Catalytic intramolecular hydroamination of aminoallenes using titanium and tantalum complexes of sterically encumbered chiral sulfonamides

Abstract

Catalysis using earth abundant metals is an important goal due to the relative scarcity and expense of precious metal catalysts. It would be even more beneficial to use earth abundant...

Jumping in the Chiral Pool: Asymmetric Hydroaminations with Early Metals

“…Aminoalcoholates of titanium ( 21a – c , 22a – c , 23a – i , 25a – i , 27a – j and 29a – h ), zirconium ( 32a – g , 33a – e ) and tantalum ( 24a – l , 26a – l and 28a – j ) ( Table 5 , Table 6 , Table 7 , Table 8 , Table 9 , Table 10 , Table 11 and Table 12 ), as well as the cyclopentadienylbis(oxazolinyl)borate group IV metal complexes 30a – c and 31 are admirable enantioselective hydroamination/cyclization catalysts for a variety of different aminoalkenes, aminodialkenes and aminoallenes, as shown by Johnson [ 90 , 91 , 92 , 93 , 94 , 95 ] and Sadow [ 56 , 58 , 96 ]. Complexes 21 – 33 commonly feature natural chiral-pool-derived ligands based on either L-valine ( 21a – c , 23a – i , 24a – l , 29e , 30a – c , 32a – g ), L-phenylalanine ( 22a – c , 25a – i , 26a – l , 27a – j 28a – j , 29a – d ), L-tert-leucine ( 31 ), L-proline ( 33a – d ) and L-pipecolic acid ( 33e ) ( Table 5 , Table 6 , Table 7 , Table 8 , Table 9 , Table 10 , Table 11 and Table 12 ).…”

“…Crowded sulfonamides featuring a benzyl group as a bulky chiral backbone (L-phenylalanine-derived) with different steric and electronic properties were successfully introduced as ligands for the in situ generation of titanium ( 27a – j ) and tantalum ( 28a – j ) catalysts [ 93 , 95 ]. The respective titanium catalysts convert 6-methylhepta-4,5-dienylamine 34b solely to 2-(2-methylpropenyl)pyrrolidine 37 with an enantiomeric excesses of max.…”

“…The respective titanium catalysts convert 6-methylhepta-4,5-dienylamine 34b solely to 2-(2-methylpropenyl)pyrrolidine 37 with an enantiomeric excesses of max. 11% ( 27a – f ) ( Table 7 , entries 1–12) [ 93 ] or 18–41% ( 27g – j ) ( Table 7 , entries 13–17) [ 95 ] with conversions of 18–100%. The corresponding tantalum catalysts generally showed an ee of 5–34% ( 28a – f ) ( Table 7 , entries 18–29) [ 93 ] and 33–39% ( 28g – j ) ( Table 7 , entries 30–33) [ 95 ] more selective with generally higher conversions than 27g – j .…”