Steric factors controlling the formation of allenyl- vs. propargyl cobaloxime complexes

Reactions of Cobaloxime Anions and/or Hydrides with Enynes as a New, General Route to 1,3- and 1,2-Dienylcobaloxime Complexes

“…This effect is presumably due to the increased solubility of these complexes in basic solution. In contrast to one reaction of an enyne with cobaloximes communicated by Pasto and Timmers, we saw no evidence for alkene hydrocobaltation for any of the enynes above 1b,c The E and Z isomers of dienyl complexes ( 35 and 36 ) were easily distinguished by their alkene proton coupling constants of 14.1 and 10.7 Hz, respectively.…”

“…(Possible mechanisms for allene and diene formation will be discussed in more detail in the next section.) Conversely, acidification to ensure the presence of a large amount of cobalt hydride had no effect on product outcome for an internally methylated alkene in an enyne (entries 5 and 6). 16d, Enyne 19 containing a terminally unsubstituted alkene was the only other enyne to yield allene 30 . The allene structure for all three of these complexes ( 27 , 29 , and 30 ) was indicated by the 190−200 ppm allene sp carbon resonance seen in the 13 C NMR …”

“…Conversely, acidification to ensure the presence of a large amount of cobalt hydride had no effect on product outcome for an internally methylated alkene in an enyne (entries 5 and 6). 16d, Enyne 19 containing a terminally unsubstituted alkene was the only other enyne to yield allene 30 . The allene structure for all three of these complexes ( 27 , 29 , and 30 ) was indicated by the 190−200 ppm allene sp carbon resonance seen in the 13 C NMR 2 Reactions of Cobaloxime Complexes with Enynes Co source a glyenyneR 1 R 2 R 3 yield, %R 4 R 5 R 6 R 7 R 8 yield, % (1) A b dmg 17 H Me Me 45 ( 27 ) (2) B dmg 17 H Me Me 31 ( 27 ) (3) A c dmg 17 H Me Me 68 ( 27 ) H H Me H H 8 ( 28 ) (4) C dmg 17 H Me Me 11 ( 27 ) (5) B dmg 18 Et Me Me 69 ( 29 ) (6) D dmg 18 Et Me Me 59 ( 29 ) (7) B dmg 19 Me Me H 57 ( 30 ) (8) B dmg 20 H H H Me H 43 ( 31 ), 10.5:1 E : Z (9) E dpg 20 H H H Me H 33 ( 32 ) (10) B dmg 21 …”