Borane complexes neutral

Reduction of aldehydes and ketones. Earlier work on amine borane reagents was conducted mainly with tertiary amines and led to the conclusion that these borane complexes reduced carbonyl compounds very slowly, at least under neutral conditions, and that the yield of alcohols is low. Actually complexes of borane with primary amines, NHj or (CH3)3CNH2, reduce carbonyl compounds rapidly and with utilization of the three hydride equivalents. BH3 NH3 is less subject to steric effects than traditional complex hydrides. A particular advantage is that NH3 BH3 and (CH3)3CNH2 BH3 reduce aldehyde groups much more rapidly than keto groups, but cyclohexanone can be reduced selectively in the presence of aliphatic and aromatic acyclic ketones. 

Tricoordinated boron compounds (boranes) are coordinatively unsaturated and their chemistry is dominated by reactions in which complexes are formed. These complexes are either neutral molecules (borane complexes), anions (borates) or boron cations. Space limitations mean that little or no attention will be paid to complexes containing several boron atoms and to species of the type L-BH3, [BH,]- and [L2BH2]+ (L = neutral ligand), discussed in detail in several books and reviews. Similarly, little attention will be paid to the plethora of metal borates and the cyclic and polymeric amino- and phosphino-boranes. 

NEUTRAL BORANE COMPLEXES 24.2.1 General Considerations and Stability... 

Since the discovery of NH3-BF3 by Gay-Lussac and Thenard12 in 1809, many neutral borane complexes have been described, mainly with N, P, As, O and S donor molecules. Their stabilities vary considerably many are formed at low temperatures only, whereas in other cases... 

The stability and equilibrium properties of neutral borane complexes are not well known. Instead, the recent chemistry of the borane complexes is predominantly characterized by preparative and structural investigations. This is even more obvious for the charged borane complexes. 

Prom neutral borane complexes via proton abstraction ... 

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... 

Cryptand (10) Cooled 9 (10 g. 26 mmoO THF (100 mL) was treated with 1 5 M diborane (100 mL). the mixture was heated to 25 C over 30 mm and refluxed lor 2 h The solvent was removed m vacuum, the crude amine borane complex was refluxed with 6 N HCI (100 mL) for3 h and evaporated to dryness The residue was dissolved m a minimum of water and passed through Oowex 1 (50 100 mesh as HO) Concentration to dryness ol the altalme and neutral ettient and azeotropic eiimmatton of water gave 9 28 g of 10 (95%). mp 68 69°C (hexane)... 

In the meantime, we studied the coordination of the bisphosphine-alane of type 7 to gold and obtained thereby the zwitterionic complex 32 (Scheme 3). The transfer of chloride from gold to aluminum was established crystaUographically. The coordination mode of 32 markedly contrasts with that of the related bisphosphine-borane complexes 17 (neutral square-planar complexes with Au B interactions). The difference between... 

The ligand is neutral and the asymmetric side-on bonding geometry with Ti-H-B near 100° is more like that in f/2-silanes than in the borohydrides or W(CO)5-(BH3 PMe3) (Figure 13.2, Table 13.1). The complexes are initially synthesized as bis(borane) complexes [Eq. (13.3)] from which one borane can be displaced as in Eq. (13.4) to form, e.g., an analogous a complex (15) with an t/2-silane instead of an >/2-borane. 

Borane, 1-methylbenzylaminocyanohydropyrrolyl-, 84 Borane, thiocyqnato-halogenohydro-, 88 Borane, trialkoxy-amine complexes, 88 Borane, trifluoro-complexes Lewis acids, 87 van der Waals complexes, 84 Borane complexes aminecarboxy-, 84 aminehalogeno-, 84 amines, 82,101 B-N bond polarity, 82 preparation, 83 reactions, 83 bonds B-N, 88 B-0,88 B-S,88 31 bonds, 82 carbon monoxide, 84 chiral boron, 84 dimethyl sulfide, 84 enthalpy of dissociation, 82 halogenohydro, 84 hydro, 83 monocyano, 84 neutral, 81-90 stability, 81 NMR, 83... 

Neutral carboranes and boranes react with transition-metal complexes forming metallocarboranes or metalloboranes, respectively. However, most metallocarboranes and metalloboranes are prepared from transition-metal halides and anionic carborane and borane species ( 6.5.3.4) or by reacting metal atoms and neutral boranes and carboranes. These reactions are oxidative addition reactions ( 6.5.3.3). 

The differences in the steric effect between catecholborane and pinacolborane, and the valence effect between a cationic or neutral rhodium complex reverse the re-gioselechvity for fluoroalkenes (Scheme 1-4) [26]. The reaction affords one of two possible isomers with excellent regioselectivity by selecting borane and the catalyst appropriately, whereas the uncatalyzed reaction of 9-BBN or SiaiBH failed to yield the hydroboration products because of the low nucleophilicity of fluoroalkenes. The regiochemical preference is consistent with the selectivity that is observed in the hydroboration of styrene. Thus, the internal products are selectively obtained when using a cationic rhodium and small catecholborane while bulky pinacolborane yields terminal products in the presence of a neutral rhodium catalyst. 

Closely related to, but distinct from, the anionic boron and aluminum hydrides are the neutral boron (borane, BH3) and aluminum (alane, A1H3) hydrides. These molecules also contain hydrogen that can be transferred as hydride. Borane and alane differ from the anionic hydrides in being electrophilic species by virtue of the vacant p orbital and are Lewis acids. Reduction by these molecules occurs by an intramolecular hydride transfer in a Lewis acid-base complex of the reactant and reductant. 

This is one of two articles in this volume concerned with the borane-carborane structural pattern. In the other (see Williams, this volume, p. 67) Williams has shown how the pattern reflects the coordination number preferences of the various atoms involved. The purpose of the present article is to note some bonding implications of the pattern, and to show its relevance to a wide range of other compounds, including metal clusters, metal-hydrocarbon n complexes, and various neutral or charged hydrocarbons. 

Although boranes and hydrocarbons are more notable for their differences than for any similarities, there are several important hydrocarbon systems that adopt structures that conform to the borane pattern (202). They include tnetal-hydrocarbon v complexes, various aromatic systems, and certain other neutral or charged hydrocarbons. Representative examples are listed in Table VII. 

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