Palladium-Catalyzed Asymmetric Phosphination. Enantioselective Synthesis of PAMP−BH₃, Ligand Effects on Catalysis, and Direct Observation of the Stereochemistry of Transmetalation and Reductive Elimination

Abstract

The complexes Pd(diphos)(o-An)(I) (o-An = o-MeOC6H4; diphos = dppe (3), (S,S)-Chiraphos (4), (R,R)-Me-Duphos (5), (R,S)-t-Bu-Josiphos (6), (R)-Tol-Binap (7)) were prepared. Complex 6 catalyzed the coupling of PH(Me)(Ph)(BH3) (2) with o-AnI in the presence of base to yield PAMP−BH3 (P(Me)(Ph)(o-An)(BH3) (1)) in low enantiomeric excess. The course of stoichiometric reactions of 3−7 with 2 and NaOSiMe3 depended on the diphosphine ligand. Complexes 6 and 7 gave PAMP−BH3 (1) and Pd(0) species; no intermediates were observed. With 3, the intermediate Pd(dppe)(o-An)(P(Me)(Ph)(BH3)) (10) was observed by 31P NMR, while 4 gave the isolable diastereomeric palladium complexes (S P )-Pd((S,S)-Chiraphos)(o-An)(P(Me)(Ph)(BH3)) (11a) and (R P )-Pd((S,S)-Chiraphos)(o-An)(P(Me)(Ph)(BH3)) (11b), whose absolute configurations were determined by X-ray crystallography after separation. The analogous Pd((R,R)-Me-Duphos)(o-An)(P(Me)(Ph)(BH3)) diastereomers (12a,b) were also separated and isolated. Treatment of 4 with highly enantioenriched 2 (R or S) gave 11a or 11b in high diastereomeric excess with retention of configuration at phosphorus. P−C reductive elimination from either isomer of highly diastereoenriched 11 in the presence of excess diphenylacetylene yielded Pd((S,S)-Chiraphos)(PhC⋮CPh) (14) and highly enantioenriched PAMP−BH3 (1), with retention of configuration.

Recent Advances in Metal-Catalyzed C–P Bond Formation

“…Slowing reductive elimination with a Pd-C 6 F 5 group enabled isolation and separation of the diastereomers of an analog of the key intermediate (Scheme 56) [94]. The stereochemical details of Pd-P bond formation and P-C reductive elimination, which both proceed with retention at phosphorus, had been elucidated earlier in related Pd(Chiraphos) complexes [95,96].…”

Mechanistic Insights into the Palladium‐Catalysed Asymmetric Phosphination of Cyclohexenyl Triflate

“…Thin-layer chromatography (TLC) was performed on silica gel 60 F-254 plates (0.1 mm) with iodine or UV detection. 1 H, 13 C, 19 F and 31 P NMR spectra were obtained on Bruker DPX 250 or AC 400 spectrometers, 11 B NMR spectra were recorded on a Bruker AC 400 spectrometer.…”

“…Hence, the putative catalyst 3 was prepared following the route described in Scheme 3, through a synthesis inspired by the chemistry developed by Brown on arylic cationic oxidative complexes for the study of reactive intermediates in the Heck reaction. [10] The first step is the preparation of the new iodide complex 7 following an adapted procedure inspired by Glueck [11] and Drago's [12] works on the preparation of aryliodide oxidative addition complexes. Pd 2 (dba) 3 ·CHCl 3 was first treated with TMEDA and iodocyclohexene, [13] to give, after work-up, the air stable complex Pd(TMEDA)(cyclohexenyl)(I) 7 in 77 % yield.…”

“…Similar behaviour was reported in the related Pt(DIOP)(o-An)(I) [21] and [Pd(DUPHOS)(o-An)-{PPhMe(BH 3 )}] complexes. [11] Table 2. 31 P{H} NMR spectroscopic data for 5 at -30°C.…”

“…This observation also highlights the high electrophilic reactivity of 3 even at low temperature. The group of Glueck having previously demonstrated in the synthesis of the Pamp ligand that P-C reductive elimination from Pd II derivatives proceeded through retention of configuration, [11] (S P ) configuration was assigned to the major enantiomer of phosphaneborane 2 obtained in the coupling. The correlation with HPLC, thus enabled us to assign (S P ) configuration to enantiomer e 2 and (R P ) to enantiomer e 1 .…”