Nucleophilic attack concerted mechanism

Substitution nucleophilic bimolecular (Sn2) mechanism (Sec tions 4 12 and 8 3) Concerted mechanism for nucleophilic substitution in which the nucleophile attacks carbon from the side opposite the bond to the leaving group and assists the departure of the leaving group... 

FIGURE 16.27 A mechanism for the aspartic proteases. In the first step, two concerted proton transfers facilitate nucleophilic attack of water on the substrate carbonyl carbon. In the third step, one aspartate residue (Asp" " in pepsin) accepts a proton from one of the hydroxyl groups of the amine dihydrate, and the other aspartate (Asp" ) donates a proton to the nitrogen of the departing amine. 

No single mechanism accounts for all the reactions. One pathway involves a concerted one-step process involving a cyclic transition state. This of necessity affords a c -product. Another possibility, more favoured in polar solvents, involves a cationic 5-coordinate intermediate [IrX(A)(CO)L2]+, which undergoes subsequent nucleophilic attack by B-. Other possibilities include a SN2 route, where the metal polarizes AB before generating the nucleophile, and radical routes. Studies are complicated by the fact that the thermodynamically more stable isolated product may not be the same as the kinetic product formed by initial addition. 

In all of the above cases involving decompositions of vinyl diazonium ions, the observed products are consistent with a vinyl cation formulation, but extensive mechanistic studies of these reactions have not been reported. It is difficult, for instance, to establish to what extent reaction proceeds through the diazonium ion via a backside nucleophilic attack and concerted loss of nitrogen rather than through the free vinyl cation. In the absence of kinetic data, it is also difficult to rule out competing or alternative mechanisms not involving vinyl cations. 

In general, allenyl cations 38 attack at the sp2-carbon atom of 1,3-dienes and form vinyl cations 39 and 40 (R = H, alkyl) or (R = aryl). Although a concerted cycloaddition mechanism is possible, a stepwise mechanism is preferred34. If a nucleophilic attack at the sp-carbon atom of the allenyl cation takes place, then cation 41 and the resulting cations 42 and 43 are formed. Some examples of bicyclic products obtained from cyclic 1,3-dienes and propargyl chlorides are given in equation 1534. 

Alkyne dipolarophiles such as methyl propiolate or DMAD reacted with ylides derived from [l,2,3]triazolo[l,5-tf]-pyridines, but the mechanism proposed involved a Michael addition and subsequent nucleophilic attack rather than a concerted [4+2] cycloaddition <1996T10519> (see Section 11.13.8). 

The (phosphino)(silyl)carbene 2a readily and cleanly adds to benzalde-hyde and cinnamaldehyde, affording the oxiranes 27 and 28, as single diaste-reomers.40 These results strongly suggest a concerted mechanism, since the formation of a zwitterionic intermediate, such as 29, would result in the formation of a phosphoryl alkene via oxygen atom attack at the phosphorus center. Note that 2a does not react with ketones, which is in line with its nucleophilic character. 

The regio- and stereo-chemistry of the nucleophilic attack of (5)-trani -hex-3-en-2-ol and (5)-trani-hex-4-en-3-ol on the corresponding O-protonated or -methylated derivatives have been examined in the gas phase at 40 °C and 720 Torr. Firm evidence of various kinds was obtained for the concerted 5ivr2 pathway accompanying the classical 5n2 mechanism. Competition between the two processes is essentially governed by the orienting properties of the oxonium intermediate towards the approaching nucleophile. Many other details were elucidated. 

Apparently, Eq. (29) represents a polar nonradical addition. If a two-step mechanism is conceived, intermediates of the type [XB=NRH] will be reasonable, though such cations proved to be rather unstable as isolated species (unless X represents a x-electron donating group) (33). Intermediates of the type HY—B(X)=NR would explain the fast reaction with protic bases of vanishing Bronsted acidity. The results, however, mentioned in Sections V, A, and V, C, favor to some extent the picture of iminoboranes as preferring electrophilic to nucleophilic attack. The high activity of amines can also be rationalized in terms of a concerted process, with a transition state of type VI. 

A cis-coordinating ligand is apparently required to bind and activate MeOH so that a methoxy group is transferred to the polyketone chain and a hydride remains on palladium. Two mechanisms are possible for this reaction (i) nucleophilic attack by the oxygen at the acyl carbonyl with concerted formation of Pd-H (ii) formation of a Pd(acyl) (methoxy) complex and H, followed by reductive elimination and subsequent proton attack on a Pd center. No experimental evidence favoring either mechanism in ethene/CO copolymerisation has been provided so far. 

The mechanism for the stereoselective polymerization of a-olefins and other nonpolar alkenes is a Ti-complexation of monomer and transition metal (utilizing the latter s if-orbitals) followed by a four-center anionic coordination insertion process in which monomer is inserted into a metal-carbon bond as described in Fig. 8-10. Support for the initial Tt-com-plexation has come from ESR, NMR, and IR studies [Burfield, 1984], The insertion reaction has both cationic and anionic features. There is a concerted nucleophilic attack by the incipient carbanion polymer chain end on the a-carbon of the double bond together with an electrophilic attack by the cationic counterion on the alkene Ti-electrons. 

A recent theoretical study has nicely addressed the question of mechanism on the silicon surface. Minary and Tuckerman carried out an ab initio molecular dynamics (MD) study of the [4 + 2] cycloaddition reaction on Si(100)-2 x 1 [251]. Because the previously reported ab initio DFT models were static , these were not able to address in detail the mechanisms by which the [4 + 2] product was formed. The results of the MD study indicate that rather than being concerted, the dominant mechanism is a stepwise zwitterionic process in which an initial nucleophilic attack of one of the C=C bonds by the down atom of the dimer leads to a carbocation. This carbocation exists for up to 1-2 ps, stabilized by resonance, and depending on which positively charged carbon atom reacts with which Si surface atom, can form... 

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