Dispersion-Energy-Driven Wagner-Meerwein Rearrangements in Oligosilanes.

Judith Baumgartner,Saskia Rathjen,Christoph Marschner,Thomas Müller,Lena Albers

doi:10.1021/jacs.6b03560

Abstract

The installation of structural complex oligosilanes from linear starting materials by Lewis acid induced skeletal rearrangement reactions was studied under stable ion conditions. The produced cations were fully characterized by multinuclear NMR spectroscopy at low temperature, and the reaction course was studied by substitution experiments. The results of density functional theory calculations indicate the decisive role of attractive dispersion forces between neighboring trimethylsilyl groups for product formation in these rearrangement reactions. These attractive dispersion interactions control the course of Wagner–Meerwein rearrangements in oligosilanes, in contrast to the classical rearrangement in hydrocarbon systems, which are dominated by electronic substituent effects such as resonance and hyperconjugation.

Highlights

A major impetus for the development of microelectronics is the constant quest for ever smaller integrated circuits defined by Moore’s law.[1]
Scheme 2 provides an example for this type of reaction in which the linear oligosilane 2 is transformed initially into its branched isomers 3 and 4.25,31 We have studied the reaction shown in Scheme 2 by ionizing close derivatives of oligosilane 2 at low temperature and characterized important cationic intermediates, which has allowed the formulation of a detailed mechanism for this reaction
The most stable hydrogen-bridged cation, the branched cation 8, is more stable by 16 kJ mol−1 than the intermediate cation 14 and more stable by 30 kJ mol−1 than cation 7, and it is the most stable cation that we located on the potential energy surface (PES). We found this result rather surprising because the quantitative evaluation of substituent effects on the stability of cations having Si−H−Si units indicated that substitution with four trimethylsilyl groups at the two silicon atoms (α-substitution, as in cation 7) stabilizes this type of cation by 37 kJ mol−1 compared to tetramethyl substitution, as detailed in the Supporting Information.[48]

Summary

■ INTRODUCTION

A major impetus for the development of microelectronics is the constant quest for ever smaller integrated circuits defined by Moore’s law.[1]. Silicon clusters of diverse structural complexity with intriguing electronic and bonding properties have been synthesized by reductive oligomerization of polyhalosilanes[2−20] or disproportionation reactions of Si2Cl6.21 A different synthetic approach to silicon cluster structures is the rearrangement of linear poly- or oligosilanes catalyzed by Lewis acids.[22] Pioneering work of Kumada’s and West’s groups showed that by using AlCl3 as the catalyst, linear oligosilanes can be transformed to structures of higher complexity such as branched or cyclic systems.[23−27] Recently, we have been able to document the activity of carbocations, such as the trityl cation, in these catalytic rearrangement reactions.[28] the use of a trityl cation paired with a weakly coordinating anion in stoichiometric amounts allowed the detection of oligosilanylsilyl cations under carefully controlled reaction conditions.[28,29] The most prominent example for this type of Lewis acid catalyzed sila-Wagner− Meerwein rearrangement is the synthesis of persilaadamantane 1 from a bicyclic precursor that was reported recently by two of us (Scheme 1).[30] During this transformation, the complexity of the bicyclic compound increases significantly by increasing the number of tetrasila-substituted silicon atoms. Article computational studies have provided clear indications that favorable attractive London dispersion forces[32] in these intermediates drive these types of skeletal rearrangement reactions in the direction of branched products such as 4

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES