Synthesis, structures and reactivity studies of P-bis(trimethylsilyl)methyl-substituted</nobr> σ³λ³-oxaphosphirane complexes

Abstract

This thesis was focused on the investigations of the scope and limitations of the new method of synthesis of oxaphosphirane complexes of the group 6 metal triade based on recently discovered Li/Cl phosphinidenoid complexes. Another important part of this work was focused on ring-opening and ring-expansion reactions using Brønstedt acids and Ti(III) complexes, which were accompanied by theoretical studies.<br /> First, the synthesis of new P-bis(trimethylsilyl)methyl-substituted-oxaphosphirane complexes synthesized from dichloro(organo)phosphane complexes; via chlorine/lithium exchange in the presence of 12-crown-4 and carbonyl derivatives is presented. Using the same methodology also the first spiro-oxaphosphirane complexes were synthesized, which showed an unusual form of isomerism.<br /> Although the transient Li/Cl phosphinidenoid complexes showed a high selectivity in all cases mentioned beforehand, the limits of this method came to the fore if very bulky ketones or N-functionalized aldehydes or ketones were employed. In these cases the formation of oxaphosphirane complexes was not observed and unknown side-reactions took place.<br /> In the second chapter the chemical behavior of oxaphosphirane complexes was studied first with special focus on ring enlargement reactions. Acid-induced ring expansion reactions of the chromium, molybdenum and tungsten oxaphosphirane complexes was achieved using triflic acid, in the presence of carbonyl derivatives, and subsequent treatment with triethylamine to form the 1,3,4-dioxaphospholane complexes. The formation of by-products and the reaction mechanism were proposed based on low temperature 31P{1H} NMR reaction monitoring.<br /> In chapter IV.2, experimental and theoretical studies were undertaken to get insight into the acid induced ring-opening reactions of the P-bis(trimethylsilyl)methyl-substituted-oxaphosphirane oxaphosphirane complex in the absence of trapping reagents. Here, the reactions with triflic acid, a toluenium carbaboranid, hydrochloric acid and fluoroboronic acid were investigated.<br /> Of particular interest is the surprising formation of the side-on complexes after O-protonation of the oxaphosphirane complex using TfOH or [C7H9][CHB11Cll1], which was proven by NMR studies and the crystal structure of [C19H26O6PSi2W][CHB11Cll1]; DFT calculations were performed onto the formation of the η2 coordinated W(CO)5 complex.<br /> Protonation of the oxaphosphirane complex with HCl yielded a mixture of chlorophosphane complexes via P-O bond cleavage. Interestingly, a coalescence process was observed, which was investigated and the process analysed with respect to its underlying kinetics by low temperature 31P{1H} NMR spectroscopic measurements.<br /> In the reaction of the oxaphosphirane complex with HBF4 two different products were obtained, the fluorophosphane complexes and the η2-[(phenyl)methylene(bis(trimethylsilyl)methyl)fluoro-hydroxyphosphorane] side-on pentacarbonyltungsten(0) complex; the latter represented the first example of a η2-Wittig ylide complex. The formation of these complexes could be explained on the basis of DFT calculations and provided information about two possible reactions pathways that involve P-O and C-O bond cleavages.<br /> A novel synthetic route to oxaphosphirane complexes was discovered, presented in chapter IV.2.2.2, which is the deprotonation of the chlorophosphane complexes. The reaction yielded a mixture of diastereomeric oxaphosphirane complexes.<br /> First investigations of SET reactions of oxaphosphirane complex using in situ prepared "CpTiCl2" are presented in chapter V. These reactions led to a deoxygenation of the oxaphosphirane complex and formation of phosphaalkene complexes, which were not stable towards traces of air and reacted further to give a chloro(diorgano)phosphane complex as final product, which was also structurally confirmed.