Borate hydrotris -, lithium

Synthetically useful stereoselective reductions have been possible with cyclic carbonyl compounds of rigid conformation. Reduction of substituted cyclohexanone and cyclopentan-one rings by hydrides of moderate activity, e.g. NaBH (J.-L. Luche, 1978), leads to alcohols via hydride addition to the less hindered side of the carbonyl group. Hydrides with bulky substituents 3IQ especially useful for such regio- and stereoselective reductions, e.g. lithium hydrotri-t-butoxyaluminate (C.H. Kuo, 1968) and lithium or potassium tri-sec-butylhydro-borates or hydrotri-sec-isoamylborates (=L-, K-, LS- and KS-Selectrides ) (H.C. Brown, 1972 B C.A. Brown, 1973 S. Krishnamurthy, 1976). 

Sand-sodium base. See Sodium-sand base a-Santalene, 42 Sobering process, 118-119 Schiff bases. See Imines 9,10-Secosteroids (calciferols and tachysterols), 31-32, 34, 281-283, 289 Seebach-Frater alkylation, 27. AT-Selectride = potassium hydrotris(l-mcthylpropyl)borate(l —) (KBHBVl), 107 XS-Selectride 1 — potassium tris(l,2-dimethyl-propyl)hydroborate(l —) (KBHSiay), 107 Z-Selectride = lithium hydrotri (l-... 

Fehlhammer and coworkers [34] reported the synthesis of a homoleptic AThet-erocychc carbene-hthium complex by deprotonation of hydrotris(3-ethylimida-zohum)borate tetrafluoroborate with -butyllithium at -78 °C in diethyl ether, after loss of hthium tetrafluoroborate and -butane. X-ray quality crystals were obtained upon cooling. Interestingly, the two lithium atoms are complexed only by the NHC carbon atoms, whereas most other examples of Li-NHC complexes include some coordinated solvent. 

Fehlhammer et al also demonstrated that the lithium complex of the monoan-ionie, tridentate hydrotris(3-methyl-imidazol-2-ylidene)borate carbene 8, could be transmetallated to Fe , Co and Re , affording cationic complexes 9, 10, and 11, respectively (Seheme 5.2). Lithium complex 8 was implieated in the formation of 9-11 as these eomplexes formed only when n-BuLi was used to generate the carbene in situ, with Na[N(SiMej)2] or KOt-Bu affording boron-substitution products. The strueture of the purported Li carbene complex was verified by the isolation and structural characterization of 8 (R = Et). 

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