1.3- Diselenanes

The metallation of 1,3-diselenanes is complex. When potassium diisopropylamide is used as base, deprotonation and alkylation affords the 2-equatorially substituted derivative <96TL2667>. However, with rertbutyllithium, Se-Li exchange is observed in preference to H-Li exchange in the reaction with 2-ox-methylseleno derivatives <96TL8015>. The reaction with nBuLi either forms the anion or cleaves a C-Se bond depending on the substituents present at the 2-, 4- and 6- positions <96TL8011>. 

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. 

Reich et al. have shown that the equatorial C-Se bond of the 1,3-diselenane derived from ds-3,5-dimethylcyclohexanone can be cleaved kinetically by t-BuLi in THF and the resulting carbanion trapped by chlorotrimethylsilane. This high stereoselectivity was reversed in a sequential experiment. The Seax/Se q isomerization occurred at - 78 °C leading to a 24 1 ratio in favor of the alkylselanyl group in the equatorial position [15] (Scheme 16). The half-life was estimated to be 7 min. 

Diselenanes are formed from 1,2-diselenolanes and dimethyl diazomethanephosphonate... 

Diselenanes (27) are available through the reactions of 1,3-propanediselenols with aldehydes and ketones in the presence of a Lewis acid such as zinc(II) chloride (Equation (3)) <85T4793, 93TL8517>. The diselenols are normally synthesized from the corresponding diselenocyanates by reduction with, for example, sodium borohydride, or with sodium in liquid ammonia <71BSB639>. 

The crystal structure of and two other crystal structures of 1,3-diselenanes 15 and 16 have been reported previously <1991TL4189> however, the... 

Conformations of a variety of 1,3-diselenanes were discussed in detail in CHEC-II(1996), 6.24.2.1.1. The structure of m-4,6-dimethyl-2-phenyl-l,3-diselenane 36 has been determined by X-ray crystallography <1998AXC1505>... 

Conformational effects seem to be important in deprotonation of 1,3 diselenanes <1996TL2667> (see Table 1). 

Detailed investigations of conditions for metallations of 1,3-diselenanes by Krief and co-workers have led to useful procedures for these transformations <1996TL2667, 1996TL8011, 1996TL8015>. 1,3-Diselenanes are metallated by lithium diisopropylamide (LDA) or potassium diisopropylamide (KDA) in tetrahydrofuran (THF) at low temperature. The latter reagent appears to be more efficient, particularly in conformationally rigid systems (see Table 1, entries iii and iv). 

The stereochemistry of the metallation reactions of 2-substituted 1,3-diselenanes has also been investigated <1996TL2667>. An experiment using the deuterated substrate 41 showed that KDA stereoselectively removes the equatorial deuterium despite an unfavorable isotope effect. This has been explained by the strong preference of an unshared pair of electrons to take the equatorial position in cyclohexanes. Methylation of this carbanion introduces the new methyl substituent into an equatorial position (Scheme 4). 

Similar equatorial preferences have been noted in metallation reactions of other substituted 1,3-diselenanes (Table 2). 

Alkylation reactions affording substituted 1,3-diselenanes have been discussed in the previous section. The desired precursor 1,3-diselenanes can be prepared by modifications of standard synthetic procedures (see CHEC-II(1996), 6.24.2.3.2). Treatment of bis-selenocyanates 45 with excess H3PO2 in the presence of aldehydes (or aldehyde equivalents) and ZnCl2 provides a convenient route to the target compounds (Scheme 6) <1996TL2667>. 

Diselenanes have been isolated as by-products in the attempted formation of selenium corrands by reaction of... 

Thiophene-substituted 1,3-diselenanes 192 can be directly prepared by reaction of propane-1,3-diselenol in chlorotrimethylsilane, which appears to promote the reaction <2006RJGC1123>. The detailed mass spectra of these diselenanes are also reported. 

It has been indicated by Rusakov et al that the experimental Vnse couplings measured for sterically crowded phenylselanylalkenes can be applied in configurational analysis of these compounds only in combination with detailed theoretical calculations. As follows from the analysis performed by the authors, the typical Karplus dependence of Juse be strongly afiected by the orientation of the terminal substituent bearing selenium atom. Another paper has been devoted by these authors to stereochemical behaviour of Vnse > HSe couplings in five pyrazolyl-1,-3-diselenanes and 1,2-diselenolane. 

The conversion of active-methylene compounds into 2-alkylidene derivatives of 1,3-dithiolans by base-catalysed condensation with carbon disulphide followed by treatment with 1,2-dihalogenoalkanes has been further exemplified, and extended to the synthesis of substituted alkyl-idene derivatives of 1,3-diselenolans and 1,3-diselenans. With dimethyl malonate, carbon diselenide, and either 1,2-dibromoethane or 1,3-dibromo-propane, for example, the compounds (142) and (143), respectively, were formed. Analogously, the dithiolate anion obtained from deoxybenzoin... 

The relationship between spectroscopic properties and conformation and configuration of 1,3-dithians and 1,3-dithiolans continues to attract interest, and some recent reviews contain relevant information. - Anteunis and his associates have shown that the geminal coupling constant at C(2) in 1,3-dithians and 1,3-diselenans can be predicted by an equation which takes into account the electronegativity of the heteroatoms, the bond lengths... 

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