Ylidene-sulfur adduct

Scheme 7. Process of the ring-closure of ylidene-sulfur adduct 8 or 10 during the Gewald reaction [59],...

In the base-promoted addition the elemental sulfur reacts with amines to yield polysidfide anions [63, 64], that can behave as nucleophiles. The methylene group of appropriate oc, / imsaturated nitrile 9 is being deprotonated first and then sulfur addition takes place (Scheme 8). The most suitable base for the activation with sulfur and the subsequent sulfur addition morpholine has been proved [59]. The morpholine exhibits the best solubility of sulfur from the entire organic base used in Gewald reaction. Additionally, by mixing the morpholine with sulfur at 150 °C the morpholine polysufide (MPS) is formed, which structure is presumed to contain from 2-5 sulfur atoms within two morpholine molecules [57, 65]. MPS acts then in two ways - as a base needed in each reaction step, and also as a S-nucleophile in the addition of sulfur step to the o yff-imsaturated substrate 9 to create reactive ylidene sulfur adduct 10 (Scheme 8). 

Scheme 8. Base-promoted formation of the ylidene sulfur adduct 10 using polysulfide-like reagent MPS [57-59].

Comparing the process of electrochemical activation to standard base-promoted addition of sulfur, the ylidene sulfur adduct 12 is formed by addition of a S -cyanomethyl anion onto t ano group (Scheme 9), while in the previous version the poly sulfide-like anion affects the methylene group of the a,p-unsaturated nitrile 10 (Scheme 8). However, if the activation with sulfur does not occur properly, the ylidene-sulfur adduct of presumed structure 10 or 12 is not formed and the side-reaction takes place. 

Calorimetric measurements in solution support these observations A relative enthalpy scale for different NHCs coordinated to ruthenium(II) [Eq. (40)] shows that NHCs have a higher donor capability than the best phosphine donor ligands. The only exception is the sterically extremely demanding l,3-di(l-adamantyl)imida-zolin-2-ylidene (Table III). The enthalpies for the formation of sulfur adducts... 

Halogen abstraction from hypervalent sulfur halides has also been reported for the imidazol-2-ylidenes (IV) (Scheme 8.22). This reaction gives a nice example of the synthetic utihty of V-heterocyclic carbenes. Indeed, this adduct is the first structurally characterized derivative featuring the chlorosulfite ion (SO2CI ). ... 

Quinazoline-4(3// )-thione condenses with ethyl bromocyanoacetate " and ethyl 2-chloro-3-oxobutanoate in the presence of sodium ethoxide in ethanol with sulfur extrusion to form ethyl 2-cyano-2-(quinazolin-4-ylidene)acetate (5, R = CN) and ethyl 2-(quinazolin-4-ylidene)-acetate (5, R = H), respectively. It has been suggested that the initial adduct of quinazoline-4(3//)-thione and a methylene compound is deprotonated to give the corresponding ylide which undergoes electrocyclic closure to a thiirane derivative and then desulfurization. A C - C double bond is formed between the methylene carbon atom and the electrophilic carbon C4, providing an alternative to the Wittig reaction. 

---