Highly enantiomerically enriched chlorophosphine boranes: synthesis and applications as P-chirogenic electrophilic blocks

The stereoselective synthesis of P-chirogenic chlorophosphine boranes 4 was investigated by HCl acidolysis of the corresponding aminophosphine boranes 10. The reaction afforded the P-N bond cleavage with inversion of the configuration at the phosphorus center, leading to the chlorophosphine boranes 4 with high to excellent enantiomeric purities (80-99% ee), except in the case of the chloro-1-naphthylphenylphosphine borane 4d. Reaction conditions and workup significantly influence the enantiomeric purity of the product, with the exception of the o-anisyl- and o-tolylchlorophenylphosphine boranes, 4b and 4c, which were found to be particularly stable even after purification by chromatography on silica gel. Reaction of the chlorophosphine boranes 4 with various nucleophiles, such as carbanions, phenolates, thiophenolates, or amides, afforded the corresponding organophosphorus borane complexes via P-C, P-O, P-S, and P-N bond formation, respectively, in 34-93% yield and with up to 99% ee. This work demonstrates the importance of chlorophosphine boranes 4 as new and powerful electrophilic building blocks for the highly stereoselective synthesis of P-chirogenic organophosphorus compounds.     

The Chemistry of Amino Imino Boranes

Dicoordinated boron compounds can no longer be considered as “exotic” species in boron chemistry: this is proved by the extensive chemistry of bis(dialkylamino)boron(1+) ions, alkylidene boranes, and imino boranes. The latter are characterized by high reactivity and chemical versatility. A special position is taken by amino imino boranes, whose chemistry is reported here. The presence of an amino group enhances the reactivity of imino boranes on the one hand and increases their kinetic stability on the other. New types of diamino boranes accessible via amino imino boranes often undergo intramolecular ring closure. These heterocycles are well suited to generate new types of cations of boron. Cycloaddition reactions using amino imino boranes open up a new heterocyclic chemistry of boron. The chemistry of N-functional amino imino boranes, which is still its infancy, shows much promise.     

Hydroboration of Arynes with N‐Heterocyclic Carbene Boranes

Arynes were generated in situ from ortho‐silyl aryl triflates and fluoride ions in the presence of stable N‐heterocyclic carbene boranes (NHC? BH₃). Spontaneous hydroboration ensued to provide stable B‐aryl‐substituted NHC‐boranes (NHC? BH₂Ar). The reaction shows good scope in terms of both the NHC‐borane and aryne components and provides direct access to mono‐ and disubstituted NHC‐boranes. The formation of unusual ortho regioisomers in the hydroboration of arynes with an electron‐withdrawing group supports a hydroboration process with hydride‐transfer character.     

Defective vertices in arachno borane networks

Triangulated boron networks can be described in terms of the deviation of their local vertex environments from the degree 5 vertices found in ideal icosahedra. Vertices of degrees other than 5 or equivalent are considered to be defective vertices. This method, which was previously applied to deltahedral borane anions B(n)H(n)(2-) and nido-B(n)H(n+4) boranes, has now been applied to arachno boranes of the types B(n)H(n+6) and B(n)H(n+5)(-) (4 < or = n < or = 10). The known structures of the neutral arachno boranes B(4)H(10), B(8)H(14), and n-B(9)H(15) consist of triangulated boron networks with no defective vertices in accord with their higher stabilities relative to other neutral arachno boranes. In other structures of known arachno boranes, there are relatively small numbers of defective vertices, and these are isolated as far as possible from each other.     

Structural relationships among two vertex sharing macropolyhedral boranes

Various two vertex sharing macropolyhedral boranes were computed at the B3LYP/6-311 + G**//B3LYP/6-31G* level of theory to determine the preferred fragments for the thermodynamically most stable isomers. These are nido-10 and arachno-9 vertex fragments for neutral macropolyhedral boranes. The thermodynamically most stable isomers of the nido:nido-, arachno:nido- and arachno:arachno-macropolyhedral borane classes are structurally related to each other by the successive removal of one open face vertex as in the case of simple polyhedral boranes. For these classes, the stabilities of the thermodynamically most stable macropolyhedra relative to isomeric simple polyhedra follow similar trends with respect to the number of skeletal electrons.     

Mild reduction of chlorophosphine boranes to secondary phosphine boranes

A number of reducing reagents were assessed in the transformation of chlorophosphine boranes to secondary phosphine boranes. The efficiency of the process requires judicious matching between steric and electronic requirements of reductant and the substrate. The stereochemistry of the reduction was investigated by using a chiral precursor.     

