Benzo selenophene, deprotonation

The first proton to be removed from N-methylpyrrole by n-butyllithium is from an a-position a second deprotonation occurs to give a mixture of 2,4- and 2,5-dilithiated derivatives. The formation of a 2,4-dilithio derivative is noteworthy since in the case of both furan and thiophene initial abstraction of a proton at C-2 is followed by proton abstraction from C-5 (77JCS(P1)887). Af-Methylindole, benzo[ >]furan and benzo[ >]thiophene are also deprotonated at C-2. Selenophene and benzo[ >]selenophene and tellurophene and benzo[6]tellurophene similarly yield 2-lithio derivatives (77AHC(2l)ll9). 

The reactions of the lithio derivatives of benzo[ >]-fused systems indole, benzo[6]furan and benzo[h]thiophene are similarly diverse. Since indole and benzo[h]thiophene undergo electrophilic substitution mainly in the 3-position, the ready availability of 2-lithio derivatives by deprotonation with n-butyllithium is particularly significant and makes available a wide range of otherwise inaccessible compounds. The ready availability of 3-iodoselenophene and hence of 3-lithioselenophene (73CHE845) provides a convenient route to 3-substituted selenophenes. 2-Lithiotellurophenes are especially important precursors of tellurophene derivatives because of the restricted range of electrophilic substitution reactions which are possible on tellurophenes (77AHC(2l)ll9). 

An inorganic selenium electrophile, selenium oxychloride (SeOCl2), has been utilized to prepare fused selenophenes <2002JOC2453>. Deprotonation of bis(cyanide) 104 with LDA followed by treatment with selenium oxychloride gave the benzo[c]selenophene 105 (Equation 15). This sequence also allowed for the preparation of a thieno[3,4-f]selenophene. 

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