Recent advances in the chemistry of heavier group 14 analogues of carbonyls

Heavier analogues of carbonyls, in the form of "R2E=O" (E = Si, Ge, Sn, Pb), feature a high polar E=O double bond. In contrast to carbonyl compounds, heavier analogues are extremely unstable and prone to proceed head-to-tail oligomerization. Thus, the isolation of such species under ambient conditions is a challenging synthetic target in main group chemistry. In recent years, much progress has been achieved in the synthesis and isolation of a variety of Lewis base/acid, Lewis base-stabilized and even Lewis acid/base free heavier analogues. These compounds exhibit interesting reactivities, such as small molecule activation and metathesis reactions, indicating the potential of heavier analogues in synthetic chemistry. This review summarizes the recent achievements in the chemistry of Lewis base and/or acid stabilized heavier analogues of carbonyls, including synthetic approaches, structural parameters and reactivity of these isolable compounds.

Perfluorinated Trialkoxysilanol with Dramatically Increased Brønsted Acidity

The Brønsted acidity of the perfluorinated trialkoxysilanol {(F₃C)₃CO}₃SiOH is more than 13 orders of magnitude higher than that of orthosilicic acid, Si(OH)₄, and even more for most previously known silanols. It is easily deprotonated by simple amines and pyridines to give the conjugate silanolates [OSi{OC(CF₃)₃}₃]⁻, which possess extremely short Si−O bonds, comparable to those of silanones.

An Isolable Three-Coordinate Germanone and Its Reactivity

A rare three‐coordinate germanone [IPrN]₂Ge=O (IPrN=bis(2,6‐diisopropylphenyl)imidazolin‐2‐imino) was successfully isolated. The germanone has a rather high thermal stability in arene solvent, and no detectable change was observed at 80 °C for at least one week. However, high thermal stability of [IPrN]₂Ge=O does not prevent its reactivity toward small molecules. Structural analysis and initial reactivity studies revealed the highly polarized nature of the terminal Ge=O bond. Besides, the addition of phenylacetylene, as well as O‐atom transfer with 2,6‐dimethylphenyl isocyanide make it a mimic of nucleophilic transition‐metal oxides. Mechanism for O‐atom transfer reaction was investigated via DFT calculations, which revealed that the reaction proceeds via a [2+2] cycloaddition intermediate.

Strained and Reactive Donor/Acceptor-Supported Metallasilanone

A novel stable donor/acceptor-supported Mn^I -metallasilanone 3 was synthesized. The intramolecular silanone-Mn^I interaction induces a highly strained three-membered cyclic structure, leading to an exceptionally high reactivity of 3 as a donor/acceptor complex of silanone. Indeed, metallasilanone 3 readily reacts with various small molecules such as H₂ or ethylene gas in mild conditions.

Acid-Base Free Main Group Carbonyl Analogues

Main group carbonyl analogues (R2 E=O) derived from p-block elements (E=groups 13 to 15) have long been considered as elusive species. Previously, employment of chemical tricks such as acid- and base-stabilization protocols granted access to these transient species in their masked forms. However, electronic and steric effects inevitably perturb their chemical reactivity and distinguish them from classical carbonyl compounds. A new era was marked by the recent isolation of acid-base free main group carbonyl analogues, ranging from a lighter boracarbonyl to the heavier silacarbonyls, phosphacarbonyls and a germacarbonyl. Most importantly, their unperturbed nature elicits exciting new chemistry, spanning the vista from classical organic carbonyl-type reactions to transition metal-like oxide ion transfer chemistry. In this Review, we survey the strategies used for the isolation of such systems and document their emerging reactivity profiles, with a view to providing fundamental comparisons both with carbon and transition metal oxo species. This highlights the emerging opportunities for exciting "crossover" reactivity offered by these derivatives of the p-block elements.