Boron, lithium complexes

A tricyclic aromatic system closely related to borole, 9-boratafluorene, might be expected to form ( -coordinated transition metal sandwich complexes, but so far only the lithium complex has been characterized.19 Monohapto aluminum adducts of neutral 9-borafluorene, in which A1 is bound only to the boron atom, have been prepared.20... 

Flow fluorination of the 4,4 -bipyridine —boron trifluoride complex gives only mono-fluorinated l-fluoro-4-(4-pyridyl)pyridinium boron trifluoride tetrafluoroborate (31), while fluorination of l-methyl-4-(4-pyridyl)pyridinium triflate in the presence of lithium triflate provides l-fluoro-l -methyl-4,4 -bipyridinium ditriflatc (32) 67... 

Novel nonchelation phenomena are observed with a steroidal a-hydroxy aldehyde. The reaction of a lithium or magnesium alkynide with the aldehyde gives the (20/, 22/ )-diastereomer piedominantiy, the formation of which was explained by Cram s cyclic model. When BF3-OEt2 is added to the lidiium alkynide prior to the addition of the aldehyde, the stereoselectivity is inverted, and the (20) ,225)-isomer is obtained as the principal product. Transformation of a-alkoxy aldehyde to the boron ate complex is suggested. Other l wis acids, such as B(OMe)3, AlCh, etc., are less effective (equation 29). °... 

Later, Sasai et al. found that a combination of the heterobimetalhc complex boron-lithium-mono(binaphthoxide) (BLB) (380) and (Bu")3P is effective in promoting the enantioselective MBH reaction of cyclic enones with aldehydes via double activation to afford the adduct in good chemical yield with moderate to high enantioselectivity (Scheme 2.216) ... 

In 1995, Barthel and Gores reported a new class of inexpensive and chemically, elec-trochanically, and thermally stable salts based upon boron chelate complex anions with aromatic or aliphatic diols or carboxylic acids. The first such salt reported was lithium bis(l,2-benzenediolato(2-)-0,0 )borate (LiBBE) (Fig. 1.26a) [372]. The acid with this anion was originally reported in 1949 by Schafer [388]. The LiBBB salt has a high solubility in aprotic solvents (>1 M), but the oxidative stability is relatively low. A number of other nonfluorinated lithium salts with benzenediol nonfluorinated... 

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). 

Boron tnfluonde also reacts with hydroxy acndones to form difluoro boron complexes [111] (Table 28) Another route to fluoroboraties involves transhalogen-ation by fluonnation of the correspondmg chloro- and bromoboranes with lithium or potassium fluonde under mild conditions [112] (Table 29)... 

The alkynyl reagent 9 was recently introduced for the dia stereoselective synthesis of tertiary propargylic alcohols144. 9 can be prepared as a solid 1 1 complex with tetrahydrofuran by treatment of 9-methoxy-9-borabicyclo[3.3.1]nonane with (trimethylsilylethynyl)lithium, followed by addition of boron trifluoride-diethyl ether complex. The nucleophilic addition of reagent 9 to (R)-2-methoxy-2-methylhexanal (10) afforded a mixture of the diastereomers 11 with a considerable preference to the nonchelation-controlled (3S,4R)-isomer144. 

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