Competitive Intramolecular/Intermolecular Chelation Options Operative during Indium-Promoted Additions to Pyridyl Aldehydes and to Glyoxylic Acid under Aqueous Conditions
作者:Leo A. Paquette、Roger R. Rothhaar
DOI:10.1021/jo981717w
日期:1999.1.1
The stereochemical course of the 1,2-addition of six allylindium reagents to 2- and 3-pyridinecarboxaldehyde and to glyoxylic acid has been investigated in order to assess the level and direction of diastereoselectivity in these coupling reactions. When 2-PyCHO is involved, the results strongly suggest that the ring nitrogen becomes chelated to the indium atom in the aqueous environment. One striking observation is the crossover in stereoselectivity seen relative to the use of 3-PyCHO. A second revealing fact is the significantly faster rate of reaction of 2-PyCHO, as long as steric effects are not allowed to interfere. The varying product distributions observed in the latter experiments are attributed to other control elements such as intramolecular chelation within the indium reagent and nonbonded steric restrictions resident in either or both reaction partners. In the absence of extramolecular chelating events (e.g., when 3-PyCHO is involved), adherence to Felkin-Anh transition-state alignments is presumably exercised. The previous working assumption that indium(III) is capable of chelation to flanking heteroatomic centers in water is supported by the present investigation.
Factors Influencing 1,4-Asymmetric Induction during Indium-Promoted Coupling of Oxygen-Substituted Allylic Bromides to Aldehydes in Aqueous Solution
作者:Leo A. Paquette、George D. Bennett、Adnan Chhatriwalla、Methvin B. Isaac
DOI:10.1021/jo970158a
日期:1997.5.1
operation of chelation effects during oxidative addition of the metal into the carbon-bromine bond, but not during the coupling stage. Once the aldehyde enters into the coordination sphere of the indium, internal chelation to the proximal oxygen is disrupted and conformational restrictions are released. These effects, in combination with the absence of a powerful steric control element in the latter