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Designed Coupling of C1 Ligands Organometallic Model Reactions.

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Designed Coupling of CI Ligands : Organometallic Model Reactions
By Helmut G. Alt*
Carbon to carbon coupling is the most important step for
the manufacture of organic fine and industrial chemicals.
Metallic compounds, mostly catalysts, play an indispensible
role in this process. Yet there is still a long way to go to
understand the mechanism of metal-centered coupling of C,
units such as in the Fischer-Tropsch synthesis. Now in this
issue Filippou et al.['] of the Technische Universitat Munchen
reports astounding progress in this fundamentally important field. He not only achieved the planned synthesis of
mononuclear bis(carbyne) metal complexes (2), but also the
C, + C, coupling of the carbyne ligands to form an alkyne
ligand (2 + 3/4, see Scheme 1).
stituents.['-31 Exactly this is the case in Filippou's complex:
the limiting resonance formulas A and B, for which spectroscopic evidence exists,['*4-61show that in such bis(carbyne)
complexes a "partially reduced" metalkarbon multiple
bond is present.
The alkyne that results from the coupling reaction functions as a four-electron ligand, and the metal center thus
must accommodate a deficit of only two electrons, which can
be compensated by the entering isonitrile ligand or by the
two bromine ligands.
CH,CI,, - 80 "C
This classic coupling reaction was not coincidental, but
the result of a well-planned strategy: previously Filippou et
al. had assigned a key role to the ynamines in the acid-induced C-C coupling of carbyne and isocyanide ligands and
to bis(carbyne) complexes in the reductive C-C coupling of
two isocyanide ligands to yield ynediamines.[']
In the light of Filippou's findings,"] it is logical to postulate the existence of bis(carbyne) intermediates for other
metal-centered coupling reactions of C, unitst8,91 as well,
such as in the coupling of two carbonyl ligands to form
bis(siloxy)acetylenes.[''] The exemplary results described
here suggest a series of prospects: The designed construction
of unsaturated C, ligands not capable of existence in unbound form opens new possibilities for the metal-centered
synthesis of heterocycles. Furthermore, the transfer of this
concept to transition-metal centers with other electron configurations gives a positive outlook for the directed cleavage
of alkynes into unsaturated, metal-fixed C, fragments. The
first successes have already been recorded.[7d1
Scheme 1. Synthesis of the bis(carbyne) complex 2 by alkylation of 1 with
Meerwein's reagent and C, + C, coupling in the ligand sphere of 2 to afford 3
and 4.
How can two metalkarbon multiple bonds be opened
simultaneously under mild conditions when thereby an electron deficiency arises at the metal center? Filippou's concept
is based on extended Huckel calculations, which predict that
the coupling of two carbyne ligands is symmetry-allowed at
a d4 metal center and should proceed spontaneously, whereas a stabilization of the bis(carbyne) complexes is attained
when the carbyne carbon atom is attached to n-donor sub[*] Prof. Dr. H. G. Alt
Laboratorium fur Anorganische Chemie
Universitat Bayreuth
Universitatstrasse 30, W-8580 Bayreuth (FRG)
Angew. Chrm. In[ Ed. Engl. 30 (1991) No. 9
German version: Angew. Ckem. 103 (1991) 1139
[l] A. C. Filippou, W. Griinleitner, C. Volkl, P. Kiprof, Angew. Ckem. 103
(1991) 1188; Angew. Ckem. Int. Ed. Engl. 31 (1991) 1167.
[2] H. Fischer, P. Hofmann, F. R. KreiBI, R. R. Schrock, U. Schubert, K.
Weiss: Carbyne Complexes, VCH, Weinheim 1988.
[3] C. N. Wilker, R. Hoffmann, 0. Eisenstein, Nouv. J Chim. 7 (1983) 5 3 5 .
[4] J. Chatt, A. J. L. Pombeiro, R. L. Richards, J. Cbem. SOC.Dalton Trans.
[5] A. J. L. Pombeiro, R. L. Richards, Transifion Met. Chem. ( Weinkeim) 5
(1980) 55.
[6] A. C. Filippou, W Griinleitner, J Organomet. Ckem. 407 (1991) 61.
[7] a) A. C. Filippou, W. Grunleitner, Z. Naturforsck. 8 4 4 (1989) 1023;
b) A. C. Filippou, Polyhedron 9 (1990) 727; c) A. C. Filippou, W.
Griinleitner, J Organomef. Ckem. 393 (1990) C 10; d) A. C. Filippou. C.
Volkl, W.Grunleitner, P. Kiprof, Angew. Chem. 102 (1990) 224; Angew.
Ckem. Inf.Ed. Engl. 29 (1990) 207; e) A. C. Filippou, W. Griinleitner, 2.
Nalurforsck. 8 4 6 (1991) 216.
[XI G. A. McDermott, A. Mayr, L Am. Ckem. Soc. 109 (1987) 580.
[9] A. Mayr, C. M. Bastos, J. Am. Ckem. SOC.112 (1990) 7797.
[lo] R. N. Vrtis, S. J. Lippard, Isr. J Ckem. 30 (1990) 331.
0 VCH Verlagsgesellsckaft mbH, W-6940 Weinheim, 1991
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organometallic, mode, reaction, designer, couplings, ligand
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