Four-center mechanisms

In a few cases, SnI reactions have been found to proceed with partial retention (20-50%) of configuration. Ion pairs have been invoked to explain some of these. For example, it has been proposed that the phenolysis of optically active a-phenyl-ethyl chloride, in which the ether of net retained configuration is obtained, involves a four-center mechanism ... 

There are some addition reactions where the initial attack is not at one carbon of the double bond, but both carbons are attacked simultaneously. Some of these are four-center mechanisms, which follow this pattern ... 

The dynamic olefin insertion process has been modeled using various quantum mechanical methods. A concerted four-center mechanism involving a frontal copla-nar attack of the C=C unit on the Zr-H bond of 1 is associated with a low activation energy of 0-15 kcal mol and has been proposed for the reaction of ethylene (Scheme 8-2) [37]. 

The relative reactivity profile of the simple alkenes toward Wacker oxidation is quite shallow and in the order ethene > propene > 1-butene > Zi-2-butene > Z-2-butene.102 This order indicates that steric factors outweigh electronic effects and is consistent with substantial nucleophilic character in the rate-determining step. (Compare with oxymercuration see Part A, Section 5.8.) The addition step is believed to occur by an internal ligand transfer through a four-center mechanism, leading to syn addition. 

The copper d-orbital being very low-lying (hence no redox chemistry available) [92], MeCu can undergo addition only through a four-centered mechanism (Eq. 10.8). 

The pH-rate constant profile for hydrolysis of phthalamic acid showed participation by the undissociated carboxyl group, and an anhydride intermediate was detected (Bender, 1957 Bender et al., 1958b). Hydrolysis was 10 times faster than in the case of p-carboxybenzamide. A four-center mechanism [58] was postulated. 

Scheme 3.28 depicts the proposed reaction mechanism that may happen on these alloys. The first step is the coordination of the ester to the alloy via its oxygen atoms (it is likely that hn favors this coordination due to its electrophilicity). In a second step there is a four-center mechanism where the hydridic hydrogen makes a nucleophilic attack at the electrophilic carbon atom of the carbonyl with forma-... 

In a study of the addition of nitrosyl chloride or nitrosyl bromide to norbor-nene and norbomadiene, it was observed that (a) there was no structural rearrangement during the reaction, (b) a cis addition had taken place, (c) nucleophilic solvents such as ethanol or acetic acid were not incorporated in the products. These facts seem to speak against an ionic addition mechanism, while a free radical initiated by NO radicals was considered unlikely since nitric oxide is inactive toward norbomadiene. Therefore a four-center mechanism has been suggested [70], However, when a relatively simple, unstrained olefin such as A9-octalin was subjected to the reaction, only blue, crystalline, monomeric 9-nitroso-10-chlorodecalin was produced. This product had a trans configuration. Thus it is evident that the structure of the olefin has a significant bearing on the steric course of the addition [71]. 

Of greater relevance catalytically is that the combined use of l3C enrichment and 13C nutation NMR spectroscopy can distinguish between proposed rival mechanisms for the Ziegler-Natta catalyzed polymerization of acetylene. In the four-center insertion mechanism the enriched acetylene (HC =C H) is incorporated as shown in Scheme 6. It is to be noted that the, 3C—13C bond label is here incorporated into a carbon-carbon double bond, the length of which is significantly smaller than that of a carbon-carbon single bond, which is how the enriched acetylene would be incorporated in the two-center mechanism shown in Scheme 7. The results of nutation experiments leave little doubt that the Ziegler-Natta polymerization of acetylene proceeds by a four-center mechanism. 

If the concerted four-center mechanism for formation of chloromethane and hydrogen chloride from chlorine and methane is discarded, all the remaining possibilities are stepwise reaction mechanisms. A slow stepwise reaction is dynamically analogous to the flow of sand through a succession of funnels with different stem diameters. The funnel with the smallest stem will be the most important bottleneck and, if its stem diameter is much smaller than the others, it alone will determine the flow rate. Generally, a multistep chemical reaction will have a slow rate-determining step (analogous to the funnel with the small stem) and other relatively fast steps, which may occur either before or after the slow step. 

The attack of a chlorine atom on a methane hydrogen is not expected to require a precisely oriented collision. Moreover, the interatomic repulsions should be considerably smaller than in the four-center mechanism discussed previously for the reaction of molecular chlorine with methane because only two centers have to come close together (Figure 4-8). The methyl radical resulting from the attack of atomic chlorine on a hydrogen of methane then can remove a chlorine atom from molecular chlorine and form chloromethane and a new chlorine atom ... 

Monocyclic pyrylium salts react with azomethines according to a postulated four-centered mechanism yielding products of exchange between the oxonium atom and the amine fragment of the Schiff base (73ZOR1079 82KGS465). 

Sakurai and coworkers75 generated the five-membered silene 44 by a photochemical 1,3-silyl shift in the cyclic divinyldisilane 45 (Scheme 13). Since the silene 44 is constrained to be planar, no bond rotation is possible during the reaction. Contrary to the previous observations i.e. a simple two-step or a concerted four-centered mechanism, alcohols add to 44 nonstereospecifically, although in the cyclic silene bond rotation is prohibited. 

Properties of the attacking group 45,46 If the attacking ligand is a Lewis acid, the basic character of the nitrogen is decreased, as shown by the four center mechanism in Scheme 20. 

Initially the process leads to almost exclusive formation of dimethyl iodophosphite which reacts in a consecutive step with the other product trimethyl methoxysilane to form dimethyl(trime-thylsilyl) phosphite and methyl iodide. The results are explained by four-centered mechanism involving the nucleophilic attack on phosphorus according to the following scheme ... 

Clark and Cook 71) disproportionated [1-14C] propylene and [2-14C] propylene over cobalt oxide-molybdate-alumina catalyst. At 60 °C their results were consistent with those reported Mol and coworkers, confirming the four-center mechanism. At temperatures above 60 °C, double-bond isomerization activity of the cobalt-molybdate catalyst became a factor and at 160 °C nearly one-half of [l-l4C] propylene had isomerized to [3-14C] propylene prior to disproportionation. The authors note that at temperatures where isomerization does not occur, the possibility of a jr-allyl intermediate appears to be excluded however, at higher temperatures, the 77-allyl mechanism cannot be so easily dismissed. 

A four-center mechanism will be assumed in this as in similar reactions. Cyanates are also known ... 

This unusual reaction has been proposed to occur via a four-center mechanism. While no structural information was derived from the original experiments, calculations at the HF level predict that a linear structure corresponding to protonated germanium oxide, GeOH+, is considerably more stable than the alternative linear HGeO+ structure . ... 

We then proposed, first, a mechanism close to the one already given by Bar-Eli and Klein (1). As shown by conductimetric data, NH2 is solvated. A N-H bond of the solvation ammonia molecule is polarized by NH2" and then weakened. The exchange occurs between a HD molecule and the solvated NH2 ion, through a four center mechanism, in the following way ... 

Our previous molecular dynamics simulations showed that under the unusual combination of conditions made possible within an impact heated cluster, the N2 + O2 -t- 2NO reaction can be made to proceed (on the computer) via a four-center mechanism. The high yield was found not only... 

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