Secondary Selection rules

The hydrogenolyaia of cyclopropane rings (C—C bond cleavage) has been described on p, 105. In syntheses of complex molecules reductive cleavage of alcohols, epoxides, and enol ethers of 5-keto esters are the most important examples, and some selectivity rules will be given. Primary alcohols are converted into tosylates much faster than secondary alcohols. The tosylate group is substituted by hydrogen upon treatment with LiAlH (W. Zorbach, 1961). Epoxides are also easily opened by LiAlH. The hydride ion attacks the less hindered carbon atom of the epoxide (H.B. Henhest, 1956). The reduction of sterically hindered enol ethers of 9-keto esters with lithium in ammonia leads to the a,/S-unsaturated ester and subsequently to the saturated ester in reasonable yields (R.M. Coates, 1970). Tributyltin hydride reduces halides to hydrocarbons stereoselectively in a free-radical chain reaction (L.W. Menapace, 1964) and reacts only slowly with C 0 and C—C double bonds (W.T. Brady, 1970 H.G. Kuivila, 1968). 

Due to limitations in signal-to-noise ratio available for the then common dispersive IR instruments, peptide and protein vibrational spectroscopic studies shifted to emphasize Raman measurements in the 1970s 29-32 Qualitatively the same sorts of empirical correlations as discussed above have been found between frequencies of amide bands in the Raman and secondary structure. However, due to the complementary selection rules for Raman as compared to IR and to the multi-component nature of these polymeric spectral bands, the... 

Kinetic resolution of chiral aUylic alcohols.7 Partial (at least 60% conversion) asymmetric epoxidation can be used for kinetic resolution of chiral allylic alcohols, particularly of secondary allylic alcohols in which chirality resides at the carbinol carbon such as 1, drawn in accordance with the usual enantioface selection rule (Scheme I). (S)-l undergoes asymmetric epoxidation with L-diisopropyl tartrate (DIPT) 104 times faster than (R)-l. The optical purity of the recovered allylic alcohol after kinetic resolution carried to 60% conversion is often > 90%. In theory, any degree of enantiomeric purity is attainable by use of higher conversions. Secondary allylic alcohols generally conform to the reactivity pattern of 1 the (Z)-allylic alcohols are less satisfactory substrates, particularly those substituted at the /1-vinyl position by a bulky substituent. 

With this model, we need only apply the method already used to derive the selection rules for electrocyclic reactions (p. 53). From the Coulson equations, we can deduce that in the in conrotatory cyclization of pentadiene, the MO generates a destabilizing C5-C4 secondary interaction, a stabilizing and Fg a destabilizing interaction. The absolute values of these contributions rise steadily because the terminal coefficients increase from Fg to Fg. Therefore, the sign of their sum is given by the HOMO contribution. If R is an attractor, the HOMO is Fg and rotation inwards is favored. If R is a donor, the HOMO is 4T and rotation inwards is disfavored. As the Coulson equations are valid only for polyenes, these conclusions are correct insofar as R can be modeled by a carbon 2p orbital. It follows that the Rondan-Houk theory works better for conjugative than for saturated substituents. 

Termmology of Cycloadditions Selection Rules for Cycloadditions Secondary Interactions Cycloadditions of Charged Species Sigmatropic Rearrangements Termmology... 

Before passing to the formulation of these postulates, we will try to answer the question why the postulates are meant and not the selection rules. When one speaks about the postulates, something primary is meant, whereas the selection rules imply something secondary , i.e. that the rules themselves are the consequences of some more fundamental principles. For instance, the relationship between the heuristic principle of orbital symmetry conservation formulated by Woodward and Hoffmann and a number of selection rules derived on its basis is of this type. On the qualitative level, these rules make it possible to predict the stereochemical direction of concerted reactions of various types [18]. 

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