Pentaborane, dicarba

When a boron atom of a borane is replaced by a heteroelement, the compounds are called carbaboranes, phosphaboranes, thiaboranes, a2aboranes, etc, by an adaptation of organic replacement nomenclature. The numbering of the skeleton in heteroboranes is such that the heteroelement is given the lowest possible number consistent with the conventions of the parent borane. Thus C2B2H is dicarba- /(9j (9-pentaborane(5) and could occur as the 1,2-, 2,3-, or 1,5-isomeric forms (l,2-dicarba- /(9j (9-pentaborane(5) [23777-70-0] 2,3-dicarba- /(9j (9-pentaborane(5) [30396-61-3] and... 

But the discovery of the carboranes in the early 1960s revealed that bonding possibilities other than simple o-- or 7r-bonds between B and C centers were necessary to understand the structure of such compounds as 1,5-dicarba-closo-pentaborane(5) [6 in Eq. (1)], which is obtained in low yield in an electrical discharge.6 Ordinary valence conventions cannot account for the bonding of boron to five other atoms, and hence the concept of electron-deficient bonding must be invoked for boron. Although carbon seems to adhere to normal tetravalence, again it should be remembered... 

The doso-carboranes with a C2 B3 skeleton are of particular interest because their structure and bonding have been described as classical and non-classical. Koster et al. [49] reported the preparation of pentaalkyl-1,5-dicarba-doso-pentaboranes by hydroboration of dialkyl(l-alkynyl)boranes with tetraalkyl-diboranes(6) (R2BH)2. By using a large excess of (Et2BH)2 as a hydride bath the route to pentaethyl-1,5 -d ic ar ba- doso-pe n tabo ra n e(5) was found to proceed via the 1-carba-aradwo-pentaborane(lO) [50] (see also Section 3.2.2.4 [33]). 

Scheme 3.2-20. Formation of 1,5-dicarba-c/oso- derivative 39c. Its methylation leads to 39b, pentaborane derivatives 39 (R = CH2CMe3) via and reaction with RC2U to 39d,e (b X = Me c hydroboration of a borylalkyne with HBCI2 to X — Cl, d R — CMe3 e R — Ph). give 40a, which on heating yields the chloro...

Thermal elimination of Me3SiH transforms lb (8 SE) into the folded 1,3-dihydro-1,3-diborete 28g (10 SE), which undergoes cluster expansion to give the dicarba-doso-pentaborane ) 39) (12 SE) (Scheme 3.2-22). Note that one of the duryl substituents bound to boron in lb and 28g is attached to carbon in 39). This describes... 

Scheme 3.2-21. Various routes to 1,5-dicarba-cIoso-pentaboranes 39f-i. 2e reduction of 39h,i furnishes 42h,i (X = Dur, NR2).

Many derivatives of isomeric 2,3-dicarba-nido-hexaborane(8) are known, available from the reaction of pentaborane(9) with alkynes [59, 60]. In contrast, the carboranes of type 43 with non-adjacent carbon atoms are rare. 

Reduction of the pentaalkyl-1,5-dicarba-closo-pentaboranes(5) 44 with elemental K opens the cluster and subsequent reaction with I2 enforces oxidative fusion of two C2B3 units to give a C4B6 cage (Scheme 3.2-36). The peralkylated hexaboraadamantane derivatives 70 (R = Me, Et X-ray structure analysis for R = Me [89]) rearrange irreversibly into the carboranes 71 with a wido-C4B6 framework [89] (X-ray structural analysis for R = Et [90]). The structure of the nido-C Bf, cluster is fluxional in solution [91]. 

PORONCOMPOUNDS - BORON HYDRIDES, HETEROBORANES AND THEIR METALLADERIVATIVES] (Vol 4) Dicarba-closo-pentaborane(5)... 

The tetraborane B4H2R4 (R = rBu) was found to react with 2-butyne to give the 1,5-dicarba-c/oso-pentaborane Me2C2B3R3 in 63 % yield [Eq. (8)]. The hypothetic second product (RBH2)2 is certainly not stable under the reaction conditions (160 °C, 6 bar, excess of butyne).8 The structural identification of the carbaborane by the NMR data followed well known data of analogous carbaboranes, e.g. Me2C2B3Et3.u... 

PENTAETHYL-l,5-DICARBA-cfoso-PENTABORANE(5) AND DECAETHYL-2,6,8,10-TETRACARBA- irfo-DECABORANE(IO)... 

Pentaethyl-l,5-dicarba-c/oso-pentaborane(5) (A) is prepared from diethyl-1-propynylborane and tetraethyldiborane(6) /triethylborane mixtures in a 2-L flask, equipped with a magnetic stirrer, a thermometer tube, and a pressure-equalized dropping funnel (1 L). 

Compound C is synthesized from the pentaethyl-1,5-dicarba-c/oso-pentaborane(S) (A) by a dimerization that has not yet been fully clarified mechanistically 1 mol of A (5"B = + 13.5) reacts with 1.2 mol of metallic potassium in tetrahydrofuran (THF) at room temperature according to the above equation, slowly forming a dark brown solution. The metal does not react faster when the mixture is heated. If > 0.5 mol of potassium per mole of A ( "B = 13.5) are taken up, four new "B NMR signals of the weakly paramagnetic solution (ESR measurementsf) appear at = 26.8, — 14.2, — 16.2, and — 37.1 (see Fig. 1). The intensity of the "B NMR signal of A decreases to ca. 50% after reaction of > 1.0-1.2 mol of potassium per mol of A. Correspondingly, we presume that a potassium salt of the dianionic species B is formed in an equilibrium with A and a small amount of the radical anion salt B (see equation). 

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