A calcium‐catalyzed direct substitution of π‐activated alcohols with different organosilanes under very mild reaction conditions is presented. The high reactivity of the calcium catalyst allows efficient conversion of secondary and tertiary allylic, secondary benzylic, and tertiary propargylic alcohols with allyltrimethylsilane at room temperature. Furthermore, the first direct substitution of an alcohol with (E)‐ as well as (Z)‐alkenylsilanes was achieved under mild reaction conditions.

A calcium-catalyzed direct reduction of propargylic alcohols and ethers has been accomplished by using triethylsilane as a nucleophilic hydride source. At room temperature a variety of secondary propargylic alcohols was deoxygenated to the corresponding hydrocarbons in excellent yields. Furthermore, for the first time, a catalytic deoxygenation of tertiary propargylic alcohols was generally applicable. The same protocol was suitable for an efficient reduction of secondary as well as tertiary propargylic methyl, benzyl and allyl ethers. Substrates containing an additional keto-, ester or secondary hydroxyl function were reduced with exceptional chemoselectivity at the propargylic position.

Complexation of the oxygen atom in 2-butylphenylethers and sulfur in 2-butylphenylthioethers to a rhodium atom in dirhodium tetracarboxylate Rh((II)) (2)[(R)-(+)-MTPA](4) is compared. Oxygen atoms complex via electrostatic attraction exclusively leading to an increase in alpha effects on C-2 complexation shifts in the sequence OCH(3) > F > Br > NO(2). However, that trend is opposite in thioethers. This can be rationalized by an additional highest occupied molecular orbital (HOMO)-LUMO interaction and the response of this interaction upon complex formation shifts. Thereby, an experimental evidence was found for the existence of the HOMO-LUMO binding mechanism which has been proposed previously based on theoretical considerations and indirect spectroscopic evidence. Sulfones hardly bind to Rh((II)) (2)[(R)-(+)-MTPA](4). Diastereomeric dispersion effects at (13)C and (1)H signals can be observed for all compounds indicating that enantiodifferentiation is easy in all classes of functionalities.

A cycloisomerization of enynes with a benign calcium catalyst is presented exploring a complementary reactivity to that usually found in transition and noble metal-catalyzed reactions. Thereby, a systematic investigation of the p-activation of alkynes with reactive carbocations has been realized and ketones of various ring sizes were easily accessed. We are certain that these basic investigations will pave the way for the elaboration of further reactions based on the reaction principles discovered in the area of noble metal catalysis.The efficient introduction of scaffold diversity and complexity, while preferentially departing from simple and readily available starting materials in a minimum of chemical operations, is one of the most challenging missions of modern organic synthesis. Addressing this objective, the field of noble metal catalysis has witnessed a veritable boom within the last decade. Due to the peculiar abilities of noble metals as p-acids for carbophilic activation, a wide range of conceptually new reaction types has been discovered. [1] Reactive intermediates in gold-catalyzed reactions have been described either as cationic or carbenoid species. [2] In the cationic rendition, the initially formed reactive species is a gold-stabilized vinyl cation, generated by the attack of the gold catalyst onto the alkyne moiety. Skeletal rearrangements of this non-classical vinyl cation then account for the accessibility of a multitude of unusual frameworks such as vinylcyclopropanes or strained bridgehead cyclobutenes. One possibility to generate a similar reactivity is the activation of the alkyne moiety with one or more equivalents of electrophilic iodine sources, thus generating an analogous iodine-stabilized vinyl cation species. [3] Even higher degrees of complexity from acyclic polyunsaturated precursors, also in the ab-sence of any transition metal or stoichiometric reagent, might be achieved by the p-activation of alkynes with reactive carbocations (cf. Figure 1), thereby again generating a highly reactive vinyl cation that sets the stage for subsequent cycloisomerization reactions in analogy to the parent gold-and iodine-mediated reactions. We have recently demonstrated that skeletal rearrangements normally granted by noble metal bound non-classical cations can be highly prolific also in the absence of the metal. [4] It has been demonstrated by some scattered literature precedents that the generation of a vinyl cation via attack of a carbocation onto a triple bond is indeed feasible. [5] Nevertheless a systematic investigation of the observed reactivity is lacking despite its high value as a starting point for the rich follow-up chemistry that is conceivable.As a new branch of sustainable metal catalysis, the application of early main group metals as an alternative to traditionally used, expensive, rare and often highly toxic transition metal catalysts has emerged within the last couple of years. Among the other early main group metals, calcium seems to be particularly privileged, which is reflected...

Thirty-eight derivatives of 3-hydroxy-2-methylpropanoic acid, each with two different oxygen functionalities, were synthesized and subjected to the standard dirhodium experiment (1 H NMR in the presence of an equimolar amount of the chiral dirhodium tetracarboxylate complex Rh*). Their structures represent ester, amide, carbonate, ether, alcohol and/or epoxy groups. Significant selectivity in the binding of those oxygen groups to the complex were determined. From these results, a priority list in binding to a rhodium atom of Rh* was established: epoxides > primary alcohols > ethers ≥ esters ≥ amides > carbonates > tertiary alcohols This sequence allows the prediction of the preferred binding site of oxygen-containing groups in polyfunctional compounds, which frequently occur among natural products, and, particularly, in asymmetric synthesis of such compounds. Differentiation of the enantiomers by the dirhodium experiment is easily accomplished due to numerous signal dispersions in nearly all cases.