Small Inorganic Ring Systems: Understanding Cyclization and Bond Activation Processes

Abstract

Within this work, the modification of the substituents at silicon of heterocyclic four-membered silyl phosphonium ions was investigated. In chapter 1 a general overview of the current state of research, and the design principles, to which the structure of the four-membered phosphonium ions adhere, are presented. The focus of chapter 3 is the modification of previously known silyl phosphine chalcogenides with different substituents and Lewis basic chalcogenides. New synthesis routes towards these novel compounds were developed, the structures synthesized and thoroughly investigated by means of single-crystal X-ray diffraction analysis, NMR analysis and DFT calculations. Through this procedure a deep understanding of the governing principles of steric and electronic effects within these four-membered silyl phosphonium chalcogenides was obtained. When all substituents are replaced by sterically demanding tert-butyl substituents, the choice of chalcogen used as a Lewis base becomes crucial for successful ring formation. The angular strain within the four-membered rings was found to be well balanced in all four-membered CPChSi rings investigated. Thermochemical investigations showed that the substituents on the silicon and phosphorus atoms play an important role for the strength of the intramolecular Ch–Si coordination. In the absence of large steric repulsions through bulky substituents (methyl groups on silicon, tert-butyl groups on phosphorus), a stability sequence depending on the chalcogen atom in the direction Se ≤ S < O can be observed. However, the order is reversed (O < S < Se) in case of strong repulsions between sterically demanding substituents (tert-butyl groups on both silicon and phosphorus atoms). Due to the shorter Si–O bond length compared to the Si–S and Si–Se bond lengths, the substituents of the phosphorus and silicon atom are forced in closer proximity in the four-membered cations. In the case of all-tert-butyl substituted compound I this leads to a significant increase in steric repulsion in this cation, therefore hampering its synthesis. Building upon this knowledge, another second-row element, nitrogen, was investigated as a donor in chapter 4. The nitrogen atom was introduced as a phosphinimine moiety into the systems. In contrast to phosphine chalcogenides, phosphinimine donor moieties allow for further modification of donor strength by the influence of steric or electronic parameters. The introduction of either a trimethylsilyl or a bis(3,5-trifluoromethylphenyl)boryl moiety allows for additional ring strain in these systems. Furthermore, an electronic destabilization of the Si–N bond was expected to be achieved by either resonance stabilization by the bis(3,5-trifluoromethylphenyl)boryl moiety or by hyperconjugative n(N)→σ*(SiMe) interactions from the –SiMe3 moiety. Taken together, these effects are expected to facilitate ring opening and enable small molecule activation or even catalytic transformations. The rational synthesis of these systems, the influence of steric and electronic factors, and the reactivity in terms of catalytic applications and FLP-like behavior were investigated. The silylated phosphinimines can be easily obtained in moderate to good yields. The ring-closure reactions of the methyl- and iso-propyl-substituted silanes (X, XI) proceeded smoothly, however the case of the all tert-butyl substituted silyl phosphinimine IX no ring closure was possible. Like the P–O bond length [1.4958(8) Å] in compound I the P–N bond length [1.538(2) Å] in compound IX is also very short, therefore the same effects hampering the ring formation in compound I were found to also affect the ring formation in compound IX. The Si–N bonds in the SiMe3-substituted four-membered phosphonium ions XII[B(C6F5)4] and XIII[B(C6F5)4] were found to be covalent in nature, and did not possess the ability to reversibly open as anticipated. Nevertheless, we were able to gain valuable insights from these results and therefore we set out to design a new system with a maximum of steric hinderance, while also preserving the desired ability to remove the hydride from the Si¬–H moiety. Therefore, an electrophilic boryl moiety was introduced to study the effect of electron-withdrawing groups on the reversible opening ability of these ring systems. With the zwitterionic compound XIV reversibility was finally achieved which was evidenced by the ability of compound XIV to catalyze the hydrosilylation of nitriles. This validated our approach of combining steric and electronic factors to weaken the Si¬–N bond in compound XIV. Upon receiving preliminary confirmation through this experiment, that reversibility is in principle possible within these systems our attention shifted away from the strongly Lewis acidic silylium-based Lewis acids towards neutral silicon-based Lewis acids. The rational was, that neutral silicon-based Lewis acids are of lower Lewis acidity compared to silylium ions, and therefore reversibility within these systems can be easier achieved. The question whether this really is the case was addressed in chapter 5. Several novel neutral Lewis acidic silanes were prepared and incorporated within the previously described CPSSi cycles (chapter 3). By varying the electron-withdrawing groups from pentafluorophenyl [-C6F5] over pentafluorophenoxy [-OC6F5] to tetrafluorocatecholato [-O2C6F4] moieties, a discernible difference in Lewis acidity at the silicon center was observed (Scheme 8.3). While the pentafluorophenyl [-C6F5] substituted compound showed no Lewis acidity, the tetrafluorocatecholato [-O2C6F4] substituted silane was strongly prone towards formation of the pentacoordinate state making the desired Si–H substituted tetravalent silane inaccessible. A good balance between Lewis acidity and stability of the tetravalent state was found with the pentafluorophenoxy [-OC6F5] substituted compound XV. Through in depth NMR spectroscopic experiments a equilibrium between compounds XV and XVI could be observed. Compound XV represents an intriguing structure as it combines a highly reducing anionic Si–H function, aswell as a protic cationic N–H function in close proximity to each other. To the best of our knowledge this is the only example were both motifs can be found within one molecule. Through additional time resolved NMR experiments it was also found that compound XV is not stable over a prolonged amount of time and slowly reacts with the loss of dihydrogen towards compound XVII. In conclusion the introduction of neutral Lewis acidic silanes indeed proved to ease the ability of the silicon Lewis acids to reversibly attach to Lewis bases. In a second part of this doctoral project, chiral silanethiols were synthesized and investigated as enantioselective hydrogen-atom transfer (HAT) catalysts together with the group of Prof. König. The deracemization was achieved by a sequence of photocatalytic hydrogen-atom transfer, reductive radical-polar crossover (RRPCO), and protonation. Our goal was to design silanethiols which were able to act as potent and enantioselective HAT-catalysts. To achieve this goal, different strategies were employed. The first attempt was made with a silanethiol which was equipped with chiral substituents. Therefore the (–)-menthol substituted silanethiol XVIII was prepared and tested, and while compound XVIII performed well as a HAT-catalysts no enantioselectivity was observed. Next, the Si-chiral silanethiol XIX was prepared and tested. The idea was to bring the chiral information closer to the reactive Si¬–S– function, and therefore enhance enantioselectivity. This approach was successful and compound XIX performed well as a HAT catalyst and gave an enantiomeric excess of 16 % in the final product. While this finding served as proof that chiral silanethiols can indeed be used as enantioselective HAT-catalysts, the enantiomeric excess obtained was not good enough for practical use. In order to increase the enantioselectivity even further, the successful concept of using Si-chiral silanethiols was combined with a chiral backbone. A suitable motif, which combined both desired features was found in a class of silanthiols with a ferrocene backbone [(SSi,SP)-XX and (SSi,SP)-XXI]. While both compounds are Si-stereogenic, they also exhibit planar chirality in the ferrocenyl backbone. However, when employing compounds (SSi,SP)-XX and (SSi,SP)-XXI in the RRPCO-HAT-protonation sequence, decomposition was observed. No catalytic activity nor enantioselectivity was observed for both compounds, therefore concluding that the ferrocene backbone is not suitable for this application. In summary, this lays the foundation for the design of silanethiol HAT catalysts based on the general structure of compound XIX