[BO2 ]- as a Synthon for the Generation of Boron-Centered Carbamate and Carboxylate Isosteres

Reactivity of free N-heterocyclic carbenes in dichloromethane

The reactivity of free 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (MeIPr) and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IDipp) in dichloromethane was investigated. MeIPr reacts relatively slowly and selectively with the solvent under the formation of imidazolium salt [MeIPrH]Cl, which was characterised through NMR and IR spectroscopy as well as single crystal X-ray diffraction. Through deuterium labelling experiments the reaction rate was determined. In contrast IDipp reacts unselectively to various imidazolium salts. Due to the slow decomposition rates of MeIPr and IDipp in CH2Cl2, reactions of the free carbenes with BeBr2 to [(MeIPr)BeBr2], [(MeIPr)2BeBr2] and [(IDipp)BeBr2] can be performed.

Acid/Base-Free Acyclic Anionic Oxoborane and Iminoborane Bearing Diboryl Groups

Anionic oxoboranes and neutral iminoboranes, which are isoelectronic to ketones and alkynes, respectively, have attracted much attention because of their unique structures and various reactivity. However, acid/base-free oxoboranes and iminoboranes are still limited, and readily accessible examples with diverse electronic and steric characteristics are highly desirable. Herein, we report the first syntheses of the acyclic anionic oxoborane 2 and iminoborane 4 bearing two boryl ligands, both of which are acid/base-free. Spectroscopic analysis, X-ray crystallography, and theoretical calculations reveal that 2 and 4 possess a polarized terminal B═O double bond and central B≡N triple bond, respectively.

Boraiminolithium: An Iminoborane-Transfer Reagent

BN/CC isosterism can give rise to attractive molecules with unique physical or chemical properties. We report here the synthesis, characterization, and reactivities of the boraiminolithium species 2, a room-temperature-stable crystalline solid accessible through a facile dehydrohalogenation/deprotonation reaction. This species, bearing a polarized B≡N triple bond and an anionic N center, is the first example of a BN analogue to the well-known alkynyllithium molecules (lithium acetylides). It has demonstrated a remarkable ability for iminoborane-transfer reactions, which allows for the isolation of a series of unprecedented N-functionalized iminoboranes as well as novel main-group heterocycles. Stable boraiminolithium reagents may become powerful tools in the exploration of new BN-containing building blocks for synthetic chemistry and materials science.

Synthesis, Structure, and Reactivity of Acid-Free Neutral Oxoborane

An efficient synthesis of an acid-free neutral oxoborane of the type carboranyl-B(carbene)=O has been developed via a serendipitous discovery from the reaction of 1,2-[BBr(carbene)]-o-carborane with AgOTf. This represents a new type of oxoborane. The stabilization of this oxoborane may be attributed to 1) kinetic stabilization provided by a bulky 3D carboranyl ligand and 2) thermodynamic stabilization offered by a carbene ligand. Crystallographic analyses support the presence of the shortest terminal B=O double bond ever reported thus far. Its reactivity has also been examined.

Anionic oxoborane and thioxoborane molecules supported by a 1,2-bis(imino)acenaphthene ligand

The isolable anionic oxoborane 3 and thioxoborane 4 have been assembled using a 1,2-bis(imino)acenaphthene ligand (Dip-BIAN). Structural characterization and DFT calculations confirmed that two compounds contain terminal doubly bonded B[double bond, length as m-dash]E (E = O, S) groups, respectively, in which only the B[double bond, length as m-dash]O group is associated with imidazolium via a hydrogen bond.

Expected and Unexpected Reactivities of Homoleptic LiNacNac and Heteroleptic NacNacMg(TMP) β-Diketiminates toward Various Small Unsaturated Organic Molecules

Homoleptic LiNacNac forms simple donor-acceptor complexes with N,N'-dicyclohexylcarbodiimide (CyN═C═NCy), triphenylphosphine oxide (Ph₃P═O), and benzophenone (Ph₂CO). These crystallographically characterized compounds could be regarded as model intermediates en route to reducing the N═C, P═O, and C═O bonds of unsaturated substrates. Heteroleptic NacNacMg(TMP) intriguingly functions as a TMP nucleophile both with t-BuNCO and t-BuNCS, producing a urea or thiourea derivative respectively attached to Mg, though the NacNac ligand in the former reaction also engages noninnocently with a second t-BuNCO molecule via insertion at the reactive NacNac backbone γ-carbon site.

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.

[BO2 ]- as a Synthon for the Generation of Boron-Centered Carbamate and Carboxylate Isosteres

Oxoborane carbamate and carboxylate analogues result from the in situ trapping of [BO₂]⁻ produced by elimination of 2,3‐dimethyl‐2‐butene from a pinacolatoboryl anion.