Density functional theory investigation of the remarkable reactivity of geminal dizinc carbenoids (RZn)(2)CHI (R = Et or I) as cyclopropanation reagents with olefins compared to mono zinc carbenoids RZnCHI(2), EtCHIZnR (R = Et or I).
Density functional theory calculations for the cyclopropanation reactions of several mono zinc carbenoids and their corresponding gem-dizinc carbenoids with ethylene are reported. The mono zinc carbenoids react with ethylene via an asynchronous attack on one CH2 group of ethylene with a relatively high barrier to reaction in the 20-25 kcal/mol range similar to other Simmons-Smith type carbenoids previously studied. In contrast, the gem-dizinc carbenoids react with ethylene via a synchronous attack on both CH2 groups of ethylene and substantially lower barriers to reaction (about 15 kcal/mol) compared to their corresponding mono zinc carbenoid. Both mono zinc and gem-dizinc carbenoid reactions can be accelerated by the addition of ZnI2 groups as a Lewis acid, and this lowers the barrier by another 1.0-5.1 kcal/mol and 0.0-5.5 kcal/mol, respectively, for addition of one ZnI2 group. Our results indicate that gem-dizinc carbenoids react with C=C bonds with significantly lower barriers to reaction and in a noticeably different manner than Simmons-Smith type mono zinc carbenoids. The three gem-dizinc carbenoids have a substantially larger positive charge distribution than those in the mono zinc carbenoids and, hence, a stronger electrophilic character for the gem-dizinc carbenoids.
Zhao
Cunyuan
C
Department of Chemistry, University of Hong Kong, Pokfulam Road, P. R. China.
Wang
Dongqi
D
Phillips
David Lee
DL
D003521:Cyclopropanes / Q000737:chemistry*; D005030:Ethylenes / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000737:chemistry*; D013816:Thermodynamics; D015032:Zinc / Q000737:chemistry*
D003521:Cyclopropanes; D005030:Ethylenes; D009942:Organometallic Compounds; C036216:ethylene; C030797:cyclopropane; D015032:Zinc