Air-Initiated Radical Polymerization of Lithium Salts of ω-(Undecamethylcarba-closo-dodecaboran-1‘-yl)alk-1-enes, CH₂CH(CH₂)_n-2C(BMe)₁₁^- Li⁺

Abstract

We report an easy access to the salts of the LiC(BMe)11- anion, which greatly simplifies the synthesis of compounds carrying the -C(BMe)11- substituent, including the title anions. The previously recognized and puzzling spontaneous oligomerization of the solid lithium salts CH2=CH(CH2)(n-2)C(BMe)11- Li+ upon storage under ambient conditions is now shown to proceed by a radical mechanism, with the "naked" Li+ cation acting as a catalyst. The degree of polymerization is higher in solution, especially when azoisobutyronitrile (AIBN) is used as initiator (up to approximately 50). Initiation by the thermal decomposition of AIBN is also catalyzed by naked Li+, and this initiator is effective at room temperature. Di-tert-butyl peroxide and UV irradiation can also be used. The observation of Li+ catalysis agrees with a prior prediction from ab initio calculations, according to which Li+ complexation of ethylene strongly lowers the activation energy for methyl radical addition. The results bear on the current discussion of the possible sensitivity of radical clocks to their molecular environment and suggest that naked Li+ will catalyze the radical polymerization of simple terminal alkenes.

The Effect of a Complexed Lithium Cation on a Norcarane‐Based Radical Clock

“…However, for a ring-closure-type radical clock, we [19] were able to show that complexation of the double bond of the intermediate hex-1-en-6-yl radical [20] to a lithium cation decreases the calculated activation barrier for the ring-closing rearrangement significantly. This effect is supported indirectly by recent work of Michl and co-workers, [21,22] who showed that "naked" lithium cations have a catalyzing effect on the radical polymerization of terminal olefins, in accord with a proposal made 20 years ago on the basis of ab initio calculations [23] and a recent more-detailed study at higher theoretical levels. [24] However, all these observations pertain to radicals that contain a double bond, which can complex the lithium cation moderately well.…”

Chemistry of the Carba-closo-dodecaborate(−) Anion, CB11H12-

“…48,164 Typical examples of introduction of substituents into the CB 11 H 12 -cage using electrophilic reagents are shown in Schemes 6 and 7. 15 The mechanism of these substitutions has not been studied in detail.…”

“…The undecamethylated 1-iodo (222) and 1-bromo (221) anions are available 48 and have been converted with n-butyllithium or tert-butyllithium to the 1-lithio derivative of 12. 164 The lifetime of this extremely strong base in THF is limited to a few hours but is sufficient for ω-alkenylation with CH 2 dCH-(CH 2 ) n-2 -OTos (n ) 3-7) 164 and presumably for other electrophilic substitution reactions. Unfortunately, among the 1-halogenated (7-12)-hexamethylated anions only the chloro and fluoro derivatives are easily accessible, 48 and they do not undergo halogen-metal exchange, so this access to the deprotonated 11 currently remains blocked.…”

“…37 The initial hope that the 1-halogenated anions will serve as useful substrates for palladium-catalyzed coupling has not been fulfilled, and the unique synthetic value of the iodo and bromo derivatives is to provide access to deprotonated 12. 164 Addition Reactions. Addition of the deprotonated carbon vertex to double bonds and rings has been investigated much less.…”

Ethynylmonocarba-closo-dodecaborates: M[12-HCC-closo-1-CB11H11] and M[7,12-(HCC)2-closo-1-CB11H10] (M=Cs+, [Et4N]+)

“…Functionalized monocarba-closo-dodecaborate anions are used as building blocks in a variety of applications [1][2][3][4][5], for example in ionic liquids [6,7] catalysis [8][9][10][11][12][13][14][15], pharmaceuticals [16,17] and polymers [18,19]. A consequence of the constantly growing demand for functionalized monocarba-closo-dodecaborate anions is the increasing search for efficient procedures for the derivatization of the {closo-1-CB 11 } cage [1][2][3][4][5].…”