a. DBFOX/Ph Complex Catalyzed Diels-Alder Reactions
Nickel(II) Aqua Complex of DBFOX/Ph Catalyzed Reaction:
Nickel(II) perchlorate hexahydrate, Ni(ClO4)2´6H2O,
is totally insoluble in dichloromethane, but dissolves in the presence of
DBFOX/Ph ligand. Surprisingly, the bluish green complex derived from commercially
available Ni(ClO4)2´6H2O, possessing six aqua ligands, exhibits a high catalytic
activity. Diels-Alder reactions with 10 mol% of the aqua complex at -40
ÁC give a single enantiomer of endo-cycloadduct. Both the chemical yield
and the endo/exo ratio are also excellent (96% yield, endo:exo = 97:3).
High enantioselectivity remains with a catalytic loading of as little as
2 mol% of the nickel complex (96% ee). However, with a small catalytic loading,
the reaction rate is too low to neglect an undesired effect from the uncatalyzed
reaction.
Reactions with beta-Substituted Dienophiles:
Beta-substituted dienophiles are much less reactive
than the unsubstituted one, therefore, reactions were performed at room
temperature. The observed enantioselectivities of 93 and 94% ee, for the
dienophiles having a primary alkyl substituent, are satisfactory enough
for the conditions employed. However, the phenyl substituted substrate shows
rather a low chemical yield with a poor enantioselectivity even when the
reaction is catalyzed by the anhydrous complex in situ prepared from R,R-DBFOX/Ph,
NiBr2 and AgSbF6. Use of the aqua complex is less effective (rt, 72 h, 20%,
51% ee for endo-cycloadduct).
Catalysis of Other DBFOX/Ph Complexes:
The red complex derived from Co(ClO4)2´6H2O shows
a similar catalytic activity as well as excellent stereoselectivities. To
compare with the aqua complex, we prepared the anhydrous nickel and cobalt
complexes by treatment of DBFOX/Ph ligand with an equimolar amount of NiBr2
or CoBr2 in dichloromethane, followed by action of two equivalents of AgClO4.
It is interesting that the nickel and cobalt aqua complexes are even more
effective, both for catalytic activity and enantioselectivity, than the
corresponding anhydrous complexes. Addition of three equivalents of water
to the anhydrous nickel complex recovers the catalytic efficiency; the effect
of water and other additives will be discussed below. DBFOX/Ph complexes
derived from manganese(II), iron(II), copper(II), and zinc(II) perchlorates,
both anhydrous and "wet", exhibit high catalytic activities resulting
in excellent enantioselectivities in the Diels-Alder reactions, where the
"wet" catalysts were prepared by addition of 3 equivalents of
water to the anhydrous catalysts. On the other hand, the DBFOX/Ph complexes
of Fe(III), Cu(I), Rh(III), Pd(II), Ag(I), Sn(II) salts as well as some
lanthanoids are much less effective. When the results observed in the Diels-Alder
reactions using anhydrous and aqua complexes are compared, it is clear that
the aqua complexes are again more fruitful in most cases. One exception
is the iron(III) complex, which provides improved enantioselectivity when
the catalyst is prepared in the presence of 3-acryloyl-2-oxazolidinone and
MS 4A.
Diels-Alder Reactions of Other Substrates:
Reactions of other substrates can be catalyzed
by the metal complexes of DBFOX/Ph ligand . Dienophiles such as N-acryloyl-2-pyrrolidinone
and methyl (E)-2-oxo-4-phenyl-3-butenoate can be similarly activated with
the magnesium complex to give the corresponding cycloadducts, the former
reaction giving an excellent enantioselectivity. Isoprene, as sluggish acyclic
diene, reacts in the presence of the anhydrous nickel catalyst, but the
selectivity is not satisfactory. Although 1-methoxy-1,3-butadiene and 1-trimethylsilyloxy-1,3-butadiene
are expected in general to be more reactive dienes than cyclopentadiene,
this expectation is not upheld in the reactions catalyzed by DBFOX complexes.
The oxygen substituents at 1-position of these dienes are sterically hindered
by the plane of dibendofuran ring in the transition state. Monodentate dienophile,
2-bromoacrolein, shows a high enantioselectivity in the reaction with cyclopentadiene
when catalyzed by the copper(II) complex.
