Synthesis, Structure, and Reactivity of Palladacycles That Contain a Chiral Rhenium Fragment in the Backbone: New Cyclometalation and Catalyst Design Strategies

Abstract

The bromocyclopentadienyl complex [(eta5-C5H4Br)Re(CO)3] is converted to racemic [(eta5-C5H4Br)Re(NO)(PPh3)(CH2PPh2)] (1 b) similarly to a published sequence for cyclopentadienyl analogues. Treatment of enantiopure (S)-[(eta5-C5H5)Re(NO)(PPh3)(CH3)] with nBuLi and I2 gives (S)-[(eta5-C5H4I)Re(NO)(PPh3)(CH3)] ((S)-6 c; 84 %), which is converted (Ph3C+ PF6 -, PPh2H, tBuOK) to (S)-[(eta5-C5H4I)Re(NO)(PPh3)(CH2PPh2)] ((S)-1 c). Reactions of 1 b and (S)-1 c with Pd[P(tBu)3]2 yield [{(eta5-C5H4)Re(NO)(PPh3)(mu-CH2PPh2)Pd(mu-X)}2] (10; X = b, Br, rac/meso, 88 %; c, I, S,S, 22 %). Addition of PPh3 to 10 b gives [(eta5-C5H4)Re(NO)(PPh3)(mu-CH2PPh2)Pd(PPh3)(Br)] (11 b; 92 %). Reaction of (S)-[(eta5-C5H5)Re(NO)(PPh3)(CH2PPh2)] ((S)-2) and Pd(OAc)(2) (1.5 equiv; toluene, RT) affords the novel Pd3(OAc)4-based palladacycle (S,S)-[(eta5-C5H4)Re(NO)(PPh3)(mu-CH2PPh2)Pd(mu-OAc)2Pd(mu-OAc)2Pd(mu-PPh2CH2)(Ph3P)(ON)Re(eta5-C5H4)] ((S,S)-13; 71-90 %). Addition of LiCl and LiBr yields (S,S)-10 a,b (73 %), and Na(acac-F6) gives (S)-[(eta5-C5H4)Re(NO)(PPh3)(mu-CH2PPh2)Pd(acac-F6)] ((S)-16, 72 %). Reaction of (S,S)-10 b and pyridine affords (S)-[(eta5-C5H4)Re(NO)(PPh3)(mu-CH2PPh2)Pd(NC5H5)(Br)] ((S)-17 b, 72 %); other Lewis bases yield similar adducts. Reaction of (S)-2 and Pd(OAc)2 (0.5 equiv; benzene, 80 degrees C) gives the spiropalladacycle trans-(S,S)-[{(eta5-C5H4)Re(NO)(PPh3)(mu-CH2PPh2)}2Pd] (39 %). The crystal structures of (S)-6 c, 11 b, (S,S)- and (R,R)-132 C7H8, (S,S)-10 b, and (S)-17 b aid the preceding assignments. Both 10 b (racemic or S,S) and (S)-16 are excellent catalyst precursors for Suzuki and Heck couplings.

Recent Advances in Palladium‐Catalyzed Cross‐Coupling Reactions at ppm to ppb Molar Catalyst Loadings

“…Other types of carbene have successfully been used in promoting the reaction at catalyst loadings of 10 mol ppm or less. A half‐sandwich rhenium cyclopentadienyl palladacycle, a dimeric C,N‐bidentate complex, a P,C‐bidentate ligand, and a NHC with a remote heteroatom, all reported less than 5 results with at least 80% yield using 10 mol ppm or less. In 2008, Joshaghani et al.…”

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

“…We have been interested in developing new types of “organic” chiral hydrogen bond donors that are templated by transition metals. ,− This is part of a larger program that employs functionally inert or “spectator” metals as architectural units in catalysts with orthogonal active sites. , Such organometallic or coordination compounds can exhibit very high activities and often enantioselectivities. However, our initial results with hydrogen bond donors were disappointing.…”

Versatile Behavior of the Schiff Base Ligand 2,5-Me2C6H3C(H)═N(2,4,6-Me3C6H2) toward Cyclometalation Reactions: C(sp2,phenyl)−H vs C(sp3,methyl)−H Activation

“…However, in a few cases this reaction gives cyclometalated complexes with an unusual trinuclear structure 15,16 which exhibits interesting catalytic applications. [17][18][19][20] We have found that this is the case with ligand a when the reaction is carried out at room temperature. Thus, ligand a can be metalated at an aliphatic or at a sterically hindered aromatic carbon and, in the latter case, it may give di-or trinuclear species, showing an unprecedented and versatile behavior.…”