The Metallation‐Elimination Reaction, II. Monocyclic Anions and Polyanions

Abstract

Equimolar mixtures of n-butyllithium and potassium tert-amyloxide effect metallation and metal hydride elimination of cyclic olefins directly to give conjugated anions and polyanions in a single operation. 1-Methyl-1-cycloalkenes or methylenecycloalkanes are particularly well suited and give highly unsaturated products. Odd-membered rings eliminate better than their even-membered counterparts, and eight-membered rings eliminate particularly slowly. Reactions with methyl iodide and trimethylsilyl chloride proceed smoothly in high yield. Die Metallierungs-Eliminierungs-Reaktion, I1 l) Monocyclische Anionen und PolyanionenAquimolare Mischungen von n-Butyllithium und Kalium-tert-amyloxid bewirken Metallierung und Metallhydrid-Eliminierung an cyclischen Olefinen und fiihren direkt in einer Eintopf-Reaktion zu konjugierten Anionen und Polyanionen. 1-Methyl-1-cycloalkene oder Methylencycloalkane sind besonders gut geeignet und geben stark ungesattigte Produkte. Ringe mit ungerader Kohlenstoffzahl eliminieren besser als die geradzahligen. Achtgliedrige Ringe eliminieren extrem langsam. Reaktionen mit Methyliodid und Trimethylsilylchlorid verlaufen glatt und in guter Ausbeute.Mixtures of n-butyllithium (BuLi) with potassium tertiary alkoxides are highly active metallation reagents. Lochmann's base (BuLi/KOtBu) a) has been used extensively to generate anions and dianions2-'), but other potassium tertiary alkoxides such as potassium menthoxide') or potassium tert-amyloxide") have the advantage .of solubility in hydrocarbon solvents. In the first paper of this series") a BuLi/potassium tert-amyloxide (BuLi/KOtAm) mixture was shown to be effective for the catalysis of metal hydride eliminations, which normally require higher temperat~res'~-l') or are carried out using hydride acceptor^'^-^^). By combining the metallation and elimination reactions we were able to prepare the heptafulvene dianion") (l), which is unaccessible by direct metallation (BuLi adds to cycloheptatrienes). Bates' calculations 26) indicate 1 to be one of the least stable dianions which have been prepared.Path A involves a dianion intermediate formed directly by deprotonation of a preformed ally1 anion. Path B is more conventional and involves a neutral diene inter- Chem. Ber. 126(1983) The Metallation-Elimination Reaction, I1

Limonene

“…disiamylborane and limonene with acetic acid at 100 "C gives completely racemized menth-1 -ene.lH3 A preliminary note by Brown and Pfaffenberger in 1967IH4 was followed by a rather different full paper in 1975l"j in which the preparation of the cyclic boron compounds (67) and ( 68) is described. The diols (69) were obtained from (67) (the stereochemistry at C-8 again not being specified) while 83 YO of D-( -)-( 1 R,2R,4R)-carvornenthol (70) was obtained from (68), this being the most efficient way of making a single carvomenthol from limonene. Hydroboration of ( + )-limonene with BH,Cl .OEt, and then reduction with lithium aluminium hydride gives a borane (71) that is said to be useful for asymmetric induction in the hydroboration of alkenes.1N6 Acetoxyborohydride is made from NaBH, and Hg(OAc), in tetrahydrof~ran.'~'?…”