Enantioselective Main Group Catalysis: Modern Catalysts for Organic Transformations

Abstract

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].

Mono- and Polynuclear Boron Dications

“…30 The lack of cationanion interaction in the solid state supports the molecular formula proposed by Ryschkewitsch. However, the average BN bond distances of 1.571 ¡ for [3][Br] 2 and [4][Br] 3 are found to be identical within experimental error in spite of the fact that the formal charge of the boron center in [4] 3+ is higher than that of [3] 2+ . In addition to the hydrido boron dications, Braunschweig and co-workers have prepared a ferrocenyl-substituted boron dication [FcB(4-Mepy) 3 Figure 2).…”

“…However, the average BN bond distances of 1.571 ¡ for [3][Br] 2 and [4][Br] 3 are found to be identical within experimental error in spite of the fact that the formal charge of the boron center in [4] 3+ is higher than that of [3] 2+ . In addition to the hydrido boron dications, Braunschweig and co-workers have prepared a ferrocenyl-substituted boron dication [FcB(4-Mepy) 3 Figure 2). However, the PEt 3 analogue of [7] 2+ cannot be prepared by the same method, potentially due to the increased steric repulsion between PEt 3 ligands.…”

“…In general, diboron dications are more reactive than the mononuclear analogues due to the presence of the relatively weak BHB 3c 2e or B(sp 3 )B(sp 3 ) bonds, which may dissociate in solution spontaneously or upon treatment of nucleophiles. The importance of the flexible coordination sphere at boron is also demonstrated in the Cp*-substituted boron dications, in which the boron center can be released from the π-coordination of Cp* anion.…”

“…Besides, treatment of [6] 2+ with LiNMe 2 , LiOMe, and pyridine all result in dissociation of the phosphine. However, both [3] 2+ and [6] 2+ are stable towards strong bases including LiH, n-BuLi, and KHMDS.…”

“…Since B(C 6 F 5 ) 3 ) (Scheme 12), in which four coplanar boron atoms share five electrons. 68 Accordingly, [36] 3+• features a relatively intense EPR signal at g = 2.006.…”