Catherine Weetman

Alkaline earths as main group reagents in molecular catalysis

The past decade has witnessed some remarkable advances in our appreciation of the structural and reaction chemistry of the heavier alkaline earth (Ae = Mg, Ca, Sr, Ba) elements. Derived from complexes of these metals in their immutable +2 oxidation state, a broad and widely applicable catalytic chemistry has also emerged, driven by considerations of cost and inherent low toxicity. The considerable adjustments incurred to ionic radius and resultant cation charge density also provide reactivity with significant mechanistic and kinetic variability as group 2 is descended. In an attempt to place these advances in the broader context of contemporary main group element chemistry, this review focusses on the developing state of the art in both multiple bond heterofunctionalisation and cross coupling catalysis. We review specific advances in alkene and alkyne hydroamination and hydrophosphination catalysis and related extensions of this reactivity that allow the synthesis of a wide variety of acyclic and heterocyclic small molecules. The use of heavier alkaline earth hydride derivatives as pre-catalysts and intermediates in multiple bond hydrogenation, hydrosilylation and hydroboration is also described along with the emergence of these and related reagents in a variety of dehydrocoupling processes that allow that facile catalytic construction of Si-C, Si-N and B-N bonds.

Selective reduction of CO2 to a methanol equivalent by B(C6F5)3-activated alkaline earth catalysis

Treatment of β-diketiminato Mg and Ca amidoborane compounds with B(C6F5)3 induces hydride elimination and formation of alkaline earth hydrido-tris(pentafluorophenyl)borate derivatives. Both species react with CO2 to provide formate complexes, one of which has been structurally characterised, and may be applied to the highly selective reductive hydroboration of CO2 with pinacolborane (HBpin) to provide the methanol equivalent, CH3OBpin.

Magnesium hydrides and the dearomatisation of pyridine and quinoline derivatives

Reactions of the β-diketiminato n-butyl magnesium complex, [HC{(Me)CN(2,6-(i)Pr(2)C(6)H(3))}(2)Mg(n)Bu], with a range of substituted pyridines and fused-ring quinolines in the presence of PhSiH(3) has been found to result in dearomatisation of the N-heterocyclic compounds. This reaction is proposed to occur through the formation of an unobserved N-heterocycle-coordinated magnesium hydride and subsequent hydride transfer via the C2-position of the heterocycle prior to hydride transfer to the C4-position and formation of thermodynamically-favoured magnesium 1,4-dihydropyridides. This reaction is kinetically suppressed for 2,6-dimethylpyridine while the kinetic product, the 1,2-dihydropyridide derivative, was isolated through reaction with 4-methylpyridine (4-methylpyridine), in which case the formation of the 1,4-dihyropyridide is prevented by the presence of the 4-methyl substituent. X-ray structures of the products of these reactions with 4-methylpyridine, 3,5-dimethylpyridine and iso-quinoline comprise a pseudo-tetrahedral magnesium centre while the regiochemistry of the particular dearomatisation reaction is determined by the substitution pattern of the N-heterocycle under observation. The compounds are all air-sensitive and exposure of the magnesium derivatives of dearomatised pyridine and 4-dimethylaminopyridine (DMAP) to air resulted in ligand rearomatisation and the formation of dimeric μ(2)-η(2)-η(2)-peroxomagnesium compounds which have also been subject to analysis by single crystal X-ray diffraction analysis. An unsuccessful extension of this chemistry to N-heterocycle hydrosilylation is suggested to be a consequence of the low basicity of the silane reagent in comparison to the pyridine substrates which effectively impedes any further interaction with the magnesium centres.

Easy access to nucleophilic boron through diborane to magnesium boryl metathesis

Organoboranes are some of the most synthetically valuable and widely used intermediates in organic and pharmaceutical chemistry. Their synthesis, however, is limited by the behaviour of common boron starting materials as archetypal Lewis acids such that common routes to organoboranes rely on the reactivity of boron as an electrophile. While the realization of convenient sources of nucleophilic boryl anions would open up a wealth of opportunity for the development of new routes to organoboranes, the synthesis of current candidates is generally limited by a need for highly reducing reaction conditions. Here, we report a simple synthesis of a magnesium boryl through the heterolytic activation of the B–B bond of bis(pinacolato)diboron, which is achieved by treatment of an easily generated magnesium diboranate complex with 4-dimethylaminopyridine. The magnesium boryl is shown to act as an unambiguous nucleophile through its reactions with iodomethane, benzophenone and N,N′-di-isopropyl carbodiimide and by density functional theory.

Magnesium Catalysis for the Hydroboration of Carbodiimides

A β-diketiminato magnesium alkyl complex, [CH{C(Me)NDipp}2 }MgnBu] (Dipp=2,6-iPr2 C6 H3 ), was shown to be an effective pre-catalyst for the first reported catalytic hydroboration of alkyl- and aryl-substituted carbodiimides with pinacol borane (HBpin). The catalytic reactions proceed under mild conditions to afford the corresponding N-borylated formamidine compounds in good yields. The reactions were observed to proceed through the intermediacy of magnesium amidinate and formamidinatoborate intermediates and an example of one of these latter species has been structurally characterised by an X-ray diffraction analysis. Crucially, no formation of the N-boryl formamidine products was observed in the absence of additional equivalents of the carbodiimide and HBpin substrates. This observation, supported by the evolution of a sigmoidal kinetic profile for the hydroboration of dicyclohexylcarbodiimide, has been rationalised as the consequence of an allosteric effect of the pinacol borane and carbodiimide on the magnesium formamidinatoborate intermediates.

The Road Travelled: After Main-Group Elements as Transition Metals

Since the latter quarter of the twentieth century, main group chemistry has undergone significant advances. Powers timely review in 2010 highlighted the inherent differences between the lighter and heavier main group elements, and that the heavier analogues resemble transition metals as shown by their reactivity towards small molecules. In this concept article, we present an overview of the last 10 years since Powers seminal review, and the progress made for catalytic application. This examines the use of low oxidation state and/or low coordinate group 13 and 14 complexes towards small molecule activation (oxidative addition step in a redox based cycle) and how ligand design plays a crucial role in influencing subsequent reactivity. The challenge in these redox based catalytic cycles still centres on the main group complexes’ ability to undergo reductive elimination, however considerable progress in this field has been reported via reversible oxidative addition reactions. Within the last 5 years the first examples of well‐defined low valent main group catalysts have begun to emerge, representing a bright future ahead for main group chemistry.

Experimental Realisation of Elusive Multiple‐Bonded Aluminium Compounds: A New Horizon in Aluminium Chemistry

The synthesis and isolation of stable main group compounds featuring multiple bonds has been of great interest for several decades. A plethora of such multiply bonded complexes have been obtained by using sterically demanding substituents that provide both kinetic and thermodynamic stability. Most of these compounds have unusual structural and electronic properties that challenge the classical concept of covalent multiple bonding. In contrast, analogous aluminium compounds are scarce in spite of its high natural abundance. The parent dialumene (Al2 H2 ) has been calculated to be extremely unstable, thus making compounds containing Al multiple bonds a challenging synthetic target. This Review provides an overview of the recent advances in the cutting edge synthetic approaches and the careful ligand design used to obtain aluminium homo- and heterodiatomic multiply bonded complexes. In addition, the reactivity of these novel compounds towards various small molecules and reagents will be discussed herein.