The Enantioselective Pauson-Khand Reaction - The Pauson-Khand Reaction - page 162

Tab l e 6 . 2 Ligand exchange reactions and diastereoselectivites with hemilabile
bidentate ligands.

*

P S

*

* P S

CO CO

S P

BH 3

CO
CO

OC
OC

CO
CO

OC
OC

CO
CO

Co Co OC

OC

Co Co

Co Co

DABCO

Toluene, 80 °C

R

R

R

1 31 31'

SM ∗ R Ligand Product dr

1i TMS PuPHOS 31ia 3:1

1i TMS CyPuPHOS 31ib 1.6:1

1i TMS CamPHOS 31ic 5:1

1i TMS MeCamPHOS 31id 12:1

1i TMS CyCamPHOS 31ie 1.5:1

1i TMS t-BuCamPHOS 31if 1.3:1

1i TMS TolCamPHOS 31ig 1:1

1i TMS epiCamPHOS 31ih 2:1

1i TMS epiCyCamPHOS 31ii 3.4:1

1j tert -Bu PuPHOS 31ja 3:1

1j tert -Bu CamPHOS 31jc 2:1

1j tert -Bu MeCamPHOS 31jd 20:1

1c CMe 2 OH PuPHOS 31ca 4.5:1

1c CMe 2 OH MeCamPHOS 31cd 13:1

∗ starting material

bonding interaction between the amide carbonyl group and the highly polarized methine
group in the ligand (connected to three heteroatoms) is established in only one of the
diastereomers. This weak stabilizing interaction leads to much higher diastereomeric ratios,
and in some cases, near total diastereoselection (see Table 6.3). Other hydrogen bond
acceptors (e.g. sulfonyl groups) are also suitable directing groups. 38
The resulting diastereomerically pure complexes 31 smoothly react with norbornadiene
to yield the enantioenriched cyclopentenones 3 (see Table 6.4 for selected results). For the
complexes derived fromunfunctionalized alkynes ( 31ia and 31id ), N -oxide activation yields
the corresponding adducts in excellent yields and moderate to excellent enantioselectivities;
however, thermal activation gives slightly lower enantiomeric excesses, probably due to
the relatively rapid isomerization of the complex. Conversely, when propynamides are used
as starting alkyne ( 31ka and 31kb ), thermal activation provides comparable or even better
results than N -oxide activation, although the reaction is not completely stereospecific (90
to 92% ee).
The unusually high diastereoselectivities attained in the formation of the intermediate
chiral dicobalt-alkyne complex when a non-classical H-bond forms between the ligand
and the alkyne-substrate prompted Riera, Verdaguer et al. to run catalytic asymmetric
PKRs with alkynylamides (Scheme 6.22). 39 When 5 mol% of complex 31lc , which gave
the highest diastereoselectivity in the complexation (200:1) as a catalyst, was used, the