Reactivity of B‐Xanthyl N‐Heterocyclic Carbene‐Boranes

The synthesis and reactivity of mono‐ and bis‐S‐xanthyl NHC‐boranes is reported. The new NHC‐boranes are prepared through nucleophilic exchange at boron from either mono‐ or bis‐triflyl NHC‐boranes, themselves obtained by protolysis of the NHC‐BH₃ starting compounds. The B?H bond of the S‐xanthyl NHC‐boranes can be cleaved both homolytically and heterolytically, albeit the latter is more synthetically useful. The S‐xanthyl NHC‐boranes can reduce both aldehydes and imines. The B?S bond can also be cleaved homolytically. Under UV irradiation, the S‐xanthyl NHC‐boranes generate NHC‐boryl radicals that can initiate radical polymerizations of acrylates.     

Mechanistic variations in ionic hydrogenation of unsaturated phosphine and amine boranes

Internal hydride transfer occurs when tethered carbocations are generated from unsaturated phosphine or phosphinite boranes. 3-Methylenecyclohexyl-derived boranes 12 or 18 react with MsOH to give ionic hydrogenation products with high syn-selectivity. With unsaturated amine boranes, initial hydrogen evolution gives BH(2)(OMs) complexes, but IH occurs using excess MsOH in a slower second stage. A diastereoselective reaction occurs from 26b using camphorsulfonic acid (first stage) and MsOH (second stage), affording 33 (68% ee) after hydrolysis.     

硼中子俘获治疗肿瘤的研究 I. 合成硼化核酸的几种化学方法

本文以小牛胸腺DNA和酵母TRNA为载体, 对核酸与硼笼化物的几种偶联方法进行了研究结果表明, 活化后的核酸通过交联剂MBS和DCCI分别能与Na2B12H11SH和(Et4N)2B1OH9R相偶联, 找到了反应条件温和, 偶联程度较高的合成方法。     

New boryl radicals derived from N-heteroaryl boranes: generation and reactivity

Recently, boryl radicals have been the subject of revived interest. These structures were generated by hydrogen-abstraction reactions from the corresponding boranes (i.e., from amine or phosphine boranes). However, the classical issue remains their high B--H bond-dissociation energy (BDE), thereby preventing a very efficient hydrogen-abstraction process. In the present paper, new N-heteroaryl boranes that exhibiting low B--H BDE are presented; excellent hydrogen-transfer properties have been found. Both the generation and the reactivity of the associated boryl radicals have been investigated through their direct observation in laser flash photolysis. The boryl radical interactions with double bonds, oxygen, oxidizing agent, and alkyl halides have been studied. Some selected applications of N-heteroaryl boryl radicals as new polymerization-initiating structures are proposed to evidence their high intrinsic reactivity.     

N‐Heterocyclic Carbene Boranes as Reactive Oxygen Species‐Responsive Materials: Application to the Two‐Photon Imaging of Hypochlorous Acid in Living Cells and Tissues

N‐Heterocyclic carbene (NHC) boranes undergo oxidative hydrolysis to give imidazolium salts with excellent kinetic selectivity for HOCl over other reactive oxygen species (ROS), including peroxides and peroxynitrite. Selectivity for HOCl results from the electrophilic oxidation mechanism of NHC boranes, which stands in contrast to the nucleophilic oxidation mechanism of arylboronic acids with ROS. The change in polarity that accompanies the conversion of NHC boranes to imidazolium salts can control the formation of emissive excimers, forming the basis for the design of the first fluorescence probe for ROS based on the oxidation of B?H bonds. Two‐photon microscope (TPM) ratiometric imaging of HOCl in living cells and tissues is demonstrated.     

Studies on the structural rule of boranes

The authors discuss the structural rule of different kinds of polyhedral boranes and propose a general topological formula for calculating the number of bonding molecular orbitals. In the case of single polyhedral boranes, results obtained from the formula are the same as those from the famous Wade rule. Futhermore, the formula can also be used to calculate the number of bonding molecular orbitals of joined-type polyhedral boranes. We have also made some quantum chemistry calculations to verify the concept that the number of bonding molecular orbitals of boranes can be determined by considering only the corresponding boron polyhedral frameworks.     

Simple unprecedented conversion of phosphine oxides and sulfides to phosphine boranes using sodium borohydride

A variety of phosphine oxides and sulfides can be efficiently converted directly to the corresponding phosphine boranes using oxalyl chloride followed by sodium borohydride. Optically active P-stereogenic phosphine oxides can be converted stereospecifically to phosphine boranes with inversion of configuration by treatment with Meerwein's salt followed by sodium borohydride.     

