The synthesis and properties of a number of very strong iminophosphorane bases up to an extremely high level of steric hindrance are described. They cover a range of ca. 4 pK units in basicity and a range of more than 11 orders of magnitude in their rates of methylation with methyl iodide. Most of the systems are readily prepared in up to molar quantities, conveniently recovered from their salts and are of high chemical and thermal stability. Crystal structures were determined in ~~ order to parametrize a force field, which is utilized in molecular modeling studies offering a rationalization of the observed differences in steric hindrance and basicity. Depending on the degree of steric protection of the basic center, these novel bases are proposed as unprecedented, versatile auxiliary bases in E2 eliminations and in reactions involving deprotonation in the presence of more or less strong electrophiles.
Abstract:The cyclic silicon-stereogenic silane (SiR)-5 decorated with three different substituents of distinct steric demand is an exceptionally useful chiral reagent in asymmetric organosilicon chemistry. Several approaches for its large-scale preparation in optically pure form have been investigated. These hinge upon the resolution of racemic silane rac-5 which, in turn, is accessible in multi-gram quantities by a straightforward one-pot two-step reaction sequence. For this, a classical as well as a novel kinetic resolution via its diastereomeric silyl ethers derived from enantiopure secondary alcohols as resolving agents has been elaborated: (1) the use of (À)-menthol [(À)-7] allowed for a quantitative separation of silyl ethers (SiS)-10 and (SiR)-10 by practical fractional crystallization and (2) a diastereoselective copper-catalyzed dehydrogenative silicon-oxygen coupling using pyridyl alcohols (S)-16 or (R)-16 capable of two-point binding has been devised and assessed as a novel kinetic resolution strategy for the synthesis of a silane with silicon-centered chirality. Subsequent stereospecific reductive cleavage of the silicon-oxygen bond enabled the preparation of (SiR)-5 and (SiS)-5 in excellent enantiomeric excesses of up to 99 % ee.
A series of peralkylated polyaminophosphazenium cations exhibiting extraordinary base resistance under phase-transfer conditions were efficiently synthesized from readily available starting materials. Their half lives under these conditions exceed those of the most stable conventional organic cations by factors of up to 3000.
An iterative polyphosphorylation approach is described, which is based on ap hosphoramidite (P-amidite) derived reagent (c-PyPA) obtained from the cyclization of pyrophosphate with areactive diisopropylaminodichlorophosphine.T his type of reagent is unprecedented as it represents ar eactive P-amidite without protecting groups.T he reagent proved to be stable in solution over several weeks.Its utility is described in the context of iterative monodirectional and bidirectional polyphosphorylations.T he ensuing functionalized cyclotriphosphate can be opened with av ariety of nucleophiles providing ready access to diverse functionalized polyphosphate chains of defined length with several tags, including both P-N and P-O labels.Their interaction with exoand endopolyphosphatases is described.
If there is an aryl substituent on the acetylene terminus of enyne‐allenes, then its reaction mode may be changed from the Myers‐Saito cyclization to a novel C2–C6 cyclization resulting in a net intramolecular Diels‐Alder or ene reaction. As a consequence, the thermal cyclization of readily accessible acyclic enyne‐allenes can be utilized for the synthesis of complex benzofulvene and benzofluorene derivatives. Kinetic results of the C2–C6 cyclization reaction indicate a two‐step reaction pathway with a benzofulvene biradical intermediate.
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