Aromatic thioketones, cycloaddition

Whenever benzyne is expected to undergo a [2+2] cycloaddition as with ketones,109 it behaves as a dienophile with aromatic thioketones. Reaction of thiobenzophenones with benzyne yields the [4+2] cycloadducts and 1H-2-benzothiopyrans.110,111 Sterically congested thiones,111 thiopivalophenones and 2, react with benzyne to give the [2+2] adducts, 2H-l-benzothietes. 

Photocycloaddition of thiones to alkenes is the most popular and fruitful method for the preparation of the thietane system. In analogy to the formation of the oxetanes by cycloaddition of the electronic excited ( ,tc ) carbonyls, thietanes can be expected to arise photochemically from aromatic thioketones and substituted olefins as well as 1,2- and 1,3-dienes. ° Thiobenzophenone serves as a source of a sulfur atom and, because of its blue color, which disappears on photocycloaddition, permits exact control over the reaction time. A mixture of thiobenzophenone and a-phellandrene must be irradiated for 70 hr before the blue color disappears (Eq. 2) and... 

Conjugation of a C=S moiety with a C=C or a C=0 bond brings the possibility of the compounds playing the role of dienes in cycloaddition (Tables 4 and 5). This is indeed observed with aromatic thioketones as demonstrated by the extensive work of Saito et al. at the Science University of Tokyo. Reaction of furyl phenyl thioketones or thiophenyl phenyl thioketones with... 

The stability of the 2,5-dihydrothiadiazole 5-oxides depends on the nature of each substituent. For example, cycloaddition of aromatic thioketone 5-oxides, such as thiobenzophenone 5-ox-ide or thiofluorenone 5-oxide, with diazopropane results in the formation of 2,5-dihydrothiadiazole 5-oxides in high yield. Diazomethane reacts more sluggishly and a complex mixture containing various products is formed78,79. 

A (1/7-imidazol-l-yl) silver species (55) has been postulated as the key intermediate in the 3-l-2-cycloaddition reaction of diazoalkanes (54) with benzynes yielding 2-aryl-2H-indazoles (56) (Scheme 18). The 3-I-2-cycloaddition reaction of 3-trifluoromethyl-4-diazopyrazolinones with dialkyl acetylene dicarboxylates, in refluxing toluene, produced spiro 3/f-pyrazole adducts that rearranged to the trifluoromethyl-substituted pyrazolo[l,5-fi(][l,2,4]triazin-7-ones. ° The 1,3-dipolar cycloaddition reaction of aromatic thioketones (58) with 2-aza-1,3-dicarbonyl compounds (57), at 20-50 C, yielded thiadiazoline adducts (59) that readily eliminate nitrogen to produce oxathioles (60) in moderate yields (up to 70%) (Scheme 19). ... 

When the C=S bond is conjugated with double or triple bonds, thioketones can also behave as heterodienes93 104 towards dienophiles. If thioketone contains an aromatic ring, the [4+2] cycloaddition can be followed by 1,3-protot-ropy to restore the ring aromaticity,105 108 forming lH-2-benzothiopyrans as shown in Scheme 13, where 4,4 dimethoxythiobenzophenone reacts with the dienophile dimethyl acetylenedicarboxylate (DMAD).105... 

For preparative purposes, the reaction of thiocarbonyl ylides with carbonyl compounds can be considered as an alternative method for the synthesis of 1,3-oxathiolanes. Aromatic aldehydes, chloral, glyoxalates, mesoxalates, pyruvates as well as their 3,3,3-trifluoro analogues are good intercepting reagents for thioketone (5)-methylides (36,111,130,163). All of these [3 + 2] cycloadditions occur in a regioselective manner to produce products of type 123 and 124. 

The following types of dipolarophiles have been used successfully to synthesize five-membered heterocycles containing three heteroatoms by [3 + 2]-cycloaddition of thiocarbonyl ylides azo compounds, nitroso compounds, sulfur dioxide, and Al-sulfiny-lamines. As was reported by Huisgen and co-workers (91), azodicarboxylates were noted to be superior dipolarophiles in reactions with thiocarbonyl ylides. Differently substituted l,3,4-thiadiazolidine-3,4-dicarboxylates of type 132 have been prepared using aromatic and aliphatic thioketone (5)-methylides (172). Bicyclic products (133) were also obtained using A-phenyl l,2,4-triazoline-3,5-dione (173,174). 

Some examples dealing with the [4 + 2] cycloaddition of ketenimines have been recorded (Scheme 58). Thus, thioketones and ynamines reacted with N-aryl ketenimines 257 through the carbon—nitrogen and the conjugated aromatic carbon—carbon double bonds to yield benzothiazine derivatives 258 (80JOC3766 82JOC3998) and substituted quinolines 259 (73JA5417), respectively. Simple ketenimines 261 were formed by reaction... 

Two types of reaction are observed for thioketones that do not have parallels in ketone photochemistry. The first is photocyclization in thioketones with polycyclic aromatic groups (4.104), where the sulfur atom forms a new bond to the aromatic system. The second is photo-oxidation by singlet oxygen to give the corresponding carbonyl compound 4.10S), possibly via a 1,2,3-dioxathietane formed as a result of initial cycloaddition. 

This reaction was intensely investigated by Mloston et al. In a classical, but not general reaction, they observed that heating aromatic or aliphatic thioketones [251, 252], thionoesters [253] and dithioesters [252, 253] with aryl-or benzylazides led to the formation of a carbon-nitrogen double bond. A [3+2] cycloaddition was assumed, with successive elimination of nitrogen and sulfur. 

Similarly small rate factors were obtained for 1,3-dipolar cycloadditions between diphenyl diazomethane and dimethyl fumarate [131], 2,4,6-trimethylbenzenecarbonitrile oxide and tetracyanoethene or acrylonitrile [811], phenyl azide and enamines [133], diazomethane and aromatic anils [134], azomethine imines and dimethyl acetylenedi-carboxylate [134a], diazo dimethyl malonate and diethylaminopropyne [544] or N-(l-cyclohexenyl)pyrrolidine [545], and A-methyl-C-phenylnitrone and thioketones [812]. Huisgen has written comprehensive reviews on solvent polarity and rates of 1,3-dipolar cycloaddition reactions [541, 542]. The observed small solvent effects can be easily explained by the fact that the concerted, but non-synchronous, bond formation in the activated complex may lead to the destruction or creation of partial charges, connected... 

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