Click Dehydrogenation of Carbon-Substituted nido-5,6-C₂B₈H₁₂ Carboranes: A General Route to closo-1,2-C₂B₈H₁₀ Derivatives

Abstract

Triethylamine-catalyzed dehydrogenation of carbon-disubstituted dicarbaboranes 5,6-R2-nido-5,6-C2B8H10 [1, where R = H (1a), Me (1b), and Ph (1c)] in refluxing acetonitrile leads to a high-yield (up to 85-95%) formation of a series of dicarbaboranes 1,2-R2-closo-1,2-C2B8H8 (2). The monosubstituted 6-R-nido-5,6-C2B8H11 (3) analogues [where R = Ph (3a), naph (1-naphthyl; 3b), Bu (3c)] afforded 1-R-1,2-closo C2B8H9 (4) isomers [where R = Ph (4a), naph (4b), n-Bu (4c)] as the main products; compounds 4a and 4c were accompanied by 2-R-1,2-C2B8H9 (5) isomers (total yields up to 90%), with the 4/5 molar ratio being strongly dependent on the nature of R (4:1 and 1:1, respectively). All of these cage-closure reactions are supposed to proceed via the stage of the corresponding Et3NH(+) salts of nido anions [5,6-R2-5,6-C2B8H9](-) (1(-)) and [6-R-5,6-C2B8H10](-) (3(-)), which lose H2 and Et3N upon heating (dehydrodeamination). The cage-closure mechanisms leading to closo isomers 2, 4, and 5 have been substantiated by B3LYP/6-31+G* calculations of the reaction profile for a simple 1a(-) → 2a + H(-) conversion. All of the compounds isolated have been characterized by multinuclear ((11)B, (1)H, and (13)C) NMR spectroscopy, mass spectrometry, and elemental analyses, and the structure of 1-Ph-closo-1,2-C2B8H9 (4a) was established by an X-ray diffraction study.

Synthesis of closo-1,2-H2C2B8Me8 and 1,2-H2C2B8Me7X (X = I and OTf) Dicarbaboranes and Their Rearrangement Reactions

“…There are several methods for cage substitution on the closo -1,2-H 2 C 2 B 8 H 8 dicarbadecaborane(10) that may parallel those employed for the larger C 2 B 10 H 12 icosahedral carboranes. The CH hydrogens in these carboranes are sufficiently protic in character to undergo lithiation with butyllithium in ethereal solvents, generating the mono- and dilithio derivatives .…”

Unusual Closure of the Ten-Vertex Nido Cage via Alkylation: Regiospecific Synthesis of 3-Alkyl Derivatives of closo-1,2-C2B8H10

“…As an example of NMR measurements, see those of the n -Pr derivative in Figures and (for a broader selection of other NMR spectra, see the SI, Figures S3–S21). The 11 B NMR spectra of the asymmetrical 3-R-substituted closo derivatives 4 display one B(3)-singlet and seven doublets assigned by [ 11 B– 11 B]-COSY NMR measurements to boron vertexes B10, B4, and B5 and to the less resolved B7, 8, and B6/9 signals (reading upfield); the spectra excellently correlate with that of the parent 1a . Moreover, 1 H-{ 11 B­(selective)} measurements led to complete assignments of BH resonances to individual cage positions, at least in the well resolved area.…”

“…While the long-known alkylation of the [ nido -B 10 H 13 ] − anion takes place mostly at the apex B6 site, − a comparable B-alkylation of the isoelectronic [ nido -5,6-R 1 2 C 2 B 8 H 9 ] − anions ( 2 – ) with primary alkyl halides proceeds most likely at the equivalent apex B9-position, too. However, the anticipated 9-R substituted nido intermediates 3 tend to spontaneously loose dihydrogen, affording regioselectively 3-alkyl substituted closo- 1,2-dicarbadecaboranes 4 (for structural work in the area of type- 1 carboranes, see refs and ), even under relatively mild conditions employed in this work (refluxing THF). In contrast, a similar reaction of the parent 2a – anion with t -BuBr represents an alternative access to the unsubstituted closo -1,2-C 2 B 8 H 10 dicarbaborane 1a .…”

“…The dicarbaborane closo -1,2-C 2 B 8 H 10 and its C-substituted derivatives (general structure 1 ) were isolated as side products from alkyne insertion into nido -B 8 H 12 and 4- l - arachno -B 9 H 13 , , or from the oxidation of the [ nido -C 2 B 9 H 12 ] − anion. , Apart from other chemistry of nido -5,6-R 2 C 2 B 8 H 10 carboranes ( 2 ) (where R 1 = H or alkyl) of type 2 , − we have just recently reported that they undergo a high-yield dehydrogenation in refluxing acetonitrile in the presence of a catalytic amount of triethyl­amine to afford closo derivatives 1 (see Scheme ). This reaction is supposed to proceed via triethyl­ammonium salts which in turn undergo dehydrogenation under skeletal closure: …”

Reactions of Experimentally Known Closo-C2B8H10 with Bases. A Computational Study

“…The 1,6-isomer has recently been examined experimentally [8]. In addition, a new synthetic pathway for the preparation of the 1,2-isomer has also been outlined, including some mechanistic considerations [9]. Mutual isomerizations of all the possible closo-C 2 B 8 H 10 have been studied computationally [10].…”