Stability of polyhedral borane anions and carboranes

Polyhedral borane anions and carboranes that can be constructed formally from the interaction of rings and caps will be stable with six interstitial electrons. Interstitial electron count is obtained by summing the number of π electrons of the ring and the electrons of the caps involved in ring cap binding. Thus B₇H₇ ⁻² (D_5h) has 6 interstitial electrons (none from the B₅H₅ ring, two each from the twobh caps and two negative charge),mndo calculations on isoelectronic pyramidal molecules B₆H₆ ⁻⁴ (C_5v), B₅H₅CH⁻³ (C_5v), B₅H₅ ⁻⁴ (C_4v), B₄H₄CH⁻³ (C_4v), B₄H₄ ⁻⁴ (T _d) and B₃H₃CH⁻³ (C_3v) suggests a criterion based on the out-of-plane bendings of the ring B-H bonds to select the best combination of borocycles and BH or CH caps. Three-membered borocycle prefers CH cap, five-membered borocycle prefers BH cap. The preference of four-membered ring for BH or CH cap is not as pronounced. The extra stability of B₁₂H₁₂ ⁻² arises from the geometry of the icosahedron. The relative stabilities ofnido andcloso carboranes follow from these rules.     

Ammonia borane as a metal free reductant for ketones and aldehydes: a mechanistic study

Without a catalyst ketones and aldehydes were reacted in THF with ammonia borane (AB) to proceed hydroboration forming alkyl borates. Mechanistic studies revealed that dissociation of ammonia from AB occurred before the hydroboration step. When methanol was used as the solvent, metal free methanolysis of AB would take place with the ketone/aldehyde being directly hydrogenated by the MeOH·BH₃ complex.     

P-stereogenic wide bite angle diphosphine ligands

Two modular synthetic approaches for the preparation of novel wide bite angle diphosphine ligands containing stereogenic P-atoms have been developed, leading to compounds (S,S)-2,2′-bis(methylphenylphosphino)diphenyl ether (L1) and (S,S)-2,2′-bis(ferrocenylphenylphosphino)diphenyl ether (L2) in very good diastereomeric ratios. Both protocols involve diphenyl ether as backbone and (2R_P,4S_C,5R_C)-(+)-3,4-dimethyl-2,5-diphenyl-1,3,2-oxazaphospholidine borane (R_P)-5 as initial auxiliary to induce chirality at phosphorus. The absolute configuration of intermediates (S,S)-9-(BH₃)₂ and (R,R)-10-(BH₃)₂ as well as the ligands (S,S)-L1-BH3 and (S,S)-L2 was determined by X-ray crystallographic analysis.     

Tetraglymes as Prochiral Host Reagents for Ammonia Borane

The molecular structure of the complex of ammonia borane (AB) with acyclic ether tetraglyme Me(OCH₂CH₂)₄OMe ( 1 ), 1· (AB)₂ was determined by single‐crystal X‐ray structure analysis for the first time. The crystal structure features two AB molecules, bonded by dihydrogen bonds, per one tetraglyme unit. The intermolecular BH ··· HN distances of 1.94 Å are shorter than those in the solid ammonia borane (2.02–2.32 Å). A comparison of the hydrogen and dihydrogen bonds in 1· (AB)₂ and in the complexes of AB with crown‐ethers (CE) was carried out. The complex formation with both the CE and the acyclic polyether 1 activates the B–H bond in AB via N–H ··· O hydrogen bonds and therefore increases the reducing activity of AB. Supposedly, the structure of 1· (AB)₂ is related to the initial steps of the AB activation in a polyether solution. The effect of the substituents on the complexation of the substituted derivatives of 1 comes down to a structural adjustment minimizing steric repulsion. Computations reveal that the complexation of diastereomeric disubstituted glymes with AB leads to the formation of diastereomeric complexes that differ noticeably in stability. This is a prerequisite for inducing stereoselectivity, which makes such complexes attractive for potential synthetic applications.     

Effect of Lewis base coordination on boryl radical reactivity: investigation using laser flash photolysis and kinetic ESR

The effect of Lewis base coordination on boryl radical reactivity (L→BH where L is triethylamine, 2‐picoline, 4‐dimethylaminopyridine, quinoline and diphenyl phosphine) is examined. Direct detection of the different boryl radicals, their transient absorption spectra, their interaction with double or triple bonds, oxygen, oxidizing agent, alkyl halides and their hydrogen lability have been studied using laser flash photolysis (LFP), kinetic ESR (KESR), ESR spin trapping and MO calculations. For example, a strong decrease of both the bond dissociation energy (BDE)(B? H) (33.8 kcal mol^?1) and the addition rate constant to MA (1300 10⁵ to >10⁵ M^?1 s^?1) was noted when going from the triethylamine borane to the quinoline borane complex, in line with the spin delocalization (1.04 vs. 0.19). These structures are also proposed as new highly efficient co‐initiators for the acrylate photopolymerization. Copyright © 2009 John Wiley & Sons, Ltd.     

Structures and Stability of Metal Amidoboranes （MAB）： Density Functional Calculations

Molecule geometry structures, frequencies, and energetic stabilities of ammonia borane （AB, NH3BH3 ） and metal amidoboranes （MAB, MNH2BH3）, formed by substituting H atom in AB with one of main group metal atoms, have been investigated by density-functional theory and optimized at the B3LYP levels with 6-311G＋＋ （3dr, 3pd） basic set. Their structural parameters and infrared spectrum characteristic peaks have been predicted, which should be the criterion of a successfully synthesized material. Several parameters such as binding energies, vibrational frequencies, and the energy gaps between the HOMO and the LUMO have been adopted to characterize and evaluate their structure stabilities. It is also found that the binding energies and HOMO-LUMO energy gaps of the MAB obviously change with the substitution of the atoms. MgAB has the lowest binding energy and is easier to decompose than any other substitutional structures under same conditions, while CaAB has the highest chemical activity.     

A Method for the Synthesis of 2,3-Disubstituted 2,3-Dihydrobenzofurans

Summary.  The synthesis of 2,3-disubstituted 2,3-dihydrobenzofuran diastereomers is described. The key step in the reaction sequence is the chemoselective reduction of a tert. alcohole with tert.-butylamine-borane/AlCl₃. The relative configuration of the substituents on the dihydrofurane moiety was assigned via NMR spectroscopy. Received September 7, 1999. Accepted November 9, 1999     

Practical enantioselective reduction of ketones using oxazaborolidine catalyst generated in situ from chiral lactam alcohol and borane

Reduction intermediate prepared in situ from chiral lactam alcohol 3 and borane at room temperature was found to catalyze the borane reduction of various prochiral ketones with high enantioselectivity up to 98% ee.