Other Base-boranes

Other base-borane adducts which can be obtained by base displacement of tetrahydrofuran from tetrahydrofuran-borane include triethylamine-borane (65% yield obtained), morpholine-borane (72% yield obtained), triethylphosphine -borane (91% yield), and di(n-propyl) sulfide-borane (61% yield). Triethylamine-borane and di(n-propyl)sulfide-borane are best purified by vacuum sublimation or distillation from the crude product. 

Niedenzu and J. W. Dawson, Boron-Nitrogen Compounds, Academic Press, Inc., New York, 1965, and references therein. 

Becke-Goehring and H. Thielemann, Z. Anorg. Allgem. Chem., 308, 33 (1961). 

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If similar processes could be developed at energy conversion efficiency levels that are comparable to the present day SMR-based NH3 synthesis plants, then it would be possible to realize a major reduction in the production costs of ammonia-borane complex. We note that a concept similar to that discussed above has already been developed for nitric acid synthesis process based on boron nitride analogous to the Haber-Bosch route for nitric acid production from NH3. Finally, recent results have shown that unusual parallel behavior exists between hydrocarbons and their corresponding B-N analogues. Thus, hydrogenation of benzene to cyclohexane may also provide a model for the reformation of borazine to other amine-boranes. 

The unique reactivity of the above system with H2 appears to arise from the unquenched Lewis basicity and acidity of the respective donor P and the acceptor B centers. This inference prompted questions about the nature and reactivity of other phosphine-borane systems and, more broadly, of Lewis acid/base combinations. Is it necessary to have a link between the donor and acceptor sites Could similar H2 activation arise from combinations of donors and acceptors in which steric encumbrance frustrates Lewis acid-base adduct formation If indeed such frustrated Lewis pairs could be uncovered, could one exploit them for the activation of small molecules and applications in catalysis ... 

Among chiral dialkylboranes, diisopinocampheylborane (8) is the most important and best-studied asymmetric hydroborating agent. It is obtained in both enantiomeric forms from naturally occurring a-pinene. Several procedures for its synthesis have been developed (151—153). The most convenient one, providing product of essentially 100% ee, involves the hydroboration of a-pinene with borane—dimethyl sulfide in tetrahydrofuran (154). Other chiral dialkylboranes derived from terpenes, eg, 2- and 3-carene (155), limonene (156), and longifolene (157,158), can also be prepared by controlled hydroboration. A more tedious approach to chiral dialkylboranes is based on the resolution of racemates. /n j -2,5-Dimethylborolane, which shows excellent enantioselectivity in the hydroboration of all principal classes of prochiral alkenes except 1,1-disubstituted terminal double bonds, has been... 

Similarly to the P-CHj group, secondary phosphine-boranes react smoothly in the presence of a base (BuLi, NaH) under mild conditions to afford other kinds of functionalized phosphine-boranes in good to high yields, without racemi-zation. Yet the success of deprotonation/treatment with an electrophile process to afford substituted phosphine derivatives without any loss in optical purity may depend on the deprotonation agents employed. Use of butyllithium usually provides the products with high enantiomeric excess in good to high yields [73]. 

Other S/N ligands have been investigated in the enantioselective catalytic reduction of ketones with borane. Thus, Mehler and Martens have reported the synthesis of sulfur-containing ligands based on the L-methionine skeleton and their subsequent application as new chiral catalysts for the borane reduction of ketones." The in situ formed chiral oxazaborolidine catalyst has been used in the reduction of aryl ketones, providing the corresponding alcohols in nearly quantitative yields and high enantioselectivities of up to 99% ee, as shown in Scheme 10.60. 

The borane adducts of phosphines having hydrogen, methyl or methylene groups adjacent to the phosphoms can be easily deprotonated by strong bases and the resulting anions react with various nucleophiles affording borane-protected tertiaiy phosphines as air stable, crystalline materials [23], Quantitative deprotection of the phosphoms can be achieved by treatment with morpholine at 110 °C followed by evaporation to dryness. Dissolution of the solid residue and addition of THF results in precipitation of the products such as -among others-19. 

