Four-center reaction mechanism

The effect of phenyl substitution on the rate of Grignard addition to IV-benzylidinetmiline has been examined. The reaction rate in ether for ethylmagnesium bromide conforms to r = it[R 2Mg-MgX2][Schiff base]. A four-centered reaction mechanism (Scheme 3) has been suggested. ... 

Zn(H)(Tpph,Me)], obtained from the reaction of [Zn(F)(Tpph Me)] with Et SiH, and [Zn(OH)(Tpph,Me)] undergo insertion reactions with heterocumulenes. CO, CS2, and RCNS insert into the Zn-H bond giving [Zn(OCH(0))(Tpph Me)], [Zn(SCH(S))(Tpph Me)], and [Zn(SCH(NR))(Tpph,Me)], respectively. [Zn(OH)(Tpph Me)] reacts with RCNS yielding [Zn(SC(0)NHR)(Tpph Me). In the presence of EtOH the reaction of [Zn(X)(Tpph,Me) with RNCS yields [Zn-(SC(NR)OEt)(Tpph Me). These compounds have been characterized by X-ray crystallography, supporting a four-center reaction mechanism proposed for hydrolytic zinc enzymes.145 [Zn(OH)-(Tpph,Me)] reacts with simple aminoacids to form [Zn(AB)(Tpph,Me)]... 

Aggregation adversely affects metalation reactivity. With 2CHLi TMED in toluene at 25°C and 820 mm hydrogen the rate at 0AM was only one-fifth that at 0.025M (46). One possible explanation is that this is related to the increased formation of separated ion pairs in the aggregates. One could rationalize more facile hydrogenolysis in a contact ion pair via a four center reaction mechanism with lithium participation than in a simple anionic attack on unpolarized hydrogen. 

The Castro reaction as shown in eq. (22.22), is thought to proceed through a four-center reaction mechanism as shown in Scheme 22.2 [64]. If the compound has a hetero atom at the ortho position, the cyclization proceeds to afford the heteroalkenyl compound in the Castro reaction as shown in eq. (22.23) [43,64,65]. For example, isocoumarin, benzofliran and indole are synthesized respectively regarding the kinds of XH in eq. (22.23) [64,65]. 

In view of the present calculated results, the SET mechanism would be described as follows. Basically, the polar four-center reaction in Scheme 14 leads to C—C bond formation. However, when the alkyl group is bulky, only the two-center (Mg—O) reaction takes place. The aUcyl-Mg bond is cleaved homolytically owing to the persistent Mg tetravalency and the stability of the resultant radical species. Hence, biradical intermediates are formed not by a single electron transfer but by the C—Mg homolytic scission. 

If a concerted four-center reaction won t work, then an alternative possibility is a stepwise mechanism ... 

The second step in this scheme involves a four-center concerted mechanism to allow the crotyl structure to be retained in the bromomagnesio alkoxide. After a reaction time of 5 min the oc-methylallyl derivative was formed in almost 79% yield, together with 10% of the cis- and rmns-crotyl derivatives. After 193 hr only 0.8% of the a-methylallyl compound had remained among the reaction products, whereas the combined cis- and trans-products were formed in a yield of 83% 10% unreacted rm-butyl isopropyl ketone was found in the reaction mixture in the presence of presumed excess Grignard reagent. Some years later, such a reversibility was also demonstrated for the reaction of allylmagnesium bromide with fcrf-butyl isopropyl ketone and di-fcrt-butylketone [62, 63]. 

The reaction of dimethylmagnesium with excess ketone consists of a series of pseudo first-order reactions involving the formation of two intermediate products. Pi and P2 before the formation of the final product P3. Interpretation of the kinetic data did not necessarily lead to the conclusion that a complex between the ketone and the organomagnesium species was required to bring about a reaction (case II) a bimolecular collision not involving a complex (case I) also fit the data. Nevertheless, in the abstract of the paper the authors showed the three equations that did involve complex formation, which may reflect their preference for the traditional concept of the preliminary formation of a "Meisenheimer complex. The paper continued as follows Inability to distinguish between case I and case II is relatively minor compared to the more essential features of the reaction path which have been clearly established. A four-center concerted mechanism was presented in each of the carbon carbon bond formation steps in the detailed mechanism depicted in the final scheme. 

