Asymmetric pinacol coupling of aromatic aldehydes with TiCl2/enantiopure amine or hydrazine reagents

Low-Valent Titanium-Mediated Reductive Coupling of Carbonyl Compounds

“…TiCl 2 in the presence of TMEDA mediated coupling of PhCHO to furnish dl-hydroxybenzoin with high diastereoselectivity; the diol could be obtained with modest enantioselectivity (<65% ee) by substitution of a chiral amine for TMEDA in the reaction [36,37]. Mukaiyama et al recently utilized intramolecular pinacol coupling of diketone 1 with a lowvalent titanium reagent, prepared from TiCl 2 and LiAlH 4 , as a key step in stereoselective construction of ABC-ring system of the diol 2 (Scheme 1).…”

“…The catalyst system derived from 15 mol% of this titanium complex, Mn, and Me 3 SiCl was applied for pinacol coupling of aromatic aldehydes to afford 1,2-diols in high yield with high dldiastereoselectivities (dl:meso > 87:13) and enantioselectivities as high as 91% [67]. Asymmetric induction through the use of enantiomerically pure metallocene catalysts have so far met with limited success, yielding only poor to moderate enantioselectivitites [36,37,55,57].…”

Enantioselective Radical Reactions

“…It seems that the cluster leads to lower ee's: addition of tetrahydrothiophene resulted in higher selectivity (58% ee) [158]. Enders has used SMP, 228, under similar condi-Scheme 59 Pinacol coupling using titanium catalysts tions with low yields and moderate ee's (entry 4) [159]. Brintzinger's ansa metallocene 229 is the most successful chiral catalyst to date in pinacol coupling (entry 5) [160].…”

Katalytische selektive Synthese

“…[17] [18] gegen die inverse Temperatur (1/T). [19] Je nach Reaktionsbedingungen kann eine katalytische enantioselektive Umwandlung von der Enthalpie (d. h. Selektivität als Funktion nichtkovalenter Wechselwirkungen), [20] der Entropie (d. h. Selektivität als Funktion von konformativer Flexibilität) [21] oder beiden kontrolliert sein (d. h. Selektivität als Funktion des Lçsungsmitteleffekts). [22] Genau wie die enantioselektive Katalyse folgen regio-, chemo-und produktselektive Reaktionen dem gleichen Satz von Parametern zur Bestimmung der Selektivität [Gl.…”