Hydrogen abstraction by thiocarbonyl

Hydrogen Abstraction by Thiocarbonyl Groups in the Solid State... 

Sakamoto et al. provided the first example of hydrogen abstraction by thiocarbonyl sulfur in the solid state [37,38]. Irradiation of AT-diphenylacetyl-AT-iso-propylthioaroylamides 6 in the solid state gave thioketones 7 (P-hydrogen abstraction products by thiocarbonyl sulfur), mercaptoazetidin-2-ones (y-hy-drogen abstraction products by thiocarbonyl sulfur), and aroylthioamides (y-hydrogen abstraction products by thiocarbonyl sulfur followed by cleavage). 

Sakamoto et al. provided an example of absolute asymmetric synthesis involving hydrogen abstraction by thiocarbonyl sulfur (Scheme 6). [24] Achiral A -diphenylacetyl-iV-isopropylthiobenzamide 33 and Y-diphenylacetyl-A-isopropyl(p-chloro)thio-benzamide 33 crystallize in chiral space group P2 2 2. Photolysis of the chiral crystals in the solid state gave optically active azetidin-2-ones whereas achiral thioketones were obtained as main products. When 33a was irradiated in the solid state at -45°C followed by acetylation (at -78°C), 2-acetylthio-3,3-dimethyl-l-diphenylacetyl-2-phenylaziridine (34a 39% yield, 84% ee), 4-acetylthio-5,5-dimethyl-2-diphenylmetyl-4-phenyloxazoline (35a 10% yield, 50% ee), 3,3-diphenyl-1-isopropy 1-4-... 

Scheme 13 Asymmetric synthesis using hydrogen abstraction by thiocarbonyl sulfur.

Dithioamides behave similarly186 (equation 131) but, in the presence of steric hindrance in addition to the thiocarbonyl groups, the photoaddition will become regioselective187 (equation 132). Intramolecular photocycloaddition will lead to the formation of strained polycyclic thietanes188 (equation 133). Hydrogen abstraction by the thioamide functional group is also possible189 (equation 134). 

Sakamoto et al. reported the X-ray crystallographic data and the solid-state photoreaction of eight Ar,AT-disubstituted a,p-unsaturated thioamides 23a-h, which involves hydrogen abstraction by the alkenyl carbon atom conjugated with thiocarbonyls (Fig. 9) [44-46]. 

Scheme 78 Photochemical [3-lactam synthesis by y-hydrogen abstraction by a thiocarbonyl group...

Monocyclic 3-lactams (411), containing a sulphur substituent at C(4) can be formed in variable yields from thioimides (410) via a photochemical Norrish Type II mechanism involving y-hydrogen abstraction by the thiocarbonyl group. 

Thio-substituted /8-lactams are also obtained from N,N-dialkylthioxamides by photo-irradiation, when selective y-hydrogen abstraction by the thiocarbonyl group is observed (Scheme 122). ... 

Photochemical hydrogen abstraction of a thiocarbonyl function is a common reaction. While thioamides generally do not show this photochemical reactivity, thioimides do undergo hydrogen abstraction by the thiocarbonyl sulfur of (C=S)-N chromophore from the P-, f, and 8-positions to yield various heterocycles. 

Cyclohexyl xanthate has been used as a model compound for mechanistic studies [43]. From laser flash photolysis experiments the absolute rate constant of the reaction with (TMS)3Si has been measured (see Table 4.3). From a competition experiment between cyclohexyl xanthate and -octyl bromide, xanthate was ca 2 times more reactive than the primary alkyl bromide instead of ca 50 as expected from the rate constants reported in Tables 4.1 and 4.3. This result suggests that the addition of silyl radical to thiocarbonyl moiety is reversible. The mechanism of xanthate reduction is depicted in Scheme 4.3 (TMS)3Si radicals, initially generated by small amounts of AIBN, attack the thiocarbonyl moiety to form in a reversible manner a radical intermediate that undergoes (3-scission to form alkyl radicals. Hydrogen abstraction from the silane gives the alkane and (TMS)3Si radical, thus completing the cycle of this chain reaction. 

The corresponding photocyclization of S-arylvinyl sulfides to dihydro-thiophens has been shown to proceed via short-lived thiocarbonyl ylide intermediates, formed in turn by cyclization of triplet excited states. 1-Phenylthio-3,4-dihydronaphthalene (49), for example, affords the colored thiocarbonyl ylide (50)44 the isolated products are derived by processes involving hydrogen abstraction and migration. The greater efficiency of the photocyclization of 2-thioaryloxyenones (51) to dihydrothiophens (52) has been attributed to the additional stabilization afforded to the ylide (53) by the carbonyl group.45 Other similar photocyclizations have been reported.46... 

On irradiation in MeOH, (3 R = Me or Et) undergoes y-hydrogen abstraction followed by intramolecular radical reaction to give a jS-lactam. However, excitation in the nn region of the thiocarbonyl group does not induce this transformation, suggesting that photocyclization of the thioxoacetamides proceeds from upper excited states as in the case of thiones. ... 

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