BCl3 is an inexpensive electrophile which induces the borylative cyclization of a wide range of substituted alkynes to regioselectively form polycycles containing synthetically versatile C(sp2)—boronate esters. It proceeds rapidly, with good yields and is compatible with a range of functional groups and substitution patterns. Intermolecular 1,2‐carboboration of alkynes is also achieved using BCl3 to generate trisubstituted vinyl boronate esters.

In contrast to the established dogma that B(C 6 F 5 ) 3 is irreversibly poisoned by excess H 2 O/amine (or imine) bases, B(C 6 F 5 ) 3 is actually a water-tolerant catalyst for the reductive amination of primary and secondary arylamines with aldehydes and ketones in "wet solvents" at raised temperatures and using only 1.2 equiv of Me 2 PhSiH as reductant. Arylamines/ N-arylimines do not result in the irreversible deprotonation of H 2 O−B(C 6 F 5 ) 3 , allowing sufficient B(C 6 F 5 ) 3 to be evolved at raised temperatures to effect catalytic reductions. Stronger Brønsted basic amines such as t BuNH 2 (and derived imines) result in irreversible formation of [HO−B(C 6 F 5 ) 3 ] − from H 2 O−B(C 6 F 5 ) 3 , precluding the formation of B(C 6 F 5 ) 3 at raised temperatures and thus preventing any imine reduction. A substrate scope exploration using 1 mol % nonpurified B(C 6 F 5 ) 3 and "wet solvents" demonstrates that this is an operationally simple and effective methodology for the production of secondary and tertiary arylamines in high yield, with imine reduction proceeding in preference to other possible reactions catalyzed by B(C 6 F 5 ) 3 , including the dehydrosilylation of H 2 O and the reduction of carbonyl moieties (e.g., esters).

Bis-boronic esters are versatile intermediates that enable the rapid elaboration of simple alkene precursors. Previous reports on their selective mono-functionalization have targeted the most accessible position, retaining the more hindered secondary boronic ester. In contrast, we have found that photoredoxcatalyzed mono-deboronation generates primary β-boryl radicals that undergo rapid 1,2-boron shift to form thermodynamically favored secondary radicals, allowing for selective transformation of the more hindered boronic ester. The pivotal 1,2-boron shift, which has been demonstrated to be stereoretentive, enables access to a wide range of functionalized boronic esters and has been applied to highly diastereoselective fragmentation and transannular cyclization reactions. Furthermore, its generality has been shown in a radical cascade reaction with an allylboronic ester.

The synthetically versatile pinacol boronic ester group (Bpin) is generally thought of as a bulky moiety because of the two adjacent quaternary sp3‐hydribized carbon atoms in its diol backbone. However, recent diastereoselective reactions reported in the literature have cast doubt on this perception. Reported herein is a detailed experimental and computational analysis of Bpin and structurally related boronic esters which allows determination of three different steric parameters for the Bpin group: the A‐value, ligand cone angle, and percent buried volume. All three parameters suggest that the Bpin moiety is remarkably small, with the planarity of the oxygen‐boron‐oxygen motif playing an important role in minimising steric interactions. Of the three steric parameters, percent buried volume provides the best correlation between steric size and diastereoselectivity in a Diels–Alder reaction.

Difunctionalization reactions of C−C -bonds have the potential to streamline access to molecules that would otherwise be difficult to prepare. However, the development of such reactions is challenging because C−C -bonds are typically unreactive.Exploiting the high ring-strain energy of polycyclic carbocycles is a common strategy to weaken and facilitate the reaction of C−C -bonds, but there are limited examples of highly strained C−C -bonds being used in difunctionalization reactions. We demonstrate that highly strained bicyclo[1.1.0]butyl boronate complexes (strain energy: ca. 65 kcal/mol), which were prepared by reacting boronic esters with bicyclo[1.1.0]butyl lithium, react with electrophiles to achieve the diastereoselective difunctionalization of the strained central C−C -bond of the bicyclo[1.1.0]butyl unit. The reaction shows broad substrate scope, with a range of different electrophiles and boronic esters being successfully employed to form a diverse set of 1,1,3-trisubstituted cyclobutanes (>50 examples) with high diastereoselectivity. The high diastereoselectivity observed has been rationalized based on a combination of experimental data and DFT calculations, which suggests that separate concerted and stepwise reaction mechanisms are operating depending upon the migrating substituent and electrophile used. This material is available free of charge via the Internet at http://pubs.acs.org.

