The challenging control of the absolute configuration of chiral-at-metal complexes is efficiently achieved using the tripodal tetradentate ligand L. The optical resolution of rac-[RhCl(κC,N,N',P-L)] mediated by (S)-α-phenylglycine provides access to enantiopure complexes of general formula [Rh(κC,N,N',P-L)A(Solv)][SbF] that enantioselectively catalyze the Diels-Alder reaction between methacrolein and HCp with enantiomeric ratio of up to >99/1. The nature of the active species, the origin of the enantioselectivity and mechanistic details are disclosed by means of NMR spectroscopy and DFT studies.
[Cp*Rh(κ3N,N′,P‐L)][SbF6] (Cp*=C5Me5), bearing a guanidine‐derived phosphano ligand L, behaves as a “dormant” frustrated Lewis pair and activates H2 and H2O in a reversible manner. When D2O is employed, a facile H/D exchange at the Cp* ring takes place through sequential C(sp3)−H bond activation.
Heating the six-membered ring N-heterocyclic
carbenes 6-Me and
6-Et with Rh(PPh3)4H afforded the rhodium monocarbene
hydride complexes Rh(6-NHC)(PPh3)2H as a mixture
of cis- and trans-P,P isomers (4a/b, NHC
= 6-Me; ratio = 1:20; 5a/b, NHC = 6-Et;
ratio = 1:9). Reaction of 4a/b with Et3N·3HF gave only the trans-P,P isomer of the bifluoride
complex Rh(6-Me)(PPh3)2(FHF) (6b), whereas 5a/b reacted to form Rh(6-Et)(PPh3)2(FHF) as a mixture of cis- and trans-phosphine
isomers (7a/b). Variable temperature 1H and 19F NMR spectroscopy showed that 6b and the previously reported 6-iPr carbene analogue cis-Rh(6-iPr)(PPh3)2(FHF) (2a; Organometallics2012418584) were fluxional
in solution. 19F Magnetization transfer experiments revealed
F exchange in both compounds and afforded similar ΔH
⧧ values (2a, 51 ± 5 kJ mol–1; 6b, 60 ± 6 kJ mol–1) but somewhat different values of ΔS
⧧ (2a, −70 ± 17 J mol–1 K–1; 6b, −27 ± 18 J mol–1 K–1). The fluoride complexes cis-Rh(6-Me)(PPh3)2F (8a), cis-/trans-Rh(6-Et)(PPh3)2F (9a/b), and the previously
reported 6-iPr analogue 3a could be formed
upon C–F activation of CF3CFCF2 by 4a/b, 5a/b, and Rh(6-iPr)(PPh3)2H (1a/b), respectively. Complex 3a reacted slowly
with H2 to partially reform 1a/b but rapidly with CO to give Rh(6-iPr)(PPh3)(CO)F (10) and Rh(PPh3)2(CO)F,
and also quickly with Me3SiCF3 to form cis-Rh(6-iPr)(PPh3)2(CF3) (11a). Complexes 4b, 5b, 6b, 7b, and 11a were structurally characterized.
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