M.C. White/Q. Chen Chem 153 Hydrogenation -148- Week of october 15. 2002 High catalyst loadings diminished yields and selectivities CH] Ch, H2(1 atm),rt decrease in selectivity is observed at higher catalyst selectivity loadings. It is possible that higher catalyst loadings yield (ratio A: B) promote the formation of dimeric( Crabtree suggested LH-M) species that no longer have the"oper 2.5mo/% coordination site necessary for providing effect directing effects in olefin hydrogenation. NO 48% experimental data exists thus far to support this Stork JACS 1983(105)1072 Crabtree JOC 1986(51)2655 Dimished yields are observed with higher catalyst loadings. This can be rationalized on the basis that higher catalyst loadings promote the irreversible trimerization of the coordinatively unsaturated catalysts to yield inactive triiridium hydride bridged complexes. Such complexes have been isolated by Crabtree from reaction mixtures of more sterically hindered olefins that did not proceed to 2.775 Crabtree Acc. Chem. Res. 1979(12)331M.C. White/Q. Chen Chem 153 Hydrogenation -148- Week of October 15, 2002 High catalyst loadings: diminished yields and selectivities Dimished yields are observed with higher catalyst loadings. This can be rationalized on the basis that higher catalyst loadings promote the irreversible trimerization of the coordinatively unsaturated catalysts to yield inactive triiridium hydride bridged complexes. Such complexes have been isolated by Crabtree from reaction mixtures of more sterically hindered olefins that did not proceed to completion. Crabtree Acc. Chem. Res. 1979 (12) 331. CH2Cl2, H2 (1 atm), rt Me OH Ir(I) PCy3 N (PF6-) + 2.5 mol% Me OH H Me OH H 99% 139:1 yield selectivity (ratio A:B) 20 mol% 48% A B 74:1 Stork JACS 1983 (105) 1072. Crabtree JOC 1986 (51) 2655. A decrease in selectivity is observed at higher catalyst loadings. It is possible that higher catalyst loadings promote the formation of dimeric (Crabtree suggested M-H-M) species that no longer have the "open" coordination site necessary for providing effective directing effects in olefin hydrogenation. No experimental data exists thus far to support this hypothesis