4- diethylboryl

Diethylboryl)pyridines and (3-Aminophenyl)boronic Acid Derivatives... 

Although 3-(diethylboryl)pyridine has been known since 1983 [20], it was not until 1994 that it was discovered to have a tetrameric structure both in solution (CHCI3, THF) and in the solid state (Fig. 3) [21, 22]. 

Recently, Wrackmeyer /. synthesized 2,3-dihydro-l//-l,3-stannaborole 29b by treating ( )-2-chloro(dimethyl)-stannyl-3-diethylboryl-A, A -dimethyl-2-pentenylamine 48 with LDA in THF (Equation 5) <2003JOM188>. The cyclization owes its success to the high basicity of LDA that abstracts a proton from one of the ethyl groups at boron the resulting carbanion acts as a nucleophile attacking the tin atom. Diisopropylamine and lithium chloride are eliminated from the reaction. 

Fig. 3. Dimeric and tetrameric macrocyclic structures obtained from (diethylboryl)pyridines...

These changes are more consistent with an /-effect, rather than an R-effect, on the B—C bond. That such diborylacetylenes have a diminished electron density at boron relative to alkyl- and vinylboranes is evident from their enhanced Lewis acidity bis(diethylboryl)acetylene (40) forms a bis(tetrahydrofuran) complex, as is evident from the1 B NMR spectrum37 (Eq. 11). Alkyl- and vinylboranes exhibit no such shifts in their11B signals in THF solution. 

The reaction leading to 20 was used to prepare 25 (Scheme 3.2-13) [35], starting from diethyl(propyn-l-yl)borane which served as a versatile reagent for the synthesis of other carboranes (vide infra). l,l-Bis(diethyl-boryl)propene 23 can be detected and isolated, whereas it was not possible to identify l,l,l-tris(diethylboryl)propane 24 in the reaction mixture. 

It turned out that the [B-H]-catalyzed condensation of two molecules of the intermediate 24 proceeds slowly with elimination of BEt3, when compared with bis(diethylboryl)-substituted hydrocarbons. Thus, a route to the arachno-carboranes 27 is provided [36] by heating the in situ formed 24 and 26 in the "hydride bath . The carboranes 27 were obtained as mixtures of isomers (Scheme 3.2-14). 

Scheme 3.2-27. Combined hydroboration and Et2BH-catalyzed condensation of diethyl(propyn-l-yl)borane (via 29) and (Z) bis(diethylboryl)alkenes lead to the 2,3,5-tricarba-n/do-hexaborane(7) derivatives 55...

Considering the success of the condensation route to carboranes in the hydride bath (vide supra), other alkynylboranes than diethyl(propyn-l-yl)borane might be equally suitable. By heating a mixture of bis(diethylboryl) ethyne (65) and excess of (Et2BH)2 ( hydride bath ) at 110-120 °C, l,2,3,4-tetraethyl-5,6,7,8-tetracarba-mdo-octaborane(8) 64c was obtained by distillation in ca. 20% yield as a colorless liquid, stable to air and H2O (Scheme 3.2-35) [87]. Possible intermediates in this reaction can be proposed as 67 and 68, where 67 results from double hydroboration of bis(diethylboryl)ethyne (65), which dimerizes to 68 and finally yields the carbo-rane 64c by elimination of Et3B. 

Scheme 3.2-35. Hyd roboration of bis(diethylboryl)ethyne 65 in the hydride bath to 66 and 67, followed by Et2BH-catalyzed condensation to 68 and self-assembly to give the nido-C4B4 carborane...

Scheme 3.2-39. The spirocarbaborane 55g is formed as one component (see Scheme 3.2-27) in the hydride bath starting from bis(diethylboryl)ethyne as the result of hydroboration and Et2BH-catalyzed condensations.

Several investigations have addressed the synthesis of boryl eno-lates by carbonyl enolization. Kbster has examined in detail the thermal reaction of triethylborane with substituted ethyl ketones catalyzed by diethylboryl pivalate (58) (eq. [40]) (61). The boryl pivalate 58 is undoubtedly the active reagent in this system, and it is regenerated by the illustrated protonolysis (eq. [41]) (62). The vigorous conditions employed in this procedure probably result in the generation of the equilibrated boryl enolates. The enolate ratios obtained by way of this procedure are summarized in Table 18. 

Stereochemistry of the Enolates Generated by the Reaction of Ketones and Diethylboryl Pivalate-EtsB (eq. [40]) (61)... 

A(A -Bis(diethylboryl)methylamine, 3223 Bis(dipropylborino)acetylene, 3670 Dibutyl-3-methyl-3-buten-l-ynlborane, 3618... 

V, /V- B i s(diethylboryl)meth y I am i ne, 3217 Bis(dipropylborino)acetylene, 3663 Dibutyl-3-methyl-3-buten-1 -ynlborane, 3612... 

Charge the flame dried and argon flushed three-necked flask with 1,3-b/s (diethylboryl)propane (0.88 g, 4.89 mmol). Cool to 0°C and add diethylzinc (2 mL, 20 mmol) (Caution ). 

German workers independently found that 1,2- 1,3-, and some 1,4-diols can be converted into bis(diethylborinates) which, on heating to 200°, lose triethylboron to give cyclic ethylboronates. Otherwise, these boronates can be prepared directly from the diols by heating with triethylboron in the presence of diethylboryl pivalate, or by dismuta-tion from equimolar proportions of diols and their bis(diethylbori-... 

There is but one brief report on the formation of boron spiro-cations in which three or more boron atoms are bridged by pyrazolyl groups. The first such species was obtained on interaction of the [B(pz)4] ion with two molar equivalents of diethylboryl tosylate, (C2H5)2B(0S02C6H4-4-CH3)... 

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