An NMR study of mixed, tartrate-containing Ti^IV complexes

Abstract

chelating diolate units and to decide if the side-chain groups are coordinated (3).The Katsuki-Sharpless catalysts (1) are fluxional because the side-chain ester groups are reversibly and weakly bound, complicating the study of their structures and their interactions with substrates. We recently undertook to examine certain mixed complexes for two reasons: Firstly, the loosely bound ester groups could be supplanted by more stably bound groups from companion ligands, permitting an easier characterization. Two existing X-ray structures (6) were to serve as controls. Secondly, acting as "proxies" for the catalytic substrates, companion ligands could provide mixed species that model the catalytic intermediates.A rich chemistry has emerged, which this paper assembles and discusses. As well as presenting more complete details of the mixed ligand species already communicated (4, 5), many new examples and an entirely new and unique tartrate binding mode are reported for the first time. Overall, the products were formed with surprising selectivity, providing three new tartrate binding modes and evidence of transient O'Pr bridges. Further insights have also been gained into reactivity, the Katsuki-Sharpless catalyst structure, and new catalyst designs. Experimental sectionGeneral Diisopropyl R,R-or S,S-tartrate, diethyl R,R-tartrate, and titanium tetraisopropoxide were freshly distilled. CDCl, was dried over Mg(ClO,),. Unless otherwise stated, NMR spectra were obtained at 19-20°C on a Bruker AM-300 instrument operating at 300 MHz for 'H or 75.1 MHz for 13c, using pulse sequences from the Bruker library. Integration was performed digitally, at times after digital enhancement of the resolution, which was achieved by zeroing the last half of a 16-kbyte-long FID and zero-filling an additional 48 kbytes before Fourier transformation. Baseline correction was at times used to integrate AB quartets. Spectra were also obtained with simultaneous irradiation of the CH, region to simplify the OCH region and to establish the numbers and positions of OCH heptets. Single-frequency homo-decoupling ('H-'H) and heterodecoupling ('H-',c) and 2-dimensional shift correlation experiments were used to establish connectivities. 'H-',c couplings were measured by gated decoupling. Chemical shifts are reported in ppm from TMS and coupling constants are in hertz (Hz). For AB quartets, the JAB and calculated A6 values are reported. Abbreviations include v for very, b for broad, h for heptet, and m for multiplet. J, , denotes apparent couplings. Mass spectrometry was pedormed by Dr. B. Khouw on a VG instrument (York) or on a ZAB-80 instrument at the McMaster Regional Centre for Mass Spectroscopy. EI experiments were conducted at 70 eV. CI experiments with NH, and FAB in various matrices were less informative.Unless specifically indicated in the results section, reaction mixtures were obtained in one or more ways, depending on which reagent, if any, needed to be added last or whether the proportion of a reagent was to be varied. Solutions 37-42 mM in...

Titanium(IV) Complexes of Disulfide-Linked Schiff Bases

“…Monobasic bidentate ligands such as acac − normally form 2:1 ligand−Ti complexes, adopting cis arrangements with chiral metal centers. 22 The same arrangement has been reported for N-phenylsalicylidenimine 23 with, moreover, cis nitrogen atoms. This arrangement is retained in the trinuclear product of its partial hydrolysis, analogous to the case for [(4)TiO] 3 , and is replicated with the present ligands.…”

“…Monobasic bidentate ligands such as acac – normally form 2:1 ligand–Ti complexes, adopting cis arrangements with chiral metal centers . The same arrangement has been reported for N -phenylsalicylidenimine with, moreover, cis nitrogen atoms.…”

Asymmetric Reduction of Ketones with Catecholborane Using 2,6-BODOL Complexes of Titanium(IV) as Catalysts

“…Complexes involved in the Sharpless Ti ± tartrate-catalyzed asymmetric epoxidation of allylic alcohols have been studied with NMR spectroscopic methods by Potvin et al [19] These structures have the possibility of extra coordination to Ti from the carbonyl of the ester or amide groups. Despite the fact that there is no X-ray data available for the Ti ± diethyltartrate esters, it is assumed that they coordinate in a tridentate fashion.…”

Synthesis of Uniform Anatase TiO2 Nanoparticles by Gel–Sol Method

“…Ti(IV) ion is known to form complexes with a wide variety of anions and other complexing agents, such as hydroxide (3)(4)(5)(6)(7)(8)(9)(10)(11)(12), fluoride (12)(13)(14)(15)(16), sulfate (3,4,17,18), phosphate (11,19,20), chloride (4,13,21), carbonate (22), hydrogen peroxide (23,24), oxalate (25)(26)(27)(28), citrate (29)(30)(31), lactate (31), tartrate (29,32,33), malate (34,35), ammonia (11,12), amines (36), 8-hydroxyquinoline (9,24,29,37,38), thenoyltrifluoroacetone (9, 10), etc. Relevant textbooks (39), reviews …”

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