Borane complexes bonds

Reduction of 3,5,5-tris-aryl-2(5// )-furanones 115 (R, R, R = aryl) with dimethyl sulfide-borane led to the formation of the 2,5-dihydrofurans 116 in high yields. However, in the case of 3,4-diaryl-2(5//)-furanones 115 (R, R = aryl R = H or r = H R, R = aryl), the reduction led to a complicated mixture of products of which only the diarylfurans 117 could be characterized (Scheme 36) (88S68). It was concluded that the smooth conversion of the tris-aryl-2(5//)-furanones to the corresponding furan derivatives with the dimethylsulfide-borane complex in high yields could be due to the presence of bulky aryl substituents which prevent addition reaction across the double bond (88S68). 

Borane, 1-methylbenzylaminocyanohydropyrrolyl-, 3, 84 Borane, thiocyanato-halogenohydro-, 3,88 Borane, trialkoxy-amine complexes, 3, 88 Borane, triaryl-guanidine complexes, 2,283 Borane, trifluoro-complexes Lewis acids, 3,87 van der Waals complexes, 3, 84 Borane complexes aminecarboxy-, 3,84 aminehalogeno-, 3, 84 amines, 3, 82, 101 B-N bond polarity, 3, 82 preparation, 3, 83 reactions, 3, 83 bonds B-N, 3, 88 B-O, 3, 88 B-S, 3, 88 Jt bonds, 3, 82 carbon monoxide, 3, 84 chiral boron, 3, 84 dimethyl sulfide, 3, 84 enthalpy of dissociation, 3, 82... 

A comprehensive review of the preparation, reactions, and n.m.r. spectra of phosphorus-fluorine compounds has appeared. This year s literature has been notable for the first detailed applications of ab initio SCF-MO calculations to the problems of bonding in halogenophosphines and their derivatives. - Comparison of the results of such theoretical calculations with experimental data obtained from photoelectron spectra shows a good correlation in the case of phosphorus trichloride and phosphoryl chloride, and of phosphorus trifluoride and its borane complex. ... 

The catalytic cycle for hydroboration is now widely accepted and direct examples of several intermediate species have been isolated and well characterized (Scheme 3).5-7 These now include (j-borane complexes, which have in some instances been found to be catalytic precursors for hydroboration.8-10 Oxidative addition of an H—B bond to a coordinatively unsaturated metal fragment... 

Miscellaneous spectral studies include e.s.r. spectra of chloro-phosphines (2)6 and vibrational spectra of various phosphines.6-8 N.m.r. studies include those of borane complexes of aryldichlorophosphines,9 and studies of the conformation of dichloro-(isopropyl)phosphine (3)10 and dichloro(ethyl)phosphine (4).10 These phosphines have the preferred conformations shown. Conformation has also been the theme of electron-diffraction work on the anhydride (5), which appears to have a staggered conformation.11 The electronic implications of a relatively short P—O bond and wide POP angle have been discussed.11... 

The basicity of halogenophosphines has attracted some attention this year. A careful study of the protonation of several halogenophosphines (6) has led to the characterization by 31P n.m.r. of phosphonium species, such as (7).12 These salts show 31P shifts to relatively high fields, and large Vph values e.g., (7) has 1190 Hz.12 Attempts have been made to correlate phosphine basicity with Vpb values in borane complexes,13 and, in turn, to correlate Vpb with electrostatic interactions in the phosphorus-boron bond.13 From such correlations, the authors deduce that this bond is dominated by electrostatic contributions, and not by other factors, such as -bonding. 

S)-(-)-CITRONELLOL from geraniol. An asymmetrically catalyzed Diels-Alder reaction is used to prepare (1 R)-1,3,4-TRIMETHYL-3-C YCLOHEXENE-1 -CARBOXALDEHYDE with an (acyloxy)borane complex derived from L-(+)-tartaric acid as the catalyst. A high-yield procedure for the rearrangement of epoxides to carbonyl compounds catalyzed by METHYLALUMINUM BIS(4-BROMO-2,6-DI-tert-BUTYLPHENOXIDE) is demonstrated with a preparation of DIPHENYL-ACETALDEHYDE from stilbene oxide. A palladium/copper catalyst system is used to prepare (Z)-2-BROMO-5-(TRIMETHYLSILYL)-2-PENTEN-4-YNOIC ACID ETHYL ESTER. The coupling of vinyl and aryl halides with acetylenes is a powerful carbon-carbon bond-forming reaction, particularly valuable for the construction of such enyne systems. 

The stability of the coordinative bond in the borane complexes is determined by the electrophilicity of the borane, by the nucleophilicity of the ligand and by steric factors. The electrophilicity of the borane, in turn, depends on the electronegativity of the groups bonded to the boron and on the overlap between the unoccupied p orbital of the boron and the occupied p orbitals of the atoms directly bonded to it (p -p interaction). The predominance of the latter is reflected by the fact that the relative acidities of halogenoboranes increase in a sense opposite to that expected from electronegativity considerations and is supported by the very weak Lewis acidity of the alkoxy- and amino-boranes. The importance of steric factors is pointed out by Brown and co-workers.13-15... 

