Competing concepts in electronic control of face selection

Hydrosilation of Carbonyl-Containing Substrates Catalyzed by an Electrophilic η-Silane Iridium(III) Complex

Hydrosilation of a variety of ketones and aldehydes using the cationic iridium catalyst, (POCOP)Ir(H)(acetone)(+), 1, (POCOP = 2,6-bis(di-tert-butyl phosphinito)phenyl) is reported. With triethyl silane, all but exceptionally bulky ketones undergo quantitative reactions employing 0.5 mol% catalyst in 20-30 min at 25 °C. Hydrosilation of esters and amides results in over-reduction and cleavage of C-O and C-N bonds, respectively. The diastereoselectivity of hydrosilation of 4-tert-butyl cyclohexanone has been examined using numerous silanes and is highly temperature dependent. Using EtMe(2)SiH, analysis of the ratio of cis:trans hydrosilation products as a function of temperature yields values for ΔΔH(‡) (ΔH(‡) (trans) - ΔH(‡) (cis)) and ΔΔS(‡) (ΔS(‡) (trans) - ΔS(‡)(cis)) of -2.5 kcal/mol and -6.9 e.u., respectively. Mechanistic studies show that the ketone complex, (POCOP)Ir(H)(ketone)(+), is the catalyst resting state and is in equilibrium with low concentration of the silane complex, (POCOP)Ir(H)(HSiR(3))(+). The silane complex transfers R(3)Si(+) to ketone forming the oxocarbenium ion, R(3)SiOCR'(2) (+), which is reduced by the resulting neutral dihydride 3, (POCOP)Ir(H)(2), to yield product R(3)SiOCHR'(2) and (POCOP)IrH(+) which closes the catalytic cycle.

Complete pi-facial diastereoselectivity in Diels-Alder reactions of dissymmetric 2,4-cyclohexadienones

[reaction: see text] The studies in the Diels-Alder reactions of 5-methoxy-masked o-benzoquinone (1a, R = OMe) and simple dissymmetric 2,4-cyclohexadienones 1b-e with methyl vinyl ketone, styrene, and benzyl vinyl ether are described. The dienones 1b-e reacted with dienophiles to form syn adducts (dienophile approach is syn to allylic methoxy group) exclusively.

Stereoelectronic and conformational effects on the stereochemical course of reduction of bicyclo[3.3.1]nonane 1,3-diketones

The stereoselectivity of the reduction of bicyclo[3.3.1]nonane 1,3-diketones was studied. The experimental data of π-facial stereoselection of the reduction of the carbonyl group were successfully rationalized by the application of the exterior frontier orbital extension (EFOE) model. The observed facial diastereoselectivity of the carbonyl reduction of bicyclo[3.3.1]nonane diketones could be reasonably explained by the ground-state facial anisotropy of the frontier orbital extension, steric effects, and the intrinsic reactivity of carbonyl groups. Although the EFOE density and PDAS values predicted the enhanced reactivity at the C-2 carbonyl compared to C-9 carbonyl, transition-state calculations at the B3LYP/6-31+G(d,p) level showed that the reactivity of C-9 and C-2 in a hydride addition to both carbonyl groups should be nearly the same. The EFOE analysis also strongly indicated that in the second hydride reduction step the corresponding oxobicyclononanolates are most likely to be involved in the reduction as the major species rather than the free hydroxyketones. The antiperiplanar effects in the transition states of the LiAlH4 reduction as measured by both the elongation and the natural bond population of the antiperiplanar bonds clearly indicated they should not be an essential factor of the facial diastereoselection of studied diketones.Key words: bicyclo[3.3.1]nonanedione, EFOE model, reduction, stereoselectivity, transition state.