Pyridine-2-thiones, formation

Photochemical addition of electron deficient alkenes (72) to the pyridine thione (73a) affords the substituted pyridines (74). This reaction arises by the formation of an unstable spiro-thietane (75) which fragments to produce the isolated products. In the case of the Al-methylpyridine thione (73b)... 

In toluene solution, the thione tautomer predominates.84 In spite of that, oxidative addition was found to occur in a manner similar to that of the thiol tautomers, even for 4-pyridine thione,85 surprisingly it was found to occur much faster than for thiols. This could be explained by a much more favorable enthalpy of formation of the initial 19 e by coordination of the metal radical to the S=C bond as shown in Scheme 10.7... 

SCHEME 10.7 Two-step H-atom transfer mechanism between Cr(CO)3Cp and 4-pyridine thione involving formation of a 19 e thione complex followed by H-atom transfer from the 4-position. 

In contrast to the 4-hydroxy isomers, the thermally stable 5-hydroxy-THISs add to the C=C bond of cyclopropenylidenes (4. 18, 27. 28). The adducts eliminate carbonyl sulfide, and the strained bond breaks resulting in ring-expansion with formation of pyridin-4-ones. -thiones, or -imines. or 4-alkylidenedihydropvridines (20, X = 0. S.NR. or CRR ) (Scheme 19). 

Carboxylic acid esters of thiols are considerably more reactive as acylating reagents than the esters of alcohols. Particularly reactive are esters of pyridine-2-thiol because there is an additional driving force in the formation of the more stable pyridine-2-thione tautomer. 

The ring expansion reaction of diaryl cyclopropenones and cyclopropene thiones occurring with pyridinium, sulfonium, and phosphonium enolate betaine 427268-270) is related to 1,3-dipolar cycloadditions. This process results in formation of 2-pyrones 428 by loss of pyridine (or sulfide or phosphine) and insertion of the remaining fragment C=C-0 to the C1(2)/C3 bond of the cyclopropenone ... 

Spectroscopically invisible carbenes can be monitored by the ylide method .92 Here, the carbene reacts with a nucleophile Y to form a strongly absorbing and long-lived ylide, competitively with all other routes of decay. Although pyridine (Py) stands out as the most popular probe, nitriles and thiones have also been used. In the presence of an additional quencher, the observed pseudo-first-order rate constant for ylide formation is given by Eq. 2.92,93 A plot of obs vs. [Q] at constant [Y ] will provide kq. With Q = HX, complications can arise from protonation of Y and/or the derived ylides. The available data indicate that alcohols are compatible with the pyridine-ylide probe technique. 

A Thorpe-Ziegler intramolecular cyclization starting with 3-cyanopyridine-2-thione derivatives leads to the formation of 3-amino-2-(organylsulfinyl)thieno[2,3- ]pyridines in high yields <2006RCB529>. 

The reaction of 2,2,4,4-tetramethyl-l,3-cyclobutanedione with tetraphosphorus decasulfide (P4S10) led to ring cleavage and the formation of the intermediate dimethylthioketene, which underwent [2+2] cycloaddition after 12 h in refluxing pyridine to give 4-isopropylidene-3,3-dimethylthietane-2-thione 88 in nearly quantitative yield (Equation 28) <2004BCJ187>. 

Cyclization occurs directly through catalysis by the acid liberated when a pyridine-2(lH)-thione is heated with an a-halo acid ester. The most convenient method for preparing the thiazole, however, seems to be the cyclization of (2-pyridinethio)acetic acids in acetic anhydride in the presence of pyridine. Without base catalysis the reaction is slow, which suggests a mixed anhydride intermediate. Mixed anhydride formation with ethyl chlorofor-mate in pyridine, or carboxyl activation by DCC in pyridine, gives the mesoionic product. The cyclization reaction and the chemical stability of the thiazole are adversely affected by a pyridine 6-substituent. The initially formed acylpyridinium salt (407) undergoes rapid tautomerization to the aromatic thiazole form equilibrium between the forms (407) and (408) is verified by rapid deuteration at C-2 (R1 = H) in AcOH-d (81H(15)1349). 

The reaction of methylketones with sulfur and ammonia produces A 3-imidazoline-5-thiones (37) (especially in the presence of pyridine).157 The initial stage of the reaction is formation of a phenylglyoxylic thionamide (38), followed by ring closure, and thus the reaction... 

Carboxamides form rather unstable 1 1 adducts with HFA insertion into one N—H bond occurring as with urea 202). Addition of another molecule of HFA can only be effected by salt formation with pyridine. The addition is reversible. A stable product 125a is obtained from urea in the presence of acetic acid anhydride 257). Thiourea forms the corresponding oxadiazin-thione 125b with HFA (257). 

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