REACTIVITY OF BICYCLIC N-PYRROLYLBORANES

Bicyclic B-alkyl-N-pyrrolylboranes (1-3) react with alkyl lithium or a lkyl Grignard reagents to give the corresponding berates 5 which, in m ost cases can be protonated to the intramolecular 2 H-pyrrole-borane a dducts 4. The molecular structure of 4d was determined by X-Ray struct ural analysis. The adducts 4 can be deprotonated to the berates 5. Cle avage of the B-N bond in 1a by EtOH to give 7a is reversible, and le r eact with CF3SO3H by protonation of the pyrrole ring. The 2H-pyrrole-b orane adducts 4a,d react with CF3SO3H by cleavage of the B-N bond to g ive trialkylboranes as the 2H-pyrrolium salts 8a,d. Cyclopentadiene re acts with the 2H-pyrrole borane adduct 4d selectively by [4 + 2]cycloa ddition to give 10 with endo-configuration. The boranes 1a and 2a reac t stereoselectively with mono-1-alkynyltin compounds in a 1:1 stoichio metry by an 1,1-organoboration to give the organometallic substituted alkenes 11-13, in which the six-membered ring present in 1a and 2a is retained. In contrast, 3a reacts under the same conditions exclusively by ring enlargement and in a 1:2 stoichiometry. The product from the reaction of 3a with two equivalents of trimethyl(1-propynyl)tin is the 1,3-butadiene derivative 14 which rearranges selectively to its isome r 15 by changing the configuration at both double bonds. The reaction of 3a with two equivalents of bis(trimethylstannyl)ethyne leads select ively to the organometallic substituted allene 16, the result of an ir reversible allylic rearrangement of a 1,3-butadiene derivative analogo us to 14 or 15. All products were characterized by H-1, B-11, C-13, N- 14 and Sn-119 NMR spectroscopy. (C) 1997 Elsevier Science S.A.