Orientation in Pyrolytic Eliminations

MECHANISMS AND ORIENTATION IN PYROLYTIC ELIMINATIONS Meehan isms ... 

Enby 6 is an example of a stereospecific elimination reaction of an alkyl halide in which the transition state requires die proton and bromide ion that are lost to be in an anti orientation with respect to each odier. The diastereomeric threo- and e/ytAra-l-bromo-1,2-diphenyl-propanes undergo )3-elimination to produce stereoisomeric products. Enby 7 is an example of a pyrolytic elimination requiring a syn orientation of die proton that is removed and the nitrogen atom of the amine oxide group. The elimination proceeds through a cyclic transition state in which the proton is transferred to die oxygen of die amine oxide group. 

The term stereoselective is often confused with the term stereospecific, and the literature abounds with views as to the most satisfactory definition. To offer some clarification, it is perhaps timely to recall a frequently used term, introduced a decade or so ago, namely the stereoelectronic requirements of a reaction. All concerted reactions (i.e. those taking place in a synchronised process of bond breaking and bond forming) are considered to have precise spatial requirements with regard to the orientation of the reactant and reagent. Common examples are SN2 displacement reactions (e.g. Section 5.10.4, p. 659), E2 anti) elimination reactions of alkyl halides (e.g. Section 5.2.1, p.488), syn (pyrolytic) elimination reactions (Section 5.2.1, p.489), trans and cis additions to alkenes (e.g. Section 5.4.5, p. 547), and many rearrangement reactions. In the case of chiral or geometric reactants, the stereoisomeric nature of the product is entirely dependent on the unique stereoelectronic requirement of the reaction such reactions are stereospecific. 

Under conditions of kinetic control, the dehydration of 2-butanol follows the Saytzeff rule, but a greater yield of m-2-butene than trans-l-batenc is obtained. These observations have no parallel in acid- or base-catalysed or pyrolytic eliminations. However, the dehydration of 2,3-dimethyl-2-butanol gives 2,3-dimethyl-l-butene (88.4%) and 2,3-dimethyl-2-butene (9.9%) and is thus oriented towards the Hofmann rule despite being more probably a carbonium ion process. Under similar reaction conditions the quite distinctly different products arising from the secondary alcohol, 3,3-dimethyl-2-butanol [3,3-dimethyl-1-butene (70%), 2,3-dimethyl-l-butene (23.5%), 2,3-dimethyl-2-butene (3.9%), l,l-dimethyI-2-methylcyclopropane (2.1%)] are accommodated in terms of concerted rather than a carbonium-ion mechanism. 

The aromatization of streptamine by pyrolysis supported the evidence for the cyclic carbon skeleton, although it is to be noted that structural conclusions drawn from pyrolytic data are ordinarily open to question. Since the aromatization was achieved without elimination of nitrogen, the degradation also yielded information regarding the orientation of the nitrogen atoms. It was found that when hexaacetylstreptamine was heated in a sealed tube at 350 for one hour, 3.5 moles of acetic acid were produced and two crystalline products were obtained. One of the products was shown to be identical with 2,4-diacetamidophenol (X). ... 

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