Dimerization of Cubene. 1-Iodoadamantane as a Probe for Radical Intermediates

A Boron Alkylidene–Alkene Cycloaddition Reaction: Application to the Synthesis of Aphanamal

“…[13] While these experiments suggest that the boron-stabilized carbanion does not have ap roclivity to react by single-electron-transfer pathways,t he concerted carbometalation pathway by which hexenyllithium [7a] undergoes cyclization does not appear to operate.C yclization/ iodination of 33 (Figure 2c)l eads to a1 :1 mixture of deuterium-labeled diastereomers 34 (epimeric at the iodination site). Assuming that iodination of the ate complex is stereospecific, [8a] this observation suggests that the cycloaddition is an on-concerted stepwise process,p roceeding through an intermediate capable of bond rotation prior to closure to the boretane.W ec onsidered that at hermally accessible open-shell structure [14] such as 35-formed by aprocess analogous to that proposed for the dimerization of strained olefins [15] and for allene-yne intramolecular [2+ +2] reactions [16] -and closed-shell system 36 (Figure 2c,i nset) were most plausible intermediates;h owever,b oth raise questions.T he radical-based pathway is not supported by the experiments in Figure 2b,w hile addition of as tabilized carbanion to an unactivated alkene appears to have little precedent. [17] Nonetheless,t he capacity for stereochemical scrambling at the reactive olefin site extends to other systems: cyclization/alkylation of the cis and trans substrates 37 and 21 ( Figure 2d)o ccurs with stereoconvergence-the intermediate "ate" complex is presumed to undergo stereoinvertive S E 2 alkylation [8] via 39 to furnish 38 in good yield and stereoselectivity.…”

“…Assuming iodination of the ate complex is stereospecific [8a] , this observation suggests that the cycloaddition is a non-concerted step-wise process, proceeding through an intermediate capable of bond rotation prior to closure to the boretane. We considered that a thermally-accessible open shell structure [14] such as 35 – formed by a process analogous to that proposed for dimerization of strained olefins [15] and for alleneyne intramolecular 2+2 reactions [16] – and closed-shell system 36 (see inset, Figure 2C) were most plausible intermediates; however, both raise questions. The radical-based pathway is not supported by experiments in Figure 2B, while addition of a stabilized carbanion to an unactivated alkene appears to have little precedent.…”

A Boron Alkylidene–Alkene Cycloaddition Reaction: Application to the Synthesis of Aphanamal

“…Assuming that iodination of the ate complex is stereospecific, this observation suggests that the cycloaddition is a non‐concerted stepwise process, proceeding through an intermediate capable of bond rotation prior to closure to the boretane. We considered that a thermally accessible open‐shell structure such as 35 —formed by a process analogous to that proposed for the dimerization of strained olefins and for allene–yne intramolecular [2+2] reactions—and closed‐shell system 36 (Figure c, inset) were most plausible intermediates; however, both raise questions. The radical‐based pathway is not supported by the experiments in Figure b, while addition of a stabilized carbanion to an unactivated alkene appears to have little precedent .…”

Effect of strain on gas-phase basicity of (E )-1-methyl-2-(1-methyl-2-adamantylidene)adamantane

“…[1] In addition, deviations of the usual planar geometry can occur, [2] as was observed in many olefins. [3][4][5][6][7][8][9][10] As these effects may have significant impact on alkene reactivity, the phenomenon of steric strain deserves serious theoretical and experimental concern. [3,10,11] There are strained alkenes that still retain planar geometry of their double bond (for example cyclopropene).…”