Triethylamine—Borane

Certain base adducts of borane, such as triethylamine borane [1722-26-5] (C2H )2N BH, dimethyl sulfide borane [13292-87-OJ, (CH2)2S BH, and tetrahydrofuran borane [14044-65-6] C HgO BH, are more easily and safely handled than B2H and are commercially available. These compounds find wide use as reducing agents and in hydroboration reactions (57). A wide variety of borane reducing agents and hydroborating agents is available from Aldrich Chemical Co., Milwaukee, Wisconsin. Base displacement reactions can be used to convert one adduct to another. The relative stabiUties of BH adducts as a function of Group 15 and 16 donor atoms are P > N and S > O. This order has sparked controversy because the trend opposes the normal order estabUshed by BF. In the case of anionic nucleophiles, base displacement leads to ionic hydroborate adducts (eqs. 20,21). 

Preparation of l,5-diborabicyclo[3.3.3]undecane 142, the diboraanalogue of manxane 143 (Figure 15), was reported as early as 1964 <1964JCS4753>. It was declared that hydroboration of triallylboranes with triethylamine-borane followed by the pyrolysis of the initially formed polymer afforded a complicated mixture of products with a high abundance of 142 <1964JCS4753, 1996CHEC-II(8)889>. 

Although dimeric (t-butylmethyleneamino)dibutylborane can be prepared by the reaction of tri-t-butylborane with pivalonitrile at temperature above 150 °C by elimination of butene. preparation at lower temperatures involving triethylamine-borane according to Eq. (11), illustrates that the formation of this iminoborane proceeds via di-n-butylborane as an intermediate 6 ... 

The thioesterimide-haloboranes, upon thermal dehydrobromation, yield C-S substituted iminoboranes, whereas treatment with trie thy lamine leads to the rather unstable thioesterimides and triethylamine-borane adducts. 

Cluster expansion of the Bn Hu cluster is possible on heating with triethylamine borane to give the dodecahydro-closo-dodecaborate (Eq. 62) [104] ... 

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. 

Calcium bis[tetrahydroborate(l-)] may be prepared from sodium tetra-hydroborate( 1 -) and calcium dichloride by cation exchange11 in a suitable solvent such as dimethylformamide,12 an amine13,14 or an alcohol.13,14 A particularly good preparation of very pure Ca[BH4]2 involves the reaction of calcium dihydride with the triethylamine-borane adduct.15,16 This method may also be used for the preparation of other tetrahydroborates of alkali and alkaline earth metals. The triethylamine-borane adduct17 may be synthesized in a variety of ways, e.g., from triethylamine, sodium tetrahydro-borate(l —), and trichloroborane 18 from a trialkoxyborane, aluminum metal, and hydrogen in the presence of triethylamine 19 or by hydrogenation under pressure of a mixture of triethylborane and triethylamine.20 The triethylamine-borane adduct is a colorless liquid (mp — 2°). It is stable to air and moisture at room temperature and it is easily purified by vacuum distillation (bp 95-96°/12 torr). 

Similar claims are also made for diphenylamine-borane, which is a solid and relatively stable re-agent. Aliphatic amine-borane complexes, however, require elevated temperatures in order to effect hydroboration.Although this is in general a serious drawback, on occasions it may be advantageous by allowing slow liberation of borane, which leads to cyclization products rather than polymers. An example of the use of triethylamine-borane in this way is shown in equation (11). ° Alternatively, methyl iodide or a Lewis acid may be added to complex the amine and thus liberate the reactive, free borane... 

Dithiadiboretanes are obtained by treatment of boron tribromide with hydrogen sulfide (six-membered heterocycles also are formed), by thermolysis of a six-membered borthiin, and by treatment of triethylamine borane with hydrogen sulfide. ... 

Kosinova ML, Fainer NI, Rumyantsev YM, Golubenko AN, Kuznetsov FA (1999) LPCVD boron carbonitride films from triethylamine borane. J de Physique IV, 9, Pr8-915-921... 

Preparation. The reaction of trans,trans,trans-cyc ododccatncnc (1) with triethylamine borane at 140° in perhydrocumene gives m,ct s,fra/ti-perhydro-9b-boraphenalene (2) in 73% yield.1-2... 

Another method of the synthesis of bicyclic boranes (1), (2), (10), and (11) involves hydroboration of trienes. Thus, the hydroboration of 1,4,8-nonatriene (12) with triethylamine-borane at 1 SOHO °C in Diglyme followed by heating for 6 h at 160°C affords isomerically pure 9-boradecalin (2) in 77% yield (Scheme 2) <85JOC520>. 

Diphenylmethyl bromide allowed to react 5 hrs. at room temp, with triethylamine-borane complex in nitromethane -> diphenylmethane. Y ca. 100%. - The yields depend on the stability of the carbonium ions derived from the halides. Prim, halides can not be reduced by this method. F. e., also in liq. SO2, s. S. Matsumura and N. Tokura, Tetrah. Let. 1968, 4703 with diborane in nitromethane s. Tetrah. Let. 1969, 363. 

B(NC4H4)3, tris(pyrrol-1-yl)borane, has been prepared from pyrrole and triethylamine-borane (3 1 mole ratio) by elimination of hydrogen and triethylamine. NMR data (in ppm solvent CDCI3) 6 1B = 27.8, b H = -207, 6 3C = 126.6, 113.6 [12]. 

Polymer formation usually arises from side reactions, in which the diene, because of hydrogen migration, serves as a monofunctional and trifunctional, as well as a difunctional, group. For example, isoprene and triethylamine-borane react to form a distillable compound whose molecular weight suggests a diborolanylalkane, but whose structure is (XIX). Further heating causes disproportionation to (XX) and (XXI) (see Section II.B.). 

Trialkylamine-boranes will also react with polyolefins to form boron heterocyclics in special cases, the products can be distilled. Triethylamine-borane reacts with cyclododeca-l,5,9-trienes to form perhydro-9b-bora-phenalenes 30, 31, 31a, 45, 47, 48). 

Triethylamine borane mixed with tri-n-butylphosphine under argon, distilled through a Vigreux column until two-thirds of the resulting triethylamine are removed, and the remaining triethylamine distilled off in vacuo tri-n-butylphosphine borane. Y 79%. F. e., also borazanes by transamination, s. R. A. Baldwin and R. M. Washburn, J. Org. Ghem. 26, 3549 (1961). 

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