On the mechanism and kinetics of radical reactions of epoxyketones and epoxynitriles induced by titanocene chloride.
The reactions of a series of epoxynitriles and epoxyketones induced by titanocene chloride have been studied. The kinetics of the decyanogenation of beta,gamma-epoxynitriles with Ti(III) corresponds to a radical reaction (k25 approximately 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 approximately 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 beta-scission. Iminyl radicals generated by 4-exo cyclizations onto nitriles undergo reduction with Ti(III) and beta-scission reaction in a ratio of 96:4 when the alpha-substituent is CN. Alkoxyl radicals from 4-exo cyclizations onto ketone carbonyls undergo reduction with Ti(III) and beta-scission in a ratio of 60:40 when the alpha-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
A
A
Departamento de Química OrgAnica, Facultad de C. Químicas, Universidad de Salamanca, Plaza de los Caídos 1-5, 37008 Salamanca, Spain. afmateos@usal.es
Teijón
P Herrero
PH
Burón
L Mateos
LM
Clemente
R Rabanedo
RR
Gonzalez
R Rubio
RR
D000475:Alkenes / Q000138:chemical synthesis / Q000737:chemistry; D003500:Cyclization; D004852:Epoxy Compounds / Q000737:chemistry*; D005609:Free Radicals / Q000138:chemical synthesis / Q000737:chemistry; D007659:Ketones / Q000737:chemistry*; D007700:Kinetics; D015394:Molecular Structure; D009570:Nitriles / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D013237:Stereoisomerism
D000475:Alkenes; D004852:Epoxy Compounds; D005609:Free Radicals; D007659:Ketones; D009570:Nitriles; D009942:Organometallic Compounds; C020528:titanocene dichloride