Selenocarboxylic acid

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... 

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. 

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]. 

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]. 

Selenocarboxylic acid esters Selenolic acid esters... 

M onoselenoacetals Selenolic acids and esters Selenocarboxylic acid esters Selenonocarboxylic — 35... 

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>. 

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... 

Aromatic acylsulfenyl bromides 154 have been synthesized by reacting thio-carboxylic acid phenylmercury [148], diphenyltin [149], silver [149], or 0-tri-methylsilyl derivatives [232] with bromine or AT-bromosuccinimide (NBS) (Scheme 20). In contrast, the selenium isologues are very sensitive toward oxygen and are also thermally labile. In the reaction of the diphenylarsanyl seleno-carboxylates 107 with NBS, only the formation of the 4-methylbenzoylselenenyl bromide 120 (R=4-CH3C6H4 X=Br) has been proved, by converting it into the jS-bromoethyl selenocarboxylates 121 (X=Br) (see Scheme 18) [135]. 

---