In a number of cases, e.g. for CO, PH3, PF3 and thioethers, the existence of complexes or their surprising stability was explained in terms of a special n bond.16 However, experimental data and theoretical considerations seem to contradict the assumption that such kinds of n bonds exist.25 Borane complexes in which donor atoms of appropriate nucleophilicity are part of the group attached to the boron, e.g. BCl2(NMe2), BH2(PMe2), BBr2SMe, constitute a special case and exist due to intermolecular coordination, in the form of dimers, trimers or polymers.17-19... 

New mechanistic studies with [Cp2Ti(CO)2] led to the observation that the tita-nocene bis(borane) complex [Cp2Ti(HBcat)2] (Hbcat = catecholborane) generated in situ is the active catalyst.603 It is highly active in the hydroboration of vinylarenes to afford anti-Markovnikov products exclusively, which is in contrast to that of most Rh(I)-catalyzed vinylarene hydroboration. Catecholborane and pinacolborane hydroborate various terminal alkynes in the presence of Rh(I) or Ir(I) complexes in situ generated from [Rh(COD)Cl2] or [Ir(COD)Cl2] and trialkylphosphines.604 The reaction yields (Z)-l-alkenylboron compounds [Eq. (6.107)] that is, anti addition of the B—H bond occurs, which is opposite to results found in catalyzed or uncatalyzed hydroboration of alkynes ... 

Computational studies performed on model complexes in collaboration with Hall and coworkers suggest that alkane borylation may occur by a ej-bond metathesis pathway (Scheme 3) [48]. The proposed mechanism for the borylation of alkanes by 1 begins with elimination of HBpin to generate the 16-electron complex Cp Rh(Bpin)2. This complex then forms a <7-complex (3) with the alkane. The vacant p-orbital on boron then enables cr-bond metathesis to generate a o-borane complex (4). Reductive elimination of the alkylboronate ester product and oxidative addition of B2pin2 then regenerate 1. 

Complex 26 is among the first examples of intermolecular coordination of a single B-H bond in a neutral borane to a transition metal, and these species can be regarded as models for alkane coordination. The crystal structures of borane complexes such as 26 are of interest to compare with those for other octahedral a complexes, particularly... 

The boryl complex Mn(CO)4(PR3)(BH2PMe3) is proto-nated to the borane analogue, which also contains a cr-bond -ligand (HBH2PMe3) similar to that obtained with H2 and HSiRs in Scheme 3. In a similar vein, the ct-borane complex... 

CpMn(CO)2(HBH2PMe3) has been characterized, with the conclusion that the bonding consists primarily of a-donation from the B-H bond with little d orbital back donation into the B-H a orbital. An upper hmit of 100 kJ has been estimated for the r] bond in the a-borane complex (8). The borane ligand in (8) is readily replaced by CO, PhC CPh, HSiMePh2, and HSnPhs, with the latter two hgands forming H-Si and H Sn a-bond complexes, as indicated in Scheme 4. 

Free oxazaborolidine (16), by itself, will not reduce ketones. Furthermore, (16) is not particularly stable, reacting with moisture (H2O), air (O2), unreacted amino alcohol, other alcohols, or, depending on the substituents, with itself to form various dimers. This instability is due to the strain of a partial double bond between nitrogen and boron (eq 4). Formation of the oxazaborolidine-borane complex (17) tends to release some of this strain. As such, (16) and (17) are generally prepared and used in situ without isolation in many cases, they have not been fully characterized. ... 

Substituted oxazaborolidines also react with borane (B2H6, H3B THF, or H3B SM62) to form an oxazaborolidine-borane complex (19) (eq 9). The oxazaborolidine-borane complex, by releasing the strain of the partial double bond between the ring boron and nitrogen, is more stable than the free oxazaborolidine, and in many cases exists as a stable crystalline solid. ... 

Kabalka and co-workers have shown that an attenuated form of BI3, BI3 NEt2Ph, will cleave a variety of compounds containing C-O single bonds at elevated temperatures [20]. Solutions of this reagent are prepared by reacting the commercially available amine-borane complex with I2 in benzene at 80 °C for several hours. This reagent cleaves ethers [21], esters [20], and geminal diacetates [21]. Esters [20] are cleaved to an activated acyl intermediate RCOX which can be used to prepare acids, other esters, and amides (Eq. 10). 

The formation of heterocycles 25-35, which are internal Lewis base-borane complexes, is dependent on the borane substituents. An increase in the Lewis acidity of the borane can result in ring formation. For example, bis(methoxy)borane 101 has no B-P bond, but treatment with boron trichloride results in the formation of 1,2-phosphoniaboratole 156, presumably via 155 (Scheme 12) <1997ZFA1093>. 

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