Lithium, organo- compounds structure

Recent structural investigations on lithium organo(fluorosilyl)amides have revealed that the lithium cation can form aggregates with internal lithium coordination to fluorine, and mixed aggregates of the amide and LiF. Structural types such as (205), (206), (207) and (208) have been found for these compounds. 

Sonication of l,l-dichloro-2,3,4,5-tetraethylgermole with an excess of lithium in THF and TMEDA gave a red solution of trigermole dianion which was crystallized and characterized by X-ray analysis. It showed that one lithium cation is engaged in a lithocene structure, while the second one is in an environment similar to those in common organo-lithium compounds complexed by THF and TMEDA (equation 47)62. 

Fig. 10.32. Competition between 1,2- and 1,4-additions in the reaction of organo-lithium compounds with a,/3-unsaturated ketones, depending on the structure of the organolithium compound.

The most stable structures of alkyl and alkenyl anions predicted with the VSEPR theory are supported by reliable calculations. There are no known experimental structural data. In fact, up to recently, one would have cited the many known geometries of the lithium derivatives of these carbanions as evidence for the structure. One would simply have dropped the C—Li bond(s) from these geometries. However, it is now known that the considerable covalent character of most C—Li bonds makes organo-lithium compounds unsuitable models for carbanions. 

Because comprehensive literature [1-8] covering various aspects of organo-lithium chemistry has recently become available, the purpose of fhis chapter is to highlight powerful synthetic tools involving organohfhium compounds. The definition of organohfhium is here limited to those compounds in which fhere is a clear C-Li bond compounds with enolate or ynolate structures or wifh heteroatom (Y)-Li bonds, etc., have been excluded. 

Structure of a novel spirotin compound. Wrackmeyer and co-workers reported an interesting multinuclear NMR study in which they demonstrated that 2,3-dichloro-l,3-bis-(trimethylsilyl)-l,3,2-diazastanna-[3]ferrocenophane reacts with lithium alkynides to give the corresponding di(alkyn-l-yl)tin derivatives, 246, that on subsequent organo-boration with triethylborane affords a novel spirotin compound that contains both ferrocenophane and a stannacyclopentadiene ring. 

Introduction The structure of organo-lithium compounds Lithium and Magnesium Complexes... 

The difficulties of direct oxidative insertion with metals other than Mg or Li mean that o-complexes are often made from organo-lithium or Grignard reagents by metal exchange. This reaction amounts to a nucleophilic substitution at the metal without a change of oxidation state so the metal is used in whatever oxidation state is finally needed. Attack of methyl lithium on a Cu(I) halide gives methyl copper 50, a o-complex of Me- and Cu(I). If an excess of MeLi is present an ate complex is formed, lithium dimethylcuprate 51. This is formally a compound of a copper anion 51a, just as BF4 is a borate. The term ate complex refers to such formally anionic complex in which the metal has one extra anionic ligand. Its true structure is dimeric 51b and it exists as an equilibrium with 52 in solution.20... 

Reactions of substituted prop-2-ynyl lithium compounds can similarly give acetylenic and allenic products. Huynh and Linstrumelle prepared the lithium acetylide 11 and confirmed its structure by NMR spectroscopy. Reaction with propylene oxide then gave a mixture of 12 (45%) and 13 (55%), showing that both organo-lithium species can react to give either allenic or acetylenic products. 

The NMR spectrum of the pure hydroxypropylated polystyrene showed peaks in the regions of d 21.5-26 ppm and d 64-67.5 ppm. Analysis by the attached proton test (APT) in conjunction with model compounds indicated that the region between 21.5 and 26 ppm corresponds to the methyl carbon resulting from attack of the polymeric organo-lithium at the least hindered carbon to form a secondary alcohol chain-end functional group (see (a) in eqn [7]). The area between 3 64 and 67.5 ppm was assigned to the carbon bonded to oxygen for two diastereomerically different products as shown by structure 1, where the chiral carbon atoms are labeled with asterisks. 

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