Metallacyclopropane ring

The extent of back-donation to R2C=CR2 is influenced by the nature of R, and is enhanced by electron-withdrawing groups such as CN. In the extreme, the 7r-contribution to the C—C bond is completely removed and the complex becomes a metallacyclopropane ring. Structures 23.19a and 23.19b show limiting bonding schemes. In 23.19a,... 

The question as to whether a transition metal complex of type 4 is best described as an alkene 7T-complex 4A or as a metallacyclopropane 4B, which is of practical importance, has been addressed in several computational studies on the relationship between alkene 7T-complexes and three-membered rings [48—52]. It has been concluded that the titanium complexes of type 4 are best represented as titanacydopropanes, i.e. by resonance structure 4B, if one is willing to accept the notion that 4A and 4B are limiting resonance forms [52],... 

Oxidative cyclization is another type of oxidative addition without bond cleavage. Two molecules of ethylene undergo transition metal-catalysed addition. The intermolecular reaction is initiated by 7i-complexation of the two double bonds, followed by cyclization to form the metallacyclopentane 12. This is called oxidative cyclization. The oxidative cyclization of the a,co-diene 13 affords the metallacyclopentane 14, which undergoes further transformations. Similarly, the oxidative cyclization of the a,co-enyne 15 affords the metallacyclopentene 16. Formation of the five-membered ring 18 occurs stepwise (12, 14 and 16 likewise) and can be understood by the formation of the metallacyclopropene or metallacyclopropane 17. Then the insertion of alkyne or alkene to the three-membered ring 17 produces the metallacyclopentadiene or metallacyclopentane 18. 

In this molecule ethylene lies in the same plane as the other ligands, unlike the case in Zeise s salt where planarity is sterically prevented. By analogy to a three-membered ring of carbon atoms, this compound may be viewed as a metallacyclopropane. 

Oxidative addition can also occur at C(sp2)-X bonds (i.e., at vinyl and aryl halides). It always occurs with retention of configuration about the double bond. Oxidative addition at C(sp2)-X obviously cannot proceed by an Sn2 mechanism. The SRN1 mechanism is a possibility. Another possibility is that the Pd coordinates first to the 77 bond of, say, vinyl iodide, to form a 7r complex. The metal-lacyclopropane resonance structure of the 77 complex can be drawn. An electro-cyclic ring opening of the metallacyclopropane (Chapter 4) can then occur. One Pd-C bond breaks, and I- leaves to give a new cationic Pd-vinyl complex. When I associates with the Pd, the overall result is oxidative addition. 

The M-Cj-age (C age- hypervalcnt carbon) a bonds in metal-carboranyl complexes (Fig. 7.1) are generally inert toward various electrophiles for steric reasons, as evidenced in our previous work [9]. It indicates that the chemistry of M-C(-age a bonds is significantly different, in terms of reactivities, from that of traditional M-C a bonds. To overcome this problem, we thought that the construction of a metallacyclopropane (metal-carboryne, Fig. 7.1) would reduce the steric hindrance around the M-C(.age bond and create the ring strains, thus facilitating the reactivity of the M-C age bonds. 

C-C triple bond cleavage has some similarity to the C-C double cleavage. Several examples are known for the C-C triple bond cleavage in the multi-metallic system. A C-C triple bond easily coordinates to transition metals giving an alkyne complex. The alkyne complex can also be formally described as a met-allacyclopropene. The second coordination of the C-C double bond moiety of the metallacyclopropene to another transition metal center provides a more strained two metallacyclopropane fused ring system. When the C-C bond is cleaved, the fragment can be stabilized in the multi-metallic system. Most reported reactions of this type used the Cp ligand in the system. 

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