Synthetic Reactions Using Low-valent Titanium Reagents Derived from Ti(OR)₄or CpTiX₃(X = O-i-Pr or Cl) in the Presence of Me₃SiCl and Mg

Abstract

In the presence of Me3SiCl, Ti(OR)4 or CpTiX3 (X = O-i-Pr or Cl) is reduced by Mg powder in THF to gradually generate a specific low-valent titanium (LVT) species that mediates several synthetic reactions. The LVT-catalyzed C-O bond-cleaving reactions of allyl and propargyl ethers and esters generate parent alcohols and carboxylic acids, respectively. O-allyl and propargyl carbamates are also readily deprotected by the LVT to afford parent amines. In addition, the respective reductive N-S or O-S bond cleavage of sulfonamides or sulfonyl esters mediated by the LVT was developed as a novel facile deprotection method. The reagent catalyzes intra- and intermolecular alkyne or alkyne/nitrile cycloaddition to produce substituted benzenes and pyridines, while epoxides and oxetanes are reduced to alcohols via an LVT-mediated homolytic ring opening. The McMurry coupling of aryl aldehydes and ketones proceeds with the LVT under homogeneous and mild reaction conditions and is effective for the polymerization of aromatic dialdehydes, generating conjugated polymers. Finally, imino-pinacol coupling of imines is mediated by the LVT to provide 1,2-diamines.

Modern applications of low-valent early transition metals in synthesis and catalysis

“…The most common routes for generating low-valent early transition metal complexes involve the reduction of metal halides with strong alkali metals 27–32 ; reduction with other (often weaker) reducing metals, such as Mg (REFS 33,34 ), Mn (REF. 35 ), Al (REFS 36,37 ) or Zn (REF.…”

“…Catalytic alkyne cyclotrimerization is also common with low-valent group 4–5 transition metals 34,110,153 . There are two limiting mechanisms for cyclotrimerization: triple insertion followed by reductive elimination through a metallacycloheptatriene ( 37 ) or [4+2] cycloaddition of the 3rd unsaturated partner to make a metallanorbornadiene ( 38 ) (FIG.…”

Dative Directing Group Effects in Ti-Catalyzed [2+2+1] Pyrrole Synthesis: Chemo- and Regioselective Alkyne Heterocoupling

“…In contrast to 2-C linkers, shorter (1-C) linkers such as propargylic ethers ( 1e ) and acetals ( 1f ) do not yield productive reactivity due to C–O bond decomposition. 40 Longer (3-C) linkers ( 1g ) display no directing-group effect, but are still competent for heterocoupling to the 2-unsubstituted pyrrole ( 4g ).…”

Remote Central‐to‐Axial Chirality Conversion: Direct Atroposelective Ester to Biaryl Transformation

“…[11] We next investigated the in situ reduction by adding Ti(OiPr) 4 ,w hich formed ablack slurry with the residual elemental magnesium, thus indicating the formation of low-valent titanium species. [12] Gratifyingly,the formation of the desired product was observed in Et 2 Oa nd the axially chiral biaryl (R a )-5a was isolated in 16 %y ield with an excellent e.r. value of 98:2 [conversion percentage (cp) = 100].…”