spectroscopic data suggest the spirocyclic 1,3,2,4-diazadiboretidinium bromide 6. This assignment is corroborated by the addition of AIBr3: The 'H-, I3C-, and "B-NMR parameters of 6 hardly change, since the Lewis acid AIBr, only converts the bromide 6 into the tetrabromoaluminate 7 . L J 3: R' = CH3, X = Br 4: R' = Ph, X = C1 11: R' = CH,, X = Br 12: R ' = Ph, X = C1 6 7 G1 P P-i3 B b CN~ ~N -C R~ CN: )N-CR, CN: )N-CR, B B B Ph' \C1 R' \Br R' 'Br 5 8 9 c N = R CH3 Hf '"'These results indicate that the expected adduct 8 is unstable because of the insufficient acidity of the CH3BN2 moiety, and at the same time explains the fact that 1 un-
In Anlehnung an bereits publizierte Verfahren zur Synthese von polylithiierten K o h l e n w a s~e r s t o f f e n~~~~~, die mehrere Li-Atome a n einem C-Atom (,,isocentrisch") Vie1 bessere Ergebnisse erhalt man, wenn man zur Ummetallierung von 4 tert-Butyllithium im UberschuB verwendet, wie dies fur analoge V e r b i n d~n g e n I~.~~ ebenfalls bereits beschrieben wurde. In diesem Falle andert sich der Mechanismus der Ummetallierung von radikalisch nach ionisch, so daB keine Kupplungsprodukte entstehen. In Cyclopentan bildet sich ein amalgamfreies, jedoch lithium- Die I3C-und 'Li-NMR-spektroskopische Untersuchung von 'Li-markiertem 5 in [D,,]Cyclohexan brachte aufgrund der Begleitsubstanzen (EtLi, tBu,Hg, fBuLi), der geringen Konzentration an CLi, und der zu erwartenden geringen Empfindlichkeit dieses Kohlenstoffatoms (hohe Multiplizitat, fehlender NOE) keine sicheren Resultate.Ein qualitativ sehr gutes Produkt 5 -einen tiefrotbraunen, glueerst pyrophoren Fesrstoff -erhalt man, wenn man aus CLi4-Losungen das Solvens und die leicht fluchtigen und sublimierbaren Begleitprodukte abzieht1'21. In der urspriinglichen Liisung kann ein Komplex aus CLi, 5 und Di-tert-butylquecksilber vorgelegen haben, insbesondere d a die Losungen sehr lichtempfindlich sind (empfindlicher als tBu,Hg) und 5 nach dem Abziehen von tBuzHg in Hexan nicht mehr in Losung gebracht werden kann.
Until now, dilithiomethane 5 was the only known geminal dilithioalkane. It was first obtained by West and Rochow['"' from dibromomethane 2 and lithium in 6% yield. The yield was improved by Lagow et al.['bl using a specially devised apparatus; however, mixtures of polylithio compounds were always formed that could not be separated. The best synthesis of 5 is the pyrolysis of salt-free methyllithium 4 described by Ziegler et al.l'cl, but, unfortunately, this method does not lend itself to the preparation of higher dilithioalkanes. J 2 2 0 T 2 CH,Li -CHzLi2 C -CHa(HgI)z -cH, -2LiI 4 -LdHg 6 i 4 rBuLi -2 Ltl, -2 IBu&We have now found that the reaction of bis(iodomercurio)methane 6[21 with lithium powder gives access to 5 ; the method also permits the synthesis of higher 1,l-dilithioalkanes for the first time. 1,l-Bis(ch1oromercurio)ethane 12, which is accessible by two routes, serves as starting material for the preparation of 1,l-dilithioethane 13. Rather than starting from vinyl bromide 7I3"l, 12 can be obtained more straightforwardly from 1,l-dichloroethane 9. This synthesis was elaborated from a known method[3b1.
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