Reactivity of bicyclic N-pyrrolylboranes

Abstract

Bicyclic B-alky- N-pyrrolylboranes ( 1–3) react with alkyl lithium or alkyl Grignard reagents to give the corresponding borates 5 which, in most cases can be protonated to the intramolecular 2 H-pyrrole-borane adducts 4. The molecular structure of 4d was determined by X-Ray structural analysis. The adducts 4 can be deprotonated to the borates 5. Cleavage of the B-N bond in 1a by EtOH to give 7a is reversible, and 1e react with CF 3SO 3H by protonation of the pyrrole ring. The 2 H-pyrrole-borane adducts 4a,d react with CF 3SO 3H by cleavage of the B-N bond to give trialkylboranes as the 2 H-pyrrolium salts 8a,d. Cyclopentadiene reacts with the 2 H-pyrrole borane adduct 4d selectively by [4 + 2]cycloaddition to give 10 with endo-configuration. The boranes 1a and 2a react stereoselectively with mono-1-alkynyltin compounds in a 1:1 stoichiometry 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(l-propynyl)tin is the 1,3-butadiene derivative 14 which rearranges selectively to its isomer 15 by changing the configuration at both double bonds. The reaction of 3a with two equivalents of bis(trimethylstannyl)ethyne leads selectively to the organometallic substituted allene 16, the result of an irreversible allylic rearrangement of a 1,3-butadiene derivative analogous to 14 or 15. AH products were characterized by 1H, 11B, 13C, 14N and 119Sn NMR spectroscopy.