1,6-Diynes, reactions, carbon disulfide

In contrast to isocyanates, isothiocyanates have hardly been examined as cycloaddition components, because the strong coordination of organosulfur compounds frequently deactivates a catalytic species [21]. Some organoruthe-nium complexes, however, recently proved to be efficient catalysts for the formation of carbon-sulfur bonds [21]. The catalytic cycloaddition of diynes with isothiocyanates was also successfully achieved using Cp RuCl(cod) as a precatalyst [22]. Importantly, the cycloaddition took place at the C=S double bonds of the isothiocyanates to afford thiopyranimines 26 (Eq. 13). This reaction requires 10 mol % of the precatalyst as well as the diynes possessing a quarternary carbon center at the 4-position. When excess amounts of carbon disulfide were also employed in place of the isothiocyanates, a bicyclic dithiopyrone 26 [X is C(C02Me)2, Z is S] was obtained in 50% yield. 

Reaction of a four-carbon unit with sulfur sources such as hydrogen sulfide, carbon disulfide, and elemental sulfur is one of the traditional thiophene syntheses that belong to this category (Equation 18). A wide variety of hydrocarbons, for example, alkanes, alkenes, dienes, alkynes, and diynes, serve as four-carbon units. Another practical method is the sulfuration of 1,4-dicarbonyl compounds (Paal synthesis). The method has become very popular with development of sulfuration reagents such as Lawesson s reagent. The reaction of a,/3-unsaturated nitriles with elemental sulfur in basic media, Gewald synthesis, is also useful for the preparation of 2-aminothiophenes which are important compounds in dyestuff and pharmaceutical industries. 

Acylations have generally been carried out using aluminum chloride as catalyst, although on occasions no catalyst was required." Titanium tetrachloride proved superior to aluminum, tin(IV) and zinc chlorides in reactions of 4-substituted-l-trimethylsilylbuta-l,3-diynes and the derived (Z)-enynes." The most frequently used solvents are chlorinated hydrocarbons," - " in preference to carbon disulfide and nitrobenzene." ... 

The neutral rhodium(I)/BINAP complex is a highly effective catalyst for the [2 -f- 2 -I- 2] cycloaddition of terminal 1,6-diynes with isothiocyanates to give the corresponding bicyclic thiopyranimines in good yields (Scheme 4.79) [63]. In these reactions, the aid of the Thorpe-Ingold effect induced by the tertiary center at the 4-position of 1,6-diynes is necessary. Carbon disulfide could also be employed in this reaction (Scheme 4.80) [63]. 

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