Lithium norcaranylidenoids. Alkylation and epimerization

Reactivity of 13,13-Dibromo-2,4,9,11-tetraoxadispiro[5.0.5.1]tridecane toward Organolithiums: Remarkable Resistance to the DMS Rearrangement

“…This indicates again that the rate for LiBr elimination from 23a to form carbene 22a is significantly lower than those for the competing processes, including a reaction of the anion with PhLi ( k 6a ≫ k 2a , Scheme ). It may appear that the observed product 10a−Li is formed according to a metal-assisted ionization (MAI) of the C−Br bond in the anion 23a as it was postulated for vinyl α-haloanions and later for some cyclopropyl α-haloanions. , The computational results show however, that there is no significant loosening of the C−Br bond in the dioxane derivative 23a as postulated in MAI and experimentally observed for some carbenoids . Instead, the C−Br bond in 23a is shorter by 0.15 Å than that in the cyclohexane analogue 23b .…”

“…In rare cases, the α-elimination step is slow and the anion is alkylated by an alkyl halide generated in a lithium−halogen exchange reaction to form VI in quantities ranging from 2 to 13%. – An electron-withdrawing heteroatom is usually present in these cases, and the alkylation has been attributed to the stability of the bromolithium intermediate IV resulting from intramolecular coordination with oxygen ,, or sulfur . In other cases, an alkyl halide was added to the reaction mixture at temperatures <−70 °C to avoid the DMS rearrangement and to obtain the desired alkylation product. , …”

“…20 In other cases, an alkyl halide was added to the reaction mixture at temperatures <-70 °C to avoid the DMS rearrangement and to obtain the desired alkylation product. 25,26 In the course of our work on functionalized spirocyclic systems, we focused on protected tertakis(hydroxymethyl)allene 1a and envisioned its formation by the DMS rearrangement of the corresponding dibromocyclopropane 2a (Scheme 1). This expectation was based on the reported efficient rearrangements of the cyclopropyl 27 and cyclobutyl 28 analogues of 2a to the corresponding allenes.…”

The Carbenoid Dihalotriangulane Rearrangement: A Mechanistic Mystery

“…For example, reactions quenched by the addition of methanol-d led to the generation of cyclopropane products containing X-C-D moieties. 14,29 This means that organolithium species are sufficiently stable. However, this observation could signify that the reactions of these intermediates are slow and/or that improper stoichiometric amounts have been employed leading to shortages of the reagents involved in alkyllithium consumption.…”

Substituent Effects on the Ring-Opening Mechanism of Lithium Bromocyclopropylidenoids to Allenes

“…Occasionally, the formation of allenes ( 5 ) from gem -dibromocyclopropanes ( 1 ) competes with the formation of intramolecular CH-insertion products, usually bicyclobutanes ( 4 ), obtained in quantities ranging from traces to exclusive products. − The result of this competition is usually determined by the nature of substituents, − conformational change in the skeleton, , and electronic effects . Bicyclobutanes are obtained when allene formation is difficult, for example, with bicyclo[4.1.0]heptan-7-ylidene affording 7 and 8 , , with tetrasubstituted cyclopropylidenes due to steric crowding, ,, or with polycyclic cyclopropylidenes in order to avoid the generation of a bridgehead allene. , However, it is interesting to note that the Doering−Moore−Skattebøl route does succeed for the methoxy derivative 9 of 7,7-dibromobicylo[4.1.0]heptane. Compound 11 , presumably formed via dimerization of 1-methoxycylohepta-1,2-diene ( 10 ), was isolated in 85% yield (Scheme ) …”