Sulfur-based rearrangements

The worries about the instability of sulfur-based signaling systems such as 13, 14, 16, and 17 have crystallized in the following cautionary tale. Ute Resch-Genger s colleagues in Berlin and Leipzig, Germany, and Kiev, Ukraine, now find that the Cu2+-induced fluorescence enhancement previously seen with 18 [62] is not due to simple complex formation but to oxidative rearrangement into 19 [63], However, the 9-anthroyl unit in 18 seems to be the main culprit. 

To organize the material to be presented in this review, a classification based on the formal oxidation state of the sulfur atom will be used. As already discussed, in the course of the rearrangement a 1,2-shift of an electron pair from Y to X takes place entailing a formal reduction of X (the atom that the allylic moiety was bound to). If we identify X with sulfur, five rearrangements accompanied by a S(VI) to S(IV) transition can be constructed (Chart 1, top... 

As with other rearrangements, the Payne rearrangement also has its variants. Similar transformations of substrates with nitrogen- or sulfur-based functionalities at the carbon atom adjacent to the epoxide are known as aza- or thia-Payne rearrangements, respectively, and there exist a nurnber of noteworthy previously reported examples of each. Although study of these systems has been somewhat limited in recent years, they are briefly surveyed in the following sections. 

It is found that the sulfur-based groups are problematic in preparation and rearrangement of the functionahzed allyl vinyl ethers. It became apparent that the phosphorus-based groups as alternative anion-stabUizing moieties, mainly because of the ease of preparation of aUenylphosphoms precursors and a well-established chemistry of phosphorus-stabilized allyl anions [77]. 

The Pd-catalyzed C-S/C-H bond activation of simple and diverse bromothio-phenes with alkynes via ring rearrangement to prepare sulfur-based heterocycles and fulvenes was described by Duan and coworkers (Scheme 4.7) [11]. This novel transformation provided a new approach to afford not only the DBTs but also the thiopyrans. 

Diallylsulfonium salts undergo intramolecular allylic rearrangement with strong bases to yield 1,5-dienes after reductive desulfurization. The straight-chain 1,5-dienes may be obtained by double sulfur extrusion with concomitant allylic rearrangements from diallyl disulfides. The first step is achieved with phosphines or phosphites, the second with benzyne. This procedure is especially suitable for the synthesis of acid sensitive olefins and has been used in oligoisoprene synthesis (G.M. Blackburn, 1969). 

The high nucleophilicity of sulfur atoms is preserved, even if it is bound to electron withdrawing carbonyl groups. Thiocarboxylales, for example, substitute bromine, e.g. of a-bromo ketones. In the presence of bases the or-acylthio ketones deprotonate and rearrange to episulfides. After desulfurization with triphenylphosphine, 1,3-diketones are formed in good yield. Thiolactams react in the same way, and A. Eschenmoser (1970) has used this sequence in his vitamin B]2 synthesis (p. 261). 

The converse situation in which ring closure is initiated by the attack of a carbon-based radical on the heteroatom has been employed only infrequently (Scheme 18c) (66JA4096). The example in Scheme 18d begins with an intramolecular carbene attack on sulfur followed by rearrangement (75BCJ1490). The formation of pyrrolidines by intramolecular attack of an amino radical on a carbon-carbon double bond is exemplified in Scheme 19. In the third example, where cyclization is catalyzed by a metal ion (Ti, Cu, Fe, Co " ), the stereospecificity of the reaction depends upon the choice of metal ion. 

Sulfur tnoxide adds to 2,2 difluoroethylenesulfonyl fluoride to afford the P sultone and its rearrangement product, bis(fluorosulfonyl)acetyl fluoride Potassium fluoride acts as a base and reacts with the acetyl fluoride to eliminate the elements of hydrogen fluoride and produce bis(fluorosulfonyl)ketene [IS] (equation 6)... 

