Highly Selective Addition of Organic Dichalcogenides to Carbon-Carbon Unsaturated Bonds Akiya Ogawa and Noboru Sonoda Department of Applied Chemistry, Faculty of Engineering, Osaka University, Abstract: Highly chemo-, regio- and/or stereoselective addition of organic dichalcogenides to carbon-carbon unsaturated bonds has been achieved based on two different methodologies for activation of the chalcogen-chalcogen bonds, i.e., by the aid of transition metal catalysts and by photoirradiation. The former is the novel transition metal-catalyzed reactions of organic dichalcogenides with acetylenes via oxidative addition of dichalcogenides to low valent transition metal complexes such as Pd(PPh3)4. The latter is the photoinitiated radical addition of organic dichalcogenides to carbon-carbon unsaturated bonds via homolytic cleavage of the chalcogen-chalcogen bonds to generate the corresponding chalcogen-centered radicals as the key species. 1. Introduction The clarification of the specific chemical properties of heteroatoms and the development of useful synthetic reactions based on these characteristic features have been the subject of continuing interest (ref. 1). This paper deals with new synthetic methods for introducing group 16 elements into organic molecules, particularly, synthetic reactions based on the activation of organic dichalcogenides, i.e., disulfides, diselenides, and ditellurides, by transition metal catalysts and by photoirradiation. In transition metal-catalyzed reactions, metal sulfides (RS-ML) are formed as the key species, whereas the thiyl radicals (ArS•E) play important roles in photoinitiated reactions. These species exhibit different selectivities toward the addition process to carbon-carbon unsaturated compounds. The intermediates formed in situ by the addition, i.e., vinylic metals and vinylic radicals, could successfully be subjected to further manipulation leading to useful synthetic transformations. 2. Transition Metal-Catalyzed Addition of Organic Dichalcogenides to Carbon-Carbon Unsaturated Bonds Although the utility of transition metal catalysts for effecting a wide variety of synthetic transformations using heteroatom compounds such as organic silicon, tin, and boron compounds is well established, use of these catalysts for synthetic reactions of group 16 heteroatom compounds has remained largely unexplored. This might be partly due to the widespread prejudice that chalcogen compounds often bind strongly to the catalysts, thus poisoning them and making the catalytic reactions ineffective. On the contrary, we have recently revealed a series of reactions made up of such mismatched combinations of transition metal catalysts and chalcogen compounds like organic disulfides and diselenides. These novel reactions clearly demonstrate the efficacy of the transition metal catalysts in synthetic reactions of organic disulfides and diselenides. 894 ( 20 ) J. Synth. Org. Chem., Jpn. 2.1. Palladium-Catalyzed Addition of Organic Dichalcogenides to Acetylenes Organic disulfides and diselenides have widely been employed as the sources of ligands for various transition metals. For example, the stoichiometric reaction of diphenyl disulfide with Pd(PPH3)4 gave the complex (1) formulated as a dimer bearing both terminal and bridged sulfide groups (ref. 2). During the course of our study on the elucidation of the characteristic features and novel reactivities of chalcogen compounds, we have found that stoichiometric reaction of complex (1) with acetylenes such as 1-octvne affords vicinal dithioalkenes in moderate yields. 1 Further detailed investigation of this reaction system led to the finding of the stereoselective addition of diaryl disulfides to acetylenes catalyzed by a palladium(0) complex (ref. 3). The method could be Table 1. Palladium-Catalyzed Addition of Organic Disulfides and Diselenides applied to a wide variety of to Acetylenes a terminal acetylenes bearing hydroxy, amino, and silyl groups, as shown in Table 1. Organic diselenides also added to acetylenes smoothly, giving the corresponding vicinal diselenoalkenes in good yields. The addition proceeded stereo- selectively to give Z isomers almost exclusively, except for the addition to phenylacetylene. The fact that the reaction of the diselenide with phenylacetylene in the absence of a palladium catalyst provided the adduct in 76% yield with EIZ = 83/17 suggests the E isomer might be formed by a competitive thermal addition. The present addition to terminal acetylenes also proceeded smoothly in THF (67 ℃), CH,CN (82 ℃) and C,eCH3 (80℃). In the case of the acetylenes bearing a carbon- carbon double bond, the addition took place chemo- selectively at the triple bond site, and no cyclization product was obtained. Vol.54, No.11 (November 1996) ( 