Borates, cyclic

Synthetically useful stereoselective reductions have been possible with cyclic carbonyl compounds of rigid conformation. Reduction of substituted cyclohexanone and cyclopentan-one rings by hydrides of moderate activity, e.g. NaBH (J.-L. Luche, 1978), leads to alcohols via hydride addition to the less hindered side of the carbonyl group. Hydrides with bulky substituents 3IQ especially useful for such regio- and stereoselective reductions, e.g. lithium hydrotri-t-butoxyaluminate (C.H. Kuo, 1968) and lithium or potassium tri-sec-butylhydro-borates or hydrotri-sec-isoamylborates (=L-, K-, LS- and KS-Selectrides ) (H.C. Brown, 1972 B C.A. Brown, 1973 S. Krishnamurthy, 1976). 

Catechols can be protected as diethers or diesters by methods that have been described to protect phenols. However, formation of cyclic acetals and ketals (e.g., methylenedioxy, acetonide, cyclohexylidenedioxy, diphenylmethylenedioxy derivatives) or cyclic esters (e.g., borates or carbonates) selectively protects the two adjacent hydroxyl groups in the presence of isolated phenol groups. 

A cyclic borate can be used to protect a catechol group during base-catalyzed alkylation or acylation of an isolated phenol group the borate ester is then readily hydrolyzed by dilute acid. ... 

Figure 9 shows the first and second cycle of a cyclic voltammogram of a 0.2 molal (mol kg"1) solution of lithium bis[2,2 biphenyldiolato(2-)-0,0 ]borate in PC at a stainless steel electrode. The sweep covers the potential range from open circuit potential ER versus a lithium reference electrode up to 4500 mV versus Li and back to ER. The first cycle shows... 

Thereby, the presence of tertiary monophosphines is critical, because both polymeric (26) and cyclic tetrameric (24 and 25) complexes can be obtained depending on the phosphine added to the reaction mixture. Furthermore, with some of the phosphines rapid decomposition and deposition of metallic silver occurs. In the macrocyclic molecules 24 and 25 a pair of silver atoms is bridged by two bis(l-imidazolyl)borate moieties, with a transannular Ag Ag distance of 8.61 A for 24 and 8.89 A for 25. The conformations of 24 and 25 are... 

The obvious next step was oxidation of the tris(pyrazolyl)borate chromium alkyls to the catalytically active -t-III oxidation state. However, cyclic voltammetry experiments did not show a reversible oxidation in any case, and all attempts to prepare complexes of the type [Tp Bu,Meci-R]+X by chemical oxidation failed, yielding [Tp Cr(THF)n] X instekl. TTie reasons for the apparent instability of TpCr alkyls are not clear, and we arc continuing our efforts to isolate related compounds,... 

FIG. 6 Typical cyclic voltammogram obtained for the transfer of pilocarpine hydrochloride at the water-DCE interface. The organic phase contains 0.01 M tetrabutylammonium tetrakis(4-chloro-phenyl)borate, the aqueous solution is 0.01 M HCl + 0.2 mM pilocarpine hydrochloride, and the sweep rate is fixed at 10, 25, 75, 100, and 150mV/s. (Reprinted from Ref. 229.)... 

The monocation tris(diethylether) ethylzinc is formed from diethyl zinc as a tetrakis(penta-fluorophenyl)borate salt.85 Longer-chain linear and cyclic ether complexes (2) of zinc alkyls have also been observed. The reaction between zinc dialkyls and primary amines gives a number of structurally diverse products dependent on the reaction conditions and the amine.86 The... 

Melendres et ai (1991) reported the in-situ study of the electrode/oxide and oxide/electrolyte interfaces for a copper electrode in pH 8.4 borate buffer under potential control. The grazing-angle incidence arrangement employed by the authors is shown in Figure 2.81(a) and a cyclic voltammogram of the Cu electrode in the buffer is shown in Figure 2.81(b). 

