Iodine azide addition reactions and the thallium(I) Hunsdiecker reaction
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. The action of iodine(I)azide on 3-methylene-5?-androstane and 1-methylene-4-t-butylcyclohexane was examined using various systems for formation of the iodine (I)azide reagent. The addition to both alkenes using iodine(I)azide generated from sodium azide-iodine(I)chloride in acetonitrile was regio- but not stereo-selective in the presence of oxygen, giving adducts containing an iodomethyl group. Formation of the products were rationalised as arising from ?- and ?-iodonium ions which are opened by azide ion in a Markovnikov manner, or in terms of azide ion attack on a C-3 carbocation. An increase in the stereoselectivity of the addition was achieved by use of dichloromethane as solvent. The addition in acetonitrile gave mainly one iodo-azide adduct in addition to an iodotetrazole if thallium(I)azide-iodine(I)chloride was used to prepare iodine(I)azide. Iodo-azide adducts containing an azidomethyl group were obtained from the addition of iodine(I)azide to both alkenes in acetonitrile in the absence of oxygen. The products were assumed to arise from an initial attack of an azide radical on the terminal carbon of the double bond, followed by stereoselective abstraction of iodine. A re-examination of the addition of iodine(I)azide to 3,3-dimethyl-2-methylenenorbornane (camphene) in acetonitrile gave a vinyl iodide, and two iodo-azide adducts which both contained an iodomethyl group. The products have been rationalised as arising from a non-classical carbocationic intermediate. The action of iodine(I)azide on benzofuran in acetonitrile in the absence of oxygen gave a mixture of cis and trans-diazides, in which the latter is the major product. A re-investigation of the addition of iodine(I)azide to 5?-androst-2-ene in the absence of oxygen revealed that products originally thought to be derived from an ionic pathway actually arose from a radical pathway. The latter gave at least two iodo-azide adducts, which both contained a 3?-azide group, indicating that the initial attack of azide radical on the alkene occurs from the less hindered ?-face. However, the abstraction of iodine by the intermediate azido radical was not stereo-selective. These results suggest that reported examples of non-regio- and stereo-selective additions of iodine(I)azide may in fact be a result of two distinct reaction pathways. Solvolysis of the iodo-azide adducts of 3-methylene-5?-androstane which contained the –CH2 I group, with silver(I)-acetate in acetic acid resulted in formation of a mixture of allylic azides and azidomethyl acetates. A complete migration of the azide group occurred during the reaction. The azidomethyl azides arose in the main from a neighbouring group participation by the azide group since a near complete inversion of stereo-chemistry occurred about C-3 during the solvolysis reaction. The addition of iodine(I)azide to methyl (E)-3-phenylpropenoate in acetonitrile in the presence of oxygen gave a single iodo-azide adduct which contained the azide group in the benzylic position. In contrast, repetition of the reaction in the absence of oxygen gave a vinylazide and an iodo-azide adduct containing the iodo group in the benzylic position. Ready solvolysis of the latter compound occurred with silver(I)acetate in acetic acid to give a mixture of threo- and erythro-acetoxyazides. Two iodo-azide adducts were obtained from methylpropenoate but it was not possible to show conclusively which of the adducts arose from a radical pathway. Iodo-azides were also formed from methyl 2-methylpropenoate in different reaction atmospheres. In the absence of oxygen a single adduct was obtained which gave (Z)-vinylazide upon treatment with 1,4-diazabicyclo[2.2.2.]octane. If the reaction was carried out in oxygen an ca. 1:1 mixture of adducts was obtained. In contrast, the addition to methyl (E)-3-methylpropenoate in the presence of oxygen gave a single adduct, which was also obtained in the absence of oxygen. AdE and AdN mechanisms have been proposed to account for the addition of iodine(I)azide to ?,?-unsaturated carbonyl compounds. In an attempt to distinguish between these two mechanisms a kinetic study was initiated. Initial results indicate an electrophilic (AdE) mechanism involving attack by I+ rather than attack by azide ion (AdN) as the initial step in the reaction. The addition of iodine(I)chloride and iodine(I)azide to methyl (E)-3-phenylpropenoate also appears to involve an equilibrium step. Treatment of a thallium(I) carboxylate with an equivalent amount of bromine gave the corresponding thallium(III) carboxylate dibromide. High yields of primary alkyl bromides were obtained if the thallium(III) carboxylate dibromide was treated with 0.5 molar equivalents of bromine in refluxing carbon tetrachloride. Pyrolysis of the thallium(III) compound in the absence of added bromine gave low yields of the corresponding alkyl bromide. The use of thallium(I) carboxylates for the preparation of alkyl bromides in high yield appears to be limited to those of primary carboxylic acids. The use of thallium(III) acetate dibromide as a synthetic reagent for the oxidation of alkenes was investigated. Reaction of cyclohexene with the thallium(III) salt gave a low yield of a ring contracted diacetate while reaction with 5?-cholest-2-ene gave a low yield of the cis-?-diol diacetate.
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/1980