Stereoisomerism in Cyclic Structures

Phosphine and hydroxy groups emerge simultaneously and therefore subsequent reaction with the air-sensitive hydroxyphosphine can be avoided. The structure of the cyclic ether determines in each case the distance separating functional groups in the product this means epoxides form 1,2-hydroxyphosphines, 1,3-prod-ucts can be produced by the employment of oxetanes [5], etc. Problems may be caused by the formation of regio- and stereoisomeric byproducts during the ringopening step by application of internal nonsymmetric cyclic ethers [6],... 

It is clear from the data in Table 4.2 that the two pairs of stereoisomeric cyclic amines do not form the same intermediate. The collapse of the ions to product is evidently so fast that there is not time for relaxation of the initially formed intermediates to reach a common structure. Generally speaking, we can expect similar behavior for all alkyl diazonium ion decompositions. The low activation energy for dissociation and the neutral and hard character of the leaving group result in a carbocation that is free of direct interaction with the leaving group. Product composition and stereochemistry is determined by the details of the collapse of the solvent shell. 

Steric interactions in the cyclic transition structures for a hydrogen transfer result in different abundance of the butene radical cation fragment, obtained from the diastereoisomeric unsaturated alcohols erythro-10 and threo-10. The higher energy of the transition state of erythro-10 due to the steric interaction of the methyl and R groups, results in a lower abundance of the fragment, as compared with the stereoisomeric threo-9 (see Scheme 9). 

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