Sulphur labile forms

It has been tentatively suggested that one mechanism underlies the Willgerodt reaction and the Kindler modification of it. A labile intermediate is first formed which has a carbon—carbon bond in the side chain. The scheme is indicated below it postulates a series of steps involving the addition of ammonia or amine (R = H or alkyl), elimination of water, re addition and eUmination of ammonia or amine until the unsaturation appears at the end of the chain then an irreversible oxidation between sulphur and the nitrogen compound may occur to produce a thioamide. 

A novel extrusion of S02 is reported for the dithiine dioxides 104 to afford low yields of the thiophenes 105 and 106. The exact step at which the S02 loss occurs is not known but the proposed intermediates 107 and/or 108 have support from the formation of pyrrole derivatives when the reaction is carried out in the presence of n-butylamine103. Sunlight-induced extrusion of sulphur dioxide from the pyranone dioxide 109 is similar to the above and results in the formation of the reactive cyclopentadienone 110 which can be trapped by dienophiles or in their absence forms a trimer104. The platinum complex 111 is photochemically labile and decomposes into benzyne on irradiation105. 

Cyclization of azobenzene and its derivatives tends only to occur under acid conditions. The reaction of the conjugate acids of azobenzene with anhydrous aluminium chloride, stannic chloride, and ferric chloride in 1,2-dichIoroethane has now been reported in detail.144 Benzo[c]cinnoline is formed, but the production of benzidine suggests that the reaction involves photodisproportionation. Similar cyclization is observed with the conjugate acids of azobenzenedicar-boxylic acids.145 With these compounds the reaction also occurs in 98% sulphuric acid to yield, from the 2,2 -isomer, benzo[c]cinnoline-4,7-dicarboxylic acid (90) and indazolo[2,1 - ]indazole-6,12-dione (91) in equal amounts the 3,3 -and 4,4/-dicarboxylic acids of azobenzene likewise give the corresponding cinnolines.145 It is important to note here that benzo[c]cinnoline is light-labile in 148... 

The original crosslinking process for natural rubber, called vulcanisation, involved mixing in 2-3% of sulphur plus an accelerator. On heating to 140 °C the sulphur reacts with C=C bonds on neighbouring polyisoprene chains to form sulphur crosslinks C—(S) —C. Typically, 15% of the crosslinks are monosulphide [n = 1), 15% are disulphide and the rest are polysulphide with n > 2. The polysulphide crosslinks are partially labile, which means that they can break and reform with other broken crosslinks when the applied stresses are high. This leads to permanent creep in compressed rubber blocks. To avoid such permanent set, efficient vulcanisation systems have been developed that produce only monosulphide crosslinks. 

The units 49 and 50 used in the synthesis of vitamin A are also used in many ways in carotenoid syntheses and are produced industrially in large scale. p-Ionone (17) can be converted into vinyl-p-ionol (51) by ethynylation to 52 and partial hydrogenation [42]. This conversion is also achieved in one step by 1,2-addition of vinylmagnesium chloride 55[43]. The two routes are, in principle, equivalent, and which one is used in practice is decided by conditions on site. In this example, the main considerations are the availability of acetylene (4) and vinyl chloride, operating experience, and permits for handling these materials. The Ci5-phosphonium salt 49 is formed directly from 51 by the action of triphenylphosphine and acid [44,45]. A step involving labile P-ionylidene-ethyl halide is thus avoided. Crystalline (lE,9E)-49 is obtained in excellent yield by reaction of 51 with triphenylphosphine and sulphuric acid in isopropanol/heptane [46]. 

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