Figure 2.81 (a) Schematic of the system for in situ X-ray reflectivity measurements. Syn = synchrotron source M = monochromator S = slit /0, /R = incident and reflected X-rays beams, respectively 9 = angle of incidence W = teflon windows WE = working electrode RE = reference electrode CF = counter electrode D = scintillation detector, (h) Cyclic voltammogram of Cu-on-Si electrode in borate buffer solution (pH 8.4), scan rate = lOmVs-1. From Melendres... 

Figure 2.82 (a) Reflectivity of Cu-on-Si electrode at various potentials in borate buffer solution (pH 8.4). A.B.C and D correspond to potentials indicated in the cyclic voltammogram of Figure 2.81(b). Solid lines represent calculated curves while symbols correspond to experimental data, Open circles, A, -0,12 V open squares, B, -0.80V both y-axes are reflectivity x 10". Filled circles, C, 0.40 V, reflectivity x 10" 3 open diamonds, D, —0.80 V, reflectivity x 10 s. (b) Schematic of multi-layer mode) for Cu-on-Si electrode (not to scale). The oxide film is represented as Cu20. From Melendres et ai (1991). 

Consequently, many reports on pyrazolylborate-zinc compounds have dealt with the modification of the classical tris(pyrazolyl)borate design to better mimic specific enzyme sites,161 or to improve the performance parameters of organozinc catalysts for the polymerization of cyclic esters. This was done by ... 

Another important bioanalytical application of voltammetric ISEs is the detection of polyions (see also above). A technique using cyclic voltammetry on micropipette electrodes filled with the organic electrolyte solutions in 1,2-dichloroethane was successfully applied for the detection of protamine [65] in saline solution and heparin in undiluted sheep plasma samples [66]. Protamine transport was facilitated with dino-nylnaphthalenesulfonic acid (DNNS). As a heparin-selective component the tetrakis-(4-chlorophenyl)borate salt of trimethyloctadecyl ammonium was used. 

There are some examples of macrocyclic complexes of germanium, tin and lead reported in the recent literature. Several crown ethers73,75, tetraaza macrocycles76 [for instance dibenzotetramethyltetraaza[14]annulene (TMTAA)], cyclic polyamines (polyazacycloalkanes)77-80 or, as already mentioned above, poly(pyrazolyl)borate were... 

Trispyrazolylborates are models for tris-histidine active sites in zinc enzymes, e.g., the matrix metalloproteinases involved in breakdown of extracellular matrices. Inhibition of these metalloproteinases may prove valuable in the treatment of, inter alios, cancer and arthritis, so efforts are being made to find appropriate ligands to block the zinc active site. The search has recently moved on from hydroxamates to hydroxypyridinones - l-hydroxy-2-pyridinone is a cyclic analogue of hydroxamic acid. As reported in Section II.B.2 earlier, hydroxypyridinones form stable five-coordinate complexes on reaction with hydrotris(3,5-phenylmethylpyrazolyl)borate zinc hydroxide. Modeling studies suggest that hydroxypyridinonate ligands should be able to access the active site in the enzyme with ease (110). 

Cragg and Weston (11) in their review of the mass spectra of boron compounds have shown the great amount of work being done with cyclic boron compounds. This trend has continued. Straight-chain trialkyl-borates fragment by C—0 cleavage reactions accompanied by hydrogen transfers. 

Intermolecular Allylboration. A tandem aza[4+2] cycloaddition/allyl-boration three-component reaction has been designed based on the prece-dented carbocyclic [4- -2] cycloaddition/allylboration and a snbsequent one-pot variant. Thns, the thermal reaction between hydrazonobutadienes 138, A-substitnted maleimides, and aldehydes provides polysnbstituted a-hydroxy-alkylpiperidines 141 via the cyclic allylboronate intermediate 139 and the proposed chairlike transition stmctnre 140 (Eq. 103). Monoactivated dienophiles like acrylates fail to react with heterodienes 138 bnt the scope of aldehydes is very broad both ahphatic and aromatic aldehydes are snitable, inclnding electron-rich ones. An inverse electron-demand variant to access the corresponding dihy-dropyran derivatives via the intermediacy of enantiomerically enriched pyranyl allylic boronate 76 has been snbsequently developed (see Eq. 64). ° ... 

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