Acid-Base Free Main Group Carbonyl Analogues

Abstract

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.

Keywords: carbonyl compounds; main group elements; multiple bonding; oxide transfer; reactivity studies.

Scheme 1

Main group carbonyl analogues of types I and II.

Scheme 2

A timeline for the isolation of crystalline acid–base free main group carbonyls.

Scheme 3

Acid‐stabilized neutral boracarbonyls (Type II).

Scheme 4

Acid‐stabilized anionic boracarbonyls (Type I).

Scheme 5

A transition metal‐stabilized boracarbonyl anion.

Figure 1

Solid‐state structure of 11. For clarity, [K(2.2.2‐crypt)]⁺ and hydrogen atoms are omitted. Thermal ellipsoids set at 50 % probability.

Scheme 6

Acid‐free boracarbonyl 11 (Type I) and its reactivity as an oxide ion transfer agent.

Scheme 7

Use of the NHBO ligand as a strong O‐donor to stabilize a series of heavier dioxycarbene analogues.

Scheme 8

Acid–base stabilized neutral alumacarbonyl 13 (Type II).

Scheme 9

Dimer‐stabilized alumacarbonyl [15‐THF]₂ (Type I) and its reactivity.

Scheme 10

Dimer‐stabilized alumacarbonyl [16]₂ (Type I) and its reactivity.

Scheme 11

Driess’ acid–base stabilized silacarbonyls (Type I).

Scheme 12

Roesky's acid–base stabilized silacarbonyls (Type I).

Scheme 13

Kato and Baceiredo's acid–base stabilized silacarbonyls (Type I).

Scheme 14

Robinson's NHC‐stabilized silicon oxides (Type I).

Scheme 15

Aldridge's acid–base stabilized silacarbonyls (Type I).

Scheme 16

Inoue's acid–base stabilized silacarbonyls (Type I).

Scheme 17

Inoue's base‐stabilized sila‐acylium ions 34(Ar) (Type II) and their reactivities.

Scheme 18

Transition metal‐stabilized silacarbonyls.

Figure 2

Solid‐state structure of 41( ⁱ Pr). For clarity, hydrogen atoms are omitted. Thermal ellipsoids set at 50 % probability.

Scheme 19

Kato's acid–base free cyclic (amino)(ylide)silacarbonyl 41(R) (Type I) and its reactivity.

Figure 3

Solid‐state structure of 42( ^t Bu). For clarity, hydrogen atoms are omitted, Dipp and ^tBu groups are simplified as wireframe. Thermal ellipsoids set at 50 % probability.

Scheme 20

Inoue's acid–base free acyclic (imino)(silyl)silacarbonyl 42(R) (Type I) and its reactivity.

Scheme 21

Sila‐Wittig chemistry with acid–base free silacarbonyl 42( ^t Bu).

Figure 4

Solid‐state structure of 49. For clarity, hydrogen atoms are omitted. Thermal ellipsoids set at 50 % probability.

Scheme 22

Kato's cyclic (amino)(bora‐ylide)silacarbonyl 49 (Type I) and its reactivity.

Figure 5

Solid‐state structure of 50. For clarity, hydrogen atoms are omitted, Me and ^tBu groups are simplified as wireframe. Thermal ellipsoids set at 50 % probability.

Scheme 23

Iwamoto's cyclic di(alkyl)silanone 50 (Type I) and its reactivity.

Scheme 24

Acid–base stabilized germacarbonyls (Type I).

Figure 6

Solid‐state structure of 53. For clarity, hydrogen atoms are omitted, Et groups are simplified as wireframe. Thermal ellipsoids set at 50 % probability.

Scheme 25

Tamao and Matsuo's acid–base free germanone 53 (Type I) and its reactivity.

Scheme 26

Base‐stabilized monocationic phosphacarbonyls (Type I).

Scheme 27

Base‐stabilized dicationic phosphacarbonyls (Type II).

Figure 7

Solid‐state structure of 60(N). For clarity, hydrogen atoms are omitted, Dipp groups are simplified as wireframe. Thermal ellipsoids set at 50 % probability.

Scheme 28

Dielmann's base‐free phosphacarbonyl 60(X) (Type I) and the phosphacarbonyl–yne reaction.