TitaniumLewis Acids for Asymmetric Catalysis: Synthesis and Structural Characterization of Dichloro[diolato(2−)-κO,κO′]bis(solvent)titanium ([TiCl2(diolato)(solvent)2]) Complexes

Abstract

The complexes [TiCl 2 {(R,R)-TADDOLato}(DME)] ¥ MeCN (3), and [TiCl 2 {(R,R)-1-Nph-TADDOLato}(MeCN) 2 ] ¥ CH 2 Cl 2 (4b) (DME 1,2-dimethoxyethane; (R,R)-TADDOLato (4R,5R)-2,2-dimethyla,a,a',a'-tetraphenyl-1,3-dioxolane-4,5-dimethanolato(2 À)-kO,kO';(R,R)-1-Nph-TADDOLato (4R,5R)-2,2-dimethyl-a,a,a',a'-tetra(naphthalen-1-yl)-1,3-dioxolane-4,5-dimethanolato(2 À)-kO,kO') were prepared and isolated in high yield as stable crystalline materials (Scheme 1). They constitute ideally suited and easyto-handle catalyst precursors for a large number of Ti-catalyzed asymmetric reactions, for which they have been previously generated in situ. The X-ray crystal structures of 3 and 4b show a distorted octahedral geometry around Ti with the chloro ligands in mutual trans positions (Figs. 5 and 6). The new chiral diols a-(1S,3R)-3-hydroxy-2,2,3-trimethylcyclopentyl]-a-phenylbenzenemethanol (13a), derived from camphoric acid (5), and (M)-6,6'-dimethyl-a,a,a',a'-tetraphenyl[1,1'-biphenyl]-2,2'-dimethanol (15) were prepared (Schemes 3 and 4). These new ligands are able to form mononuclear complexes with the Ti IV Cl 2 fragment. The corresponding complex 14 derived from 13a was characterized by X-ray as a mixed THF/MeCN adduct. 1. Introduction. ± Complexes of the general formula [TiCl 2 (OR) 2 ] serve as Lewis acid catalysts or mediators (i.e., in stoichiometric amounts) in allylation reactions of aldehydes in the presence of allylstannanes, hetero-Diels-Alder reactions, ene reactions, Michael additions, Mukaiyama silyl-aldol condensations, and other reactions (see review [1]). Of particular interest are those complexes derived from an enantiomerically pure chelating diol. Thus, [TiCl 2 (BINOLato)] complexes (BINOLato [1,1'-binaphthalene]-2,2'-diolato(2 À)-kO,kO') have found specific use in enantioselective Mukaiyama aldol reactions [2], cyanosilylation of aldehydes [2], carbonyl-ene reactions [3], carbonyl allylsilane or allylstannane additions [4 ± 6], ene cyclizations [7], (hetero)-Diels-Alder reactions [8], and others (for a review, see [9]). Complexes of the type [TiCl 2 (TADDOLato)] 1 ) serve as catalysts in enantioselective cyanosilylations [10], (hetero)-Diels-Alder [11 ± 15], [2 2] cycloadditions [16 ± 18], [2 3] cycloadditions [19], 1,3-dipolar cycloadditions [20], nitro-aldol reactions [21], fluorinations [22], and others (for reviews, see [9] [23]).When inspecting the procedures for the generation of the titanium catalysts used in the reactions mentioned above, one notes that they are prepared in situ according to

[(4R,5R)-(2,2-Dimethyl-1,3-dioxolane-4,5-diyl)bis(diphenylmethanolato)-κ2O:O′]bis(N-methylmethanaminato)titanium(IV)

“…The Ti-O distances in the title complex are also slightly longer than those reported for six-coordinate TADDOLate complexes, where the distances are very similar to those found in Ti(TADDOLate) 2 (average Ti-O distance 1.78 (2) Å), with TADDOLate Ti-O bond lenghts in six-coordinate complexes 1.780 (2) and 1.786 (1) Å (Chen et al, 2007), 1.76 (1) and 1.79 (1) Å (Gothelf et al, 1995), 1.752 (5) and 1.765 (5) Å (Hintermann et al, 2002), 1.801 (1) and 1.788 (1) Å (Kongprakaiwoot et al, 2010), 1.782 (2) and 1.771 (2) Å (Shao & Gau, 1998), 1.772 (4), 1.778 (3), 1.792 (4) and 1.793 (4) Å (Shao et al 2001), and 1.782 (4), 1.793 (4) and 1.776Å (Sheen & Gau, 2004 (Chen et al, 2007), 97.2 (5)° (Gothelf et al, 1995), 98.6 (2)° (Hintermann et al, 2002), 94.77 (5)° (Kongprakaiwoot et al, 2010), 98.7 (1)° (Shao & Gau, 1998), 96.8 (2)° (Shao et al 2001), 97.9 (2) and 99.5° (Sheen & Gau, 2004 geometry.…”

“…For a related structure of a four-coordinate titanium-TADDOLate compound, see: Seebach et al (1992). For related structures of six-coordinate titanium-TADDOLate compounds, see: Chen et al (2007); Gothelf et al (1995); Hintermann et al (2002); Kongprakaiwoot et al (2010); Shao & Gau (1998); Shao et al (2001); Sheen & Gau (2004). For a report of the in-situ preparation of the title compound, see: Ackermann et al (2003).…”

“…E68, m217 ,α,α′, have been reported to be excellent asymmetric catalysts, for example in asymmetric ethylation of aldehydes (Degni et al, 2005;Seebach et al, 1992) and Diels-Alder reactions (Gothelf et al, 1995). Whereas the molecular structure of one four-coordinate Ti(TADDOLate) has been reported (Seebach et al, 1992), six-coordinate complexes are more common (Chen et al, 2007;Gothelf et al, 1995;Hintermann et al, 2002;Kongprakaiwoot et al, 2010;Shao & Gau, 1998;Shao et al 2001;Sheen & Gau, 2004).…”

TiIV‐Catalyzed Asymmetric Sulfenylation of 1,3‐Dicarbonyl Compounds

“…Materials : All solvents were distilled prior to use. Complex [TiCl 2 (( R , R )‐1‐Np‐TADDOLato)(MeCN) 2 ] ( 1 , bis(acetonitrile)dichloro [(4 R ,5 R )‐2,2‐dimethyl‐α,α,α′,α′‐tetra(naphthalene‐1‐yl)‐1,3‐dioxolane‐4,5‐dimethanolato(2‐)‐κ O ,κ O′ ]titanium),24 phenylsulfenyl chloride25 and benzylsulfenyl chloride26 were prepared following published procedures. 2,4‐Dinitrophenylsulfenyl chloride was obtained from Aldrich.…”

Relative Electrophilic Fluorinating Power as Assayed by Competitive Catalytic Halogenation Reactions

“…Complex 1, bis(acetonitrile)dichloro[(4R,5R)-2,2-dimethyl-a,a,a',a'-tetra(naphthalen-1-yl)-1,3-dioxolane-4,5-dimethanolato(2-)-kO,kO']titanium, was prepared as reported [20]. Reactions with air-or moisture-sensitive materials were carried out under Ar by using Schlenk techniques.…”