A Heteroleptic Dirhodium Catalyst for Asymmetric Cyclopropanation with α‐Stannyl α‐Diazoacetate. “Stereoretentive” Stille Coupling with Formation of Chiral Quarternary Carbon Centers

Abstract The heteroleptic dirhodium paddlewheel catalyst 7 with a chiral carboxylate/acetamidate ligand sphere is uniquely effective in asymmetric [2+1] cycloadditions with α‐diazo‐α‐trimethylstannyl (silyl, germyl) acetate. Originally discovered as a trace impurity in a sample of the homoleptic parent complex [Rh2((R)‐TPCP)4] (5), it is shown that the protic acetamidate ligand is quintessential for rendering 7 highly enantioselective. The ‐NH group is thought to lock the ensuing metal carbene in place via interligand hydrogen bonding. The resulting stannylated cyclopropanes undergo “stereoretentive” cross coupling, which shows for the first time that even chiral quarternary carbon centers can be made by the Stille–Migita reaction.

The low-angle reflections was shadowed by the beamstop and has been omitted from the data set from the final refinement cycles. CCDC-1990590.
NMR spectra were recorded on Bruker DPX 300, AV 400, AV 500 or AV III 600 spectrometers in the solvents indicated; chemical shifts are given in ppm relative to TMS, coupling constants (J) in Hz. LC analyses were conducted on a Shimadzu LC 2020 instrument equipped with a Shimadzu SPD-M20A UV/VIS detector. GC analyses were conducted on an Agilent technologies 7890B instrument with a FID detector.

Ethyl (trimethylgermyl)diazoacetate (1b):
A flame dried 25 mL Schlenk flask was charged with a solution of dimethylamine (1.2 mol•L −1 in THF, 4 mL, 4.80 mmol) and diethyl ether (10 mL). n-BuLi (1.6 mol•L −1 in hexane, 3 mL, 4.16 mmol) was added slowly at −78°C and the resulting colorless suspension was allowed to warm to ambient temperature. After stirring for 30 min, the mixture was concentrated to dryness under reduced pressure to give a white solid. A solution of Me3GeCl (496 mg, 3.24 mmol) in diethyl ether (5 mL) was added at 0°C and the resulting mixture was stirred at 50°C (bath temperature) for 1.5 h. The resulting light yellow slurry was transferred into a 25 mL two neck round bottom flask through a cannula-filter and then distilled at  110°C into a 10 mL Schlenk flask which was cooled in a dry ice/acetone bath to give N,N,1,1,1pentamethylgermanamine as a colorless oil, which was directly used for the next step.

Ethyl (trimethylsilyl)diazoacetate (1c):
A flame dried 50 mL Schlenk flask was charged with a solution of ethyl diazoacetate in CH2Cl2 (87 %, 0.5 mL, 4.14 mmol), N-ethyldiisopropylamine (0.9 mL, 4.96 mmol) and diethyl ether (20 mL). The mixture was cooed to −78°C before a solution of TMSOTf (0.8 mL, 4.14 mmol) in diethyl ether (5 mL) was added dropwise at this temperature. The resulting yellow mixture was allowed to warm to ambient temperature and stirring was continued for 20 h, during which time a colorless solid precipitated. The mixture was cannula filtrated into a flame dried 50 mL two neck flask and all volatile materials were evaporated. The resulting oily, orange residue was purified by distillation at 1•10 −3 mbar; the fraction that distilled at 27-32°C was collected to give the title compound as an orange oil (562 mg, 73 %). 1  The compound can be handled for short periods of time in air but was stored under Ar in a refrigerator.
The flask was charged with dirhodium(II) acetate dihydrate (90 mg, 0.19 mmol), (R)-4 (470 mg, 1.50 mmol), and toluene (20 mL). The mixture was stirred at reflux temperature and the refluxing solvent passed through the Soxhlet extractor. After 12 h, the mixture was cooled to room temperature and concentrated under reduced pressure. The green residue was purified by flash chromatography (silica, acetonitrile/toluene, 1:9) to give the title compound as a green solid (205 mg, 0.14 mmol, 75 %).
With this pure sample of known structure in hand, the HPLC peaks of identical mass corresponding to these two geometric isomers could be assigned. With this additional information, one can deduce from the data recorded during reaction monitoring that (