21 ) 895 2.2. Palladium-Catalyzed Carbonylative Addition of Organic Dichalcogenides to Acetylenes It is of much interest that the present addition reaction is carried out in the presence of carbon monoxide, because there is a possibility that it may cause the carbonylative addition of organic dichalcogenides to acetylenes. When the equimolar reaction of 1-octyne with diphenyl diselenide was carried out under pressurized carbon monoxide, the desired carbonylative addition took place to give the (Z)-1,3-bis(phenylseleno)-2-nonen-1-ones (4a) in high yield (Table 2) (ref. 3). Table 2. Pd-Catalyzed Carbonylation of 1-Octyne with CO and (PhS)2 The carbonylation was completely regioselective and highly stereoselective. The carbonylation using diphenyl diselenide also took place even at atmospheric Table 3. Palladium-Catalyzed Carbonylative Addition of (PhSe)2 to Acetylenes a pressure of CO. The procedure can be applied to a similar carbonylation using diphenyl disulfide, although higher pressures and prolonged reaction times are essential. Pd(PPh3)2C12is an excellent catalyst for the carbonylative addition of (PhS)2 at lower CO pressures. Representative results of the carbonylative addition of diphenyl diselenide are indicated in Table 3. While the carbonylative addition to phenylacetylene by using Pd(PPh3)4 as the catalyst was accompanied by the formation of large amounts of vicinal diselenostyrene, the carbonyl- ation using Pd(PPh3)2C12 provided selectively 13-seleno- a,13-unsaturated selenoesters in good yields. In the case of enynes, carbonylation took place chemoselectively at the triple bond site. A possible mechanism for this carbonylation may 896 ( 22 ) J . Synth . Org . Chem . , Jpn include the following: (i) oxidative addition of (PhY)2 (Y = S, Se) to low-valent palladium species;(ii) stereoselective cis-thiopalladation or cis-selenopalladation of acetylenes to-form a cis-vinylpalladium intermediate; (iii) COinsertion to form an acylpalladium intermediate; (iv) reductive elimination of the product with retention of the stereochemistry. 2.3. Palladium-Catalyzed Reduction of Thioesters and Selenoesters with "Bu,SnH Next, we examined the reduction of the carbonylative addition products with tri(n-butyl)tin hydride. Under radical conditions (AIBN / 80 •Ž), selenoesters are known to be reduced easily to the corresponding aldehydes (ref. 4), but the reduction of thioesters did not proceed; rather , it resulted in only a trace amount as shown in entry 1 of Table 4. We have found that the use of Pd(PPh3)4 as the catalyst in the reduction with nBu3SnH led to the successful formation of the desired aldehydes with excellent chemoselectivity (entry 3) (ref. 5). Table 4. Palladium-Catalyzed Reduction of a Thioester with nBu3SnH Pd(PPh1),C1, and Pd(OAc), also exhibited the catalytic activities (entries 4 and 5) . while Ni(PPh3)2C12,Pt(PPh3)4,and Rh(PPh3)3C1 did not catalyze this reduction . When Et,SiH was used instead of "Bu3SnH, the reduction of thioesters in the presence of Pd on carbon (ref. 6) or Pd(PPh3)4 Vol.54, No .11 (November 1996) ( 23 ) 897 did not take place at all (entry 6). The present palladium-catalyzed reduction can be applied to selenoesters, and the corresponding aldehydes are formed selectively in high yields, as shown below. Although longer reaction times and higher temperatures are required, AIBN-initiated radical reduction with nBu3SnH also proceeds with a similar chemoselectivity. Representative results of the palladium-catalyzed reduction of some other thioesters (ref. 7) and selenoesters (ref. 8) are summarized in Table 5. Table 5. Palladium-Catalyzed Reduction with nBu3SnH a 898 ( 24 ) J . Synth . Org . Chem . , Jpn . In each case, excellent chemoselectivity was observed and isomerization of the olefinic unit hardly took place. The palladium-catalyzed carbonylative addition of terminal acetylene and the reduction of the thus formed selenoester by 93u3SnH can be carried out successively without the isolation of selenoesters . This one-pot transformation from acetylene to ƒÀ-seleno-ƒ¿ .ƒÀ-unsaturated aldehyde is wnthetirally the equivalent to regio- and stereoselective selenoformylation of acetylene . 3. Photoinitiated Radical Addition of Organic Dichalcogenides to Carbon-Carbon Unsaturated Bonds 3.1. Photoinduced Thioselenation of Acetylenes Using Disulfide-Diselenide Mixed System Organic disulfides, diselenides, and ditellurides reach their absorption maximum in ultraviolet near UV, and visible light regions, respectively, and therefore irradiation with these light source causes homolytic cleavage of the chalcogen-chalcogen bonds to generate the corresponding chalcogen - centered radicals as label species (ref. 9). Accordingly, if the photolysis of dichalcogenides was performed in the presence