Thiobenzophenone 4,4 -dimethyl

Like many other 1,3-dipoles (e.g., nitrile ylides, imines, and oxides) (7), thiocarbonyl ylides undergo head-to-head dimerization to give sterically crowded 1,4-dithianes. The first reported example involves the formation of 2,2,3,3-tetraphenyl-l,4-dithiane (18) from thiobenzophenone (5)-methylide (16) (17,28) (cf. Scheme 5.3). Other (5)-methylides are known to form analogous 1,4-dithianes (e.g., thiofluorenone (5)-methylide yields 172) (17). The (5)-methylides of 4,4-dimethyl-2-phenyl-l,3-thiazole-5(4//)-thione (105) and methyl dithiobenzoate (60,104) dimerize to give compounds 173 and 174, respectively. 

Dimethyl acetylenedicarboxylate (DMAD) is able to trap both products of the reversible thermal 1,3-dipolar cycloreversion. With an excess of DMAD at 60 °C without solvent, the product 247 (Ar = Ph) arising from thiosulfine was formed in 83% yield, and the benzothiopyran 248 arising from the thiobenzophenone in 68% yield. With R = C1, yields were slightly lower (Scheme 77) <1997T939>. 

An outstanding comparison was made with the best known dipolarophi-le, dimethyl acetylenedicarboxylate (DMAD). The cycloaddition of N-methyl-C-phenylnitrone to adamantanethione is 1500 times faster than the addition to DMAD. As indicated in Scheme 74 the reaction is an equilibrium, leading to 56 44 ratio of product vs starting material. This equilibrium is shifted towards the left side for thiobenzophenones, as the cycloadduct formation breaks the conjugation, and thus these thiones do not appear to react with nitrones. Ab initio calculations were carried out to model the high reactivity of nitrones with thiocarbonyl compounds [257]. 

Photolysis of the bicyclic enone 57 gives a modest yield of thietane thermolysis of 5-cyano-2,2,4,4-tetraphenyl-l,3-dithiane gives 3-cyano-2,2-diphenyl-thietane by loss of thiobenzophenone. Photolysis of 3,3,6,6-tetramethyl-S-acetoxy-l-thiacycloheptan-4-one gives a small amount of 3,3-dimethyl-2-acetoxy-thietane which also can be obtained in 30% yield by treatment of 3,3-dimethyl-thietane with lead tetraacetate in pyridine. ... 

Only one example is known for the azomethine ylide trapping with thio-carbonyl dipolarophiles. Nonstabilized azomethine ylides 41 (R = PhCHj) undergo smooth cycloadditions in acetonitrile at room temperature with thiobenzophenone to give a mixture of two regioisomeric cycloadducts 223, whose ratio is independent of the C substituent R (87JOC235). Direct competition experiments using ylides 41 indicate that thiobenzophenone dipolarophile is more reactive than benzaldehyde but less than dimethyl fumarate. 

The first conclusive evidence for the intermediacy of silathiones came in 1975 from the pyrolysis of 1,1-dimethyl-1-silacyclobutane 15 with thiobenzophenone, giving a Wittig-type reaction, and forming olefin and cyclodisilthiane (equation 57)51. 2,2-dimethyl-2-... 

CL973 87BCJ4079). In other words, the competitive ylide trappings in general favor olehnic dipolarophiles rather than carbonyl dipolarophiles, indicating that olefins would be better dipolarophiles toward azomethine ylides. Padwa has established a more quantitative estimate of the relative reactivity of C-unsubstituted azomethine ylide 43 (R = PhCHj) toward a variety of dipolarophiles (87JOC235). Reactivities relative to that of benz-aldehyde (1.0) are shown in parentheses benzaldehyde is less reactive than thiobenzophenone (1.2), dimethyl fumarate (1.9), and Al-phenylmaleimide... 

