Syntheses of Enantiopure Bifunctional 2-Guanidinobenzimidazole Cyclopentadienyl Ruthenium Complexes: Highly Enantioselective Organometallic Hydrogen Bond Donor Catalysts for Carbon–Carbon Bond Forming Reactions

Abstract

2-Guanidinobenzimidazole (GBI) derivatives with a NHR group in place of NH 2 (R: a, CH 2 Ph; b, (S C )-CH(CH 3 )Ph; c, (R C R C )-CH(CH 2 ) 4 CHNMe 2 ; d, (R C R C )-CH-(CH 2 ) 4 CH-NCH 2 (CH 2 ) 3 CH 2 ) are prepared in four steps from 2-aminobenzimidazole. Reactions with [(η 5 -C 5 H 5 )Ru(CO)-(NCCH 3 ) 2 ] + PF 6 − (5 + PF 6 − ) afford the chiral-at-metal chelates [(η 5 -C 5 H 5 )Ru(CO)(GBI-R)] + PF 6 − (6a−d + PF 6 − , 39−77%), which have been characterized by NMR ( 1 H, 13 C, 31 P, 19 F) and other spectroscopy. The Ru,C configurational diastereomers of the dimethylamino containing complex 6c + PF 6 − separate upon alumina chromatography (R Ru R C R C , >99:01 dr; S Ru R C R C , <2:98 dr) but slowly epimerize at ruthenium in solution at room temperature. Configurations have been assigned by CD spectra and the crystal structure of (R Ru R C R C )-6c + (Δ)-TRISPHAT − (TRISPHAT − = P(o-C 6 Cl 4 O 2 ) 3 − ). The latter reveals hydrogen bonding between two of the GBI-R NH linkages and the TRISPHAT oxygen atoms. Both diastereomers catalyze (10 mol %, room temperature) additions of malonate esters to nitroalkenes in high yields and enantioselectivities (90−99% ee for aryl-substituted nitroalkenes). The dominant product configurations are identical, showing the chiral ruthenium center to have little influence. The free GBI-R ligand exhibits only modest activity. Unlike most transition-metal-catalyzed reactions, there is no direct interaction of the substrate with the ruthenium; rather, evidence is presented for "second coordination sphere promoted catalysis" mediated by hydrogen bonds derived from two NH groups that chelation preorganizes in a synperiplanar conformation.

N‐Heterocyclic Carbene Complexes of Cp*–Aluminum Triflate

“…Being a coordinatively and electronically flexible substituent of transition metals, the cyclopentadienyl ligand (Cp – ) has played a critical role in developing organometallic catalysts . Metal complexes featuring only one Cp moiety in the so‐called piano‐stool geometry have also been intensively studied.…”

Thiourea and Guanidine Compounds and Their Iridium Complexes in Drug‐Resistant Cancer Cell Lines: Structure‐Activity Relationships and Direct Luminescent Imaging

“…The second imidazoyl unit was displaced by a selection of amines (n‐propylamine, n‐butylamine, n‐hexylamine, and benzylamine) in the presence of dimethylaminopyridine (DMAP) in dimethylformamide (DMF) to give compounds 1S to 4S in 63–81 % yield after purification . Portions of these compounds were converted to the corresponding guanidines by reaction with mercury(II) oxide and methanolic ammonia in chloroform to give 1N to 4N in 40–65 % yield . Two iridium complexes based on these systems were previously prepared by reacting 2S and 2N separately with [Ir(ppy) 2 Cl] 2 in toluene in the presence of potassium carbonate, giving bright yellow powders Ir ‐ 2S and Ir ‐ 2N in 53 % and 44 % yield respectively as racemates (confirmed for Ir ‐ 2N by single crystal X‐ray diffraction) .…”

Werner Complexes with ω‐Dimethylaminoalkyl Substituted Ethylenediamine Ligands: Bifunctional Hydrogen‐Bond‐Donor Catalysts for Highly Enantioselective Michael Additions

“…Allied themes have been studied by Meggers and others . Importantly, the performances of hydrogen‐bond‐donor catalysts are often markedly improved when they are rendered bifunctional . For example, thioureas constitute a widely applied class of twofold NH bond donors, and the incorporation of tertiary amines can give catalysts that afford superior rates, yields, and enantioselectivities in addition reactions…”

“…[2] We have sought to broaden this field to include inorganic and organometallic hydrogen-bond donors that feature earth-abundant metals and chirality motifs not normally achievable in organic architectures. [3][4][5] Allied themes have been studied by Meggers and others. [6][7][8] Importantly,t he performances of hydrogen-bond-donor catalysts are often markedly improved when they are rendered bifunctional.…”