Contents 1. Introduction………………………………………………………………………………………………... 1 2. Group 2-catalyzed Mannich reactions…………………………………………………………………...... 3 3. Group 2-catalyzed 1,4-addition reactions…………………………………………………………………. 5 4. Silylation of ketones and imines…………………………………………………………………………... 8 5. Hydrogenation reactions using group 13 Lewis acids and frustrated Lewis pairs…………………………12 6. Hydroamination reactions with s-block elements………………………………………………………… 14 7. Aluminum-centered catalysts in phosphonylation reactions……………………………………………… 17 8. Domino reactions…………………………………………………………………..……………………….21 9. Conclusions……………………………………………………………………………………………….. 24 This review highlights a number of recent developments in the field of main group enantioselective catalysis. Many essential transformations can be effected catalytically such as hydrosilylation, hydroamination and hydrogenation reactions, amongst others, in an asymmetric fashion using earth abundant sand p-block elements such as calcium, strontium, boron and aluminum. Recent work in this area has shown that these systems are not only active in catalysis but may also have the potential to compete with transition metal based systems with the reduced cost and toxicity often associated with main group chemistry. Keywords: enantioselective main group catalysis chiral asymmetric | 7 Scheme 8. Catalytic malonate addition reaction using calcium PyBOX complex [66]. Scheme 9. Proposed catalytic cycle of calcium-catalyzed addition reaction to nitro-styrene [66].

The metal‐free catalyst tris(2,4,6‐trifluorophenyl)borane has demonstrated its extensive applications in the 1,2‐hydroboration of numerous unsaturated reagents, namely alkynes, aldehydes and imines, consisting of a wide array of electron‐withdrawing and donating functionalities. A range of over 50 borylated products are reported, with many reactions proceeding with low catalyst loading under ambient conditions. These pinacol boronate esters, in the case of aldehydes and imines, can be readily hydrolyzed to leave the respective alcohol and amine, whereas alkynyl substrates result in vinyl boranes. This is of great synthetic use to the organic chemist.

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Controlling the reactivity of transition metals using secondary, s-accepting ligands is an active area of investigation that is impacting molecular catalysis.H erein we describe the phosphine gold complexes [(o-Ph 2 P(C 6 H 4 )Acr)AuCl] + ([3] + ;A cr = 9-N-methylacridinium) and [(o-Ph 2 P-(C 6 H 4 )Xan)AuCl] + ([4] + ;X an = 9-xanthylium) where the electrophilic carbenium moiety is juxtaposed with the metal atom. While only weak interactions occur between the gold atom and the carbenium moiety of these complexes,t he more Lewis acidic complex [4] + readily reacts with chloride to afford at rivalent phosphine gold dichloride derivative (7)i nw hich the metal atom is covalently bound to the former carbocationic center.This anion-induced Au I /Au III oxidation is accompanied by aconversion of the Lewis acidic carbocationic center in [4] + into an X-type ligand in 7.W ec onclude that the carbenium moiety of this complex acts as alatent Z-type ligand poised to increase the Lewis acidity of the gold center,anotion supported by the carbophilic reactivity of these complexes.

The reactions of propargyl amides, ureas, carbamates, and carbonates with B(C 6 F 5 ) 3 proceed via an intramolecular 5-exo-dig cyclization across the alkyne unit to yield the corresponding vinyl borate species. The generated sp 2 carbocation is stabilized by the flanking heteroatoms, allowing for isolation of oxazoline intermediates. The fate of these intermediates is strongly dependent upon the propargylfunctionalized starting material, with the carbamates and carbonates undergoing a ring-opening mechanism (propargyl rearrangement) to give cyclic allylboron compounds, while prolonged heating of the urea derivatives shows evidence of oxazole formation. In a deviation away from the reactivity of carbamates stated previously, the benzyl carbamate substrate undergoes dealkylation at the benzylic position, liberating 5-methyloxazol-2-(3H)-one.

Non-iodinated arenes can be easily and selectively converted into (diacetoxyiodo)arenes in a single step under mild conditions by using iodine triacetates as reagents. The oxidative step is decoupled from the synthesis of hypervalent iodine(III) reagents, which can now be prepared conveniently in a one-pot synthesis for subsequent reactions without prior purification. The chemistry of iodine triacetates was also expanded to heteroatom ligand exchanges to form novel inorganic hypervalent iodine compounds.

The reactions of allenes with frustrated (or cooperative) Lewis acid/base pairs result in the 1,4-addition of the base pair to the allene. The reactions of allenyl ketones and esters just in the presence of the strong Lewis acid B(C 6 F 5 ) 3 afford the selective formation of the 1,2-carboboration products. In both cases the Lewis acid activates the allene to either a C 6 F 5 migration or nucleophilic attack by the Lewis base. In addition to the 1,2-carboboration pathway, which can be viewed as being triggered by activation of the ketone (σactivation), in the case of allenyl esters the corresponding cyclization products are observed in the presence of water.

This work showcases a new catalytic cyclization reaction using a highly Lewis acidic borane with concomitant C-H or C-C bond formation. The activation of alkyne-containing substrates with B(C F ) enabled the first catalytic intramolecular cyclizations of carboxylic acid substrates using this Lewis acid. In addition, intramolecular cyclizations of esters enable C-C bond formation as catalytic B(C F ) can be used to effect formal 1,5-alkyl migrations from the ester functional groups to unsaturated carbon-carbon frameworks. This metal-free method was used for the catalytic formation of complex dihydropyrones and isocoumarins in very good yields under relatively mild conditions with excellent atom efficiency.