Four-center exchange reactions

The H—D exchange between triethyltin deuteride and di-isobutylaluminum hydride has been studied by Neumann 84). A four-center transition state has been proposed for this reaction... 

Square-planar stereochemistry is mostly confined to the d8 transition metal ions. The most investigated solvent exchange reactions are those on Pd2+ and Pt2+ metal centers and the mechanistic picture is well established (Table XIV (194-203)). The vast majority of solvent exchange reactions on square-planar complexes undergo an a-activated mechanism. This is most probably a consequence of the coordinatively unsaturated four-coordinate 16 outer-shell electron complex achieving noble gas... 

The mechanism suggested for this reaction is represented in Scheme 21. The involvement of a four-centered transition state is generally accepted for many related (2+2) exchange reactions (1,63). 

FOUR-CENTER EXCHANGE REACTIONS The general form for this type of elementary step is A-B + C-D -> A-C+B-D... 

The mechanisms of these exchange reactions as well as of the related exchanges with boron halides, tris(dialkylamino)boranes, and boron alkyls has been interpreted to proceed through a four-center transition state (255, 258, 260). 

In the associative mechanism this exchange would be a side reaction not related to the polymerization process. The exchange would have to occur by a four centered transition state between two chain ends at least one of which is complexed to catalyst. 

In addition, studies indicate that Zn(CH3)2 undergoes rapid exchange with Cd(CH3)2, and various postulates concerning the mechanism of this reaction have been made (57, 87). Recent studies of this system in methyl-cyclohexane solvent have clearly shown that the reaction is first order in each of the components and proceeds with an activation energy of 17 kcal/mole (57). This study indicates that the exchange process proceeds through a four-centered transition state... 

The stereospecific cis-addition of diboron tetrachloride to alkynes and alkenes (37) may be interpreted as an interaction of the empty 7r-orbitals of the boron atoms with the 7r-orbital of the organic species. According to this picture, boron-boron bond breaking would lag behind boron-carbon bond formation. The transition state is a 4 + 2 Hiickel aromatic ( .=0), and thermal addition is allowed. If bond making and breaking were synchronous, this four-center reaction would be more like the <7-77 exchange reactions, which we shall discuss later. With regard to (37), there is a discrepant case in which an apparent trans addition of diboron tetrachloride to cyclopentadiene has been found (Saha et al., 1967). 

We complete molecular reactions with a group in which bond changes are prominent. This section will also serve to effect the transition from the no-mechanism to the some-mechanism categories. As indicated earlier, our classification serves mainly to divide up a large body of material. From the point of view of orbital correlations, there is nothing in o-o and a-ir exchange processes that restricts them to one mechanistic class or the other. This is illustrated by some of the possible reactions involving four centers, in which the formal similarities are stressed. 

Notwithstanding the fact that certain four-center additions to alkenes, e.g. equation (94) or Fig. 22, are forbidden, bromine addition to an alkene does occur. The orbital symmetry arguments which forbid cis addition to isolated bonds favor trans addition in equation (144), in which j = 4, X may be a nucleophile or radical and Y an electrophile or radical. Therefore, additions of molecular bromine or in general X—Y to alkenes, should proceed in at least two steps. Otherwise, separated X and Y, with one electron pair between them, add in concerted fashion to the alkene. Equation (144) is effectively the prototype of numerous ionic and radical a-w exchange reactions. A wealth of information has been recorded in excellent reviews covering special aspects of this general process, e.g. electrophilic additions (de la Mare and Bolton,... 

It has been speculated (5) that the olefin metathesis reaction mech-nism involves a four-centered quasi-cyclobutane transition state. The three basic steps postulated for the reaction, namely, formation of a bis-olefin-tungsten complex, transalkylidenation and olefin exchange, may account, in general, for the initiation and propagation steps in the ringopening polymerization of cycloolefins. Several modes of termination have been considered, but suitable data to test these are not yet available. 

Nevertheless, for bimolecular atom-exchange reactions it has been possible [49] to identify three categories of reactive dynamics, together with intermediate cases. On the other hand, very little is known about four-center reactions [50, 51], and no results have been reported for association reactions. The following four families of reactions are now considered in some detail ... 

Reaction of BF3 with B at 1850 °C generates the reactive intermediate species boron monofluoride, BF, which cannot be isolated. Cocondensation of BF with BF3 yields B2F4, a metastable lower fluoride of boron, and BF2-B(F) BF2, which is unstable and disproportionates according to equation (46). Mixtures of boron trihalides undergo exchange reactions, presumably via a four-centered... 

Exchanges between aryl halides and aryllithiums are second order with four-center transition states . A free-radical component, i.e., some homolytic fission, is detected in some reactions , e.g., of alkyl halides and alkyllithium. However, the complete retention of configuration, e.g., of cycloalkyL and alkenyF" halides, and the partial retention in others, e.g., of chiral alkyl halides , suggest that the radical component is small. 

Similar conjugative stabilization is unavailable in addition reactions to cyclopropane. Thus, the thermally neutral isotopic exchange of cyclopropane has a lower activation energy than hydrogenolysis, which is exothermic by 40 kcal. The hydrogenolysis probably proceeds as a four-center reaction between Cr +H and cyclopropane. 

---