- On the mechanism and kinetics of radical reactions of epoxyketones and epoxynitriles induced by titanocene chloride
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(Chemical Equation Presented) The reactions of a series of epoxynitriles and epoxyketones induced by titanocene chloride have been studied. The kinetics of the decyanogenation of β,γ-epoxynitriles with Ti(III) corresponds to a radical reaction (k25 ≈ 106 S-1), as demonstrated by competition experiments with H-transfer from 1,4-cyclohexadiene (1,4-CHD) or PhSH or conjugate addition to acrylonitrile. The 5-exo cyclization onto nitrile induced by Ti(III) is a radical reaction (k25 ≈ 107 S-1) as seen in competition experiments with H-transfer from PhSH or the titanocene-water complex. The iminyl or alkoxyl radicals generated by 5-exo cyclization onto nitriles or ketones only undergo a reduction with Ti(III). This reaction overwhelms any alternative process, such as tandem cyclization onto alkenes or β-scission. Iminyl radicals generated by 4-exo cyclizations onto nitriles undergo reduction with Ti(III) and β-scission reaction in a ratio of 96:4 when the α-substituent is CN. Alkoxyl radicals from 4-exo cyclizations onto ketone carbonyls undergo reduction with Ti(III) and β-scission in a ratio of 60:40 when the α-substituent is COOR. In nearly all the reactions studied, the role of Ti(III) is triple: a radical initiator (homolytic cleavage of oxirane), a Lewis acid (coordination to CN or C=O). and a terminator (reduction of iminyl or alkoxyl radicals).
- Fernandez-Mateos,Herrero Teijon,Mateos Buron,Rabanedo Clemente,Rubio Gonzalez
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p. 9973 - 9982
(2008/03/27)
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- Thermal and lewis acid catalyzed intramolecular ene reactions of allenylsilanes
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Intramolecular ene reactions of allenylsilanes can be effected with a variety of imines, aldehydes and alkenes as enophiles, forming five and six membered rings. These reactions are cis stereoselective in all cases studied, and appear to proceed via a concerted, pericyclic process. The cycloadditions all generally occur under mild thermal conditions and some involving imino enophiles can also be effected at lower temperatures using Lewis acid catalysis.
- Weinreb, Steven M.,Smith, Daniel T.,Jin, Jian
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p. 509 - 521
(2007/10/03)
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