Hydroboration Of Terminal Alkenes Research Articles

Pt(II) complexes bearing P,N-donor ligands as catalysts in chemoselective hydrosilylation, hydrogermylation, and hydroboration of terminal alkenes

In this report, we present the synthesis of six novel neutral platinum complexes employing readily available P,N-donor ligands through a straightforward two-step procedure. Subsequently, we investigated their catalytic activity in the hydroelementation of terminal olefins, highlighting their versatile utility in material and natural product chemistry. The efficient addition of silicon, germanium, and boron hydrides to the CC bond at low catalyst loading occurred exclusively in an anti-Markovnikov manner. Furthermore, our substrate scope encompasses a wide range of aliphatic and aromatic alkenes, featuring substituents with varying electronic properties. In contrast to many previous Pt-complexes, our catalytic system displayed exceptional chemoselectivity towards other unsaturated functional groups, such as carbonyl and internal CC bonds. Notably, it also exhibited remarkable tolerance to hydroxyl, alkoxyl, silyl, and thioether moieties. Additionally, we accentuated the prowess of our catalyst, showcasing its capability to sustain consistent activity and selectivity throughout numerous catalytic cycles.

Hydroboration of Terminal Alkenes and trans-1,2-Diboration of Terminal Alkynes Catalyzed by a Manganese(I) Alkyl Complex.

A MnI‐catalyzed hydroboration of terminal alkenes and a 1,2‐diboration of terminal alkynes with pinacolborane (HBPin) is described. For alkenes, anti‐Markovnikov hydroboration takes place; for alkynes the reaction proceeds with excellent trans‐1,2‐selectivity. The most active pre‐catalyst is bench‐stable alkyl bisphosphine MnI complex fac‐[Mn(dippe)(CO)3(CH2CH2CH3)]. The catalytic process is initiated by migratory insertion of a CO ligand into the Mn–alkyl bond to yield an acyl intermediate, which undergoes B−H bond cleavage of HBPin (for alkenes) and rapid C−H bond cleavage (for alkynes), forming the active MnI boryl and acetylide catalysts [Mn(dippe)(CO)2(BPin)] and [Mn(dippe)(CO)2(C≡CR)], respectively. A broad variety of aromatic and aliphatic alkenes and alkynes was efficiently and selectively borylated. Mechanistic insights are provided based on experimental data and DFT calculations revealing that an acceptorless reaction is operating involving dihydrogen release.

ChemInform Abstract: A Concise and Atom‐Economical Suzuki—Miyaura Coupling Reaction Using Unactivated Trialkyl‐ and Triarylboranes with Aryl Halides.

AbstractIt is shown that commercially available trialkyl/aryl boranes as well as trialkylboranes generated in situ by hydroboration of terminal alkenes with borane can be used for the coupling reactions.

Dynamics and the Failure of Transition State Theory in Alkene Hydroboration

Transition state theory fails to accurately predict the selectivity in an example where it is ubiquitously invoked, hydroboration. The hydroboration of terminal alkenes with BH(3) is moderately regioselective, affording an 88:12-90:10 ratio of anti-Markovnikov/Markovnikov adducts. High-level ab initio calculations predict too large of an energy difference between anti-Markovnikov and Markovnikov transition structures to account for the observed product ratio, and consideration of calculational error, solvent, tunneling, and entropy effects does not resolve the discrepancy. Trajectory studies, however, predict well the experimental selectivity. The decreased selectivity versus transition state theory arises from the excess energy generated as the BH(3) interacts with the alkene, and the observed selectivity is proposed to result from a combination of low selectivity in direct trajectories, moderate RRKM selectivity, and high selectivity after thermal equilibration.

Organoboranes for synthesis. 10. 1 the base-induced reaction of bromine with organoboranes. A convenient procedure for the conversion of alkenes into alkyl bromides via hydroboration

The reaction of trialkylboranes with bromine is greatly accelerated by base. Bromination in the presence of sodium hydroxide provides alkyl bromide along with a large amount of the corresponding alcohol. The use of sodium methoxide as a base eliminates this undesirable sidereaction and provides an improved yield of alkyl bromide. Consequently, hydroboration, followed by bromination in the presence of sodium methoxide, provides a convenient new procedure for the conversion of alkenes into alkyl bromides. The organoboranes, obtained via hydroboration of terminal alkenes, react with the utilization of all three alkyl groups attached to boron, providing nearly quantitative yields of alkyl bromides. This procedure also accommodates common organic functional groups, as demonstrated by the preparation of methyl 11-bromoundecanoate and 11-bromoundecyl acetate from the corresponding functionally substituted alkenes. Under these conditions, secondary and bulky primary alkyl groups react more sluggishly. However, a procedure involving simultaneous addition of bromine and methanolic sodium methoxide provides improved results for such derivatives. Surprisingly, the base-induced bromination of tri-exo-norbomylborane results in an inversion of configuration at the reaction center to give predominantly endo-2-bromonorbomane. A mechanism is proposed to account for this remarkable inversion.

Organoboranes for synthesis. 4 : Oxidation of organoboranes with pyridinium chlorochromate. A direct synthesis of aldehydes from terminal alkenes via hydroboration

The oxidation of trialkylboranes containing primary a1kyl groups with pyridinium chlorochromate (PCC) in methylene chloride provides the corresponding aldehydes in good yields. The stoichiometry for the oxidation of alcohols, borate esters and trialkylboranes with PCC has been examined. In view of the poor regioselectivity (only 94% primary alkyl groups) and functional group tolerance observed in the hydroboration with borane (BH 3.THF or BH 3.SMe 2), a more selective hydroborating agent, bis(3-methyl-2-butyl)borane (disiamylborane), was utilized for the preparation of aldehydes from terminal alkenes. However, the formation of 3-methyl-2-butanone as a by-product, and the requirement of six moles of PCC per mole of aldehyde are major disadvantages in this method. This difficulty was circumvented by employing monochloroborane-dimethyl sulfide for hydroboration. This reagent exhibits high regioselectivity (⪢ 99% primary alkyl groups) in the hydroboration of terminal alkenes. Oxidation of the resulting dialkylchloroborane following hydrolysis affords the desired aldehydes in satisfactory yields. Consequently, the hydroboration of terminal alkenes, followed by PCC oxidation, represents a direct convenient method for the transformation of alkenes into the corresponding aldehydes.