A new type of intermediate, C+(BCH3)11- <--> C(BCH3)11, in a Grob fragmentation coupled with intramolecular hydride transfer. A nonclassical carbocation ylide or a carbenoid?
In solvolysis of alkyl halides Hal-(CH(2))(n)-C(BCH(3))(11)(-) (n = 2, 5, 6, but not 3, 4, or 7) and protonation of alkenes CH(2)=CH-(CH(2))(n)(-)(2)-C(BCH(3))(11)(-) (n = 3, 6, 7, but not 4 or 5) carrying the icosahedral electrofuge -C(BCH(3))(11)(-) attached through its cage carbon atom, generation of incipient positive charge on C(alpha) (as shown in Scheme 1 in the article) leads to simultaneous cleavage of the C(beta)-C(BCH(3))(11)(-) bond. The products are a C(alpha)=C(beta) alkene and a postulated intermediate C(+)(BCH(3))(11)(-) <--> C(BCH(3))(11), trapped as the adduct Nu-C(BCH(3))(11)(-) by one of the nucleophiles (Nu(-)) present. The reaction kinetics is E1, first order in the haloalkylcarborane and zero order in [Nu(-)], and the elimination appears to be concerted, as in the usual E2 mechanism. The process is best viewed as a Grob fragmentation. The loss of the longer chains involves intrachain hydride transfer from the C(alpha)-H bond to an incipient carbocation on C(delta)(') or C(epsilon)(') via a five- or six-membered cyclic transition state, respectively. The electronic structure of the postulated intermediate is believed to lie between those of a nonclassical carbonium ylide C(+)(BCH(3))(11)(-) and a carbenoid C(BCH(3))(11) whose electronic ground state resembles the S(2) state of ordinary carbenes.
Vyakaranam
Kamesh
K
Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, USA.
Körbe
Stefanie
S
Divisová
Hana
H
Michl
Josef
J