Intermediate symmetrical

Analysis You have two 1,5- and one 1,3-dicarbonyl relationships to consider. Discoimections like a hardly simplify the problem at all, whereas discoimecting the 1,3-dicarbonyl relationship b gives a symmetrical intermediate ... 

The use of chiral ketones for the protection of diols serves two purposes first, diol protection is accomplished, and second, symmetrical intermediates are converted to chiral derivatives that can be elaborated further, so that when the diol is deprotected, the molecule retains chirality. ... 

Evidence for symmetrical intermediates such as benzyne cannot be established by quantitative analysis of the reaction mixture unless a labelled starting substance is used. By applying labeling techniques, Roberts and his collaborators obtained results which indicated that benzyne (13) occurs as an intermediate in the amination of chlorobenzene with potassium amide in liquid ammonia. From chlorobenzene-1-C (12) about equal amounts of anUine-l-C (14) and aniline-2-C (15) were formed. More or less probable alternative... 

Because an S jl reaction occurs through a carbocation intermediate, its stereochemical outcome is different from that of an S 2 reaction. Carbocations, as we ve seen, are planar, sp2-hybndized, and achiral. Thus, if we carry out an S jl reaction on one enantiomer of a chiral reactant and go through an achiral carbocation intermediate, the product must be optically inactive (Section 9.10). The symmetrical intermediate carbocation can react with a nucleophile equally well from either side, leading to a racemic, 50 50 mixture of enantiomers (Figure 11.10). 

Further mechanistic evidence comes from trapping experiments. When bromobenzene is treated with KNH2 in the presence of a diene such as furan, a Diels-Alder reaction (Section 14.5) occurs, implying that the symmetrical intermediate is a benzyne, formed by elimination of HBr from bromobenzene. Ben-zyne is too reactive to be isolated as a pure compound but, in the presence of water, addition occurs to give the phenol, in the presence of a diene, Diels-Alder cycloaddition takes place. 

Although 17 is symmetrical, intermediates in most neighboring-group mechanisms are not, and it is therefore possible to get not a simple substitution product but a rearrangement. This will happen if Y attacks not the carbon atom from which X left, hut the one to which Z was originally attached ... 

The symmetrical intermediate 5 can be attacked by the NH3 at either of two positions, which explains why about one-half of the aniline produced from the radioactive chlorobenzene was labeled at the 2 position. The fact that the 1 and 2 positions were not labeled equally is the result of a small isotope effect. Other evidence for this mechanism is the following ... 

Imanishi, T., Shin, H., Yagi, N., Hanaoka, M. (1980) l,6-Dihydro-3(2H)-Pyridinones as Symmetric Intermediates. Formal Synthesis of ( )-Tabersonine and ( )-Catharanthine. Tetrahedron Letters, 21, 3285-3288. 

The net structural change is the same for both mechanisms. The energy requirements of the cyclopropanone and semibenzilic mechanism may be fairly closely balanced.87 Cases of operation of the semibenzilic mechanism have been reported even for compounds having a hydrogen available for enolization.88 Among the evidence that the cyclopropanone mechanism operates is the demonstration that a symmetrical intermediate is involved. The isomeric chloro ketones 12 and 13, for example, lead to the same ester. 

The occurrence of a symmetrical intermediate has also been demonstrated by 14C labeling in the case of a-chlorocyclohexanone.89... 

In view of this finding, it was proposed (135-138) (Scheme 22) that in the case of cocaine (98) the ornithine (201) is incorporated through free putrescine (202), which is a symmetrical intermediate and therefore would afford the pyrrolinium salt 206 equally labeled at C-2 and C-5. As above, condensation of the (V-methyl-A1 -pyrrolinium salt (206) with acetyl coenzyme A leads to the coenzyme A ester of hygrine-1 -carboxylic acid (207), which by transester-... 

Thus oxidation of both 6- and 7-substituted 3-amino-benzo-l,2,3-triazin-4-ones with lead tetraacetate in methanol produced a mixture of p- and m-substituted benzoates, clearly indicating that a symmetrical intermediate, i.e. benzocyclopropenone (166) was formed and underwent ring opening by attack of solvent121 ... 

