Kernresonanzspektroskopische Untersuchungen an Borverbindungen, XX1)11B‐ und 13C‐NMR‐Studien an benzanellierten Heteroborolenen

Goetze, Richard; Nöth, Heinrich; Pommerening, H.; Sedlak, Dieter; Wrackmeyer, Bernd

doi:10.1002/cber.19811140526

Cited by 45 publications

(7 citation statements)

References 25 publications

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“…In the signal derived from the three equivalent MTB ligands, one of the H3(3') binaphthyl protons suffers a large downfield shift relative to the free ligand MTBH 2 (6). These electronic effects are also apparent in the 13 C NMR spectrum of [Ga 2 (MTB) 3 ][Li 2 (MTB)], in which one of the signals appears at appreciably higher frequency ( 13 C NMR: d 158.4) than the rest ( 13 C NMR: d 120.0 ± 139.3). Full assignment of the carbon spectrum was not possible because of problems in obtaining adequate signal/ noise in the 1 H: 13 C COSY spectrum.…”

Section: Introductionmentioning

confidence: 78%

Enantioselective Reduction of Prochiral Ketones by Catecholborane Catalysed by Chiral Group 13 Complexes

et al. 2000

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LiGaH4, in combination with the S,O-chelate 2-hydroxy-2'-mercapto-1,1'-binaphthyl (MTBH2), forms an active catalyst for the asymmetric reduction of prochiral ketones, with catecholborane as the hydride source. Enantioface differentiation is on the basis of the steric requirements of the ketone substituents. Aryl/ n-alkyl ketones are reduced in 90-93% ee and RC(O)Me (e.g. R = iPr, cycloC6H11, tBu) in 60-72% ee. Other borane sources and alternative catalyst structures based on indium do not form enantioselective catalysts.

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Section: Introductionmentioning

confidence: 78%

Enantioselective Reduction of Prochiral Ketones by Catecholborane Catalysed by Chiral Group 13 Complexes

et al. 2000

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“…Signals for 4-coordinate boron are typically located upfield of those for 3-coordinate boron . For example, the doublet in the 11 B NMR spectrum of free HBcat is shifted from δ 29 to δ 10.5 upon coordination by Et 3 N to form the Lewis acid−base adduct HBcat·Et 3 N. , Thus, the upfield shift in the 11 B NMR spectrum of 3 may indicate an increase in coordination number. The 1 H NMR spectrum of complex 3 displayed two hydride signals at δ −4.99 and −6.79 in addition to two Cp singlets at 4.99 and 4.95 ppm.…”

Section: Resultsmentioning

confidence: 99%

Titanocene Borane σ-Complexes

1999

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The chemistry of titanocene bisborane complexes Cp2Ti(HBcat‘)2 (1a − g) (HBcat‘ = catecholborane or a substitued catecholborane) and monoborane complexes Cp2Ti(HBcat‘)(L) (2 − 4) (L = PMe3, PhSiH3, or PhCCPh) is reported. These complexes are unusual σ-complexes. The B−H bond in the catecholborane of 1 acts as a two-electron-donor ligand. The 4-tert-butyl version 1a was studied in depth and underwent ligand substitution reactions with PMe3, CO, PhSiH3, and PhCCPh. The products of the reaction of 1a with PMe3 and PhSiH3 are the novel monoborane σ-complexes Cp2Ti(HBcat‘)(PMe3) (2a; HBcat‘ = HBO2C6H3-4-t-Bu) and Cp2Ti(HBcat‘)(PhSiH3) (3; HBcat‘ = HBO2C6H3-4-t-Bu), in which the catecholborane remains a two-electron-donating ligand. Reaction with CO formed Cp2Ti(CO)2. Reaction with PhCCPh formed Cp2Ti(HBcat‘)(PhCCPh) (4; HBcat‘ = HBO2C6H3-4-t-Bu), which was observed in solution and reductively eliminated the vinyl boronate ester (Ph)(Bcat‘)CC(Ph)(H). The rates for the reactions of 1a with these substrates showed a first-order dependence on the concentration of 1a and a zero-order dependence on the concentrations of both the departing HBcat‘ and the incoming ligand. The substitution reaction proceeded at the same rate ((3.8 ± 0.3) × 10-4) regardless of the identity of the incoming ligand. The entropy of activation was +30 ± 5 eu. These data are consistent with a dissociative substitution mechanism for the reaction of 1a with these substrates. The ΔH ⧧ value of 25 ± 3 kcal mol-1 for these reactions provides an upper limit for the strength of the borane−metal interaction. Electronic effects on the reaction rate support a bonding model involving back-donation from titanium to the borane, and the unusual steric effects allow a proposal for the geometric changes that occur upon formation of the transition state.

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“…Benzene- d 6 was dried over sodium/benzophenone ketyl and degassed before use. The complexes CpFe(CO) 2 Bcat, Cp*Fe(CO) 2 Bcat ( 1d ), [Cp*Fe(CO) 2 ] 2 , [Cp*Ru(CO) 2 ] 2 , , Cp*Ru(CO) 2 H, Cp*W(CO) 3 H, Cp*Mo(CO) 3 H, Li[Cp*W(CO) 2 PMe 3 ], Cl[Bcat( t Bu) 2 ], ClBS 2 Tol, PhBS 2 Tol, , 3,5-dimethylcatechol, and PXy 3 70 were prepared according to literature procedures. All other chemicals were used as received from commercial suppliers.…”

Section: Methodsmentioning

confidence: 99%

Functionalization of Alkanes by Isolated Transition Metal Boryl Complexes

2000

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We report the photochemical reaction of transition metal boryl complexes of the type Cp*M(CO) n B-(OR) 2 (M ) Fe, Ru, W) with alkanes to form alkylboronate esters in yields as high as 85% for reaction ofSynthesis of a series of Cp and Cp* catecholboryl complexes showed that sterically blocking or eliminating sp 2 positions on the metal boryl complex was important for alkane functionalization to occur. The metal boryl complexes reacted exclusively at the terminal C-H position of alkanes. Functionalization of 2-methylbutane occurred preferentially at the least sterically hindered terminal position with a selectivity of 10:1. This selectivity data, in addition to kinetic isotope effects measured for reaction of metal boryls with a mixture of pentane and pentane-d 12 , argues against radical chemistry. Several experiments were conducted to probe for CO dissociation. An experiment employing added 13 CO, one conducted under 2 atm pressure of CO, and one conducted in the presence of PMe 3 indicated that the mechanism involves CO dissociation to form a 16-electron intermediate that reacts faster with alkane solvent than it recoordinates CO. The effect of boryl electronics on the functionalization of alkanes was studied by examining the reactions of ruthenium dialkylboryl, dithioboryl, and dialkoxyboryl complexes with pentane. The dialkoxyboryl complexes gave the highest yields of functionalized product. A comparison between reactions of the different boryl complexes in arene and alkane solvents showed that the electronic properties of the boryl group had a greater effect on the reaction of the unsaturated intermediate with alkane than they did on the generation of the intermediate.

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Kernresonanzspektroskopische Untersuchungen an Borverbindungen, XX¹⁾¹¹B‐ und ¹³C‐NMR‐Studien an benzanellierten Heteroborolenen

Cited by 45 publications

References 25 publications

Enantioselective Reduction of Prochiral Ketones by Catecholborane Catalysed by Chiral Group 13 Complexes

Enantioselective Reduction of Prochiral Ketones by Catecholborane Catalysed by Chiral Group 13 Complexes

Titanocene Borane σ-Complexes

Functionalization of Alkanes by Isolated Transition Metal Boryl Complexes

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