Boron complex

Pentamethylcyclopentadienyl substituted boron complexes arc obtamed by the reaction of the pentamethylcyclopentadienyl anion with boron tnfluonde [109] (Table 27) Similarly, the Gngnard reagent prepared from 3,5-bis(tnfluorometh-yl)iodobenzene reacts with sodium tetrafluoroborate to form the phase-transfer eatalyst 2 under anhydrous eonditions [110] (equation 87)... 

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)... 

Some of the developments of catalytic enantioselective cycloaddition reactions of carbonyl compounds have origin in Diels-Alder chemistry, where many of the catalysts have been applied. This is valid for catalysts which enable monodentate coordination of the carbonyl functionality, such as the chiral aluminum and boron complexes. New chiral catalysts for cycloaddition reactions of carbonyl compounds have, however, also been developed. 

Chiral boron(III) complexes can catalyze the cycloaddition reaction of glyoxy-lates with Danishefsky s diene (Scheme 4.18) [27]. Two classes of chiral boron catalyst were tested, the / -amino alcohol-derived complex 18 and bis-sulfonamide complexes. The former catalyst gave the best results for the reaction of methyl glyoxylate 4b with diene 2a the cycloaddition product 6b was isolated in 69% yield and 94% ee, while the chiral bis-sulfonamide boron complex resulted in only... 

Yamamoto et al. have developed a catalytic enantioselective carbo-Diels-Alder reaction of acetylenic aldehydes 7 with dienes catalyzed by chiral boron complexes (Fig. 8.10) [23]. This carbo-Diels-Alder reaction proceeds with up to 95% ee and high yield of 8 using the BLA catalyst. The reaction was also investigated from a theoretical point of view using ab-initio calculations at a RHF/6-31G basis set. 

Boron tribromide reacts with ethers by the sequence shown (22). The boron complex... 

The anti diastereoselectivity is improved to a 6 1 ratio by the addition of triethylborane to the reaction mixture83. NMR-spectroscopic investigations indicate that a boronate complex is the decisive intermediate84, since it can also be prepared by the addition of alkyllithium to the dialkyl(2-butenyl)borane (path ). 

The formation of dimeric products is unique for the case of boron, because analogous complexes with other elements are all monomeric [95]. This can be attributed to the small covalent radius of the boron atom and its tetrahedral geometry in four-coordinate boron complexes. Molecular modeling shows that bipyramidal-trigonal and octahedral coordination geometries are more favorable for the formation of monomeric complexes with these ligands. 

If 2,6-pyridinedimethanol is condensed with arylboronic acids in non-polar solvents, the tetrameric boron complexes 74 and 75 are formed rapidly (within 15-30 min) in yields of 80 and 93% (Fig. 20). In both cases only the RSRSI SRSR enantiomeric pair with approximate S4-symmetry is obtained, so that the reaction is diastereoselective. 

Traven VF, Chibisova TA, Manaev AV (2003) Polymethine dyes derived from boron complexes of acetylhydroxycoumarins. Dyes Pigm 58 41 16... 

The boranediyl insertion with BC13 and BBr3 gives products with 1-haloboratabenzene ligands which easily undergo nucleophilic substitution at boron complex 6 has been made in this way (57,79). However, these reactions have never been published in detail. 

Boron complex azo dyes have also been reported. These include solvent soluble boron complexes, such as the red dye (32) used for dyeing polyester and coloring plastics25,26 and water-soluble dyes for the detection of boron (as boric acid) by a color change27 (Scheme 5). 

Lewis C. E. (American Cyanamid). Boron Complexes of o,o -DiHydroxyPhenylAzoNaphthyl Dyes. U.S. Patent 3726,854, April 10 1973. 

It is necessary for the intermediate cation or complex to bear considerable car-bocationic character at the carbon center in order for effective hydride transfer to be possible. By carbocationic character it is meant that there must be a substantial deficiency of electron density at carbon or reduction will not occur. For example, the sesquixanthydryl cation l,26 dioxolenium ion 2,27 boron-complexed imines 3, and O-alkylated amide 4,28 are apparently all too stable to receive hydride from organosilicon hydrides and are reportedly not reduced (although the behavior of 1 is in dispute29). This lack of reactivity by very stable cations toward organosilicon hydrides can enhance selectivity in ionic reductions. 

Nonmetallic systems (Chapter 11) are efficient for catalytic reduction and are complementary to the metallic catalytic methods. For example lithium aluminium hydride, sodium borohydride and borane-tetrahydrofuran have been modified with enantiomerically pure ligands161. Among those catalysts, the chirally modified boron complexes have received increased interest. Several ligands, such as amino alcohols[7], phosphino alcohols18 91 and hydroxysulfoximines[10], com-plexed with the borane, have been found to be selective reducing agents. 

Stereoselective reduction of some triazolodiazines (derivatives of ring systems 33 and 37) bearing chiral terpene residues has been elaborated by Groselj el al. <2006TA79>. With catalytic hydrogenation, partial saturation of the six-membered ring was experienced, while reaction with borane-methyl sulfide resulted in formation of triazole-boron complexes. 

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