Boron complexes chelates

Although some of the cationic chelate complexes have long been known,141 most of the cationic boron complexes have been synthesized in the last three decades following the... 

Novel tetrahedral boronate complexes have been prepared as potential ligands for affinity chromatography either from the amination (LiN(TMS)2) of a-haloalkylboronates followed by A-acetyl-ation (i.e., (132)) or, alternatively, from the a-iodo derivatives and acetamidine (i.e., (133)) (Scheme 17). The 1,3,2-dioxaborinanes proved to be less stable than the pinanediol derivatives to silica gel. "B NMR as well as other data confirmed their chelated nature (e.g., (132) d 6.8) <92JOM(43i)255>. Spiroborates (e.g. (134)) have also been prepared from boric acid, diols, and enolizable 1,3-dicarbonyl compounds <86JPR755>. Vinylzirconocenes (135) are converted to vinylboronates (136) with chloroboranes <910M3777>. The hydrozirconation of 5-vinylboronates (137) produces the mixed... 

Boric acid, fluoro-, 101 Boromycin, 96 Boron, 81-101 as ligand, 99 Boron complexes anionic, 90-97 antibiotics, 101 applications, 101 cationic, 97 chelates, 89... 

The cycloaddition reactions of chiral 2-amino-l, 3-dienes with Fischer-type carbene complexes have been examined by Barluenga et al. Reactions with tungsten vinyl carbene complexes A and with boron-nitrogen-chelated chromium complexes B lead very selectively to cyclohexanone derivatives. However, when employing chromium vinyl carbene complexes in acetonitrile at room temperature in these reactions, cycloheptadienes were obtained selectively by cyclopropanation and subsequent Cope rearrangement. 

Figure 15.17 An example of a C,C-chelated bis(2,4,6-trimethylphenyl)boron complex (R = 2,4,6-trimethylphenyl). A benzimidazole analog has also been prepared [67].

Other relevant publications which have appeared concern acetoxyboron derivatives of iV-substituted salicylaldimines, boron complexes of Schiif bases of pentane-2,4-dione, and the diphenylboron chelate of 1,5-diphenylcar-bazone. ... 

Excellent chelation control was observed using tributyl(2-propenyl)stannane and a-benzyloxy-cyclohexaneacetaldehyde with magnesium bromide or titanium(IV) chloride, whereas useful Cram selectivity was observed for boron trifluoride-diethyl ether complex induced reactions of the corresponding ferr-butyldimethylsilyl ether89. 

For a-benzyloxycyclohexaneacelaldehyde and 2-butenylstannanes, good chelation control was observed using zinc iodide and titanium(IV) chloride, but only weak synjanti selectivity. Better syn/anti selectivity was found using boron trifluoride-diethyl ether complex, but weak chelation control. Magnesium bromide gave excellent chelation control and acceptable syn/anli selectivity90. 

The boron trifluoride-diethyl ether complex induced reaction of 2-butenyl(tributyl)-stannane and 3-(/er/-butyldimethylsilyloxy)-2-methylpropanal gave predominantly the nonchelation-controlled yyn-product93, whereas with the analogous 3-benzyloxyaldehyde, 2-propenyl-tin trichloride, generated in situ from tributyl(2-propenyl)stannanc and tin(IV) chloride, gave the chelation-controlled product93. 

The hydroboration of enynes yields either of 1,4-addition and 1,2-addition products, the ratio of which dramatically changes with the phosphine ligand as well as the molar ratio of the ligand to the palladium (Scheme 1-8) [46-51]. ( )-l,3-Dienyl-boronate (24) is selectively obtained in the presence of a chelating bisphosphine such as dppf and dppe. On the other hand, a combination of Pdjldba), with Ph2PC6p5 (1-2 equiv. per palladium) yields allenylboronate (23) as the major product. Thus, a double coordination of two C-C unsaturated bonds of enyne to a coordinate unsaturated catalyst affords 1,4-addition product On the other hand, a monocoordination of an acetylenic triple bond to a rhodium(I)/bisphosphine complex leads to 24. Thus, asymmetric hydroboration of l-buten-3-yne giving (R)-allenyl-boronate with 61% ee is carried out by using a chiral monophosphine (S)-(-)-MeO-MOP (MeO-MOP=2-diphenylphosphino-2 -methoxy-l,l -binaphthyl) [52]. 

Summary of Facial Stereoselectivity in Aldol and Mukaiyama Reactions. The examples provided in this section show that there are several approaches to controlling the facial selectivity of aldol additions and related reactions. The E- or Z-configuration of the enolate and the open, cyclic, or chelated nature of the TS are the departure points for prediction and analysis of stereoselectivity. The Lewis acid catalyst and the donor strength of potentially chelating ligands affect the structure of the TS. Whereas dialkyl boron enolates and BF3 complexes are tetracoordinate, titanium and tin can be... 

In a molecule of 2-phenyl-l-boraadamantane there are two markedly different types of B-C bonds two of them are boron-alkyl and one is boron-benzyl. On treatment of THF complex 34 with 8-hydroxyquinoline at 20 °C, mpture of the 1-boraadamantane core takes place, resulting in a mixture of boron chelates 52-54 (Scheme 16). When trimethylamine adduct 16 is used as the starting compound, reaction takes place only in boiling toluene. Interestingly, all the products result from the protolysis of B-CH2 bonds only <2006UP1>. 

A further step towards improved selectivity in aldol condensations is found in the work of David A. Evans. The work of Evans [3a] [14] is based in some early observations from Meyers laboratory [15] and the fact that boron enolates may be readily prepared under mild conditions from ketones and dialkylboron triflates [16]. Detailed investigations with Al-propionylpyrrolidine (31) indicate that the enolisation process (LDA, THE) affords the enolate 32 with at least 97% (Z>diastereoselection (Scheme 9.8). Finally, the observation that the inclusion of potential chelating centres enhance aldol diastereoselection led Evans to study the boron enolates 34 of A(-acyl-2-oxazolidones (33), which allow not only great diastereoselectivity (favouring the 5yn-isomer) in aldol condensations, but offer a possible solution to the problem of enantioselective total syntheses (with selectivities greater than 98%) of complex organic molecules (see below, 9.3.2), by using a recyclisable chiral auxiliary. 

Although the results are easily rationalised in the case of the a-alkylation (attack of the electrophile at the Re face, i.e., attack from the less hindered a face), in the aldol condensation it is somewhat more difficult to rationalise and several factors should be considered. According to Evans [14] one possible explanation for the diastereofacial selection observed for these chiral enolates is illustrated in Scheme 9.14. In the aldol reactions, the more basic carbonyl group of the aldehyde partner interacts with the chelated boron enolate 45 to give the "complex" A which may... 

---