Dramatic effect of Lewis acids on the rhodium-catalyzed hydroboration of olefins

Catalytic Friedel-Crafts C-H Borylation of Electron-Rich Arenes: Dramatic Rate Acceleration by Added Alkenes

In the electrophilic C-H borylation of electron-rich aromatic compounds with catecholborane, the catalytic generation of the boron electrophile is initiated by heterolysis of the B-H bond by various Lewis and Brønsted acids, with a boronium ion formed exclusively. After ligand dissociation, the corresponding borenium ion undergoes regioselective electrophilic aromatic substitution on aniline derivatives as well as nitrogen-containing heterocycles. The catalysis is optimized using B(C₆ F₅ )₃ as the initiator and proceeds without the addition of an external base or dihydrogen acceptor. Temperatures above 80 °C are generally required to secure efficient turnover in these Friedel-Crafts-type reactions. Mechanistic experiments reveal that regeneration of the boronium/borenium ion with dihydrogen release is rate-determining. This finding finally led to the discovery that, with added alkenes, catalytic C-H borylations can, for the first time, be carried out at room temperature.

Transition metal free catalytic hydroboration of aldehydes and aldimines by amidinato silane

Benz-amidinato dichlorosilane [PhC(NtBu)₂SiHCl₂] has been reported to catalyze hydroboration of aldehydes at room temperature and aldimines under slightly harsh conditions.

Synthesis of Secondary and Tertiary Alkylboranes via Formal Hydroboration of Terminal and 1,1-Disubstituted Alkenes

Copper-catalyzed functionalization of terminal or 1,1-disubstituted alkenes with bis(pinacolato)diboron and methanol provides formal hydroboration products with exceptional regiocontrol favoring the branched isomer. Pairing this procedure with photocatalytic cross-couplings using iridium and nickel cocatalysis provides an effective, highly regioselective procedure for the hydroarylation of terminal alkenes.

Boron Lewis Acid-Catalyzed Hydroboration of Alkenes with Pinacolborane: BArF3 Does What B(C6 F5 )3 Cannot Do!

The transition-metal-free hydroboration of various alkenes with pinacolborane (HBpin) initiated by tris[3,5-bis(trifluoromethyl)phenyl]borane (BAr^F₃ ) is reported. The choice of the boron Lewis acid is crucial as the more prominent boron Lewis acid tris(pentafluorophenyl)borane (B(C₆ F₅ )₃ ) is reluctant to react. Unlike B(C₆ F₅ )₃ , BAr^F₃ is found to engage in substituent redistribution with HBpin, resulting in the formation of Ar^F Bpin and the electron-deficient diboranes [H₂ BAr^F ]₂ and [(Ar^F )(H)B(μ-H)₂ BAr^F₂ ]. These in situ-generated hydroboranes undergo regioselective hydroboration of styrene derivatives as well as aliphatic alkenes with cis diastereoselectivity. Another ligand metathesis of these adducts with HBpin subsequently affords the corresponding HBpin-derived anti-Markovnikov adducts. The reactive hydroboranes are regenerated in this step, thereby closing the catalytic cycle.

Tautomerization of 2,6-lutidines in the presence of B(C6F5)3 using catecholborane as a precatalyst

The Lewis pair of 2,6-lutidine and B(C₆F₅)₃ can undergo tautomerization to produce enamine B(C₆F₅)₃ adducts when catecholborane is applied as a precatalyst.

Synthesis of 2-(lutidinyl)organoboranes and their reactivities against dihydrogen and pinacol borane

Two 2,4,6-tris(trifluoromethyl)phenyl-substituted 2-(lutidinyl)organoboranes (5a and 5b) were prepared. These complexes can function as intramolecular vicinal B/N frustrated Lewis pairs to heterolytically activate dihydrogen. When these complexes were treated with HBpin, two different reaction pathways took place. Whereas the reaction between 5a and HBpin affords a formal ligand-redistribution product, the reaction of 5b with HBpin leads to a dearomative dehydroborylation product.

Application of a nucleophilic boryl complex in the frustrated Lewis pair: activation of H-H, B-H and C=C bonds with B(C6F5)3 and boryl-borate lithium

The frustrated Lewis pair comprised of B(C6F5)3 and a boryl-borate lithium salt Li[pinBB(Ph)pin] can efficiently activate dihydrogen, pinacolborane and ethylene at ambient temperature. Theoretical studies suggest that the nucleophilic sp(2) boryl moiety of Li[pinBB(Ph)pin] plays different roles in these reactions.

Diastereoselective Ni-catalyzed 1,4-hydroboration of chiral dienols

The Ni-catalyzed hydroboration of dienols occurs in a 1,4 fashion and delivers a syn-propionate motif in high diastereoselectivity and with a stereodefined trisubstituted crotylboronic ester. The boronic ester can be further manipulated to provide carbon-carbon or carbon-oxygen bonds.

FeCl2-catalyzed hydroboration of aryl alkenes with bis(pinacolato)diboron

Alkylboronates were synthesized by a ligand-free ferrous chloride catalyzed anti-Markovnikov hydroboration of un-activated aryl alkenes with bis(pinacolato)diboron.

(N-phosphinoamidinate)cobalt-catalyzed hydroboration: alkene isomerization affords terminal selectivity

Herein we establish the utility of a three-coordinate (N-phosphinoamidinate)cobalt(amido) pre-catalyst that is capable of effecting challenging alkene isomerization/hydroboration processes at room temperature, leading to the selective terminal addition of the boron group.

