Selenoketene

IR spectra of 1,2,3-selenadiazole and the pyrolysis products including selenoketene were recorded <1995CPL211>. 

The IR spectra of selenoketene and selenoketene- i, which were prepared by flow thermolysis of the 1,2,3-selenadiazoles 1 and 1-5d at 600°C, were investigated (Equation 4) <1995CPL211>. With the aid of ab initio predictions of vibrational frequencies and intensities, the spectra were assigned in detail. 

Selenoketones.1 Selenoketones can be prepared by reaction of dialkyl ketones with 1 and trapped by a Diels-Alder reaction with a diene. A sterically hindered selenoketene, selenofenchone, has been isolated from this reaction. 

Allenic selenoketenes 129, generated by [3,3] sigmatropic rearrangements, react with primary amines to afford 2-imino-277-5,6-dihydroselenines 130 (Scheme 12) <2001JOC4099>. This reaction appears to be the first example of a selenoketene-allene cycloaddition. Uncyclized allenic selenoamides 131 are by-products in the reactions and are the only products when the reaction is carried out with secondary amines. 

In the variable temperature photoelectron spectroscopy of 1,2,3-selenadiazole (101) at 5 x 1(T2 mbar between 450 and 750 °C, acetylene is always formed along with selenoketene (105). At 800 °C, acetylene is the sole product. 1,2,3-Benzoselenadiazole (106) does not produce benzyne even at 800 °C. The reaction product was found to be the highly reactive 6-fulveneselone (107). The PE spectra of (101), (106) and their decomposition products are very similar to those of their corresponding sulfur analogs (Scheme 45) (80CB3187)... 

Selenoketene produced by pyrolysis of 1,2,3-selenadiazole was identified by observation of the microwave absorption of the isotopic species including monodeuterated selenoketene (116). An rs structure has been derived with distances of C=Se 1.706 A, C=C 1.303 A and an HCH bond angle of 119.7° (78MI42000). 

The structural assignment of selenoketene was also based on an ab initio SCF calculation of its PE spectrum as well as on a radical cation state comparison with ketene and thioketene by mass spectrometry (80CB3187). 

Selenoketenes formed by pyrolysis of 4-alkyl-l,2,3-selenadiazoles in the vapour phase are isolated at -196 °C as highly colored materials which dimerize rapidly. 

When selenoketene vapour is mixed with gaseous dimethylamine, the expected formation of N,N- dimethylselenoamide takes place (Scheme 51) (79CC99). 

Homopropargylic alcohols are readily available substrates that can be used for the synthesis of 7-lactones. Cul-catalyzed selenation with PhSeBr at the alkyne terminus affords alkynyl aryl selenides. These react with an excess of /i-toluenesulfonic acid monohydrate, in dichloromethane at 60°C, to form a selenium-stabilized vinyl cation intermediate. The cation is then intramolecularly trapped by the tethered hydroxyl group to afford a cyclic selenoketene acetal, which readily adds a molecule of water to give the 7-lactone products (Scheme 55) <2006SL587>. 

Additionally, carbon diselenide, selenoketenes [17] and isoselenocyanates (RN = C = Se) [18] in which the selenium atom is attached to the sp-carbon atom are also well known. They have often been used as key starting materials of the compounds in category C of Scheme 1. 

Similarly, irradiation (2 = 2350-2800 A) of 1,2,3-selenadiazole (22) produces selenoketene (24) alkyne (25) and ethynyl selenol (26), as well as a species (23) absorbing at 3197, 3161, 3155, 1681, 1637, and 868 cm-1 (Equation (5)). The positions and shapes of these bands are strikingly similar to those of thiirene (17) and are reasonable for the analogous selenirene (23), though the assignment of these bands to selenirene is entirely tentative. 

The kinetics and detailed mechanism of the base-catalyzed decomposition of 4-aryl-1,2,3-selenadiazoles has been studied. It has been demonstrated that arylethynylselenolates (131) and selenoketenes (132) are intermediates in the diselenafulvene formation. ... 

Flash pyrolysis of 1,2,3-selenadiazoles gives selenoketenes which are stable at — 196°C and were identified by IR and microwave spectroscopy.143 On wanning, dimerization occured (Eq. 31). 

With selenium, silylated alkyne selenols can be readily prepared if R is alkyl. They are more stable than the isomeric selenoketene but react with amines through such an intermediate. If R is Me3Si however, then the bis-silylselenoketene results as the sole product at — 30 °C, but as an isomeric mixture with the thermally unstable alkyne selenol at higher temperatures (Scheme 5)61. 

The formation of 2-alkylidene-l,3-diselenoles by the photolysis of 1,2,3-selenadiazoles suggested to the investigators the intermediacy of selenoketens. The tetramerization of bis(trifluoromethyl)thioketen (100), which was found to take place only in the presence of selenium, was considered by Raasch to depend on the transient existence of the seleno-carbonyl intermediate (338). The preparation of transition-metal complexes... 

Photochemical elimination of nitrogen from a bicyclic 1,2,3-thiadiazole results in the formation of a thioketen, e.g. (79), instead of a thiiren (see Scheme 4), which is in contrast to the reaction of monocyclic thiadiazoles. When 1,2,3-selenadiazoles are subjected to thermolysis at 500—600 C, they give the selenoketens (80)— (82). The selenoketens have been trapped and characterized at —196 C. 

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