Multicentered reaction

A number of reactions do not seem to belong to any of the above mechanistic types. Such processes are referred to as multicenter reactions. The Diels-Alder cycloaddition reaction of 1,3-butadiene with maleic anhydride is an example (Scheme 5). No charged or odd election intermediates seemingly are involved in this reaction. 

Schenck and Steinmetz281 consider that the latter reaction can be explained in terms of a biradical multicenter reaction, with liberation of nitrogen. 

The catalysis of symmetry-forbidden reactions is currently of broad interest and a number of workers have expressed different points of view on the metal s role. Katz, for example, seems to take the position that these catalytic processes are very likely stepwise in nature, and, therefore, for-bidden-to-allowed arguments do not apply i5,40). This criticism is perhaps more broadly directed toward the concerted, "n complex multicenter reaction mechanism first postulated by Schrauzer to explain metal-assisted cycloaddition chemistry 54). A preference for nonconcerted mechanisms in this chemistry is also shared by Cassar, Eaton, and Halpem... 

A number of chemiluminescent studies of chemiluminescent multicenter reactions of transition metal dimers and trimers have been reported [117], namely... 

Ring stereochemistry can be efficiently created through multicenter reactions, and in this respect the Diels-Alder reaction is extremely useful. Other tricks are to use intramolecular reactions, e.g. iodolactonisation.[18]... 

In a first discussion of the possible mechanism (Schrauzer, e< al., 32, 33) it was assumed that the products are formed within tt complexes in a concerted fashion ( it complex multicenter reaction ). Norbom-adiene forms many complexes with transition metals in which it is symmetrically coordinated. Some pertinent examples are (XII) (1, 2), (XIII) (35), and (XIV) (36). Ni(CO)4 reacts with acrylonitrile to produce bisacrylonitrile nickel, Ni(CH2=CHCN)2 which adds phosphines forming 1 1 and 1 2 adducts (37, 38). Acrylonitrile is a relatively strong TT-bonding ligand and may back-accept charge from the metal via d -p bonding. In this regard it is related to duroquinone (tetra-methylquinone), which is known to form Ni(0) it complexes of the type Ni(Dqu)2 or (XIV). The most likely intermediate in the reaction... 

The stereospecific dimerization of norbornadiene catalyzed by a dimeric cobalt complex, is called a ir-complex multicenter reaction iso-153)... 

In 1967, another major paper by Pearson and Songstad (5), applying the HSAB principle to organic chemistry, appeared. Saville published a full account of his analysis of multicenter reactions (10) based on the HSAB principle. Efforts have been made to determine quantitatively the hardness (softness) of some donors and acceptors (11). Klopman presented a perturbation treatment (12) of chemical reactivity and related HSAB to molecular orbital theory. 

Saville s rules serve to indicate optimal catalytic conditions. However, there exist many facile multicenter reactions which do not fall into either of the two categories. The hydrolysis rates for acetylimidazole (62) rise with increasing imidazole buffer concentration, and this observation suggests a (S)- - - transition state. 

It might be interesting to note that the proponents of the carbene mechanism (mentioned earlier), point out that this is also consistent with their mechanism [254, 255], The reaction can consist of (a) an insertion of a metal into an a-CH bond of a metal alkyl to form a metal-carbene hydride complex. This is followed by (b) reaction of the metal-carbene unit with an alkene to form a metal-cyclobutane-hydride intermediate. The final step (c), is a reductive elimination of hydride and alkyl groups to produce a chain-lengthened metal alkyls. This assures that a chiral metal environment is maintained [254]. It is generally believed [258], however, that stereospecific propagation comes from concerted, multicentered reactions, as was shown in the Cossee-Arlman mechanism. The initiator is coordinated... 

Even though Ni(CO)4 is called liquid death, this nickel catalyst has been applied in carbonylation reactions [52]. The group of Ricart reported a nickel-catalyzed carbonylative cycloaddition of alkynes and aUyl hahdes to cyclopentanes. The desired products were obtained in high yields and with controlled stereoselectivity. Iron was used as a reductant. An extension of the reaction to new substrates led to the conclusion that, although the steric and electronic effects of the alkyne substituents are generally irrelevant in relation to the adducts and their yields, those of the allylic counterpart may have a significant influence on the outcome of the reaction. However, the presence of the amine moiety in the alkyne completely inhibited the reaction. The feasibility of a multicentered reaction was verified with a triacetylene, in which up to 12 bonds were created simultaneously and in good yield (Scheme 1.30). 

The concept on multicentered reaction system of chain conversion [9] (Section 5.1) enables to consider the conditions of both the selective inhibition and selective initiation of chain reactions from the more general viewpoint. 

The unimolecular dissociation of Mc3Si+ (78) has been studied experimentally . As indicated by CAD studies and isotopic exchange reactions, ion 78 readily interconverts upon excitation to isomer 79 (equation 34). The reaction mechanism for this interesting interconversion has been proposed to proceed via a concerted 1,2-hydrogen/l, 2-methyl migration. This dyotropic rearrangement corresponds to a thermally allowed process. In a similar fashion the ethene elimination from 78 was proposed to proceed via a dyotropic multicenter reaction mechanism (equation 35). 

Complex compounds s. a. Metal complex compounds "-Complex multicenter reactions... 

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