Palladium-Catalyzed Coupling of Ethynylated p-Carborane Derivatives:  Synthesis and Structural Characterization of Modular Ethynylated p-Carborane Molecules

Abstract

Methodology leading to a new class of rodlike p-carborane derivatives is described, involving the palladium-catalyzed coupling of B-iodinated p-carboranes with terminal alkynes. The products of these reactions contain an alkyne substituent at a boron vertex of the p-carborane cage. Reaction of closo-2-I-1,12-C(2)B(10)H(11) (1) with closo-2-(C&tbd1;CH)-1,12-C(2)B(10)H(11) (3) in the presence of pyrrolidine and catalytic quantities of bis(triphenylphosphine)palladium dichloride and cupric iodide yields 1,2-(closo-1',12'-C(2)B(10)H(11)-2'-yl)(2) acetylene (4). Oxidative coupling of 3 in the presence of cupric chloride in piperidine affords 1,4-(closo-1',12'-C(2)B(10)H(11)-2'-yl)(2)-1,3-butadiyne (5). Reaction of 2 molar equiv. of closo-2,9-I(2)-1,12-C(2)B(10)H(10) (6) withcloso-2,9-(C&tbd1;CH)(2)-1,12-C(2)B(10)H(10) (7) in the presence of pyrrolidine and catalytic quantities of bis(triphenylphosphine)palladium dichloride and cupric iodide yields closo-2,9-(closo-2'-I-9'-C&tbd1;C-1',12'-C(2)B(10)H(10))(2)-1,12-C(2)B(10)H(10) (8), a rigid, iodine-terminated carborod trimer in which the p-carborane cages are linked at the 2 and 9 B-vertices by alkyne (C&tbd1;C) bridges. The molecular structures of 5 and the previously described closo-2,9-(C&tbd1;CSiMe(3))(2)-1,12-C(2)B(10)H(10) (9) have been determined by X-ray crystallography. Crystallographic data are as follows: for 5, monoclinic, space group P2/a, a = 12.352(6) Å, b = 14.169 (6) Å, c = 12.384(5) Å, beta = 109.69(2) degrees, V = 2041 Å(3), Z = 4, R = 0.098, R(w)( )()= 0.135; for 9, monoclinic, space group C2/m, a = 22.111(4) Å, b = 7.565(2) Å, c = 6.943(2) Å, beta = 107.871(8) degrees, V = 1105 Å(3), Z = 2, R = 0.059, R(w)( )()= 0.090.

Dicarba‐closo‐dodecaboranes with One and Two Ethynyl Groups Bonded to Boron

“…The Me 3 -SiCC groups of the second independent molecule in the crystal of 2b are disordered. The bond lengths of the alkynyl groups in 2b are similar to values reported for 2,9-(Me 3 SiCC) 2 -closo-1,12-C 2 B 10 H 8 [19] ( Table 1). The experimentally determined bond lengths of the {closo-C 2 B 10 } clusters of 1a, 2b, and 3a are in good agreement with values calculated at the B3LYP/6-311++G(d,p) level of theory.…”

“…Only two structures of related {closo-C 2 B 10 } clusters with alkynyl groups bonded to boron have been previously reported: 2,9-(Me 3 SiCC) 2 -closo-1,12-C 2 B 10 H 10 and 1,4-(closo-1Ј,12Ј-C 2 B 10 H 11 -2Ј-yl) 2 -1,3-butadiyne. [19] In contrast, a number of dicarba-closo-dodecaboranes with ethynyl groups or other alkynyl substituents bonded to carbon were structurally characterized. [2,[27][28][29][30][31][32] In Figure 1 the diethynyl-substituted clusters 1a and 3a are depicted.…”

“…[15] Thereafter, other dicarba-closododecaboranes with alkynyl substituents bonded to boron have been described that were either synthesized following a Kumada-type reaction protocol or a related coupling procedure. [17][18][19][20] Furthermore, cross-coupling reactions were employed for the preparation of other {closo-C 2 B 10 } derivatives with various substituents bonded to boron, for example, by means of a carbon [9,11,15,[17][18][19][20][21][22] or a nitrogen [23] atom. In this contribution, the syntheses of the diethynyldicarba-closo-dodecaboranes 1,2-R 2 -9,12-(HCC) 2 -closo-1,2-C 2 B 10 H 8 [R = H (1a), Me (2a)] and 9,10-(HCC) 2 -closo-1,7-C 2 B 10 H 10 (3a) and their NMR and vibrational spectroscopic data are presented.…”

Relevance of the Electronegativity of Boron in η5‐Coordinating Ligands: Regioselective Monoalkylation and Monoarylation in Cobaltabisdicarbollide [3,3′‐Co(1,2‐C2B9H11)2]− Clusters

“…[15a, 28] In these earlier reports, it was said that diminished yields are observed in reactions in which the intermediate in the catalytic cycle is capable of b-hydrogen elimination. If this were the case, low coupling yields would have been expected with ethylmagnesium bromide, but the corresponding product was obtained in reasonably good yield (80 %).…”

Four Decades of Organic Chemistry of closo-Boranes: A Synthetic Toolbox for Constructing Liquid Crystal Materials. A Review

“…39) 126 organometallic reagents. First demonstrated for iodocarboranes 175 , the reaction has also been applied to charged species derived from [B 12 [173][174][175]177,178 ), and para-carborane 6 (refs 126,175,179 …”