Selenophenes metallation

Photochemical reactions of selenophene-metal complexes (Section 3.13.5.6) and photocycloadditions of selenophenes (Section 3.13.5.9) are treated later in this subsection. The photolysis of gaseous selenophene (and also tellurophene) has been studied <2000JOC2759>. Interestingly, the cleavage of both Se-C carbons gave elemental Se in the process. 

Selenophene, 2,5-diamino-3,4-dicyano-synthesis, 4, 119, 964 Selenophene, 2,4-diaryl-synthesis, 4, 967 UV spectra, 4, 941 Selenophene, 2,5-diaryl-synthesis, 4, 967, 969 UV spectra, 4, 941 Selenophene, 2,5-dichloro-metallation, 4, 949 Selenophene, dihydro-3-methylene-synthesis, 4, 963 Selenophene, 2,5-dimethoxy-lithiation, 4, 949 Selenophene, 2,4-dimethyl-... 

The 5-lithium derivative (90) obtained upon metalation of 2-tert-butyl-selenophene ether (89) was treated either with alkyl halides or with carbonyl derivatives (Scheme 8). 

In this section the reactions of selenophenes and tellurophenes with metal alkoxides, alkyls, aryls and amides will be considered. These reactions result primarily in proton abstraction and, in the monocyclic and benzo[6] fused systems, protons are removed preferentially from positions a to the heteroatom. In conformity with the general format of the reactivity sections in this work, the reactivity of the metal derivatives formed is dealt with in Section 3.16.8.8 on metalloid substituents. 

Arbuzov and Kataev40 obtained selenophene and its methyl homologs from dienes (butadiene, piperylene, or hexadiene) and metallic selenium at 380°-420°C. 

Yur ev and Magdesieva41 found a very convenient procedure to obtain selenophene and its homologs by the reaction of butylenes with metallic selenium at 580°C. The selenophene ring closure is most favored when the four-carbon chain of the initial hydrocarbon has a central double bond and the doubly bonded carbons carry as many substituents as possible. 

No /3-monohalogenated selenophenes are obtained by direct halogenation. /3-Bromoselenophene is readily obtained from 2,3,5-tribromoselenophene by debromination with zinc in acetic acid this procedure is based on the different debromination (metallation) rates of a- and j9-bromoselenophenes.56... 

Metallation followed by carbonation of 3-bromoselenophene yields, depending on the reagents and temperature,56 either 3-bromoseleno-phene-2-carboxylic acid (phenyllithium, 36°C) or selenophene-3-car-boxylic acid (butyllithium, — 50°C). When treated with butyllithium, the acetal of selenophene-2-aldehyde is metallated at the free a position subsequent carboxylation leads to 2-formyIselenophene-5-carboxylic acid.76... 

The experimental evidence thus indicates that selenophene is metallated in much the same manner as thiophene.82,83... 

Selenophene /3-diketones can be used as extractants for the separation and isolation of metals. The advantages of selenophene /3-dike-tones were revealed by comparison of their dissociation and distribution constants with those of acetylacetone, benzoylacetone, thenoyl-trifluoromethylacetone, etc. Selenophene /3-diketones containing a trifluoromethyl group and a pyridyl radical were of particular interest. 2-Acetoacetylselenophene127 is better for the extraction of thorium from water than acetylacetone, previously extensively used. 

Selenophene /3-diketones containing a-, /3-, or y-pyridyl radicals can be used for extraction of neodymium the extraction percentage is approximately 90%. The most practical is w-picolinoyl-2-aceto-selenophene which gives a high percentage of metal extraction at a... 

Removal of thiophene impurities from petroleum feedstocks is accomplished by a process called hydrodesulfurization (HDS) which involves the insertion of metals into the thiophene ring between the C-S bond. In order to better understand the mechanism of this reaction, different groups have utilized selenophene model systems due to the enhanced NMR characteristics of Se. Metal complexes of selenophenes that have been studied include rhodium <19970M2751>, molybdenum <2006POL499>, manganese <20010M3617, 19950M332>, chromium... 

A convenient one-step synthesis of fused selenophenes has been developed <1994H(38)143, 1996T471>. For example, heating ynediol 106 in the presence of selenium metal gave selenolo[3,2- ]selenophene 107 (Equation 16). 

Metal-mediated category llae cyclizations comprise the next group of selenophene syntheses to be discussed. Dibenzoselenophenes (and dibenzotellurophenes) have been prepared from 2,2 -diiodobiphenyl derivatives <1995JOC5274>. Treatment of the biphenyl 108 with selenium-copper slurry, generated from disodium diselenide and copper(l) iodide, produced 3,7-dinitrodibenzoselenophene 109 (Equation 17). 

An jj (S) equilibrium (equation 6) is observed in the benzo[fc]thiophene (BT) complexes Cp (CO)2Re(BT), where Cp is Cp or Cp. As for selenophene, the Cp ligand favors j -BT, while Cp favors the rj (S) form it should be noted that 2-MeBT and 3-MeBT form only the jj (S) complexes. The formation of the r] form from the is important for understanding the first step in the HDS of BT, which gives dihydrobenzothiophene (equation 7). This alkene hydrogenation reaction is catalyzed by homogeneous catalysts and may occur similarly on heterogeneous HDS catalysts. While there is no evidence that there is an r] (S) equilibrium in thiophene complexes, both thiophene species may exist on a catalyst surface. Low metal oxidation states would favor r] coordination, which could lead to the initially observed alkene hydrogenation products. 

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