Carbon heterobimetallic

This section is limited to complexes which have a group 1 metal in conjunction with another, different main group metal, but also includes Cu and Cd since they exhibit properties akin to their main group analogs. It is also limited mainly to those complexes in which the metals find themselves attached to different atoms and there is a particular emphasis on compounds with alkali metal-carbon bonds of various types, except where the evolution of inverse crown complexes is discussed. There are many more heterobimetallic-heteroatom complexes (e.g., mixed metal amides), but these lie outside the scope of this current review though references may be found to them in the references for the complexes described herein. 

Of great synthetic potential are demetallations with simultaneous formation of C-Sn bonds (Figure 2.20) [137,146,230,281,282]. These reactions presumably proceed via a heterobimetallic intermediate containing an Sn-M-C group. Reductive elimination of the metal M from this intermediate leads to formation of the carbon-tin bond. The resulting alkyl- or vinylstannanes are valuable synthetic intermediates. 

In order to vary the electronic situation at the carbene carbon atom a number of carbo- and heterocycle-annulated imidazolin-2-ylidenes like the benzobis(imida-zolin-2-ylidenes) [58-60] and the singly or doubly pyrido-annulated A -heterocyclic carbenes [61-63] have been prepared and studied. Additional carbenes derived from a five-membered heterocycle like triazolin-5-ylidenes 10 [36], which reveals properties similar to the imidazolin-2-ylidenes 5 and thiazolin-2-ylidene 11 [37] exhibiting characteristic properties comparable to the saturated imidazolidin-2ylidenes 7 have also been prepared. Bertrand reported the 1,2,4-triazolium dication 12 [64]. Although all attempts to isolate the free dicarbene species from this dication have failed so far, silver complexes [65] as well as homo- and heterobimetallic iridium and rhodium complexes of the triazolin-3,5-diylidene have been prepared [66]. The 1,2,4-triazolium salts and the thiazolium salts have been used successfully as precatalysts for inter- [67] and intramolecular benzoin condensations [68]. 

Ligands with a single carbon atom between phosphorus and a heteroatom have been synthesized and used to prepare heterobimetallic complexes between platinum and a second metal ion. Examples of such complexes are shown in structures (153)-(156).1464-1467... 

Finally, we reported a di-iron(III) catalyst 24 and the corresponding copolymerization activity [147]. This system was able to produce copolymer with CHO/C02 and demonstrated a TOF of 53 h 1, at 80°C, lObar and aCHO/Fe ratio of 10,000 1. The system did not yield copolymer with PO, but addition of one equivalent of [PPN]C1, per Fe centre, allowed the conversion of PO into cyclic propylene carbonate with TOFs around 10 h 1. Previously, some heterobimetallic iron tert-butoxide complexes ( (7-BuO)5FeLa] and [(f-BuO)4FeZn]) had been reported for the copolymerization of PO and C02 [153]. This catalyst was the first use of an iron complex for the homogeneous copolymerization of CHO and C02. Rieger and coworkers recently reported a mononuclear Fe system that showed similar behaviour towards PO [154] and some copolymer formation with CHO/C02 strongly dependent on the co-catalyst system [98]. 

Palladium(ll) and cobalt-rhodium heterobimetallic nanoparticles can catalyze the reaction of twtfo-iodophenols with internal alkynes and carbon monoxide to furnish 3,4-disubstituted coumarins (Equation 281) <20000L3643, 2003JOC9423, 2004SL2541>. Unsymmetrical alkynes react to form two regioisomeric coumarins the major product usually features the more bulky alkyne substituent at G-3 (Equation 281) <20000L3643, 2003JOC9423, 2004SL2541>. 

The heteroalkyne PCBu behaves in a remarkably similar fashion. Addition across the Rh=Rh bond in 5 affords the rigid metallaenone 94 (151). While dimethylcyclopropene is not an alkyne, the reaction of 5 with dimethylcyclopropene is related. The product is 95 and is derived by cleavage of the C=C of the three-carbon ring. The analogous dicobalt and CoRh heterobimetallic species are formed similarly (228,229). 

AUcynylchalcogenato ruthenium complexes react with zirconocene to give rise to heterobimetallic early-late dissymmetrically bridged complexes of family (76) (equation 39). In those specific complexes, the two metal centers are linked by /u.-chalcogenido and /x-a, jr-alkynyl moieties. The acetylenic bridge is unsymmetrical because the terminal carbon of alkynyl is ct-bonded to ruthenium, while zirconium interacts with both alkynyl carbons in a side-on fashion. 

Ligands whose chirality is the result of atropisomerism, and not of an asymmetric center on phosphorus or carbon (Table 5), are highly enantioselective when complexed to Ru °. Heterobimetallic complexes form with metal-containing chiral ligands. Stereoselective lithiation of (S)- or (R)-a-ferrocenylethyldimethylamine (an easily resolved derivative of ferrocene) allows introduction of one or two phosphino groups, to need ferrocenylphosphinite (Table 6). ... 

In the following, such a desired new and innovative multifunctional catalytic system is reviewed The chiral heterobimetallic lanthanoid complexes, developed by Shibasaki et ah, have recently been shown to catalyze a broad spectra of organic reactions including many classical carbon-carbon bond formations... 

Scheme 30. Heterobimetallic carbon-carbon coupling reactions that yield metallacyclic, head-to-head 1,3-diyne products.

Books 1. Ojima, Catalytic Asymmetric Synthesis , VCH, New York, 1993 R. Noyori, Asymmetric Catalysis in Organic Synthesis , John WUey Sons, New York, 1994 T. Hayashi, K. Tomioka, O. Yonemitsu, Asymmetric Synthesis , Kodansha/Gordon and Breach Science Publishers, Tokyo, 1998 J. Seyden-Penne, Chiral AuxUiaries and Ligands in Asymmetric Synthesis , John Wiley Sons, New York, 1995 Recent Reviews M. Shibasaki, H. Sasai, T. Aral, Angew. Chem. 1997, 109, 1290 Angew. Chem., Int. Ed. 1997, 36, 1237 (Asymmetric Catalysis with Heterobimetallic Compounds), S. Koba-yashi, PureAppl Chem. 1998, 70, 1019 (New Types of Lewis Acids Used in Organic Synthesis), K. Mikami, Pure Appl. Chem. 1996, 68, 639 (Asymmetric Catalysis of Carbo-nyl-Ene Reactions and Related Carbon-Carbon Bond Forming Reactions). 

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