Lithium biphenylide

Eight procedures highlighting some recent and practical developments in organometallic chemistry form the next group. Highly reactive calcium prepared by reduction of dissolved calcium bromide with lithium biphenylide affords access to the... 

Highly reactive calcium can be readily prepared by the reduction of calcium halides in tetrahydrofuran solution with preformed lithium biphenylide under an argon atmosphere at room temperature.5 This colored calcium species seems to be reasonably soluble in THF. However, the reactive calcium complex prepared from preformed lithium naphthalenide was insoluble in THF solution and precipitated out of solution to give a highly reactive black solid. The exact nature of this black calcium complex has not been determined. Acid hydrolysis of the black material releases naphthalene as well as THF. Accordingly, the most likely structure of the black material is a Ca-naphthalene-THF complex similar in nature to the soluble magnesium-anthracene complex recently reported.6... 

Furthermore, it seems reasonable to ascribe the regiochemistry observed to that of an unencumbered radical anion or solvent separated species in the radical anion reductions and the triethylamine photochemical runs. Various studies on lithium biphenylide have shown that it exists both as a contact ion pair and a solvent separated ion pair in THF, although at or below 20 °C mostly as a solvent separated ion pair [47,48], In the case of the exciplex between the radical anion of 1 and Et3N+, various studies of pyrene and aliphatic or aromatic amine exciplexes [49, 50] and related excited singlet states of EDA complexes... 

Various other reagents can be used for this reductive debromination. 1,2-Dipropylcyclopropane was prepared from 4,6-dibromononane using chromium(II) perchlorate in dimethylformami-de/water (yield 93%), lithium amalgam in tetrahydrofuran (75%), lithium biphenylide in te-trahydrofuran (78%), potassium-sodium alloy in tetrahydrofuran (68%), zinc dust and zinc(II) chloride in propan-2-ol/water (95%) and alkyllithiums in tetrahydrofuran (BuLi 16%, i-BuLi 18%, t-BuLi 47%). Ring closure of 1,3-dibromobutane to methylcyclopropane was achieved by treatment with zero-valent copper, which was obtained from reaction of lithium naphthalen-ide and copper(I) iodide/tributylphosphane in tetrahydrofuran (yield 91%) ... 

Coupling of allylic halides.1 In the presence of Ba, prepared in situ by reduction of Bal2 by lithium biphenylide, cis- or /rans-allylic halides undergo, a.a -coupling with retention of the double bond geometry. Unsymmctrical dienes can be prepared by a,a -cross coupling. Thus (EJi)-farncsyl chloride is converted into squalcne in 64% yield. 

Allylbarium reagents homoallylic alcohols. The reaction of anhydrous Bal2 with lithium biphenylide (2 cquiv.) in THF provides a reactive barium species that reacts with allylic chlorides at —78° to form allylbarium chlorides. These reagents react with high a-selcctivity with carbonyl compounds to form homoallylic alcohols with retention of configuration. 

Sakurai et al. [91] have developed a synthetic route towards polysilanes via masked disilenes (7,8-disilabicyclo[2.2.2]octa-2,5-dienes). The starting materials are accessible from dichlorodisilanes and lithium biphenylide [92] (RS R R R = Me, Pr, Bu, Hex) ... 

Active calcium was prepared by the lithium biphenylide reduction of CaBr2 in THE CuCN-2LiBr was used for transmetalation with organocalcium reagents. 

Lithium (9.0 mmol) and biphenyl (9.8 mmol) in freshly distilled THF (20 ml) were stirred under argon until the lithium was completely consumed (ca. 2h). To a well-suspended solution of Cal2 or CaBr2 in freshly distilled THF (20 ml), the preformed lithium biphenylide was transferred via a cannula at room temperature. The reaction mixture was stirred for 1 h at room temperature prior to use. (Note Excess calcium salt was used in the oxidative addition reactions with organic halides. Details are described later in this section.)... 

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