Selenocarboxylates

Optically active iV-unprotected-2-pyrrolidinones 194 were obtained from selenocarboxylate or allylamine via radical cyclization and subsequent one-step cleavage of the C-O and C-N bond of the inseparable mixture of the two bicyclic oxyoxazolidinones 192 and 193 with -Bu4NF. The initial radical reaction is highly stereoselective. Products were obtained with ee up to 90%. The mandelic acid 195, which served as the chiral auxiliary in this method, was recovered with no loss of optical activity (Equation 33) <2003T6291>. 

The oxidation of 4-imidazolin-2-selones with 7,7,8,8-tetracyanoquinodime-thane [96] and electrochemical oxidation of benzthiazoline-2(3H)-selone has been carried out [97]. The properties of the former products have also been tested as semi-conducting molecules. The elimination of selenocarboxyl groups and sulfonyl groups mediated by JV-acyloxypyridinethione gives olefins (Eq. 26) [98]. 

As well as acylation of selenols, alkylation of selenocarboxylates with alkyl halides is one of the most straightforward methods for synthesis of selenol esters. However, known examples of selenol esters synthesized using this strategy have been limited to a few cases, due to the difficulty of the preparation of selenocarboxylates. 

Potassium selenocarboxylates, which are useful starting materials for the preparation of selenol esters,can be prepared by the reaction of diacyl selenides with methanolic potassium hydroxide (equation 9). 

Piperidinium selenocarboxylates (15 equation 10) have been isolated in high yields from the reaction between diacyl diselenides and piperidine. The salts (15) dissolved in common protic and aprotic solvents and readily reacted with alkylating agents such as phenacyl bromide to give the corresponding esters in almost quantitative yields. ... 

Analogous trends, although to a lesser extent, appear in selenocarboxylic acid derivatives as soon as third- and/or higher-row elements (sulphur or selenium) are involved, the 8 C Se) reach up to 5 = ca. ISOO. It seems that the interaction of the lone-pairs of these chalcogen atoms with the C=Se 7i-bond (formula II in Scheme 24) is less effective.2 These trends may be demonstrated in a series of selenoesters Ph-C(=Sc) X X = NMc2, 5 = 733 X = OMe, 5 = 910 X = SeMe, 5= 205 correlations of Se... 

Amide formation. Carboxamides including peptides are S3mthesized from carboxylic acids and aUcyl azides, after converting the acids into mixed anhydrides and then selenocarboxylates. Treatment of the mixed anhydrides with a suspension of freshly prepared from LAH and Se completes the first stage of the transformation. 

Selenocarboxylate salts are not generally available due to the instability of the corresponding seleno-acids. An indiret method to... 

The acidities of thio- and dithiocarboxylic acids have been discussed in an earlier review [3]. For example, the values of PhCOOH, PhCOSH and PhCSSH are 4.20,2.48 and 1.92, respectively. The acidities of some chalcogeno-carboxylic acids have been estimated through theoretical studies. The gas-phase acidities (AH) of formic acid and its sulfur congeners are 342.1 for HC(0)0H, 332.2 for HC(0)SH, 328.8 for HC(S)OH and 325.8 kcal/mol for HC(S)SH. Therefore, the acidity appears to increase roughly in proportion to the number of sulfur atoms present [23]. For selenocarboxylic acids, gas-phase acidities are 340.4 for HC(0)0H, 327.6 for HC(0)SeH, and 321.9 kcal/mol for HC(Se)OH, indicating that selenocarboxylic acids are more acidic than their parent carboxylic acids [24]. It was also predicted that selenocarboxylic acids maybe more acidic than their corresponding thioic acids [14]. 

Hadad et al. found that XC(=0)SH (X=Me, NH2, OH, F) is preferred over XC(=S)OH by 5-14 kcal/mol using ab initio calculations [23]. The C-0 bond dissociation energy is greater than the C-S energy by 30 kcal/mol and the C=0 bond is significantly stronger than the C=S bond by about 40 kcal/mol. The C=0 bond has more polar character (C -0 ") than the C=S bond, so that the bond order of the C=0 bond is about 1.2 while the C=S bond order is -2.0. For selenocarboxylic acids, density functional theory (DFT) calculation results suggest that the syn selenol form is more stable than the other forms [24]. 

In 1994, successful isolations of various alkyl and aryl selenocarboxylic acids [14] enabled them to undergo exact reactions. Reactions of selenocarboxylic acids with dicyclohexylcarbodiimide (DCC) yield the corresponding diacyl selenides 25 and selenourea 26 quantitatively (Scheme 15) [14], as in reactions of thio- [44] and dithiocarboxylic acids [45,46]. In air,selenocarboxylic acids are immediately oxidized to afford the corresponding diacyl diselenides [13, 14]. Also, thio- and dithio-carboxylic acids readily react with aryl isocyanates to give acyl carbamoyl [47,48] and thioacyl carbamoyl sulfides [49, 50], respectively. 

Metal thiocarboxylates Metal selenocarboxylates Metal tellurocarboxylates... 

A series of thio- 46 [51,168] and seleno-carboxylato complexes 47 [175] with tmns-configurations have been synthesized from the reaction of NiCl2(R3P)2 with alkali metal or 0-trimethylsilyl selenocarboxylates (Scheme 4). Telluro-carboxylato nickel complexes 48 Ni(RCOTe)2(R3P)2 are too labile to isolate, although their formation has been confirmed by their H, and Te NMR and IR spectral data [175]. The anionic thiocarboxylato nickel complex 49 is... 

The trialkyls R3M (M=A1, Ga, In) undergo insertion reactions with COS to form alkylmetal thiocarboxylates (RCOS)xMR3 x (M=A1, Ga, In x=l-3), which are difficult to separate [327]. Treatment of trimethylaluminum with thioacetic acid, however, leads to a good yield of dimethylaluminum thioacetate 75 as yellow crystals that sublime (Scheme 6) [327]. On the basis of infrared and NMR spectra, the structure of 78 has been deduced to be a dimer [327]. Syntheses of Group 13 metal derivatives of selenocarboxylic acids have not been reported, although the formation of RCOSeAl(CH3)3 has been described as a short-lived intermediate in the reaction of RCOOR with [(CH3)2Al]2Se to form diacyl diselenide [281]. 

Se-Organogermyl selenocarboxylates 86 (M=Ge, E=Se) can easily be synthesized by reacting alkali metals [ 165] or 0-triorganosilyl selenocarboxylates... 

E=Se) have been prepared in good yields using selenocarboxylic acid sodium salts or 0-trimethylsilyl esters [132,139]. Diphenylarsanyl selenocar-boxylates 107 (E=Se,x=l) also are obtained by treating piperidinium diphenyl-selenoarsinate or -diselenoarsanate with acyl chlorides [137]. 

---