title: A critical evaluation of heteronuclear TOCSY (HEHAHA) experiments for 1H,6Li spin pairs.
abstract: Heteronuclear TOCSY (HEHAHA) experiments for (1) H,(6) Li spin pairs in organolithium compounds with adjacent strongly coupled (1) H,(1) H spin systems showed unexpected cross peak behaviour: for n-butyllithium (1) H,(6) Li cross peaks were completely missing, whereas for the dimer of (Z)-2-lithio-1-(o-lithiophenyl)ethane, a cross peak for remote protons was observed even at very short mixing times. It was assumed that strong magnetization transfer within the proton spin systems was responsible for these results, which prevented unambiguous chemical shift assignments. Selective experiments with the (6) Li,(1) H-HET-PLUSH-TACSY sequence then showed the expected (6) Li,(1) H cross peaks for the transfer via the directly coupled (1) H and (6) Li nuclei. For n-butyllithium transfer to H(Cα) via an unresolved heteronuclear coupling constant below 0.1 Hz is unambiguously observed. Cross peaks in the 2D (6) Li,(1) H-HET-PLUSH-TACSY spectra for the dimer of (Z)-2-lithio-1-(o-lithiophenyl)ethane are readily explained by the measured coupling network and the corresponding active mixing conditions.
authors.affiliation: Westfälische Wilhelms Universität, Organisch-Chemisches Institut, Corrensstraße 40, D-48149, Münster, Germany.
authors.forename: Klaus
authors.initials: K
authors.lastname: Bergander
authors.forename: Dietmar
authors.initials: D
authors.lastname: Hüls
authors.forename: Steffen J
authors.initials: SJ
authors.lastname: Glaser
authors.forename: Harald
authors.initials: H
authors.lastname: Günther
authors.forename: Burkhard
authors.initials: B
authors.lastname: Luy
keywords: 1H; 6Li; Hartmann-Hahn; NMR; PLUSH-TACSY; butyl lithium; coherence transfer; selective transfer; small coupling constant