Sulfur/nitrogen-centered heterocyclic

Sulfur/Nitrogen-Centered Heterocyclic Radicals—Thiazyls... 

Meyers and Garcia-Munoz64 assume that the cyclization of the -ketonitriles 67 by the action of concentrated sulfuric acid may be regarded as a still further extension of the Ritter reaction to the reactions of carbonyl compounds in the synthesis of nitrogen-containing heterocycles (equation 25). In this case the cyano-group reacts as a nucleophile with the positively charged center of the carboxonium intermediate J L... 

The problem of the synthesis of highly substituted olefins from ketones according to this principle was solved by D.H.R. Barton. The ketones are first connected to azines by hydrazine and secondly treated with hydrogen sulfide to yield 1,3,4-thiadiazolidines. In this heterocycle the substituents of the prospective olefin are too far from each other to produce problems. Mild oxidation of the hydrazine nitrogens produces d -l,3,4-thiadiazolines. The decisive step of carbon-carbon bond formation is achieved in a thermal reaction a nitrogen molecule is cleaved off and the biradical formed recombines immediately since its two reactive centers are hold together by the sulfur atom. The thiirane (episulfide) can be finally desulfurized by phosphines or phosphites, and the desired olefin is formed. With very large substituents the 1,3,4-thiadiazolidines do not form with hydrazine. In such cases, however, direct thiadiazoline formation from thiones and diazo compounds is often possible, or a thermal reaction between alkylideneazinophosphoranes and thiones may be successful (D.H.R. Barton, 1972, 1974, 1975). 

As reagents causing intramolecular recyclizations of 2-benzopyrylium salts, nucleophiles centered at nitrogen, oxygen, sulfur, or carbon atoms were used. As a fairly general rule, the ring opening of the heterocyclic moiety leads to an orf/io-quinonoid structure, as shown in more detail in Section III,C,4,iv (Structure 201). 

Over the last 15 years, the homogeneous studies of HDS and HDN processes have been extremely useful to understand many mechanistic details regarding the coordination of sulfur and nitrogen heterocycles to metal centers, hydrogen transfer from metal to coordinated heterocycle, metal insertion into C-S and C-N bonds, and the desulfurization/denitrogenation paths. Recently, however, there has been a qualitative leap in molecular catalysis so that crossing the border-... 

Molecules containing sulfur or nitrogen donor atoms can serve as ligands to form stable complexes with the relatively kinetically inert Pt(II) center (for a review see Ref. 60). It is likely that cisplatin binds to many cellular components, the result of which is general toxicity, but coordination to the heterocyclic nitrogen atoms of DNA nucleobases is most likely responsible for its antitumor activity. 

Although the oxygen or sulfur atom of the heterocycle part of the P,N ligand was never observed to participate in direct coordination to the metal, it may influence the donor properties of the nitrogen of the oxazoline or thiazoline rings. Only few examples in which the oxygen/sulfur atom of an oxazoline/thiazoline ring coordinated to a metal center have been structurally characterized [52], and a recent example is shown in Scheme 14.22 [52]. 

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