Amine-directed hydroboration: scope and limitations

Iodine activation induces intramolecular hydroboration of homoallylic and bis-homoallylic amine boranes with good to excellent control of regiochemistry compared to control experiments using excess THF*BH 3. Deuterium labeling and other evidence confirm that the iodine-induced hydroboration reaction of homoallylic amine boranes occurs via an intramolecular mechanism equivalent to the classical 4-center process and without competing retro-hydroboration. Longer carbon chain tethers result in lower regioselectivity, whereas the shorter tether in allylic amines results in a switch to dominant intermolecular hydroboration. Regioselectivity in THF*BH 3 control experiments is higher for the allylic amine boranes compared to the iodine activation experiments, whereas the reverse is true for homoallylic amine borane activation.     

Organoboron compounds: CDIV. A hydride transfer reacton in the series of ate complexes of 7-substituted 3-alkyl-3-borabicyclo[3.3.1]nonanes and 3-alkyl-3-borabicyclo[3.3.1]non-6-enes

The ate complexes of 7-substituted 3-alkyl-3-borabicyclo[3.3.1]nonanes and of 3-alkyl-3-borabicyclo[3.3.1]non-6-enes react with acetyl chloride under mild conditions by an intermolecular β-hydride transfer mechanism to form 5-substituted 3-methylenecyclohex-1-ylmethyl(dialkyl)boranes. The latter compounds were converted, by oxidation with alkaline hydrogen peroxide, to 3-substituted 1-methylene-5-hydroxymethylcyclohexanes. The reaction of cycloalkylmethyl(dialkyl)boranes with aromatic aldehydes was applied to the synthesis of 1,3-di- and 1,3,5-tri-methylene derivatives of the cyclohexane series.     

Diastereoselective synthesis of chiral non-racemic 2-substituted 2-boranatophosphino ethanoic acid: a potential intermediate to chiral ligands for asymmetric catalysis

Chiral α-amidophosphine boranes 7a–b can be diastereoselectively alkylated, using a phenylglycinol derivative as a chiral inducer, to furnish α-substituted α-amidophosphine boranes 8–12 with up to 99% diastereoisomeric excess. Selective reduction of the amidophosphine boranes afforded optically pure β-boranatophosphine-alcohol 13. The latter one can then be oxidized in boronatophosphine acid 14.     

Synthesis and Ambiphilic Reactivity of Metalated Diorgano-Phosphonite Boranes

Unprecedented metalated phosphonite boranes were prepared from PH-substituted precursors and silyl amides. Although potassium derivatives were thermally stable and could even be isolated and structurally characterised, lithiated analogues proved to be unstable towards self-condensation under cleavage of LiOR at ambient temperature. Reaction studies revealed that the metalated phosphonite boranes exhibit ambiphilic character. Their synthetic potential as nucleophilic building blocks was demonstrated in the synthesis of the first stannylated phosphonite representing a new structural motif in phosphine chemistry.     

Synthesis of alpha-substituted beta-amidophosphines by diastereoselective alkylation. A new access to chiral ligands for asymmetric catalysis

Chiral beta-amidophosphine boranes 7a-f can be diastereoselectively alkylated, using O-protected amino-alcohols as chiral inducers, to furnish alpha-substituted beta-amidophosphine boranes 8a-f and 9-12 with up to 72% diastereoisomeric excess. Selective deprotection afforded optically pure carboxylic derivative 13 which is a key intermediate for the synthesis of various potential chiral ligands for asymmetric catalysis.     

Modular asymmetric synthesis of P-chirogenic beta-amino phosphine boranes

A short concise route to beta-aminophosphine boranes is presented via the desymmetrization of prochiral phosphine boranes, forming P-chirogenic aldehydes that are rapidly transformed to the target compounds employing reductive amination under microwave irradiation. This sequence provides a modular route to P-chirogenic P,N ligands, and in addition, the intermediate aldehydes are versatile P-chiral building blocks for ligand design in general. An alternative pathway via the corresponding alpha-carboxyphosphines is also described. The ligands were subsequently evalutated in the asymmetric conjugate addition of diethylzinc to trans-beta-nitrostyrene.     

Wade规则在稠合型硼烷等中的应用

本文把Ｗａｄｅ规则推广应用于稠合型硼烷和稠合型金属碳硼烷中,导出计算它们价电子数（ＮＶＥ）的公式。对于稠合型硼烷,本文公式与唐敖庆等的拓扑结构规则的计算结果相同。但本文公式的适用范围比上述两个规则广。