Chemistry:Selenol
Selenols are organic compounds that contain the functional group with the connectivity C–Se–H. Selenols are sometimes also called selenomercaptans and selenothiols. Selenols are one of the principal classes of organoselenium compounds.[1] A well-known selenol is the amino acid selenocysteine.
Structure, bonding, properties
Selenols are structurally similar to thiols, but the C–Se bond is about 8% longer at 196 pm. The C–Se–H angle approaches 90°. The bonding involves almost pure p-orbitals on Se, hence the near 90 angles. The Se–H bond energy is weaker than the S–H bond, consequently selenols are easily oxidized and serve as H-atom donors. The Se-H bond is much weaker than the S–H bond as reflected in their respective bond dissociation energy (BDE). For C
6H
5Se–H, the BDE is 326 kJ/mol, while for C
6H
5S–H, the BDE is 368 kJ/mol.[2]
Selenols are about 1000 times stronger acids than thiols: the pKa of CH
3SeH is 5.2 vs 8.3 for CH
3SH. Deprotonation affords the selenolate anion, RSe−
, most examples of which are highly nucleophilic and rapidly oxidized by air.[3]
The boiling points of selenols tend to be slightly greater than for thiols. This difference can be attributed to the increased importance of stronger van der Waals bonding for larger atoms. Volatile selenols have highly offensive odors.
Applications and occurrence
Selenols have few commercial applications, being limited by the toxicity of selenium as well as the sensitivity of the Se–H bond. Their conjugate bases, the selenolates, also have limited applications in organic synthesis.
Biochemical role
Selenols are important in certain biological processes. Three enzymes found in mammals contain selenols at their active sites: glutathione peroxidase, iodothyronine deiodinase, and thioredoxin reductase. The selenols in these proteins are part of the essential amino acid selenocysteine.[3] The selenols function as reducing agents to give selenenic acid derivative (RSe–OH), which in turn are re-reduced by thiol-containing enzymes. Methaneselenol (commonly named "methylselenol") (CH
3SeH), which can be produced in vitro by incubating selenomethionine with a bacterial methionine gamma-lyase (METase) enzyme, by biological methylation of selenide ion or in vivo by reduction of methaneseleninic acid (CH
3–Se(=O)–OH), has been invoked to explain the anticancer activity of certain organoselenium compounds.[4][5][6] Precursors of methaneselenol are under active investigation in cancer prevention and therapy. In these studies, methaneselenol is found to be more biologically active than ethaneselenol (CH
3CH
2SeH) or 2-propaneselenol ((CH
3)
2CH(SeH)).[7]
Preparation
Selenols are usually prepared by the reaction of organolithium reagents or Grignard reagents with elemental Se. For example, benzeneselenol is generated by the reaction of phenylmagnesium bromide with selenium followed by acidification:[8]
Another preparative route to selenols involves the alkylation of selenourea, followed by hydrolysis. Selenols are often generated by reduction of diselenides followed by protonation of the resulting selenolate:
- 2 RSe–SeR + 2 Li+
[HB(CH
2CH
3)
3]−
→ 2 RSe−
Li+
+ 2 B(CH
2CH
3)
3 + H
2 - RSe−
Li+
+ HCl → RSeH + LiCl
Dimethyl diselenide can be easily reduced to methaneselenol within cells.[9]
Reactions
Selenols are easily oxidized to diselenides, compounds containing an Se–Se bond. For example, treatment of benzeneselenol with bromine gives diphenyl diselenide.
- 2 C
6H
5SeH + Br
2 → (C
6H
5Se)
2 + 2 HBr
In the presence of base, selenols are readily alkylated to give selenides. This relationship is illustrated by the methylation of methaneselenol to give dimethylselenide.
Safety
Organoselenium compounds (or any selenium compound) are cumulative poisons despite the fact that trace amounts of Se are required for health.[10]
See also
References
- ↑ Tanini, Damiano; Capperucci, Antonella (2021). "Synthesis and Applications of Organic Selenols". Advanced Synthesis & Catalysis 363 (24): 5360–5385. doi:10.1002/adsc.202101147.
- ↑ Lide, David R., ed (2006). CRC Handbook of Chemistry and Physics (87th ed.). Boca Raton, FL: CRC Press. ISBN 0-8493-0487-3.
- ↑ 3.0 3.1 "Selenium in Chemistry and Biochemistry in Comparison to Sulfur". Biological Chemistry 388 (10): 997–1006. 2007. doi:10.1515/BC.2007.138. PMID 17937613.
- ↑ "Methylselenol, a Selenium Metabolite, Plays Common and Different Roles in Cancerous Colon HCT116 Cell and Noncancerous NCM460 Colon Cell Proliferation". Nutrition and Cancer 64 (1): 128–135. 2012. doi:10.1080/01635581.2012.630555. PMID 22171558.
- ↑ "Methylselenol Formed by Spontaneous Methylation of Selenide Is a Superior Selenium Substrate to the Thioredoxin and Glutaredoxin Systems". PLOS ONE 7 (11): e50727. 2012. doi:10.1371/journal.pone.0050727. PMID 23226364. Bibcode: 2012PLoSO...750727F.
- ↑ "New Concepts in Selenium Chemoprevention". Cancer and Metastasis Reviews 21 (3–4): 281–289. 2002. doi:10.1023/a:1021263027659. PMID 12549766.
- ↑ "Cytotoxic and Proapototic Activities of Imidoselenocarbamate Derivatives Are Dependent on the Release of Methylselenol". Chemical Research in Toxicology 25 (11): 2479–2489. 2012. doi:10.1021/tx300306t. PMID 23043559.
- ↑ "Selenophenol". Organic Syntheses 24: 89. 1944. doi:10.15227/orgsyn.024.0089. http://orgsyn.org/demo.aspx?prep=CV3P0771.
- ↑ "Formation of methylselenol, dimethylselenide and dimethyldiselenide in in vitro metabolism models determined by headspace GC-MS". Metallomics 2 (2): 167–173. 2010. doi:10.1039/b914255j. PMID 21069149.
- ↑ "Selenium and human health". The Lancet 379 (9822): 1256–1268. 2012. doi:10.1016/S0140-6736(11)61452-9. PMID 22381456. https://mts.intechopen.com/storage/books/12331/authors_book/authors_book.pdf.
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