A boron analog - sodium borohydride - was prepared by reaction of sodium hydride with trimethyl borate [84 or with sodium fluoroborate and hydrogen [55], and gives, on treatment with boron trifluoride or aluminum chloride, borane (diborane) [86. Borane is a strong Lewis acid and forms complexes with many Lewis bases. Some of them, such as complexes with dimethyl sulfide, trimethyl amine and others, are sufficiently stable to have been made commercially available. Some others should be handled with precautions. A spontaneous explosion of a molar solution of borane in tetrahydrofuran stored at less than 15° out of direct sunlight has been reported [87]. 

From the early structural studies carried out on boranes, it appeared that most of these compounds adopted structures based on arrangements of their boron atoms that defined fragments of icosahedra. That other triangular-faced polyhedra were important became apparent during the 1960s 141, 145, 164, 166, 212), as the structures of key materials such as the borane anions and carboranes C2B 2H were determined,... 

Many further examples could be added to those already cited to support the close relationship between carbonyl clusters and borane clusters, particularly for species based on an octahedron. These other clo80 octahedral clusters include anions such as Co0(CO)i6 (34), [Fe6(CO)ieC] - (35, 40), [Rhe(CO)i5l]- (6), [Nie(CO)i2] - (31), [Co4Ni2(CO)i4] (5), and [Rhi2(CO)8o], in which two Rhg octahedra are linked by a metal-metal bond and two bridging carbonyl groups (2). 

The structures, relative stabilities, and relative Lowry-Bronsted acidities of carboranes and boranes as well as related anions, Lewis base adducts, and heteroelement analogs are rationalized primarily on the basis of rudimentary coordination numbers. The principal factors, in decreasing order of importance, are (a) the various deltahedra and deltahedral fragments, (b) the placement of bridge and endohydrogens, (c) the placement of carbon and other heteroelements, and d) the resulting coordination number of boron. 

Chiral Dialkylboranes. Several allylic boranes have been developed as chiral auxiliary reagents (Fig. 5). The introduction of terpene-based reagents such as 12 and 64-68 has been pioneered by H.C. Brown, and the most popular class remains the bis(isopinocampheyl) derivatives (structures 12, 64-66). A wide variety of substituted analogs have been reported, including the popular crotylboranes but also a number of other reagents bearing heteroatom-... 

Deuterium exchange studies on decaborane and other boranes indicate acidic character of bridge hydrogens. They react with bases undergoing proton abstraction reactions ... 

Although most of the structurally characterized gold clusters are phosphine-based systems, in recent years some studies have focused on the synthesis of gold clusters with other stabilizers coexisting with phosphines such as thiols, arsines and boranes. Certain heteroleptic gold clusters stabilized with these ligands and arylphosphines in the same molecule have been structurally characterized. 

Arachno Clusters (2 n + 6 Systems). In comparison to the number of known closo and nido boranes and heteroboranes, there are rdatively fewer arachno species. Pardy because of the lack of a large number of structures on which to base empirical rules, arachno structures appear to be less predictable than their closo and nido counterparts. For example, there are two isomeric forms of B9H15, one with the arachno [19465-30-6] framework shown in Figure 2 (33), the other with a framework more reminiscent of that shown for the nine-atom nido classification (34). Structures of arachno molecules involve the presence of even more extra hydrogens or other electron-donating heteroatoms than nido molecules. Typical examples are given in Table 1. 

With this repertoire of bonding possibilities at our disposal, we car construct the molecular structures of various boron-hydrogen compounds, both neutral species and anions. The simplest is the tetrahydroborate126 or borohydride ion, BH. Although borane is unstable with respect to dimerization, the addition of a Lewis base, H , satisfies the fourth valency of boron and provides a stable entity. Other Lewis bases can coordinate as well. 

In Chapter 15 we observed that I he l electron rule was adequate For predicting stabilities of small organometallic clusters. In this chapter we have seen that Wade s rules allow us Lo make predictions about borune structures based on the number of framework electrons. These rules ulso are adequate for most curboranes. metallacar-boranes, and other heteroboranes. i1" Furthermore, organometallic clusters that are not derived from boranes can be dealt with in a similar fashion. More sophisticated extensions are required for complex larger clusters.139... 

Its unique reactivity comes from the fact that borane first forms a Lewis acid-base complex with the acid and then a boron-carboxylate intermediate which increases the reactivity of the boron hydride and delivers the hydride by an intramolecular reaction. As such it provides a selective way to reduce acids and produce alcohols in the presence of most other functional groups. 

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