N−Me‐Benzothiazolium salts are introduced as a new family of Lewis acids able to activate Si−H σ bonds. These carbon‐centred Lewis acids were demonstrated to have comparable Lewis acidity towards hydride as found for the triarylboranes widely used in Si−H σ‐bond activation. However, they display low Lewis acidity towards hard Lewis bases such as Et3PO and H2O in contrast to triarylboranes. The N−Me‐benzothiazolium salts are effective catalysts for a range of hydrosilylation and dehydrosilylation reactions. Judicious selection of the C2 aryl substituent in these cations enables tuning of the steric and electronic environment around the electrophilic centre to generate more active catalysts. Finally, related benzoxazolium and benzimidazolium salts were found also to be active for Si−H bond activation and as catalysts for the hydrosilylation of imines.

A substoichiometric quantity of the Lewis acid B(C 6 F 5 ) 3 is sufficient to initiate the aldehyde−amine−alkyne reaction, in a one-pot methodology that enables the synthesis of a range of functionalized quinolines. Optimization studies revealed that key requirements for the high-yielding tricomponent reaction initiated by B(C 6 F 5 ) 3 at raised temperatures include an excess of the in situ generated imine (which acts as a hydrogen acceptor) and an alkyne substituent able to stabilize positive charge buildup during the cyclization. Mechanistic experiments revealed that under these conditions B(C 6 F 5 ) 3 is acting as a Lewis acid-assisted Brønsted acid, with H 2 O−B(C 6 F 5 ) 3 being the key species enabling catalytic quinoline formation. This was indicated by deuterium labeling studies and the observation that the cyclization of N-(3-phenylpropargyl)aniline using B(C 6 F 5 ) 3 under anhydrous conditions afforded the zwitterion [(N-H-3-B(C 6 F 5 ) 3 -4-Ph-quinolinium], which does not undergo protodeboronation to release B(C 6 F 5 ) 3 and the quinoline product under a range of conditions. Finally, a brief substrate scope exploration demonstrated that this is an operationally simple and effective methodology for the production of functionalized quinolines.

Lower Lewis acidity boranes demonstrate greater tolerance to combinations of water/strong Brønsted bases than B(C6F5)3, this enables Si−H bond activation by a frustrated Lewis pair (FLP) mechanism to proceed in the presence of H2O/alkylamines. Specifically, BPh3 has improved water tolerance in the presence of alkylamines as the Brønsted acidic adduct H2O–BPh3 does not undergo irreversible deprotonation with aliphatic amines in contrast to H2O–B(C6F5)3. Therefore BPh3 is a catalyst for the reductive amination of aldehydes and ketones with alkylamines using silanes as reductants. A range of amines inaccessible using B(C6F5)3 as catalyst, were accessible by reductive amination catalysed by BPh3 via an operationally simple methodology requiring no purification of BPh3 or reagents/solvent. BPh3 has a complementary reductive amination scope to B(C6F5)3 with the former not an effective catalyst for the reductive amination of arylamines, while the latter is not an effective catalyst for the reductive amination of alkylamines. This disparity is due to the different pK a values of the water–borane adducts and the greater susceptibility of BPh3 species towards protodeboronation. An understanding of the deactivation processes occurring using B(C6F5)3 and BPh3 as reductive amination catalysts led to the identification of a third triarylborane, B(3,5‐Cl2C6H3)3, that has a broader substrate scope being able to catalyse the reductive amination of both aryl and alkyl amines with carbonyls.