Comforth has reviewed literature reports and independently studied the special cases of reaction of 1 with salicylaldehyde and with 2-acetoxybenzaldehyde. Coumarins (10) are afforded in the condensation of 1 with salicylaldehyde or its imine, whereas when 2-acetoxybenzaldehyde is used, acetoxy oxazolone 12 is the major product. The initial aldol condensation product between the oxazolone and 2-acetoxybenzaldehyde is the 4-(a-hydroxybenzyl)oxazolone 11, in which base-catalyzed intramolecular transacetylation is envisioned. The product 9 (R = Ac) can either be acetylated on the phenolic hydroxy group, before or after loss of acetic acid, to yield the oxazolone 12, or it can rearrange, by a second intramolecular process catalyzed by base and acid, to the hydrocoumarin, which loses acetic acid to yield 10. When salicylaldehyde is the starting material, aldol intermediate 9 (R = H) can rearrange directly to a hydrocoumarin. Comforth also accessed pure 4-(2 -hydroxyphenylmethylene)-2-phenyloxazol-5(4//)-one (13) through hydrolysis of 12 with 88% sulfuric acid. 

The catalytic system used in the Pacol process is either platinum or platinum/ rhenium-doped aluminum oxide which is partially poisoned with tin or sulfur and alkalinized with an alkali base. The latter modification of the catalyst system hinders the formation of large quantities of diolefins and aromatics. The activities of the UOP in the area of catalyst development led to the documentation of 29 patents between 1970 and 1987 (Table 6). Contact DeH-5, used between 1970 and 1982, already produced good results. The reaction product consisted of about 90% /z-monoolefins. On account of the not inconsiderable content of byproducts (4% diolefins and 3% aromatics) and the relatively short lifetime, the economics of the contact had to be improved. Each diolefin molecule binds in the alkylation two benzene molecules to form di-phenylalkanes or rearranges with the benzene to indane and tetralin derivatives the aromatics, formed during the dehydrogenation, also rearrange to form undesirable byproducts. 

The base-induced Ramberg-Backlund rearrangement later initiated extensive mechanistically - and synthetically -oriented investigations, and played a significant role not only with respect to the study of thiirane dioxides (3b), but also contributed substantially to the present state of the art concerning three-membered rings containing sulfur (equation 2). 

Similar to the thermal rearrangements discussed in the previous subsection, the base-catalyzed rearrangements of cyclic four-membered sulfones to five-membered sulfinates have also been reported. For example, Dodson and coworkers have observed the rearrangement of cis- and trans-2,4-diphenylthiethane 1,1 dioxides to cis- and trans-3, 5-diphenyl-1,2-oxathiolane (2,3)-cis-2-oxides, respectively (equation 35), on treatment with t-butoxymagnesium bromide, which is stereospecific with respect to the phenyl group but stereoselective with respect to the oxygen atoms on sulfur. 

Finally, the most significant mechanistic feature of the Ramberg-Backlund rearrangement is the stereoselective formation of ds-olefin products, as a result of the preferential cis-positioning of the pair of R groups in the episulfone-forming transition state, variously attributed to London forces , to diastereoselectivity in carbanion formation and to steric attraction . However, with the use of stronger bases such as potassium t-butoxide °, the trans-olefin predominates (equation 52), apparently due to prior epimerization of the kinetically favoured cts-episulfone, and subsequent loss of the sulfur dioxide. Similarly, when the episulfone intermediates possess unusually acidic... 

In general, sulfur-substituted allenes are accessible starting from alkyne precursors by a variety of transformations such as isomerization, rearrangement or addition reactions. The standard method for the synthesis of donor-substituted allenes is again the base-induced isomerization of alkynes. This very first method was applied for the preparation of achiral [11, 162, 163] and chiral [164] S-functionalized 1,2-dienes (Scheme 8.78). 

An O-labelling investigation of the oxygen to sulfur transposition in the base-catalysed rearrangement of o-benzoyl-A-(diphenylphosphinothioyl)hydroxylamine (200) to (201) has been undertaken. The labelling results are outlined in Scheme 70 although further evidence is required to substantiate the mechanism... 

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