s-Block cooperative catalysis: alkali metal magnesiate-catalysed cyclisation of alkynols

Abstract

Through mixed metal cooperativity, alkali metal magnesiates efficiently catalyse the cyclisation of alkynols.

Alkali Metal (Li, Na, K, Rb, Cs) Mediation in Magnesium Hexamethyldisilazide [Mg(HMDS)2] Catalysed Transfer Hydrogenation of Alkenes

“…A plausible factor is that a larger alkali metal interacts more with the π‐system of the aromatic ring as was described for a series of monomeric alkali metal benzyl complexes by Robertson and co‐workers [42,43] . This trend has also been observed for the catalytic intramolecular hydroalkoxylation reactions utilizing other alkali‐metal magnesiates as catalysts [44] . As a result, the conjugated double bond is polarised and the insertion of the nucleophilic hydride becomes more favourable.…”

“…[42,43] This trend has also been observed for the catalytic intramolecular hydroalkoxylation reactions utilizing other alkali-metal magnesiates as catalysts. [44] As a result, the conjugated double bond is polarised and the insertion of the nucleophilic hydride becomes more favourable. Prompted by the reactivity enhancement seen on descending the first three alkali metals we eagerly investigated the behaviour of the two larger alkali metals rubidium and caesium.…”

“…As crown ethers are well known to sequester alkali metals and form solvent-separated ion pairs we expected a decrease or even shutdown of reactivity, [47] though it is known that 18-crown-6 does not always completely fill the coordination sphere of potassium. [44] Surprisingly the substrate was fully consumed within only 30 minutes, however, only 17 % of the styrene was converted to ethylbenzene while the rest underwent oligomerization. A possible explanation is that after the addition of Lewis bases like THF or a crown ether smaller aggregates are formed resulting in an increased reactivity.…”

Structural Studies of Donor‐Free and Donor‐Solvated Sodium Carboxylates

“…[11] Insight has also been gained into cyclisation reactions of alkynols, which proceed sluggishly using a mixed lithium magnesiate, but worked more efficiently using a sodium magnesiate and best of all using a potassium based system, showing the potential advantages of investigating heavier alkali metals further. [12] A catalytic system looking at hydrophosphorylation of nitriles and alkenes has also been developed utilising simple alkali metal HMDS complexes (HMDS is 1,1,1,3,3,3hexamethyldisilazide), which show superior performance when moving to the heavier alkali metals, again highlighting the advantages that sodium and potassium can offer. [13] In a final example, a NaHMDS catalyst system has also been found to perform aminobenzylation of aldehydes with toluenes in excellent yields while tolerating a wide variety of functional groups.…”

Structural and metal–halogen exchange reactivity studies of sodium magnesiate biphenolate complexes

“…Sodium magnesiates have primarily been employed as powerful bases in deprotonation chemistry 22,23 and as efficient catalysts for a range of transformations. [24][25][26][27][28] To the best of our knowledge there are no examples of metal-halogen exchange involving sodium magnesiates. Herein, we report the synthesis of two new disodium dialkylmagnesiates containing the (rac)-BIPHEN ligand (Scheme 1) and discuss their preliminary reactivity in metal-halogen exchange.…”