1,4-Addition of Nucleophiles to -Unsaturated Carbonyl Compounds - Carbohydrates: Tools for Stereoselective Synthesis - page 280

conversion dropped drastically (10%). As previously observed for 2-cyclohexenone,
the introduction of sterically demanding groups in the thioether moiety had a
negative effect on selectivity, while at low temperatures selectivities were better
than at 0 °C. The best enantioselectivities were obtained with the catalyst precursor
containing 56c (Table 12.3, entry 5).
The use of monodentate ligands 59, 61, 66, 69, and 71 a-f in the addition of
triethylaluminium to trans -3-nonen-2-one and trans -5-methyl-3-hexen-2-one has
also been studied [34]. The results with these ligands indicated that the optimum
trade-off between yields and enantioselectivity for trans -3-nonen-2-one was
obtained when diethyl ether was used as a solvent, the ligand-to-copper ratio was
4, and Cu(OTf) 2 was used as a catalyst precursor.
Under optimized conditions, unlike cyclic substrates, pyranoside ligands 69 (31)
provided better enantioselectivities than the corresponding furanoside ligands 59
(28). The best result (ee values up to 52%) was obtained with ligand 69f (31f), which
contains the best combination of the ligand parameters (Table 12.3, entry 6).
Similar trends were observed for trans -5-methyl-3-hexen-2-one. Thus, the best
enantioselectivity (ee values up to 52%) was obtained with ligand 69f (31f), which
contains an R binaphthyl phosphite moiety attached to the pyranoside backbone
(Table 12.4, entry 4).
12.4
Rhodium-Catalyzed Reactions
While copper-catalyzed 1,4-additions of diorganozinc or trialkylaluminium rea-
gents to unsaturated carbonyl compounds have been known for several decades,
the rhodium-catalyzed conjugate addition of boronic acids to enones (Hayashi-
Miyaura reaction) has only quite recently emerged as a powerful tool for stereose-
lective C-C bond formation [40, 41]. Along with this transformation chiral olefins
have evolved as a new class of stereodirecting ligands [42]. These chiral olefin
ligands are either dienes (the first examples were designed independently by
Hayashi [43] and Carreira [44]) or structures containing one olefin moiety in com-
bination with a another donor center, mainly phosphorus (first reported by Grütz-
macher [45] and later also by Hayashi [46]) and are generally superior to traditional
diphosphorus donor ligands in rhodium-catalyzed 1,4-additions.
Boysen et al . have designed a family of novel chiral diene and olefin-phosphinite
hybrid ligands derived from d-glucose (Scheme 12.14) and d-arabinose [47]. The
olefin-phosphinite hybrid based on glucose led to excellent levels of stereoselectiv-
ity in 1,4-additions of boronic acids to enones (99% ee from cyclohexenone and
cyclopentenone, Scheme 12.15), whereas the dienes only led to modest
enantioselectivity.