Heteroatom-substituted lithium organics

FIGURE 36. Solid-state stmctures of heteroatom-substituted lithium organics 

The dianionic methylene diimidosulfites CR2S(NR)2 can be regarded to be the carba/imido analogues of Thus, they are sulfur(IV) ylides. Like Wittig 

Two RC-metalated compounds were discussed earlier in Section II.B, because [(THF)LiC=CBu-f]4 (21) and [(TMHDA)2(LiC=CPh)4]oo ° (22) form donor-base-coordinated tetramers. In dimeric [(TMPDA)LiC=CPh]2 (245), a Li2C2 four-membered 

FIGURE 37. Solid-state structures of lithiated R2C and RC species 

FIGURE 39. Solid-state stmctures of lithium organic co-complexes that show retention of the Lis C motif 

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The field of R3C lithium organic structures is wide and we wiU concentrate on those with alkyl-, aryl- and silyl-substituted anions. Heteroatom-snbstitnted lithinm organics will just be mentioned briefly. 

Other organic redox shuttles based on aromatic compoimds with heteroatom substitutions include phenothiazine [18], triphenylamine [89], diarylamines with different substitutions [38], and 2-(pentafluorophenyl)-tetrafluoro-l,3,2-benzodi-oxaborole [29, 137]. Nitroxide radicals such as (2,2,6,6-Tetramethylpiperidin-l-yl) oxy (TEMPO) have been studied as a redox shuttle as well but showed inferior rate of charge transfer compared with DDB [90]. In addition, lithium borate cluster salts (Li2Bi2Fi2) have also been reported to be suitable redox shuttle additives for 4-V lithium ion chemistry (Chen et al. [26]). 

The importance of carbanions a-substituted by heteroatoms in organic synthesis explains the vast amount of literature concerning the use of a-heterosubstituted organotins in transmetallation reactions. As the tin-lithium exchange is assumed to occur with a complete retention of configuration at the carbanion centre639, the enantioselective approach of such stabilized carbanions for synthesis has been the subject of recent developments. 

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