Abstract
In our group, we have synthesized indium nanoparticles (InNPs) of 4.0±0.5 nm, through the reducing system InCl3-Li-DTBB(cat.) in THF at room temperature and in the absence of any additives or anti-caking ligand. The catalytic efficiency of these InNPs was evaluated by the allylation reaction of carbonyl compounds, giving excellent yields of the corresponding homoallylic alcohols. The reagents were selected rationally in order to complete an appropriate mechanistic knowledge of the system. Substituted allyl bromides used enabled us to establish that the reaction products come from a γ-coupling, via a six members cyclic transition state, type Zimmerman-Traxler.1 Based on this, and in order to give an explanation to the experimental results, we started a computational theoretical study using the Gaussian09 program. The initial conformational analysis was performed using the semiempirical PM3 method, then we work with the B3LYP functional, applying the LANL2DZ pseudopotential for the indium and the 6-31+G* basis set for all the other atoms and the solvent effect was evaluated with the PCM model. Regarding the reactivity, the allylation of benzaldehyde with allyl bromide gave 98% of product after 1 h reaction time, while the allylation of acetophenone, after 20 h, gave 67% yield. The computational modeling showed a very good agreement with the experimental results: the activation energy (Ea) for the first process (exothermic in 3.4 kcal/mol) was 6.9 kcal/mol, while the Ea for the allylation of acetophenone was 10.8 kcal/mol, being an endothermic process (+0.9 kcal/mol). Furthermore, the allylation of benzaldehyde with prenyl bromide, gave 93% of product, after 4 h reaction time. The reaction is slow because of the high Ea, 11.4 kcal/mol, and the endothermicity of 3.6 kcal/mol. Relative the selectivity, when using crotyl bromide as allylating agent, a mixture of diastereomer alcohols was obtained, with a higher proportion of syn regarding to anti (67:33). The computational analysis is agreed with these results, being the Eas 8.0 and 9.6 kcal/mol respectively. Besides, the allylation with crotyl bromide of ortho substituted benzaldehydes showed syn selectivity. For example, for the ortho-Cl derivative, the computational modeling indicates that 8.1 kcal/mol are required to give the anti diastereomer while only 5.7 kcal/mol for the syn product; moreover, the first process is less exothermic than the second (-2.7 vs -4.0 kcal/mol respectively). We appreciate the support received from SGCyT-UNS, CONICET and ANPCyT. 1 Dorn, V., Chopa. A., Radivoy, G. RSC Advances, 2016, 6, 23798-23803.
Highlights
Metal-mediated allylation of carbonyl compounds with allyl bromides is an interesting and convenient method to form C-C bond and to obtain homoallylic alcohols, which are significant building blocks in organic synthesis
The best results were obtained by using InCl3 (1.0 mmol) with an excess of lithium sand (3.5 mmol) in the presence of DTBB (0.1 mmol) as electron carrier, THF (2 mL) as the solvent, with allyl bromide (1.5 mmol dissolved in 1 ml THF) and benzaldehyde (0.5 mmol dissolved in 1 ml THF) at room temperature and under a nitrogen atmosphere obtaining the alcohol homoallylic 1phenylbut-3-en-1-ol (2a) in 98% yield
The less reactivity observed could be attributed to the steric hindrance caused by methyl substituents attached to the allyl moiety, which could affect the formation of the homoallylic alcohol
Summary
Metal-mediated allylation of carbonyl compounds with allyl bromides is an interesting and convenient method to form C-C bond and to obtain homoallylic alcohols, which are significant building blocks in organic synthesis. The diastereoselectivity in the addition of an allyl-metal to a carbonyl compound is a fundamental parameter to consider, and could be explained by steric and stereoelectronic or chelating effects.[1] In this sense, in our group, we have synthesized indium nanoparticles (InNPs) of 4.0 ± 0.5 nm, through the reducing system InCl3-Li-DTBB(cat.) in THF at room temperature and in the absence of any additives or anti-caking ligand. The catalytic efficiency of these InNPs was evaluated by the allylation reaction of carbonyl compounds, giving excellent yields of the corresponding homoallylic alcohols. Substituted allyl bromides allowed us to establish that the reaction products come from a γ-coupling, via a six-membered cyclic transition state, type Zimmerman-Traxler.[2] In order to extend the scope of this indiummediated allylating methodology, we employ different ortho-substituted carbonyl compounds and crotyl bromide as allylating agent to study their effect over the diastereoselectivity in this C-C bond formation. In order to give an explanation to the reactivity and the syn-anti selectivity observed from the experimental results, computational studies have been applied by the Gaussian[09] program
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