Synthetic Reactions Mediated by a Ti(O-i-Pr) / 2 i-PrMgX Reagent

Fumie Sato; Hirokazu Urabe; Sentaro Okamoto

doi:10.1055/s-2000-6689

Synlett 2000; 2000(6): 0753-0775
DOI: 10.1055/s-2000-6689

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Synthetic Reactions Mediated by a Ti(O-i-Pr) / 2 i-PrMgX Reagent

Fumie Sato^* , Hirokazu Urabe, Sentaro Okamoto

^*Department of Biomolecular Engineering, Tokyo Institute of Technology, 4295 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8501, Japan; Fax +81-45-924-5826; E-mail: fsato@bio.titech.ac.jp

Abstract

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A new titanium complex (η²-propene)Ti(O-i-Pr)₂ (1), generated in situ by treatment of Ti(O-i-Pr)₄ with 2 equiv. of i-PrMgX (X = Cl or Br), nicely acts as a versatile titanium(II) equivalent to effect the following unique transformations. Thus, alkynes react with 1 to afford the alkyne-titanium complexes of type (η²-alkyne)Ti(O-i-Pr)₂, which could be utilized as a vicinal vinylic dianion equivalent in the reactions with various kinds of electrophiles. Allyl- and allenyltitaniums could readily be prepared from allylic or propargylic alcohol derivatives by the reaction with 1 through an oxidative addition pathway. While acetylenic esters resulted in an intramolecular nucleophilic acyl substitution (INAS) reaction to afford α,β-unsaturated carbonyl compounds, olefinic esters undergo tandem INAS and intramolecular carbonyl addition reactions to give cyclopropanols. Ene-ene, ene-yne, and yne-yne coupling reactions proceed in an intra- or intermolecular way via a cyclometallation reaction, and the generated titanacyclic complexes react with a variety of electrophiles in a chemo-, regio-, stereoselective way. This account describes these reactions and their utilization in organic synthesis which include representative applications to asymmetric synthesis and natural product synthesis.

low-valent titanium - (η²-alkyne)titanium complex - allytitanium - intramolecular nucleophilic acyl substitution - cyclometallation

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