The reductive alkylation of the pyrene isomer 1 affords vided by cyclic voltammetric measurement of rate constants bridged [14]annulenes (2) and 2a,3-dihydro-2a,3-dialkyldicy-for both the ET process and the SN2 reaction. The choice of clopenta[ef,kl]heptalenes (3). The involvement of an electron the electrophile is relevant not only for the importance of electransfer (ET) in this reaction is demonstrated by optically pure tron transfer, but also for the regioselectivity of the alkylation electrophiles. Supporting evidence for ET processes is pro-reaction.There is a growing body of information that many nucleophilic substitutions which have been supposed to proceed according to a polar SN1 or s$! mechanism have some characteristics of an electron transfer (ET)[2-141. Experimental evidence for the occurrence of radical intermediates in substitution reactions stems from the application of radical q~enchers"~] or from ESR[I6] and CIDNP[171 measurements. The competition between ET and SN mechanisms is of special relevance for those cases in which it influences the product distribution of the nucleophilic substitution. This can be the case e.g. in the alkylation of carbanions ['81.Most of these studies, however, have been performed with monocharged carbanions. The alkylation of doubly charged carbanions, on the other hand, constitutes a particularly challenging example since (i) one can compare the alkylation of the dianion with that of the resulting monoalkylated monoanion; (ii) the application of enantiomerically pure alkylation reagents as electrophiles can give rise to the formation of diastereomers; (iii) the use of different alkylating agents in the first and second alkylation step allows us to compare the regioselectivity of both processes.As we have reported earlier, the reductive alkylation of the pyrene isomer l[I9] affords the 1,4,8,11 -ethanediylidene-[14]annulene 2 with substituents in the central positions C-15 and C-16[*01. The reaction proceeds by a two-electron transfer with the formation of the dianion and the kinetically controlled quenching reaction of the latter. In this paper, we investigate the dependence of this alkylation reaction on the nature of the electrophile and on the ion pairing of the charged intermediate. Crucial questions in the mechanistic description of the alkylation are the role of an ET process and its influence on the regioselectivity of the overall substitution reaction.
ResultsThe dianion salt of 1 may be obtained by both chemical and electrochemical reduction. In a cyclic voltammetric experiment (DMF, TBABF4) 1 shows two reversible one-electron reductions with EY = -0.875 V and E ; = -1.748 V (versus Ag/AgI).In a typical alkylation experiment the dianion 1'-was prepared as a lithium salt by contacting a lop2 M solution of 1 in dry degassed THF with highly active lithium wire at -30°C. When dianion formation was complete the deep green solution was treated at ambient temperature with an excess of pure dimethylsulfate to obtain an 88% yield of trans-1 5,16-dimethyl-1,4,8,1 l-e...