The formation of 1,3-oxathiolane with chloral and 1,3-dithio-lanes with cycloaliphatic thioketones occurs regioselectively to yield the sterically less hindered products. On the other hand, aromatic thioketones, e.g., thiobenzophenone or 9/ fluorene-9-thione, intercept 8 to give comparable amounts of both regio-isomeric adducts. Usually, stereoisomeric dipolarophiles such as fumaronitrile and maleonitrile as well as dimethyl fumarate and maleate form 18 in a stereoselective manner. However, in the case of extremely electron-poor dipolarophiles, e.g., dimethyl 1,2-dicyanofumarate or ( )-l,2-bis(trifluoromethyl)ethylene-l,2-dicarbonitrile, non-stereospecific formations of the corresponding tetrahydrothiophenes are described. i- s This result is interpreted in terms of a stepwise reaction mechanism with a zwitterion as the key intermediate. Alternatively, this intermediate can cyclize to form seven-membered ketenimines of type 20. With R = Cp3, this product can be isolated in a crystalline form, whereas in the case of R = CN, stable lactam 21 is obtained only after addition of water (eq 10). 

The relative rate constants for cycloaddition of thiocarbonyl -methylides with dipolarophiles show that C=S dipolarophiles are very efficient. For example, thiofluorenone exceeds tetracyanoethylene and thiobenzophenone reacts 3000 times faster than dimethyl acetylene-dicarboxylate. ... 

A degassed soln. of thiobenzophenone and 2,4-dimethyl-2,3-pentadiene warmed at 40° until the blue color has disappeared 2,4-dimethyl-3-benzhydrylthiopenta-1,3-diene. Y ca. 100%. F. e. s. H. Gotthardt, Tetrah. Let. 1971, 2343 formation of thietanes by irradiation (s. Synth. Meth. 24, 696) cf. ibid. 1971, 2345. 

The applicability of thioketones in the synthesis of heterocyclic compounds has been further demonstrated. The Diels-Alder reaction of thiobenzophenone with l,3>5-cycloheptatriene gave the (2 + 4) cycloadduct (101) in 40% yield. Cycloheptatrienethione (92) yielded the (8 + 2) cycloadduct (102) quantitatively by reaction with maleic anhydride, and the bicyclic compound (103) in 65% yield by reaction with dimethyl acetylenedicarboxylate. Compound (103) was considered to be a rearrangement product of the probably primarily formed (8 + 2) cycloadduct intermediate (104). " The in situ addition of nitrile oxides to aromatic thioketones resulted in formation of 1,4,2-oxathiazoles, " whereas 1,4,2-oxathiazolidines were the products in the reactions of some alicyclic thiones with nitrones. The formation of thio-ozonides (1,2,4-trithiolans) by treatment of thioketones with amines or chloramine T was investigated recently. ... 

The photocycloaddition of thiobenzophenone to acrylonitrile at 366 nm is known to give the thietan (130). It has now been shown that the reactive state of the thione under these conditions is the second excited (tt, n ) singlet, and that the 1,3-dithian (131) is an intermediate in this reaction. " In contrast to earlier findings, irradiation at longer wavelengths (>500 nm) has been found also to result in product formation. The heterocyclic compounds (132) and (133) were the main products in the latter photolysis reaction, and their formation was interpreted in terms of an intermediate lowest triplet state of thiobenzophenone. The irradiation of thiobenzophenone in tetramethylethylene solution at -78 C afforded two 1 1 adducts, (134) and (135), respectively. Various thietans were the products in the photocycloaddition reactions of some aromatic thioketones with tetramethylallene, 2,4-dimethylpenta-1,3-diene, dimethyl fumarate, and dimethyl maleate, respectively. The nitrile function has also been found to be susceptible to addition by the excited thione group. Thus several alicyclic and aromatic thioketones, on irradiation in acetonitrile solution, have been converted into N-thioacylketimines (136), probably via an intermediate 1,3-thiazetine (137). Two recent papers by de Mayo and his... 

The cycloaddition of 2-diazoalkanes to aromatic sulphines (thione S-ox-ides), readily accessible by peroxy-acid oxidation of thienes, provides yet another efficient route to A -l,3,4-thiadiazolines (156). Thiobenzophenone S-oxide (155) and 2-diazopropane, for example, afford 2,2-dimethyl-5,5-di-phenyl-1,3,4-thiadiazoline 1-oxide (156). Photolysis to the azines (157) and (158) establishes the orientation of the cycloaddition, and excludes the alternative formulation (159)." Thiofluorenone S-oxide (160) similarly yields spiro-compounds of type (161). " ... 

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