Three chiral diamines were synthesised and evaluated as sparteine surrogates in the lithiation-substitution of N-(tert-butoxycarbonyl)pyrrolidine. The synthesis and attempted resolution of sparteine-like diamines [(1S*,2R*,8R*)-10-methyl-6,10-diazatricyclo[6.3.1.0(2,6)]dodecane and (1S*,2R*,9R*)-11-methyl-7,11-diazatricyclo[7.3.1.0(2,7)]tridecane] (via inclusion complex formation) are reported. Unfortunately, it was only possible to resolve the diazatricyclo[7.3.1.0(2,7)]tridecane compound. An alternative route to (1R,2S,9S)-11-methyl-7,11-diazatricyclo[7.3.1.0(2,7)]tridecane starting from the natural product, (-)-cytisine, is described. This simple three-step route furnished gram-quantities of a (+)-sparteine surrogate. X-Ray crystallography of an intermediate in the route, (1R,5S,12S)-3-methoxycarbonyldecahydro-1,5-methanopyrido[1,2-a][1,5]diazocin-8-one, enabled the stereochemistry of all of the tricyclic diamines described in this paper to be unequivocally established. Two other diamines, starting from (S)-proline and resolved 2-piperidine ethanol, were prepared using standard methods. These diamines lacked the bispidine framework of (-)-sparteine and were found to impart vastly inferior enantioselectivity. It was concluded that, for the asymmetric lithiation substitution of N-Boc pyrrolidine, a rigid bispidine framework and only three of the four rings of (-)-sparteine are needed for high enantioselectivity. Furthermore, it is shown that diamine (1R,2S,9S)-11-methyl-7,11-diazatricyclo[7.3.1.0(2,7)]tridecane is the first successful (+)-sparteine surrogate.
A detailed kinetic study of the protonation and subsequent benzene elimination reactions of a (diimine)Pt II diphenyl complex (denoted as (N-N)PtPh 2 ) has been undertaken in dichloromethane solution with and without acetonitrile as a cosolvent. Spectroscopic monitoring of the reactions by UV-vis stopped-flow and NMR techniques over the temperature range -80 to +27 °C allowed the assessment of the effects of acid concentration, coordinating solvent (MeCN) concentration, temperature, and pressure. Protonation of (N-N)PtPh 2 with HBF 4 3 Et 2 O in CH 2 Cl 2 /MeCN occurs with a kinetic preference for protonation at the metal, rather than at a phenyl ligand, and rapidly produces (N-N)PtPh 2 H(NCMe) + (ΔH q = 29 ( 1 kJ mol -1 , ΔS q = -47 ( 4 J K -1 mol -1 ). At higher temperatures, (N-N)PtPh 2 H(NCMe) + eliminates benzene to furnish (N-N)PtPh(NCMe) + . This reaction proceeds by rate-limiting MeCN dissociation (ΔH q = 88 ( 2 kJ mol -1 , ΔS q = +62 ( 6 J K -1 mol -1 , ΔV q = +16 ( 2 cm 3 mol -1 ). Protonation of (N-N)PtPh 2 in dichloromethane in the absence of MeCN cleanly produces the Pt(II) π-benzene complex (N-N)PtPh(η 2 -C 6 H 6 ) + at low temperatures. Addition of MeCN to a solution of the π-benzene complex causes an associative substitution of benzene by acetonitrile, the kinetics of which were monitored by 1 H NMR (ΔH q = 39 ( 2 kJ mol -1 , ΔS q = -126 ( 11 J K -1 mol -1 ). When the stronger triflic acid is employed in dichloromethane/acetonitrile, a second protonation-induced reaction also occurs. Thus, (N-N)PtPh(NCMe) + produces (N-N)Pt(NCMe) 2 2+ and benzene with no detectable intermediates (ΔH q = 69 ( 1 kJ mol -1 , ΔS q = -43 ( 3 J K -1 mol -1 ). The mechanisms for all steps are discussed in view of the accumulated data. Interestingly, the data allow a reinterpretation of a previous report on proton exchange between the phenyl and benzene ligands in (N-N)PtPh(η 2 -C 6 H 6 ) + . It appears that the exchange occurs by a direct σ-bond metathesis pathway, rather than by the oxidative cleavage/reductive coupling sequence that was proposed.
PtII diphenyl complexes (N–N)PtPh2 [N–N = diimines Ar–N=C(An)C=N–Ar with Ar = substituted aryl groups] have been prepared and characterized by 1H, 13C, and 195Pt NMR spectroscopy. The 195Pt NMR spectroscopic data establish the electronic influence exerted by substituents at the backbone of the diimine ligand system to the metal center. When compared to diimines Ar–N=CMe–CMe=N–Ar, the electron‐withdrawing ability of the Ar‐BIAN ligand and the electron‐donating ability of the O,O‐heterocyclic Ar‐BICAT systems are demonstrated. Trends in 195Pt NMR chemical shifts suggest that electronic tuning of the metal center is better achieved through variations of the diimine backbone substituents rather than variation of the substituents at the N‐Aryl groups. Protonation of (N–N)PtPh2 in dichloromethane/acetonitrile at –78 °C furnishes the corresponding PtIV hydrides (N–N)PtPh2H(NCMe)+. The PtIV hydrides liberate benzene with the formation of (N–N)PtPh(NCMe)+ when the temperature is raised. A second protonation and rapid benzene elimination produces the dicationic PtII species (N–N)Pt(NCMe)22+ at approximately 50 °C. Protonation of (N–N)PtPh2 in the absence of acetonitrile results in the clean formation of (N–N)PtPh(η2‐C6H6)+ at temperatures that depend on the steric hindrance provided by the alkyl substituents at the diimine N‐aryl groups. These findings support the notion that the metal is the kinetically preferred site of protonation. The results qualitatively agree with a recent mechanistic study of protonation‐induced reactions of (diimine)PtPh2 complexes that bear simple methyl substituents at the diimine backbone. Several compounds have been crystallographically characterized. All complexes have the expected square planar environment at the metal. Modest variations in the metric parameters suggest that the Ar‐BICAT system has a weaker trans influence than the Ar‐BIAN and Ar‐DAB systems.
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