Concerted rearrangement stereochemistry

Table 4.1 shows the stereochemistry of substitution for five representative systems. Displacement at the primary 1-butyl system occurs mainly by inversion (Entry 1). However, there is also extensive formation of a rearranged product, 2-butanol (not shown in the table). Similarly, the 2-butyl diazonium ion gives 28% inversion in the unrearranged product, but the main product is r-butanol (Entry 2). These results indicate competition between concerted rearrangement and dissociation. Several secondary diazonium ions were observed to give alcohol with predominant... 

The stereoselectivity achieved in the synthesis in Scheme 13.12 is the result of a preferred conformation for the base-catalyzed oxy-Cope rearrangement in step C. Although the intermediate used in step C is a mixture of stereoisomers, both give predominantly the desired relative stereochemistry at C-4 and C-1. The stereoselectivity is based on the preferred chair conformation for the transition state of the concerted rearrangement. 

The di-TT-methane rearrangement is a stereospecific reaction. There are several elements of stereochemistry to be considered. It is known that the double bond that remains uncyclized retains the E or Z configuration present in the starting material. This result excludes any intermediate with a freely rotating terminal radical. The concerted... 

More mechanistic work undoubtedly will be done on the stereochemistry of solvolysis, particularly of simple alkyl-substituted acyclic vinyl systems. The questions of concerted anchimerically assisted or nonconcerted rearrangements both to the double bond and across the double bond need further development. 

The stereochemistry of dienes has been found to have a pronounced effect in the concerted cyclo-additions with benzyne 64>65h A concerted disrotatory cyclo-addition of tetrafluorobenzyne, leading for example with trans- (3-methylstyrene to (63, R = Me), is likely and in accord with the conservation of orbital symmetry 68>. However while the electro-cyclic rearrangement of (63, R = H) to (65, R = H) is not allowed, base catalysed prototropic rearrangement is possible. A carbanion (64, R = H) cannot have more than a transient existence in the reaction of tetrafluorobenzyne with styrene because no deuterium incorporation in (65) was detected when either the reaction mixture was quenched with deuterium oxide or when the reaction was conducted in the presence of a ten molar excess of deuteriopentafluorobenzene. 

The RDA reaction is often observed from steroid molecular ions, and it can be very indicative of steroidal stmcture. [107,110,113,114] The extent of the RDA reaction depends on whether the central ring junction is cis or trans. The mass spectra of A -steroidal olefins, for example, showed a marked dependence upon the stereochemistry of the A/B ring juncture, in accordance with orbital symmetry rules for a thermal concerted process. In the trans isomer the RDA is much reduced as compared to the cis isomer. The effect was shown to increase at 12 eV, and as typical for a rearrangement, the RDA reaction became more pronounced, whereas simple cleavages almost vanished. This represented the first example of such apparent symmetry control in olefinic hydrocarbons. [114]. 

Further, it is noted that most of them involve inversion of stereochemistry at the particular centre, a feature of the concerted nature of these rearrangements, so that as one group leaves another approaches from the rear. Thus, we have the features of Sn2 reactions in a carbocation mechanism. 

The formation of a very electrophilic intermediate 258 from 256 and 257 is proposed (equation 78). The hydroxyl group of the oxime adds to 259, giving a reactive cationic species 260 that rearranges and affords the nitrile 261 (in the case of aldoxime, equation 79), or the amide 262 upon hydrolytic workup (equation 80). The conversion of 260 to the nitrilium ion should occur through a concerted [1,2]-intramolecular shift. This procedure can be applied in the conversion of aldoximes to nitriles. It was observed that the stereochemistry of the ketoximes has little effect on the reaction, this fact being explained by the E-Z isomerization of the oxime isomers under the reaction conditions. 

We summarize here a procedure to predict the feasibility and the stereochemistry of thermally concerted reactions involving cyclic transition states. The 1,2 rearrangement of carbocations will be used to illustrate the approach. This is a very important reaction of carbocations which we have discussed in other chapters. We use it here as an example to illustrate how qualitative MO theory can give insight into how and why reactions occur ... 

Exercise 21-23 Show how one can predict the stereochemistry of the electrocyclic rearrangement of frans,c/ s,frans-2,4,6-octatriene to 5,6-dimethyM, 3-cyclohexadiene by a favorable concerted thermal mechanism. 

The same alcohol could be made by the Baeyer-Villiger rearrangement but the stereochemistry would have to be set up before the Baeyer-Villiger step. Hydroboration has the advantage that stereochemistry is created in the hydroboration step. We have discussed the details of this step. In drawing the mechanism it is usually best to draw it as a simple concerted four-centre mechanism providing you remember that the regioselectivity is controlled by the initial interaction between the nucleophilic end of the alkene and the empty p orbital on boron. 

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