Photolysis of arene chromium tricarbonyl complexes in presence of amine-boranes: Observation of σ-borane complexes in solution

Activation of the B-H σ-bond of amine-boranes on the chromium(0) center of arene chromium tricarbonyl complexes (η⁶-arene)Cr(CO)₃ (arene = fluorobenzene, 1a; benzene, 1b and mesitylene, 1c) has been studied. Photolysis of 1b in presence of ammonia-borane (H₃N·BH₃, AB) and tert-butylamine-borane (^tBuH₂N·BH₃, TBAB) resulted in H₂ evolution and precipitation of a BNH_x polymer. On the other hand, photolysis in the presence of trimethylamine-borane (Me₃N·BH₃, TMAB) resulted in the formation of a σ-borane complex (2) along with Cr(CO)₅(η¹-HBH₂·NMe₃) (3). The σ-borane complexes (η⁶-arene)Cr(CO)₂(η¹-HBH₂·NMe₃) (arene = fluorobenzene, 2a; benzene, 2b and mesitylene, 2c) were characterized in solution by ¹H, ¹¹B, and ¹³C NMR spectroscopy. Electron withdrawing substituents on the arene ring provide the more stable σ-borane moiety in this series of complexes.     

Macropolyhedral boron-containing cluster chemistry: two-electron variations in intercluster bonding intimacy. Contrasting structures of 19-vertex [(η-C5Me5)HIrB18H19(PHPh2)] and [(η-C5Me5)IrB18H18(PH2Ph)]

Fused double-cluster [(η⁵-C₅Me₅)IrB₁₈H₁₈(PH₂Ph)] (8), from syn-[(η⁵-C₅Me₅)IrB₁₈H₂₀] (1) and PH₂Ph, retains the three-atoms-in-common cluster fusion intimacy of 1, in contrast to [(η⁵-C₅Me₅)HIrB₁₈H₁₉(PHPh₂)] (6), from PHPh₂ with 1, which exhibits an opening to a two atoms-in-common cluster fusion intimacy. Compound 8 forms via spontaneous dihydrogen loss from its precursor [(η⁵-C₅Me₅)HIrB₁₈H₁₉(PH₂Ph)] (7), which has two-atoms-in-common cluster-fusion intimacy and is structurally analogous to 6.     

Mapping the substrate scope of monoamine oxidase (MAO-N) as a synthetic tool for the enantioselective synthesis of chiral amines

A library of 132 racemic chiral amines (α-substituted methylbenzylamines, benzhydrylamines, 1,2,3,4-tetrahydronaphthylamines (THNs), indanylamines, allylic and homoallylic amines, propargyl amines) was screened against the most versatile monoamine oxidase (MAO-N) variants D5, D9 and D11. MAO-N D9 exhibited the highest activity for most substrates and was applied to the deracemisation of a comprehensive set of selected primary amines. In all cases, excellent enantioselectivity was achieved (e.e. >99%) with moderate to good yields (55–80%). Conditions for the deracemisation of primary amines using a MAO-N/borane system were further optimised using THN as a template addressing substrate load, nature of the enzyme preparation, buffer systems, borane sources, and organic co-solvents.     

Some observations on the reductive ring opening of 4,6-O-benzylidene acetals of hexopyranosides with the borane trimethylamine-aluminium chloride reagent

Reductive ring openings of 3-O-benzoyl-4,6-O-benzylidene-d-glucopyranosides with BH₃·NMe₃–AlCl₃ are accompanied by side reactions, such as debenzoylation and reduction of the benzoate to benzyl ether. This phenomenon was rationalized by aluminium chelate formation between the O-4 acetal and the benzoyl carbonyl group oxygens. It was also shown that these side reactions can be eliminated by using BH₃·THF as the reducing agent.     

Reductive openings of benzylidene acetals. Kinetic studies of borane and alane activation by Lewis acids

The reaction kinetics for a number of reductive openings of methyl 2,3-di-O-benzyl-4,6-O-benzylidene-α-d-glucopyranoside have been investigated. Openings to give free HO-6 (using BH₃·THF-AlCl₃-THF or LiAlH₄-AlCl₃-Et₂O) follow first order kinetics, while reactions yielding free HO-4 (using BH₃·NMe₃-AlCl₃-THF or BH₃·NMe₃-BF₃·OEt₂-THF) follow higher order kinetics. The addition of water to the BH₃·NMe₃-AlCl₃-THF results in faster reactions. The BH₃·SMe₂-AlCl₃-THF system constitutes a borderline case, yielding both free HO-6 (by a first order reaction) and free HO-4 (by a higher order reaction). These results correlate well with the concept of regioselectivity by activation of borane complexes.     

Determination of Β-isomerized aspartic acid as the corresponding alcohol

The age-related formation of succinimides in proteins, through spontaneous deamidation of asparagine, and through cyclization of aspartic acid, is thought to be followed by the hydrolysis of the succinimide ring, yielding a mixture of normal aspartic acid sites and-isomerized aspartic acid sites (isoaspartic acid). The chemical reduction of an isoaspartyl site to the corresponding amino acid alcohol, isohomoserine, has now been investigated as a general approach to measuring the accumulation of isomerized residues in aging proteins. The methods employed were based on conditions previously found to be successful in reducing protein aspartic acid to homoserine. Borane was employed as the reducing agent, and was found to produce the expected amino acid alcohols in reactions with model peptides. In addition, amino acid analysis revealed a complex pattern of unknown products of these reduction reactions, some of which were also evident when a much stronger reducing agent, lithium aluminum hydride, was used. The correlation of some of these side-products with the isomerization of the peptide suggests, unexpectedly, that the reactivity of reducing agents toward aspartyl residues and perhaps other sites in the peptide may be influenced by steric factors related to aspartyl isomerization. The borane reduction method was also applied to proteins. No detectable isohomoserine was formed either in ovalbumin, a model aged protein, or in human lens proteins of advanced age, with conditions that fully reduced normal aspartyl residues to homoserine. These tests thus indicate that the percentage of aspartic acid in the isomerized form in these proteins is below the limit of detectability (below 5%). These results complement previous experimental results that have indicated a low bulk isoaspartyl content in most natural proteins.     

Macropolyhedral boron-containing cluster chemistry : Products of the reaction between [{Ru(η-MeC6H4Pr)Cl2}2] and syn-[B18H22]. A syn → anti 18-vertex configurational change

Reaction of [{RuCl₂(η⁶-MeC₆H₄^isoPr)}₂] with syn-[B₁₈H₂₂] and non-nucleophilic base results in [8-(η⁶-MeC₆H₄^isoPr)-8-RuB₁₇H₂₁], of 18-vertex anti 10-vertex-nido-10-vertex-nido configuration, as the predominant product. The syn → anti configurational change arises from a trans-cluster pseudo-vertex-substitution of a {BH} vertex by the {Ru₂(η⁶-MeC₆H₄^isoPr)} centre.     

Excited state properties of Tl2B12H12. Metal-centered photoluminescence

The compound Tl₂B₁₂H₁₂ which consists of icosahedral anions and Tl⁺ cations shows a metal-centered r.t. luminescence at λ_max = 530 nm which originates from the lowest-energy sp triplet of Tl⁺. This emission indicates a considerable covalent interaction between Tl⁺ and which is based on the hydridic nature of the boron cluster.     

Catharanthine borane – An unexpected reaction product

The reaction of catharanthine with sodium borohydride in methylene chloride was claimed to produce an isomer of the title compound. A spectroscopic re-examination of the product has revealed that it is catharanthine borane.