Unique Stereocontrol in Carborane Chemistry: Skeletal Alkylcarbonation (SAC) versus Exoskeletal Alkylmethylation (EAM) Reactions

Abstract

Reactions between the arachno-6,9-C 2 B 8 H 14 (1)d icarbaborane and acyl chlorides,R COCl (2), are subject to stereocontrol that completely changes the nature of the reaction products.W hile most chlorides produce the 8-R-nido-7,8,9-C 3 B 8 H 11 (3)t ricarbollides (by skeletal alkylcarbonation = SAC), bulky RCOCls (2; where R = 1-adamantyl, 2a;1 -mesityl, 2b;9 -anthranyl, 2c;1 -naphthyl, 2d)i n1 ,2-dichloroethane (DCE) in the presence of triethylamine at 40-60 8 8Cg ave as eries of entirely different 1-R-2-CH 3 -closo-1,6-C 2 B 8 H 8 (4)d icarbaboranes upon acidification with conc.H 2 SO 4 (by exosleletal alkylmehylation = EAM). Both types of reactions seem to proceed via ac ommon [8-R-nido-7,8,9-C 3 Recently,wehave demonstrated that reactions of the [arachno-6,9-C 2 B 8 H 13 ] À (1 À )a nion and various acyl chlorides,R COCl (2), in the presence of tert-amine bases (Et 3 N [1] or proton sponge [2] )i nC H 2 Cl 2 ,f ollowed by in situ acidification of the anion 3 À thus formed with conc. H 2 SO 4 ,g enerate as eries of neutral alkyl and aryl tricarbollides (11-vertex nido tricarbaboranes) [3] 8-R-nido-7,8,9-C 3 B 8 H 11 (3)( where R = linear alkyl and simple aryl groups, see also pathway Ao fS cheme 1). These so-called skeletal alkylcarbonation (SAC) reactions are consistent with an aldol-type condensation between the RCO group and openface hydrogen atoms on the dicarbaborane 1, [2] which is associated with the insertion of the RC unit into the structure of 1 À under elimination of three extra hydrogen atoms as H 2 O and HCl. Ther eactions thus result in an effective Rtricarbaborane cross-coupling.H owever,j ust recently we have found that reaction in Equation (1) surprisingly proceeds in an entirely different manner when acyl chlorides with bulky (sterically demanding) R-substituents are used as reaction components.Initial results of this study are presented herein.Scheme 1( pathway B) shows that treatment of the arachno-6,9-C 2 B 8 H 14 (1)dicarbaborane with bulky acyl chlorides,R COCl (2)( where R = 1-adamantyl = adm, 2a;1 -mesityl = mes, 2b;9 -anthranyl = anth, 2c;1 -naphthyl = naph, 2d)i n1 ,2-dichloroethane (DCE) in the presence of triethylamine (deprotonation agent generating in situ the 1 À Scheme 1. Stereocontrol in COCl reactions. Most chlorides (path A, SAC reactions) produce tricarbollides 8-R-nido-7,8,9-C 3 B 8 H 11 (3)w hile bulky chlorides (path B) induce cage rearrangement with resulting alkylmethylation (EAM) to form the disubstituted 1-R-2-CH 3 -closo-1,6-C 2 B 8 H 8 (4)c ompounds.