Structural Studies of the Low-Valent Titanium “Solution”:  What Goes on in the Pinacol Coupling Reaction?

Hashimoto, Yuko; Mizuno, Utako; Matsuoka, Hideki; Miyahara, Takafumi; Takakura, Masahiro; Yoshimoto, Mamoru; Oshima, Koichiro; Utimoto, Kiitirô; Matsubara, Seijiro

doi:10.1021/ja001843t

Cited by 33 publications

(12 citation statements)

References 8 publications

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“…The bigger ones had an average radius of 51 Å whereas the smaller ones had an average radius of 9 Å. The largest particles were considered to be a cluster, and the smallest might be a monomeric amine−titanium−THF complex, with the former being responsible for the low enantioselectivity . Therefore, any change in the preparation protocol which deletes the presence of large clusters could improve the enantioselectivity.…”

Section: 7 Pinacol Coupling Processesmentioning

confidence: 99%

In the Arena of Enantioselective Synthesis, Titanium Complexes Wear the Laurel Wreath

2006

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Section: 7 Pinacol Coupling Processesmentioning

confidence: 99%

In the Arena of Enantioselective Synthesis, Titanium Complexes Wear the Laurel Wreath

2006

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“…Accordingly, these metals are routinely utilized as potent Lewis acid activators or catalysts in preparative organic chemistry such as in Friedel–Crafts and Diels–Alder reactions. , On the industrial scale, the early metals are widely known for their utility in Ziegler–Natta catalysis for the polymerization of olefins such as ethylene . However, the chemistry of low-valent early metals (LVEMs), as defined by electron counts ≥ d 2 , is accessible and plays a critical role in a number of practical chemical transformations. , LVEMs have been utilized in natural product syntheses, − can induce the coupling and cyclization of alkenes and alkynes, ,− and are especially adept at McMurry and Pinacol reductive aldehyde and ketone coupling reactions. ,− …”

Section: Introductionmentioning

confidence: 99%

Redox Character and Small Molecule Reactivity of a Masked Titanium(II) Synthon

et al. 2019

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The two-electron reduction of the Ti(IV) guanidinate (Ketguan)(ImDippN)Ti(OTf)2 (2 OTf ) (Ketguan = [( t Bu2CN)C(NDipp)2]−; ImDippN– = 1,3-bis(Dipp)imidazolin-2-iminato; Dipp = 2,6-diisopropylphenyl) with an excess of KC8 generates the masked complex (Ketguan)(η6-ImDippN)Ti (1). Conversely, reduction of the chloride analogue (Ketguan)(ImDippN)TiCl2 (2 Cl ) with an excess of Na/Hg amalgam produces the Ti(III) compound (Ketguan)(ImDippN)TiCl (3), while treatment of 2 Cl with 3.0 equiv of KC8 affords a complicated mixture from which (ImDippN)(DippN)[η2-( t Bu2C)NC(NDipp)](THF)Ti (4) is isolated as the product of reductive ligand cleavage. These results clearly indicate that the success of early metal reduction chemistry is highly sensitive to the halide coligands and reaction conditions. Complex 1, despite possessing a Ti(IV) canonical form, behaves as a Ti(II) synthon and appreciable reducing agent. For instance, 1 effects the one-electron reduction of benzophenone and pyridine to give the Ti(III) products (Ketguan)(ImDippN)Ti(η1-OC·Ph2) (6) and [(Ketguan)(ImDippN)Ti]2[μ2-(NC5H5–H5C5N)] (7), providing an approximate chemical redox potential range for 1 between ca. −2.3 to −3.1 V (vs [Cp2Fe]0/+). Additionally, treatment of 1 with π-acids such as CNCy (Cy = cyclohexyl) or NC t Bu leads to the formation of the Ti(III) and Ti(IV) products (Ketguan)(ImDippN)Ti(CN)(CNCy) (9) and (ImDippN)[(DippN)(2- i PrC6H3-6-(η1-CH3CHCH2)N)C(NC t Bu2)]Ti[NC(H) t Bu] (10), respectively, via reduction of the π-acid substrate. The two-electron reduction proclivity of 1 is demonstrated by its reactivity with chalcogen sources (e.g., N2O) and organoazides to give the Ti(IV) products (Ketguan)(ImDippN)Ti(E) (E = O (13), S (14), Se (15), S2 (16), NSiMe3 (17), NAd (18)). In addition to illustrating the versatile Ti(II) synthon character of 1, the synthesis of these compounds shows that the 3N-coordinated [(Ketguan)(ImDippN)Ti] n+ manifold can readily accommodate metal–ligand multiple bonds, including relatively rare examples of terminally bound TiS and TiSe bonds. Taken altogether, the redox chemistry of 1, as a Ti(II) synthon, clearly shows the chemical diversity of low-valent early metals (LVEMs) and their ability to reductively activate a wide range of substrates.

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“…Given the fact that both high diastereoselectivity and enantioselectivity should be considered for a successful asymmetric pinacol coupling reaction, this formidable challenge highlights the demand for an efficiently catalytic system. Nonetheless, asymmetric pinacol coupling reaction has been studied mostly in the use of chiral titanium complexes [16][17][18][19][20][21][22][23][24], however, those results are still far from satisfying. In 2004, Yamamoto developed a chiral chromium complex in this reaction with good diastereoselectivity and high enantioselectivity, which afforded a practical approach to prepare the optically active 1,2-diols [25].…”

Section: Introductionmentioning

confidence: 99%

Enantioselective pinacol coupling reaction of aromatic aldehydes catalyzed by chiral vanadium complexes

Sun

Dai

et al. 2009

Journal of Organometallic Chemistry

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Structural Studies of the Low-Valent Titanium “Solution”: What Goes on in the Pinacol Coupling Reaction?

Cited by 33 publications

References 8 publications

In the Arena of Enantioselective Synthesis, Titanium Complexes Wear the Laurel Wreath

In the Arena of Enantioselective Synthesis, Titanium Complexes Wear the Laurel Wreath

Redox Character and Small Molecule Reactivity of a Masked Titanium(II) Synthon

Enantioselective pinacol coupling reaction of aromatic aldehydes catalyzed by chiral vanadium complexes

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