Selenoacetals

Treatment of selenoacetals 24 with butyllithium at 78 °C leads to the chiral a-seleno lithium compounds 25. Selenoacetals are stable compounds and can be readily prepared by selenoacetal-ization of the corresponding aldehydes25,26. In contrast to the corresponding dithioacetals, no competing deprotonation occurs on treatment with butyllithium, even with selenoacetals derived from aromatic aldehydes. 

These methoxylated and acetoxylated seknides are a-perfluoroalkyl mono-selenoacetals, which seem to be useful building blocks similar to those of the sulfur analogues described above. So far, only limited methods have been developed for the preparation of monoselenoacetals and they require rather complicated procedures or special reagents. In this regard, this electrochemical method has advantages since monoselenoacetals can be prepared in a one step reaction under mild conditions. 

Secondary alkyl selenides are reduced by (TMS)3SiH, as expected in view of the affinity of silyl radicals for selenium-containing substrates (Table 4.3) [40]. Reaction (4.23) shows the phenylseleno group removal from the 2 position of nucleoside [50]. Similarly to 1,3-dithiolanes and 1,3-dithianes, five- and six-membered cyclic selenoacetals can be monoreduced to the corresponding selenides in the presence of (TMS)3SiH [51]. The silicon hydride preferentially approached from the less hindered equatorial position to give transicis ratios of 30/70 and 25/75 for the five-membered (Reaction 4.24) and six-membered cyclic selenoacetals, respectively. 

Metallation of aryl selenides and selenoacetals.1 This base (KDA) is the most efficient for metallation of these substrates. 

When magnesium bromohydroselenide reacts with ethyl chloro-formate or acetyl chloride it gives, respectively, ethoxyseleno-formic acid, C2H5O.COSeH, and selenoacetic acid, CH3.COSeH, products in which one atom of oxygen in the carboxyl group is replaced by selenium. 

Selenoacetic acid, CH3COSeH, from magnesium bromohydroselenide and acetyl chloride, has a penetrating, irritating odour. The ammonium salt is unstable and decomposes into selenium and ammonium acetate. With salts of the heavy metals the ammonium salt gives coloured precipitates, which decompose, forming the corresponding black selenides. 

Tetranitrodiphenyl selenide, [C6H3(N02)2]2Se.1— This occurs as a by-product in the preparation of 2 4-dinitrophenyl-selenoacetic acid, p. 11, and is probably formed as shown below. 

Vinyl selenides, ketene selenoaeetals. Methyl selenoacetals (1) derived from ketones are converted into vinyl selenides on reaction with P2I4 or PI3 and triethylamine in CH2C12 at 25° (equation I). The sulfur analogs undergo the same reaction. 

Nitration and oxidation. Clay-supported Cu(N03)2, unlike clayfen (12,231), > shelf-stable for months. Like clayfen, it is a convenient source of N02+ and can leave thioacetals or selenoacetals to the carbonyl compound at 25° in high yield. It effects aromatization of 1,4-dihydropyridines in 80-92% yield. In the presence i acetic anhydride, it can effect nitration even of halobenzenes at 25° with marked, - jra-preference, which can be enhanced by use of lower temperatures. 

R-SeH R1-Se-R2 R-Se-CN Selenol Selenide Selenocyanate R1, SeR3 K SeR3 Selenoacetal... 

Acyclic bis(seleno)acetals 426 (R = H) must be deprotonated with LDA at —78 °C, because w-BuLi produces lithium-selenium exchange8,13,635 639. a-Lithioselenoacetals can also be prepared by this transmetallation from selenoorthoesters 427 (R = SeMe) with n-BuLi638,639. a-Alkyl substituted selenoacetals 426 (R = alkyl) can be deprotonated with LDA or lithium tetramethylpiperidide in the presence of HMPA at — 30 °C640. 

Cyclic selenoacetals 428 (R = H)641,642 can be lithiated with LDA and 4,6-dimethyl-1,3-diselenane 428 (R = Me)643 with n-BuLi at — 78 °C at the equatorial position. Axial functionalization has been achieved through a Se/Li exchange upon reacting 4,6-dimethyl-2-methylselanyl-l,3-diselenane with n-BuLi643. However, these 2-lithio-l,3-diselenanes have not been used as acyllithium reagents. 

Bis(phenylselanyl)methyllithiums 429 (R = H) are stable till 0 °C and were initially trapped with deuterium oxide, methyl iodide and benzophenone639. a-Substituted organolithium intermediate 429 (R = Me, w-CgH ), prepared with LiTMP in THF/HMPA at — 20 °C, reacted with alkyl bromides, ethylene oxide and benzaldehyde to give products 430 in good yields (Scheme 113)640. Bis(methylselanyl)methyllithiums 431 have been allowed to react with different electrophiles to afford products 432 (Scheme 113)640. Alkylated products have been deprotected with mercury(II) chloride or copper(II) chloride/copper(II) oxide, and by oxidation with hydrogen peroxide or benzeneseleninic anhydride644. Deprotection of selenoacetals to ketones can also be performed with sulfuric acid645. 

Phenyheleno acetals. This borane converts both aldehydes and ketones into diphenyl diselenoacetals. Addition of TFA is usually advantageous. An alternative route, which sometimes is more efficient, is acid-catalyzed exchange from oxygen acetals. The selenoacetals are precursors to useful selenium-stabilized carbanions. 

This occurs when one or more oxygen atoms in a functional group are notionally replaced by other heteroatoms. Depending on the hierarchy, this may lead to the use of functional replacement prefixes (e.g., seleno in selenoacetic acid, H3C-C(=Se)OH), infixes, or suffixes (e.g., in benzenecar-bodithioic acid, Ph-C(=S)-SH). 

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