a halidedependent manner and display divergent regioselectivity in catalytic C−C couplings of isoprene to alcohol proelectrophiles via hydrogen autotransfer. Whereas the chloride-bound ruthenium-SEGPHOS complex prefers a trans-relationship between the halide and carbonyl ligands and delivers products of carbonyl secprenylation, the iodide-bound ruthenium-SEGPHOS complex prefers a cis-relationship between the halide and carbonyl ligands and delivers products of carbonyl tert-prenylation. The chlorideand iodide-bound ruthenium-SEGPHOS complexes were characterized in solution and solid phase by 31 P NMR and X-ray diffraction. Density functional theory calculations of the iodide-bound catalyst implicate a Curtin−Hammett-type scenario in which the transition states for aldehyde coordination from an equilibrating mixture of sec-and tert-prenylruthenium complexes are rate-and product-determining. Thus, control of metal-centered diastereoselectivity has unlocked the first catalytically enantioselective isoprene-mediated carbonyl tert-prenylations.

Palladium(I)-Iodide-Catalyzed Deoxygenative Heck Reaction of Vinyl Triflates: A Formate-Mediated Cross-Electrophile Reductive Coupling with cine-Substitution

“… − Our collective data corroborate a mechanism in which Pd­(OAc) 2 and Bu 4 NI form the dianionic iodide-bridged dimer [Pd 2 I 6 ]­[NBu 4 ] 2 , which, under reducing conditions, serves as a reservoir for the active palladium­(I) complex [Pd 2 I 4 ]­[NBu 4 ] 2 . Our data also corroborate the key role of iodide counterions vis-á-vis stabilization of palladium­(I) under reductive coupling conditions. − These transformations represent the first nonphotochemical reductive C–C couplings via palladium­(I)-catalysis and highlight the distinct reactivity of dinuclear iodide-bridged palladium­(I) complexes. − …”

“…The presence of Bu 4 NI was found to be essential (Table , entries 2–4). As documented by Schoenebeck et al, iodide counterions stabilize palladium(I) species. , Indeed, the unique influence of iodide counterions in ruthenium-catalyzed C–C coupling via hydrogen transfer is what led us to palladium(I) catalysis. , The ammonium cation of Bu 4 NI is also necessary, as other iodide sources failed to animate the catalytic process (Table , entry 5). Bu 4 NI and Pd(OAc) 2 form the dianionic iodide-bridged dimer [Pd 2 I 6 ][NBu 4 ] 2 , which, in the presence of NaO 2 CH, is a latent source of the palladium(I) complex [Pd 2 I 4 ][NBu 4 ] 2 ( vide supra ), which is a competent catalyst for the reaction (Table , entry 6).…”

Cu-Catalyzed Asymmetric Dicarboxylation of 1,3-Dienes with CO2

“…Based on the previous reports − and our mechanistic studies, we proposed the following possible catalytic cycle (Figure F). First, the reaction of B 2 pin 2 and Cu­(I) complex I , which is generated in situ from the precatalyst and base, would form reactive boryl-copper species II .…”

“…With our continuing interest in asymmetric C–C bond formation with CO 2 , ,,, we wondered whether we could develop a catalytic asymmetric dicarboxylation with CO 2 . Considering the high importance of chiral adipic acids and ready availability of 1,3-dienes, , we hypothesized that catalytic asymmetric dicarboxylation of 1,3-dienes with CO 2 would be an ideal route to chiral adipic acids. Specifically, we envisioned that Cu-catalyzed boracarboxylation of 1,3-dienes with CO 2 would give carboxylated allyl boronic ester intermediates, which might undergo a subsequent Cu-catalyzed second carboxylation of C–B bonds with CO 2 to give dicarboxylates .…”

Leveraging the Stereochemical Complexity of Octahedral Diastereomeric-at-Metal Catalysts to Unlock Regio-, Diastereo-, and Enantioselectivity in Alcohol-Mediated C–C Couplings via Hydrogen Transfer

“…An especially compelling illustration of the effect of metalcentered stereogenicity on selectivity is found in the C−C coupling of primary alcohols with isoprene (Figure 9). 30 Here, pseudo-diastereomeric chiral-at-ruthenium complexes RuX-(CO)[η 3 -prenyl][(S)-SEGPHOS], where X = Cl and I, deliver products of sec-prenylation and tert-prenylation, respectively. To determine the origins of regiodivergence, 31 the parent chlorideand iodide-bound π-allylruthenium complexes modified by SEGPHOS were analyzed via single-crystal X-ray diffraction.…”