Coletti Cecilia, Marrone Alessandro, Giorgi Giacomo, Sgamellotti Antonio, Cerofolini Gianfranco, Re Nazzareno
Dipartimento di Scienze del Farmaco, Università G. D'Annunzio, Via dei Vestini, I-66100 Chieti, Italy.
Langmuir. 2006 Nov 21;22(24):9949-56. doi: 10.1021/la060013b.
We propose a new concerted mechanism for the uncatalyzed hydrosilylation of terminal alkenes and alkynes, alternative to the conventional radical-based mechanism. Density functional calculations have been carried out on these and on previously proposed alternative mechanisms for the hydrosilylation of ethylene and acetylene by suitable finite size clusters as models of the thermal functionalization of -SiH3, =SiH2, and [triple bound] SiH groups in flat Si(100) and Si(111) and porous silicon surfaces by alkenes and alkynes. For each step involved in the considered hydrosilylation pathways, we optimized the geometries of reactants and products and located the corresponding transition states. The calculated activation energies for the concerted pathways of ethylene and acetylene are, respectively, 57.6 and 60.9 kcal mol(-1) on -SiH3 and in the ranges 62-63 and 58-61 kcal mol-1 on =SiH2 and 64-66 and 56-61 kcal mol(-1) on SiH. These values are much lower than the activation energies calculated for the corresponding homolytic dissociation of the Si-H bond, which is the preliminary step in the radical path, 85.6, 82-83, and 79-81 kcal mol(-1), respectively, for -SiH3, =SiH2, and [triple bound] SiH groups. Our results thus suggest that the thermal hydrosilylation of alkenes and alkynes on silicon surfaces, for which a radical-based mechanism is currently accepted, may occur through a concerted mechanism.
我们提出了一种用于末端烯烃和炔烃非催化硅氢化反应的新协同机制，它不同于传统的基于自由基的机制。我们通过合适的有限尺寸簇对这些反应以及先前提出的乙烯和乙炔硅氢化反应的替代机制进行了密度泛函计算，这些簇作为平面Si(100)和Si(111)以及多孔硅表面上 -SiH3、=SiH2和[三键]SiH基团与烯烃和炔烃进行热官能化反应的模型。对于所考虑的硅氢化反应途径中的每一步，我们优化了反应物和产物的几何结构，并确定了相应的过渡态。乙烯和乙炔协同反应途径的计算活化能，在 -SiH3上分别为57.6和60.9 kcal mol(-1)，在 =SiH2上为62 - 63和58 - 61 kcal mol-1，在SiH上为64 - 66和56 - 61 kcal mol(-1)。这些值远低于在自由基途径的初步步骤中Si - H键相应均裂解离所计算出的活化能，对于 -SiH3、=SiH2和[三键]SiH基团，分别为85.6、82 - 83和79 - 81 kcal mol(-1)。因此，我们的结果表明，目前被认为基于自由基机制的烯烃和炔烃在硅表面的热硅氢化反应可能通过协同机制发生。
