Dimerization of bridgehead olefins formed from tricyclic cyclopropylidenes

Formation of Olefins by Eliminative Dimerization and Eliminative Cross‐Coupling of Carbenoids: A Stereochemical Exercise

“…Eliminative dimerization of an a-bromoalkyllithium generated by Skattebøl rearrangement of avinylcyclopropylidene-type carbenoid. [52] Angewandte Chemie Reviews workers were the first to explore this possibility and discovered that the eliminative dimerization of lithiated epoxides has elements of both stereoselectivity and stereospecificity. [35] Tin-lithium exchange from stannane (AE)-(E)-44 as indicated led, after acetylation, to dl-(E)-45 (with a4:1 preference for the illustrated dl form over the meso form) and meso-(Z)-45 in a3 :2 E/Z ratio (Scheme 8).…”

“…[51] In an interesting example of eliminative dimerization involving an apparent Skattebøl rearrangement, Warner et al observed the formation of the single dimeric product dl-(E)-38 upon treatment of racemic dibromopropellane (AE)-37 with methyllithium (Scheme 7). [52] Al ikely explanation for this finding would involve isomerization of the Br-Li exchange product 39 to the corresponding bicyclo-[2.2.1]heptene 40 through aS kattebøl rearrangement. The C 2 symmetry and chiral nature of the product informs us that it must come about from ah omochiral pairing of two molecules of 40.I tc an be further deduced, if one assumes that dimerization follows the usual mechanism, that the elimination mode was syn because the alkene has an E configuration.…”

“…The exo disposition of the alkoxide moiety in 40 was proposed to play ac ritical role in favoring the homochiral pairing because it was found that exposure of the C3 epimer of 37 to otherwise identical reaction conditions led to ac omplex mixture of products that included two (and not one) binorbornenylidene isomers. [52] 3.2. Eliminative Dimerization of a-Lithiooxiranes a-Metalated oxiranes [53] can be generated by ar ange of methods,i ncluding direct lithiation [20,43,[54][55][56] and various metal-metaloid exchange processes,f or example,t in-lithium [35] and sulfoxide-metal [18b,19] exchange.L ithiated epoxides are chemically and configurationally stable for short time periods at low temperature (typically < À90 8 8C) and the requisite precursors are easily obtained in enantiopure form.…”

“…40!38), it is generated in the presence of ah igh concentration of its precursor 82 and this setting favors eliminative cross-coupling of (racemic) 82 with (achiral) 83 to give the observed alkene products 80 and 81.T wo distinct regioisomeric pre-elimination adducts (84 and 84' ')may arise from rearrangement of the ate complex, either of which may undergo syn elimination to give major product 80 or anti elimination to give minor product 81.T he type of stereochemical pairing is moot in this example,b ecause one of the carbenoid coupling partners is achiral, and the observed product distribution suggests that the dominant mode of elimination is syn,a ssuming the canonical eliminative crosscoupling mechanism (Scheme 2). This conclusion is consistent with the result of the eliminative dimerization of the related carbenoid 40 (Scheme 7), [52] but with the caveat that both processes may actually involve an oncanonical mechanism (see Section 5).…”

“…Interesting cases in point are eliminative dimerization and eliminative cross-couplings involving 7-halo-7lithiobicyclo[2.2.1]hept-2-enes such as 40 and 83.T he true nature of norbornenyl carbenoids 40 and 83 is undoubtedly more complex than the simple covalent depictions shown in Schemes 7a nd 15 imply. [52,64] Thep henomenon of metalassisted ionization [76] and the proximity of an ucleophilic p CC bond to the carbenoid center means that ion-pair formulations are conceivable and several have been suggested in the literature for 83:f or example,c lassical carbocation 83 a, [51c, 64a] nonclassical carbocation 83 b, [2e,51c, 64a] and nonclassical carbocation 83 c (Figure 3). [77] Given this potential characterization of carbenoids 40 and 83,itispossible that eliminative coupling reactions involving them could proceed through net stereoretentive metalate rearrangement steps rather than the more typical stereoinvertive pathways.I ns uch as cenario,t he aforementioned observed major products, dl-(E)-38 (Scheme 7) [52] and 80 (Scheme 15), [64a] would arise from concluding anti-rather than syn-elimination steps.This alternative hypothesis is illustrated in Scheme 22 for the formation of bicyclopropylidene 80 from carbenoids 82 and 83.T hus,s tereoretentive displacement of the halide atom in 83 by nucleophilic attack from coupling partner 82 is envisioned to occur with anchimeric assistance from the p CC bond to afford pre-elimination adduct 125 via ate complex 124.Subsequent anti elimination from 125 would lead to the observed major product 80,aconclusion favored by the original authors,W arner and Herold.…”