A new area in dynamics, characterized by collision times shorter than hitherto studied, is discussed with special reference to four-center reactions. Under the unusual combination of conditions made possible within an impact heated cluster, the nominally four-center reactions can be made to proceed (on the computer) via the four-center mechanism as suggested by Bodenstein. It is even possible to achieve multi-center (> 4) reactions. The unique features of the new dynamical regime are discussed and illustrated by a variety of high-barrier processes, including the four center H2 +12 2HI and N2 + O2 2NO reactions and a multi-center "burning of air" process. 

The classical four-center reaction studied by Bodenstein was that of H2 +12 —> 2HI [17]. While in the bulk, under ordinary thermal conditions, the reaction may well proceed by a different mechanism ([18] and references therein), there is no question that in a cluster, a concerted four center reaction is the dominant mechanism [2], see Fig. 8. The novel aspect seen in Figure 8 is the rather different time scale for the H2 and I2 motion. The much faster moving H atoms scede from one another before the iodine molecule has moved much. Consequently, the newly formed HI molecules are typically rotationally excited. This is seen in the oscillation of the H-other I-distance v. time. 

So far we have discussed concerted four-center reactions. An alternative is a sequential mechanism [12,19] in which one new bond forms before the other. Such a mechanism is unlikely for a diatom-diatom reaction in a cluster but is possible for a unimolecular reaction. A discussion of how the cluster can govern the relative importance of the two mechanisms can be found elsewhere [3]. 

D. Chemical kinetics of four-center reactions. In the family of four-center reactions H2 + X2 2HX, X = halogen, there are significant variations in the bond strengths of X2. The X = I case has the simplest rate law and the others proceed by a chain mechanism (Steinfeld et ai, 1999 Houston, 2001). For H2 + I2 the proposed mechanism [J. H. Sullivan, J. Chem. Phys. 46, 73 (1967) G. HammesandB. Widom,T Chem. Phys. 96,7621 (1974)] is a rapid dissociation equilibrium I2 21 followed by I forming a weakly bound complex with H2, I -I- H2 IH2, which then reacts with another I atom, I + IH2 2HI. Show that this mechanism accounts for the overall kinetics being of the second order, first order in H2 and first order in I2. Propose experimental tests for this mechanism. 

A simplified mechanism for the hydroformylation reaction using the rhodium complex starts by the addition of the olefin to the catalyst (A) to form complex (B). The latter rearranges, probably through a four-centered intermediate, to the alkyl complex (C). A carbon monoxide insertion gives the square-planar complex (D). Successive H2 and CO addition produces the original catalyst and the product ... 

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 ... 

Due to the high rate of reaction observed by Meissner and coworkers it is unlikely that the reaction of OH with DMSO is a direct abstraction of a hydrogen atom. Gilbert and colleagues proposed a sequence of four reactions (equations 20-23) to explain the formation of both CH3 and CH3S02 radicals in the reaction of OH radicals with aqueous DMSO. The reaction mechanism started with addition of OH radical to the sulfur atom [they revised the rate constant of Meissner and coworkers to 7 X 10 M s according to a revision in the hexacyanoferrate(II) standard]. The S atom in sulfoxides is known to be at the center of a pyramidal structure with the free electron pair pointing toward one of the corners which provides an easy access for the electrophilic OH radical. 

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]. 

A theoretical study of the reaction mechanism for addition of organozincate complexes to aldehydes was recently performed using density functional theory.298 It has been suggested that the addition takes place through formation of a four-centered transition state and, therefore, it can be considered a typical nucleophilic reaction. 

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). 

Comparison within the family of carotenoid oxygenases show that the four histidines are strictly conserved as well as their close environment. Presumably, all members of this family, including the protein catalyzing central cleavage of P-carotene, share a common chain fold, possess similar active centers and follow a similar reaction mechanism, vide infra. 

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