Main Group Multiple Bonds for Bond Activations and Catalysis

Abstract

Since the discovery that the so-called "double-bond" rule could be broken, the field of molecular main group multiple bonds has expanded rapidly. With the majority of homodiatomic double and triple bonds realised within the p-block, along with many heterodiatomic combinations, this Minireview examines the reactivity of these compounds with a particular emphasis on small molecule activation. Furthermore, whilst their ability to act as transition metal mimics has been explored, their catalytic behaviour is somewhat limited. This Minireview aims to highlight the potential of these complexes towards catalytic application and their role as synthons in further functionalisations making them a versatile tool for the modern synthetic chemist.

Keywords: bond activation; catalysis; main group; multiple bonds; small molecule activation.

Figure 1

Effect of ligand steric demands on isolable structures. Mes=2,4,6‐trimethylphenyl, Cp*=1,2,3,4,5‐pentamethylcyclopentadienyl.

Figure 2

Frontier molecular orbitals of transition metals (left) and main group multiple bonds (right) for the activation of dihydrogen.

Scheme 1

Generic catalytic cycles for the activation of substrates (X–Y) by transition metal multiple bonds.

Figure 3

Selected examples of E¹³–E¹³ multiple bonds. Dipp=2,6‐di‐iso‐propylphenyl.

Scheme 2

Ligand controlled small molecule activation with diborynes.

Scheme 3

Proposed catalytic cycle for CO₂ reduction by dialumene (7).

Scheme 4

Dihydrogen activation by an aryl‐stabilised dialumene (16). Tipp=2,4,6‐tri‐iso‐propylphenyl.

Scheme 5

CO₂ reduction by a boron‐aluminium multiple bond.

Scheme 6

(a) Reactivity of borasilene (21) towards chalcogens. (b) Selected resonance structures of Lewis base stabilised borasilene (25).

Scheme 7

Reactivity of E¹³‐imides towards CO₂ (Al only) and organic azides (In only).

Scheme 8

Synthesis and reactivity of an Al‐imide (34) towards small molecules.

Scheme 9

Phosphaborenes as a synthetic reagent. Mes=2,4,6‐trimethylphenyl; Mes*=2,4,6‐tri‐tert‐butylphenyl. IMe₄=1,3,4,5‐tetramethyl‐imidazol‐2‐ylidene.

Figure 4

Different Lewis acid and base strategies for stabilisation of terminal E¹³=E¹⁶ multiple bonds.

Scheme 10

Step‐wise synthetic cycle for oxide ion transfer agent, 44.

Scheme 11

Synthesis and reactivity of monoalumoxanes.

Scheme 12

Synthesis of ethenetetraolate ligand (54).

Scheme 13

Reactivity of heavier aluminium chalcogenides.

Figure 5

First reported examples of E¹⁴–E¹⁴ triple bonds. Dipp=2,6‐di‐iso‐propylphenyl. Tipp=2,4,6‐tri‐iso‐propylphenyl.

Scheme 14

(a) Selective anti‐addition of dihydrogen to a highly trans‐bent and twisted disilene (b) Ligand controlled activation of dihydrogen.

Scheme 15

Reactivity at the disilene‐silylsilylene equilibrium.

Scheme 16

Intercepting the disilene‐silylsilylene equilibrium. ^MesTer=2,6‐bis(2,4,6‐trimethylphenyl)phenyl.

Scheme 17

Digermyne catalysed cyclotrimerisation of alkynes. Tbb=4‐tBu‐2,6‐[CH‐(SiMe₃)₂]‐C₆H₂.

Figure 6

Different types of E¹⁴=C double bonds.

Scheme 18

Synthesis and reactivity of a silyne (Si‐C triple bond).

Scheme 19

(a) Reactivity of silastannenes and (b) reactivity of germastannenes. Tipp=2,4,6‐tri‐iso‐propylphenyl.

Scheme 20

Metalloid behaviour of a germanimine (90).

Scheme 21

Heavier nitrile reversible reactivity with diphenylketene.

Scheme 22

Ligand dependant transformations of silanones and subsequent reactivity with ethylene.

Scheme 23

Sila‐Wittig reactivity of silanones, a new synthetic route to silenes.

Scheme 24

Silaphosphene synthesis from silaaldehydes. IMe₄=1,3,4,5‐tetramethyl‐imidazol‐2‐ylidene.

Scheme 25

Si^II/Si^IV formation via silicon chalcogen multiple bond formation and chalcogen abstraction.