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Enantioselective Transition Metal-Catalyzed Hydrogenation for the Asymmetric Synthesis of Amines.

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fuels. In fact, theory predicts a better efficiency for N, than
for the conventional fuel, which is based on H,and 0,.
The intention of this short report was to show the advance
of theory through a b initio calculations in the field of nitrogen rings. Although the calculated molecular properties can
still be extended and refined, already a quite clear picture
emerges. Nitrogen rings, and in particular nitrogen cages,
are extremely energy-rich and kinetically labile systems. In
comparison with the results of theoretical studies, the yield
of the experimental investigations is as yet small. N3H3 and
N, were unintentional successes. Well-targeted photochemical studies under matrix conditions will probably still expose
several surprises.
German version: Angeu.. Chem. 1993, 104. 242
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Enantioselective Transition Metal-Catalyzed Hydrogenation
for the Asymmetric Synthesis of Amines
By Carsten Balm*
Catalyzed asymmetric reductions of functionalized olefins
and ketones occur very selectively, in contrast reductions of
C = N bonds have so far only led to low enantioselectivities.''] In two recent publications Burk and Feaster"I and
also Willoughby and Buchwaldr3' reported new transition
metal catalysts for homogeneous asymmetric hydrogenation, with which optically active amines can be obtained with
high enantiomeric excesses.
Following the investigations of the enantioselective hydrogenation of functionalized olefins with chiral bis(phospholane)rhodium(I) complexes,[41 Burk and Feaster have
now used the new transition metal catalysts for the asymmetric synthesis of primary amines.[21With 0.1 mol% of the
catalyst system (Et-duphos)Rh' [Rh(5)]+, N-aroylhydrazones 3 of arylalkyl ketones and a-keto esters are reduced
under mild reaction conditions (20 "C. 1-4 atm H,.1 -36h.
in 2-propanol) to give the corresponding optically active Naroylhydrazines 4 (Scheme 1).
In comparison with other chiral diphosphanes such as
binap,''] ~ h i r a p h o s , o~r~bdpp,[']
]
5 proved to be the most
effective ligand for the transition metal catalysts; enantioselectivities of up to 9 7 % ee were achieved with 5. If compound 4 is allowed to react with samarium iodide ( 22
equiv), the N-N bond can be cleaved and the corresponding
amine 2 is obtained with no loss of optical purity. Thus,
synthetically valuable primary amines are obtained as reaction products. The chiral a-hydrazino acid derivatives are of
particular interest. These derivatives are obtained from aketo esters and their hydrogenolysis leads to the optically
active a-amino acids.
[*I
Dr. C. Bolm
Departement Chemie der Universitdt Base1
St. Johanns-Ring 19, CH-4056 Basel (Switzerland)
I
Et
Scheme 1 . [a] Reductive amination. R
aryl; R" ary,; Et-duphos = 5.
=
aryl, CO,Me, C0,Et; R'
=
alkyl.
~
In their new workc2]Burk and Feaster refer to a series of
experiments on the asymmetric reduction of C=N bonds
with rhodium(1) catalysts for which molecular hydrogen[61
and silanesr7]were used as reducing agents.[" In contrast to
these known methods, which usually lead to the formation of
secondary amines, the asymmetric reductive amination with
the [Rh'(duphos)] complexes now introduced is also distinguished by an extended substrate range, higher enantio- and
chemoselectivity as well as milder reaction conditions. Nitriles, imines, halides as well as carbonyl and nitro compounds are not reduced, nonfunctionalized olefins and alkynes are only hydrogenated in small amounts ( <2 %). Two
factors are proposed to be responsible for this high chemoselectivity. A substrate chelation at the cationic transition
metal center increases the rate of hydrogenation of the N-
aroylhydrazones selectively, and the reduction of other functional groups is inhibited by the N-aroylhydrazines formed.
The N-phenylimine and N-phenylhydrazone of acetophenone
are correspondingly not hydrogenated under these mild reaction conditions. Osborn et al. previously reportedlga.b1
similar high chemoselectivities in metal-catalyzed hydrogenations of N-arylketimines. The diphosphaneiridium complexes used here are very active, as Spindler et al. have also
shown,[9"] but result in products with lower enantiomeric
excesses.lYb.C1
The optically active secondary and cyclic
amines prepared with the iridium catalyst system are important intermediates in the synthesis of herbicides.
Willoughby and Buchwald have i n ~ e s t i g a t e d ' ~a ]fundamentally different catalyst system for the asymmetric reduction of C=N bonds. If 2-10 mol% of the chiral ansa-titanocene 6 is used as the catalyst for the hydrogenation of cyclic
imines with molecular hydrogen excellent enantioselectivites
(up to 98% ee; Scheme 2) are achieved. In contrast, the
Scheme 2 . X, = l.l'-binaphthyl-2,2'-diolate.
[a] H, pressure. 2-10 mol% 6 ;
R.R,R" = alkyl. aryl.
reactions of acyclic compounds with C = N bonds occur less
selectively; starting from N-benzylimines the products are
only obtained with ee values of between 58 and 87 %. Steric
Factors in the formation of diastereomeric transition states
and competitive hydrogenations of stereochemically different substrate/catalyst complexes are thought to be responsible for this.
The authors assume that the hydridotitanium(ii1) complex
7 is the active form of the catalyst; 7 is prepared by the
reaction of the stable complex 6 with n-butyllithium in situ
and is stabilized by phenylsilane. The hydrogenation of the
ketimines occurs in T H F at 65 "C under increased hydrogen
pressure (ca. 135 atm).
Willoughby and Buchwald have thus shown for the first
time that the ansa-metallocenes['O1developed by Brintzinger
et al. can also be used for the asymmetric hydrogenation of
ketimines and thus for the enantioselective synthesis of
amines. In the corresponding asymmetric reduction of olefins,
structurally related ansa-zirconocene binaphtholates were
previously tested by Waymouth and Pino as precursors for
homogeneous Ziegler-Natta systems (65 O h eemaxfor the
deuteration of styrene)." l a ] This reaction is also catalyzed by
a cationic ansn-zirconocene hydride structurally analogous
to 7." The idea of using ansa-metallocenes as asymmetric
o r chiral
is based on their unusually high stereochemical control for the polymerizations
and oligomerization reactions of ole fin^,^'^] which can be
used both for the formation of stereoregular
of
Angew. ('h(wn. Jnr. Ed. Engl. 1993, 32, No. 2
$3
high molecular weight and also for the synthesis of optically
active oligo- and polymers.['51
The enantioselective syntheses of amines of Burk and
Feaster,"] and Willoughby and B ~ c h w a l dintroduced
~~]
here
again illustrate the extraordinary potential of modified chiral transition metal complexes for asymmetric catalysis.
Thus, they are not only interesting contributions to modern
synthetic chemistry but also important steps on the often
difficult path to a more selective and more efficient chemistry.
German version. Angeu.. Chem. 1993. 105, 245
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VCH Verlugsgesellschafi mbH, W-6940 Wemherm. 1993
0570-0833:93:0202-0233 S 10.00f .25p
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asymmetric, synthesis, metali, enantioselectivity, hydrogenation, transitional, amines, catalyzed
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