A theoretical study of degenerate Boulton-Katritzky rearrangements concerning the anions of the 3-hydroxyimi-nomethyl-l,2,5-oxadiazole has been carried out by using semi-empirical modified neglect of diatomic overlap (MNDO) and ab initio Hartree-Fock procedures. Different transition structures and reactive pathways were obtained in the two cases. Semi-empirical treatment shows asymmetrical transition states and nonconcerted processes via symmetrical intermediates. By contrast, ab initio procedures describe concerted and synchronous processes involving symmetrically located transition states <1998JMT(452)67>. 

An elimination-addition mechanism has been suggested for the substitution of amino for chloro in chlorobenzene.95 The neutral and symmetrical intermediate is called benzyne although it can not contain an ordinary linear bond system like that of acetylene. It is not known whether it is a distorted acetylene, a triplet (diradical), or a zwitterion. 

Interesting results concerning phenyl group participation were observed with ( )-styryl(phenyl)iodonium tetrafluoroborate (26) using a deuterated substrate (eq 12)16 When 26-ad was heated in trifluoroethanol (TFE) at 60 °C, slow reaction gave die E isomer of substitution product 28 quantitatively, but the deuterium was completely scrambled between the a and p positions. This strongly indicates that a symmetrical intermediate is involved during the reaction and the most reasonable one is vinylenebenzenium ion (27) formed by phenyl participation. This intermediate also explain the exclusive formation of the retained ( )-28. 

The distorted structure predicted for 19a has been confirmed 39) by n.m.r. studies one might on this basis expect interconversion of the two isomers 20 and 22 via a symmetrical intermediate 21 to take place at a measurable rate. 

Mechanistically speaking there have been no recent advances. What is known is that, at least for 4-OH formation, the reaction is intermolecular, requires two proton transfers at some stage and that a symmetrical intermediate is involved (often described as... 

As seen in the conversion of an optically enriched vinylketone complex to the corresponding vinylketene complex (vide supra), the availability of enantiomerically pure samples of the vinylketene complexes 221 allowed the subsequent reactions of these complexes to be screened for the presence of symmetrical intermediates along the reaction coordinate. Their... 

This information confirmed that a symmetrical intermediate could not lie along the reaction coordinate, although the alkyne complex 254 was still theoretically a viable intermediate. However, the stereochemical study... 

The retention of optical activity of camphene rules out methyl migration (Nametkin rearrangement) (71) or a symmetrical intermediate. On the acidic alumina at low contact time the retention of optical activity is high, about 80%. At longer contact time, however, there is essentially complete racemization. Hence, the dehydration mechanism seems to be the same on the acidic and on the base-modified alumina. The acidic alumina, however, causes the readsorption of the dehydration product leading to isomerization and equilibration. 

It is however not ruled out that the reaction might have proceeded through the formation of a symmetrical intermediate such as phenyl-cyclopropane. 

Exchange is visualized as occurring through a symmetrical intermediate or transition state 6, which allows for interchange of Cl between Pt(II) and Pt(IV). Breakage of the Cl bridge at a produces the original... 

A theoretical study of degenerate Boulton-Katritzky rearrangements concerning the anions of 3-formylamino-l,2,4-oxadiazole and 3-hydroxy-iminomethyl-1,2,5-oxadiazole has been carried out7 The treatment has shown the participation of asymmetric transition states and non-concerted processes via symmetrical intermediates. A detailed ab initio and density functional study of the Boulton-Katritzky rearrangement of 4-nitrobenzofuroxan has indicated a one-step mechanism for the process. 

It has been suggested that a symmetrical intermediate or transition state as represented in Scheme IV.54 could be involved, in which the new and the old nitrogen-oxygen bonds are formed and broken to the same extent. A semiempirical approach for calculating the free energy of activation for the degenerate rearrangement of the anion of 3-acetylamino-5-methyl-l,2,4-... 

---