Are Methyl Groups Electron-Donating or Electron-Withdrawing in Boron Clusters? Permethylation of o-Carborane

Abstract

That methyl groups attached to carbon atoms are electron donors must not be generally assumed. In boron clusters, Me groups on boron are electron withdrawing. At the B3LYP/6-31G* level of theory, it has been proven that the Mulliken charge on each boron after substitution of -H by -Me increases by +0.18 unit. This leads to a high build-up of positive charge upon permethylation, then hampering it. Experimentally, this is proven by the synthesis of 9-I0.707H0.293-12-Cl0.566H0.434-3,4,5,6,7,8,10,11-Me8-1,2-C2B10H2, in which positions 9 and 12 were first methylated and then attacked by nucleophiles. This is substantiated by the synthesis of 3,6,8,9,10,12-Me6-1,2-C2B10H6 under the same experimental conditions but with time control.

Palladium‐Catalyzed Direct Dialkenylation of Cage BH Bonds in o‐Carboranes through Cross‐Coupling Reactions

“…The exchange reaction of 1 with Pd(OAc) 2 generates the intermediate A.S ubsequent regioselective electrophilic attack at the more electron-rich cage B(4) site gives the intermediate B. [7,10] Steric effects imposed by the substituent Ro nt he second cage carbon atom may also prevent B (3,6) positions from being attacked by Pd II ,t hough only 4a was isolated if R = H( entry 20, Table 2). Alkene insertion into the Pd À B bond proceeds to produce the seven-membered palladacycle C. [10] b-H Elimination of C affords the intermediate D. Reductive elimination coupled with oxidation by AgOAc affords the monoalkenylated compound E (analogue of 3l-COOH) and regenerates the active Pd II species.F urther regioselective electrophilic attack at the cage B(5) site followed by styrene insertion, b-H elimination, reductive elimination, oxidation, and decarboxylation gives the final dialkenylated product 4 and reproduces the catalyst Pd-(OAc) 2 .I tis noted that the carboxylic acid directing group plays ad ecisive role for the diselectivity of this olefination reaction.…”

“…[6] Such reactions proceed stepwise in the order B(9,12) > B(8,10) > B (4,5,7,11) > B(3,6), which corresponds to the calculated charge distribution on the cage (see Scheme 1f or the numbering scheme). [7] As charge differences are very small, regioselectivity in these substitution reactions is usually problematic.O nt he other hand, several stoichiometric transition-metal-mediated cage B(3,6) À Hderivatization reactions of o-carboranes have been documented. [8] Tr ansitionmetal-catalyzed monoarylation of BÀHbonds involving B(3), B(4), B(7), B(8), or B(9) centers in o-carboranes has recently been reported.…”

Additive Tuning of Redox Potential in Metallacarboranes by Sequential Halogen Substitution

“…The cluster total charge (CTC) is also shown. [35] CTC is a useful property to interpret the behaviour of boron clusters, since it computes all charges on the cluster atoms, not only these in the periphery. In this case, CTC has been computed with the natural population analysis (NPA) method.…”

Preparation of undecamethylated and hexamethylated 1‐halocarba‐closo‐dodecaborate anions

“…It is known from calculations of atomic charges in the parent (protiated) and in methylated CB 11 cluster anions [12] and neutral C 2 B 10 clusters [31] that the methyl group withdraws more electron density from the vertex atom than a hydrogen atom does. Relative to the hydrogen standard, methyl on boron or carbon is thus calculated to be an electronwithdrawing substituent, and this agrees with the order of Pauling electronegativites (2.1 for hydrogen and 2.5 for carbon).…”