Stereochemistry of Metallacycle Formation in the Double Alkylation of Bis(triphenylphosphine)nitrogen(1+) Bis(η5-cyclopentadienyl)bis(μ-carbonyl)dicobaltate with α,γ-Diiodoalkanes

Article abstract of DOI:10.1021/ja00357a015

Alkylation of the dinuclear cobalt radical anion 4 with 1,3-diiodopentane yields three new matallacycles: a methyldicobaltacyclopentane (11) and cis and trans isomers of an α,α'-dimethyldicobaltacyclopentane (6c and 6t).Alkylation of 4 with either of the diastereomeric 2,4-diiodopentanes meso-7 and dl-7 yields identical ratios of 6c and 6t.Control studies demonstrate that neither starting diiodide isomerization nor product isomerization can account for this product mixture.This is evidence that at least one of the two alkylation steps proceeds via electron transfer.In contrast, alkylation of 4 with a mixture of (2R,3R)- and (2S,3S)-1,3-diiodobutane-2-d (syn-12-d) proceeds stereoselectively: a 1:5 ratio of syn/anti metallacycles 11-d is formed.We conclude that the first (intermolecular) step in the double alkylation proceeds by initial electron transfer while the second (cyclization) step proceeds predominantly by S_N2 displacement or by a partly stereospecific electron transfer process of the type recently postulated in certain photosolvolysis reactions.