Mechanistic Insights into Carbonyl-Directed Rhodium-Catalyzed Hydroboration: ab Initio Study of a Cyclic γ,δ-Unsaturated Amide

A two-point binding mechanism for the cationic rhodium(I)-catalyzed carbonyl-directed catalytic asymmetric hydroboration of a cyclic γ,δ-unsaturated amide is investigated using density functional theory. Geometry optimizations and harmonic frequency calculations for the model reaction are carried out using the basis set 6-31+G** for C, O, P, B, N, and H and LANL2DZ for Rh atoms. The Gibbs free energy of each species in THF solvent is obtained based on the single-point energy computed using the PCM model at the ECP28MWB/6-311+G(d,p) level plus the thermal correction to Gibbs free energy by deducting translational entropy contribution. The Rh-catalyzed reaction cycle involves the following sequence of events: (1) chelation of the cyclic γ,δ-unsaturated amide via alkene and carbonyl complexation in a model active catalytic species, [Rh(L2)₂S₂]⁺, (2) oxidative addition of pinacol borane (pinBH), (3) migratory insertion of the alkene double bond into Rh–H (preferred pathway) or Rh–B bond, (4) isomerization of the resulting intermediate, and finally, (5) reductive elimination to form the B–C or H–C bond with regeneration of the catalyst. Free energy profiles for potential pathways leading to the major γ-borylated product are computed and discussed in detail. The potential pathways considered include (1) pathways proceeding via migratory insertion into the Rh–H bond (pathways I, I-1, and I-2), (2) a potential pathway proceeding via migratory insertion into the Rh–B bond (pathway II), and two potential competing routes to a β-borylated byproduct (pathway III). The results find that the Rh–H migratory insertion pathway I-2, followed in sequence by an unanticipated isomerization via amide rotation and reductive elimination, is the most favorable reaction pathway. A secondary consequence of amide rotation is access to a competing β-hydride elimination pathway. The pathways computed in this study are supported by and help explain related experimental results.

Bis(imino)pyridine cobalt-catalyzed alkene isomerization-hydroboration: a strategy for remote hydrofunctionalization with terminal selectivity

Bis(imino)pyridine cobalt methyl complexes are active for the catalytic hydroboration of terminal, geminal, disubstituted internal, tri- and tetrasubstituted alkenes using pinacolborane (HBPin). The most active cobalt catalyst was obtained by introducing a pyrrolidinyl substituent into the 4-position of the bis(imino)pyridine chelate, enabling the facile hydroboration of sterically hindered substrates such as 1-methylcyclohexene, α-pinene, and 2,3-dimethyl-2-butene. Notably, these hydroboration reactions proceed with high activity and anti-Markovnikov selectivity in neat substrates at 23 °C. With internal olefins, the cobalt catalyst places the boron substituent exclusively at the terminal positions of an alkyl chain, providing a convenient method for hydrofunctionalization of remote C-H bonds.

Palladium-catalyzed borylation of primary alkyl bromides

A mild Pd-catalyzed process for the borylation of alkyl bromides has been developed using bis(pinacolato)diboron as a boron source. This process accommodates the use of a wide range of functional groups on the alkyl bromide substrate. Primary bromides react with complete selectivity in the presence of a secondary bromide. The generality of this approach is demonstrated by its extension to the use of alkyl iodides and alkyl tosylates, as well as borylation reactions employing bis(neopentyl glycolato)diboron as the boron source.

Remarkable levels of enantioswitching in catalytic asymmetric hydroboration

TADDOL-derived phosphites and phosphoramidites are effective ligands for rhodium-catalyzed asymmetric hydroborations of β,γ-unsaturated amides, achieving up to 99% ee. However, the sense of stereoinduction, R or S, is surprisingly dependent on rather subtle features of the ligand. For example, catalysts employing a TADDOL phenylphosphite and those using the closely related N-methylaniline-derived phosphoramidite of the same configuration give opposite enantiomers of the product. Those derived from optical antipodes give the same product with virtually the same enantioselectivity as illustrated above. The different stereochemical outcomes may reflect fundamental differences in catalyst structure, reactivity, or reaction mechanism.

Electronic tuning of a carbene center via remote chemical induction, and relevant effects in catalysis

The present report develops the idea that an N-heterocyclic carbene incorporating a remote anionic functionality--here, a malonate group--as a backbone component of its heterocyclic framework, can be "post-functionalized" directly from its transition-metal complexes, upon simple addition of a variety of electrophiles interacting directly with the malonate group in the outer coordination sphere. From a palette of selected electrophilic reagents, it was thus possible to modulate the electronic donor properties of the carbene center over a rather broad range. Both the zwitterionic complex [Rh{malo-NHC}(cod)] and the cationic derivatives [Rh{malo-NHC(E)}(cod)](+) (where "malo-NHC(E)" represents the ligand modified by a selected electrophile "E") were used as pre-catalysts in two types of catalytic reactions, namely, the polymerization of phenylacetylene and the hydroboration of styrene. The results indicate that, in both cases, the zwitterionic species is by far the best catalyst, whereas a decrease in the ligand donicity induced by the added electrophile results in a concomitant reduction of catalytic activity. Apparent deviations to such a trend in the case of the hydroboration of styrene were rationalized in terms of an interaction between the reactive catecholborane substrate and the remote functionality of the N-heterocyclic carbene leading to an in situ modification of the nature of the active species. These observations serve as a useful basis to define the scope and limitations of the present conceptual approach in catalysis.

Highly regio- and enantioselective catalytic asymmetric hydroboration of α-substituted styrenyl derivatives

The catalytic asymmetric hydroboration of a variety of 1,1-disubstituted olefins has been achieved with excellent yields, perfect regioselectivity and in some cases, high levels of enantioselectivity using readily accessible iridium catalyst.