H(+), CH(3)(+), and R(3)Si(+) carborane reagents: when triflates fail

Abstract

For decades, triflic acid, methyl triflate, and trialkylsilyl triflate reagents have served synthetic chemistry well as clean, strong electrophilic sources of H(+), CH(3)(+), and R(3)Si(+), respectively. However, a number of weakly basic substrates are unreactive toward these reagents. In addition, triflate anion can express undesired nucleophilicity toward electrophilically activated substrates. In this Account, we describe methods that replace triflate-based electrophilic reagents with carborane reagents. Using carborane anions of type CHB(11)R(5)X(6)(-) (R = H, Me, X; X = Br, Cl), members of a class of notably inert, weakly nucleophilic anions, significantly increases the electrophilicity of these reagents and shuts down subsequent nucleophilic chemistry of the anion. Thus, H(carborane) acids cleanly protonate benzene, phosphabenzene, C(60), etc., while triflic acid does not. Similarly, CH(3)(carborane) reagents can methylate substrates that are inert to boiling neat methyl triflate, including benzene, phosphabenzenes, phosphazenes, and the pentamethylhydrazinium ion, which forms the dipositive ethane analogue, Me(6)N(2)(2+). Methyl carboranes are also surprisingly effective in abstracting hydride from simple alkanes to give isolable carbocation salts, e.g., t-butyl cation. Trialkylsilyl carborane reagents, R(3)Si(carborane), abstract halides from substrates to produce cations of unprecedented reactivity. For example, fluoride is extracted from freons to form carbocations; chloride is extracted from IrCl(CO)(PPh(3))(2) to form a coordinatively unsaturated iridium cation that undergoes oxidative addition with chlorobenzene at room temperature; and silylation of cyclo-N(3)P(3)Cl(6) produces a catalyst for the polymerization of phosphazenes that functions at room temperature. Although currently too expensive for widespread use, carborane reagents are nevertheless of considerable interest as specialty reagents for making reactive cations and catalysts.

Fig. 1

Carborane anions of the type CHB₁₁R₅X₆⁻.

Fig. 2

The proton-bridged linear-chain X-ray structure of H(CHB₁₁Cl₁₁).

Fig. 3

X-ray structure of the benzenium ion salt [C₆H₇][CHB₁₁Me₅Br₆] showing H-bonding.

Fig. 4

Structure of HC₆₀⁺ as determined by NMR showing key ¹³C chemical shifts.

Fig. 5

X-ray structure of a protonated phosphabenzene showing ion-pairing to the CHB₁₁Me₅Cl₆^- anion via weak H-bonding.

Fig. 6

X-ray structure of i-Pr(CHB₁₁Me₅Br₆) showing alkylation at the 7 position.

Fig. 7

X-ray structure of t-butyl cation indicating H-bonding to CHB₁₁Me₅Cl₆⁻ anion.

Fig. 8

X-ray structure of i-Pr₃Si(CHB₁₁H₅Cl₆) (H omitted) showing developing i-Pr₃Si⁺ silylium ion character (Si---Cl = 2.32 Å; ΣC-Si-C = 351.9°).

Fig. 9

X-ray structure of the trimethylsilylated phosphazene cation as a carborane salt.

Fig. 10

X-ray structure of the CF(p-C₆H₄F)₂⁺ carbenium ion as a carborane salt.

Fig. 11

X-ray structure of the five-coordinate Ir(CO)Cl(Ph)(PPh₃)₂⁺ cation as a carborane salt.

Fig. 12

X-ray structure of the B(sub-phthalocyanine)⁺ cation as a carborane salt.

Fig. 13

X-ray structure of the triethylsilylated o-dichlorobenzene cation as a carborane salt.

Fig. 14

X-ray structure of the hydride-bridged silyl cation Me₃Si-H-SiMe₃⁺ as a carborane salt.

Fig. 15

X-ray structure of a β-silyl stabilized vinyl cation (CHB₁₁H₅Br₆^- carborane anion not shown).

Scheme 1

Formation of tertiary carbocations from alkanes.

Scheme 2

Reactions of CH₃(carborane) with various weakly basic heteroatom substrates.

Scheme 1

Synthesis of a stable vinyl cation (carborane anion omitted).