“Synthesis of ionones on solid Brønsted acid catalysts”

VERÓNICA K. DÍEZ; J. ISABEL DI COSIMO; CARLOS R. APESTEGUÍA

Zaragoza (España)

Congreso; 9Th Congress on Catalysis Applied to Fine Chemicals; 2010

Ionone  isomers  (a-,  b-  and   g-ionone)  are  fine  chemicals  widely  used  as pharmaceuticals and  fragrances.  The  commercial  synthesis  of  ionone  isomers  from cyclization  of pseudoionones  is  catalyzed  by  liquid  acids,  which  entails  environmental concerns because  of  corrosion  and  waste  disposal.  Then, new  industrial  strategies  for ionone synthesis from pseudoionone cyclization demand the replacement of liquid acids by  solid catalysts.  However,  very  few  papers  have  been  published  on  the  use  of  solid acids for converting pseudoionone to ionone isomers. In  this  work,  we  prepared  solid  Brønsted  acid  catalysts,  namely  resin  Amberlyst 35W, SiO2-supported heteropolyacid (HPA) and SiO2-supported triflic acid (TFA), with the  aim  of correlating  the  catalyst  acid  site  strength  with  the  resulting  ionone  isomer distribution. Catalyst activity depended on the surface acid site strength so that the initial activity  order  for obtaining  ionones  was  TFA  >  Amberlyst  35W  = HPA.  The pseudoionone molecule is initially activated on surface Brønsted acid sites and forms a common  cyclic  intermediate  for the  consecutive  ionone  isomer  generation.  This  cyclic carbocation  intermediate  contains three  different  kinds  of  protons  that  upon  direct detachment  lead  to a -,  b-  or g-ionones as  primary  products.  Selectivity  to  the  three ionone  isomers  was  also  strongly  dependent on  the  acid  site  strength.  Under  initial conditions,  pseudoionone  transformation  gave ionone  mixtures  with  approximately statistical composition (40%   a-ionone, 20%  b-ionone and 40% g-ionone), regardless of catalyst  Brønsted  acid  site  strength.  However, with  the  progress  of the  reaction   g-ionone,  the  least  stable  isomer,  was  isomerized  to   a-ionone  on  HPA and  Amberlyst 35W,  and  to b-ionone  on  the  stronger  acid  sites  of  TFA.  Therefore,  binary mixtures  of  a-  and   b-ionone  can  be  obtained  by  properly  selecting  operational conditions  since  the  kinetics  of  ionone  isomer  interconversion  may  be  controlled  by changing the Brønsted acid site strength, temperature and reaction time.