Ethylene derivatives preferential formation

Without additional reagents Acetoxymonodrganomercury acetates from ethylene derivatives Preferential formation... 

Reactions with 9-borabicyclo[3.3.1 Jnonane Aldehydes from ethylene derivatives Synthesis with addition of 1-C-atom Preferential formation... 

Preferential formation of 1,2-dibromides from ethylene derivs. 

Preferential formation of 1,2-dihalides, also mixed — from ethylene derivatives, and reverse reaction... 

Lithium perchlorate/calcium carbonate Ring expansion of isocyclic diol monotosylates with preceding preferential formation of glycols from ethylene derivatives and preferential O-tosylation... 

Unsubstituted 20-ketones undergo exchange dioxolanation nearly with the same ease as saturated 3-ketones although preferential ketalization at C-3 can be achieved under these conditions. " 20,20-Cycloethylenedioxy derivatives are readily prepared by acid-catalyzed reaction with ethylene glycol. The presence of a 12-ketone inhibits formation of 20-ketals. Selective removal of 20-ketals in the presence of a 3-ketal is effected with boron trifluoride at room temperature. Hemithioketals and thioketals " are obtained by conventional procedures. However, the 20-thioketal does not form under mild conditions (dilution technique). ... 

In contrast to the reaction carried under an Ar atm, when (S)-49 was treated with 10 mol% la under an atmosphere of ethylene (22°C, CH2C12,24 h), (S)-41 was obtained in 81% isolated yield and >98% ee (Scheme 12). As expected, the use of ethylene atmosphere proved to be necessary for preferential monomer formation (10% of the derived dimer was also generated). These results indicate that the presence of ethylene is imperative for efficient metal-catalyzed chromene formation as well for processes involving disubstituted styrenyl ethers (25-30% yield of dimer 50 under argon). 

Addition of ethylene to an allylic anion is not involved since 1- and 2-butene would give rise to a common intermediate and identical products would result. Preferential metalation of the terminal vinylic position of 1-butene also seems unlikely since products derived from vinylic lithium intermediates are not obtained with isobutylene. To explain the product structure, ethylene insertion into the allylic carbon-lithium bond may possibly involve both a four-centered and six-centered transition state (Reactions 11a and lib) with the latter predominating in butene-2. The exclusive formation of 2-methylpentene-l and its homologs from the telomerization with isobutylene is easily understood since this product would result from both reaction pathways. 

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