Sulfur bonding imines

The carbon-sulfur bond of thiazines can be reductively cleaved (Sections 7.06.5.7 and 7.06.6.7) and 2//-thiazines, being imines, can be hydrolyzed (Section 7.06.5.5). Saturated thiazines (thiomorpholines) are stable toward alkaline hydrolysis <1981CPB1554> and Lewis-acidic boron trifluoride <1980JHC449>. 

Carbon-sulfur bond formation by oxidative cyclization of a thiourea derivative using bromine will in the case of (521) furnish a thiazolo[/,/]quinoline (522 Z = NH). The same imine is obtained from 8-mercapto-l,2,3,4-tetrahydroquinoline on treatment with cyanogen bromide with phosgene the 2-oxo derivative (522 Z = 0) is formed (63JOC2581). 

Carbon nucleophiles may react at the sulfur-sulfur bond, or at C-3. Thus Grignard reagents react with monocyclic dithiol-3-ones (3a) and -3-imines (3c) at S-2 (74LA1261,73LA247). The product (84) of reaction of a Grignard reagent with benzo-1,2-dithiol-3-one (77a) (74T4113) could be alternately formed by attack at S-2, at C-3 or at C-5. Phosphonium ylides or other stabilized carbanions react at the 3-position of dithiol-ones... 

In contrast to isocyanates, isothiocyanates have rarely been examined as cydoad-dition components, because their strong coordination of organosulfur compounds frequently deactivates catalytic species. Some organomthenium complexes, however, recently proved to be efficient catalysts for the formation of carbon-sulfur bonds [52]. The catalytic cydoaddition of diynes 20 with isothiocyanates was also successfully achieved using 17 as a precatalyst [53]. Importantly, the cydoaddition took place across the C=S double bonds of the isothiocyanates to afford thiopyran-imines 59 (Scheme 4.21). This reaction requires 10 mol% of 17, as well as the diynes possessing a quaternary carbon center at the 4-position. When excess amounts of carbon disulfide were also employed in place of the isothiocyanates, a bicyclic dithiopyrone was obtained. 

Graphite reacts with alkali metals - potassium, cesium and rubidium - to form lamellar compounds with different stoichiometries. The most widely known intercalate is the potassium-graphite which has the stoichiometry of CgK. In this intercalate the space between the graphite layers is occupied by K atoms. CgK functions as a reducing agent in various reactions such as reduction of double bonds in a,fl-unsaturated ketones [19], carboxylic acids and Schiff bases alkylation of nitriles [20], esters and imines [21] reductive cleavage of carbon-sulfur bonds in vinylic and allylic sulfones [22]. The detailed reaction mechanism of CgK is not known, and the special properties which are ascribed to the intercalate come either from the equilibrium between K+/K [23], or topochemical observations (the layer structure) [24]. 

Several reviews on the synthesis of aziridines have been published in the previous year. These publications include a review on the silver catalyzed addition of nitrenes (among other intermediates such as carbene) across a double bond <06EJOC4313> a review on sulfur ylide addition to imines to form aziridines <06SL181> a review on nitrogen addition across double bonds <06ACR194> a general review on functionalization of a,p-unsaturated esters with some discussion of aziridination <06TA1465>... 

The first step in either direction consists of addition of NaHSO-, to one of the double bonds of the ring, which gives an enol (or enamine) that tautomerizes to the keto (or imine) form. The conversion of 10 to 11 (or vice versa) is an example of 6-14 (or 6-2). Evidence for this mechanism was the isolation of 10 " and the demonstration that for p-naphthol treated with ammonia and HSOj. the rate of the reaction depends only on the substrate and on HSOi. indicating that ammonia is not involved in the rate-determining step.112 If the starting compound is a (i-naphthol, the intermediate is a 2-keto-4-suIfonic acid compound, so the sulfur of the bisulfite in either case attacks meta to the OH or NH2-m... 

Benzothiadiazines (158) are prepared by cyclization of the chlorohydrazones (212) on treatment with triethylamine (81JCS(Pl)2245). The reaction probably involves intramolecular nucleophilic attack by sulfur on a nitrile imine intermediate, since (212) does not cyclize on heating alone. Intramolecular aromatic sulfonation gives 1,2,4-benzothiadiazine 1,1-dioxides (214). Thus treatment of (213) with aluminum trichloride leads to cyclization with bond formation between sulfur and the aromatic ring (79JCS(P1)1043). 

The required chiral sulfur ylide of type 59 is formed in a reaction with a diazo compound in the presence of an achiral metal catalyst. Subsequently, asymmetric reaction of the chiral ylide 59 with the C=N double bond of the imine proceeds diastereoselectively and enantioselectively, giving the optically active aziridine 57. The chiral sulfide catalyst released is then used for the next catalytic cycle. The cat-alytically active species in the asymmetric process is the sulfide, so this concept can also be regarded as an organocatalytic reaction. 

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