Selective hydroboration and synthetic utility of organoboranes thus obtained

Suzuki, Akira; Dhillon, R. S.

doi:10.1007/3-540-15810-3_2

Cited by 44 publications

(13 citation statements)

References 220 publications

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“…As a final test for hydroboration by X 2 BH species, the reaction of 2a with HBCat‘ was performed in the presence of excess cyclooctene. The rates for uncatalyzed hydroboration of cyclooctene and α-olefins by BH 3 are competitive . Thus, any B−C bond forming pathway that does not occur at the metal should give appreciable quantities of cyclooctene hydroboration products.…”

Section: Resultsmentioning

confidence: 99%

Group 5 Metallocene Complexes as Models for Metal-Mediated Hydroboration: Synthesis of a Reactive Borane Adduct, endo-Cp*₂Nb(H₂BO₂C₆H₄), via Hydroboration of Coordinated Olefins

1997

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The olefin complexes Cp*2M(CH2CH(R))(H) (R = H (1), CH3 (2); M = Nb (a), Ta (b)) react cleanly with catecholborane (HBCat) and HBO2C6H3-4- t Bu (HBCat‘) to give Cp*2M(H2BCat) (M = Nb (5a), Ta (5b)) and Cp*2M(H2BCat‘) (M = Nb (6a), Ta (6b)) and the anti-Markovnikov hydroboration products CatBCH2CH2R and Cat‘BCH2CH2R. Compounds 2a and 2b react with DBCat‘ (DBO2C6H3-4- t Bu) to afford the deuterated analogs Cp*2M(D2BCat‘) (M = Nb (6a), Ta (6b)) where the deuterium label is incorporated exclusively in the metal complex. The hydride resonances in 6a exhibit large perturbations in chemical shift when deuterium is incorporated. On the basis of this isotopic labeling experiment, a mechanism is proposed where HBCat reacts with the 16-electron alkyl intermediates, Cp*2MCH2CH2R (R = H (3), Me (4)), via σ-bond metathesis or oxidative-addition/reductive-elimination sequences, to generate the alkylboranes and an intermediate hydride, Cp*2MH, that is trapped by additional borane to give Cp*2M(H2BCat) (5a and 5b). The solid-state structures for Cp*2Nb(η2-H2BO2C6H3-3- t Bu) (16) and Cp2*Nb(η2-BH4) (17) were determined by X-ray diffraction. The metal−boron distances in these two compounds are identical within experimental error. While related group 5 catecholateboryl compounds have pronounced boryl character, the structural parameters for the hydride and boryl ligands in 16 are consistent with formulation as either a borohydride complex or a borane adduct of “Cp*2NbH”. In contrast to other group 5 boryl complexes, 6a reacts readily with various two-electron ligands with elimination of HBCat‘. For example, H2 reacts reversibly to form Cp*2NbH3 and HBCat‘, while “BH3” and CO react irreversibly to yield Cp*Nb(BH4) and Cp*2Nb(H)(CO) with elimination of HBCat‘, respectively. Ethylene and propylene react at 40 °C to regenerate 1a and 2a, with elimination of HBCat‘. When excess olefin is present, the liberated borane is converted to CatBCH2CH2R. Solutions of 1a and 2a catalyze olefin hydroboration under mild conditions. Relationships between the reactivity of 1a and 2a and other early metal and lanthanide catalysts are discussed.

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Section: Resultsmentioning

confidence: 99%

Group 5 Metallocene Complexes as Models for Metal-Mediated Hydroboration: Synthesis of a Reactive Borane Adduct, endo-Cp*₂Nb(H₂BO₂C₆H₄), via Hydroboration of Coordinated Olefins

1997

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“…The decreased hydroboration reactivity of 4 towards alkenes is similar to that of pinacol- [60,61] and catecholborane, [62][63][64][65] both of which are known to react faster with alkynes over alkenes. [60,66] Overall, hydroboration of the less substituted carbon was favoured in all examples due to the increased bulk around the pentacoordinated phosphorus, which provides some steric to the boron atom.…”

Section: Hydroboration Reactivitymentioning

confidence: 74%

Synthesis, Structure and Reactivities of Pentacoordinated Phosphorus–Boron Bonded Compounds

O'Brien¹,

Kano²,

Havare³

et al. 2020

Preprint

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The isolation and reactivities of two pentacoordinated<br>phosphorus–tetracoordinated boron bonded compounds were explored. A strong Lewis acidic boron reagent and electronwithdrawing ligand system were required to form the pentacoordinated phosphorus state of the P–B bond. The first compound, a phosphoranyl-trihydroborate, gave a THF stabilised phosphoranyl-borane intermediate upon a single hydride abstraction in THF. This compound could undergo a unique rearrangement reaction, which involved a two-fold ring expansion, to give a fused bicyclic compound or it could act as a mono-hydroboration reagent. The hydroboration reactivity of the intermediate was found to be more reactive towards alkynes over alkenes with good to moderate regioselectivity towards the terminal carbon. The second compound, a phosphoranyl-triarylborate, was found to have different reactivity as it was highly stable towards acids and bases. This is thought to be due to the large bulk around the P–B bond as shown in the crystal structure.<br>

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“…For theses ystems, the hydroboration step proceeded as an anti-Markovnikova ddition, eventually leadingt ot erminal carboxylic acids with excellent regioselectivity. [15,17] Other regioisomers were not observed. Further studies showed that internal alkenes 1k-m are far less reactive than terminal olefins (2k-m,5 2-71 %).…”

Section: Scheme1a)mentioning

confidence: 94%

“…However,f or these substrates the reactions proceeded with excellent regioselectivities owing to the steric controlo ft he hydroboration step. [15,17] The reduced reactivity of internal alkenes allowed us to conductr egioselective hydrocarboxylation on nonconjugated dienes possessing one internal and one terminal double bond, 1n and 1o.I nt his case, we used 0.7 equivalents of 9-BBN,w hich allowed us to prepare only the hydrocarboxylation product of the terminal double bond (2n,5 8%; 2o, 73 %). These observations may explain why the hydrocarboxylation of 1a workedi nR oseOx and g-Terp, which have internal double bonds (Figure 2).…”

Section: Scheme1a)mentioning

confidence: 99%

Exploration of New Biomass‐Derived Solvents: Application to Carboxylation Reactions

2020

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Selective hydroboration and synthetic utility of organoboranes thus obtained

Cited by 44 publications

References 220 publications

Group 5 Metallocene Complexes as Models for Metal-Mediated Hydroboration: Synthesis of a Reactive Borane Adduct, endo-Cp*₂Nb(H₂BO₂C₆H₄), via Hydroboration of Coordinated Olefins

Group 5 Metallocene Complexes as Models for Metal-Mediated Hydroboration: Synthesis of a Reactive Borane Adduct, endo-Cp*₂Nb(H₂BO₂C₆H₄), via Hydroboration of Coordinated Olefins

Synthesis, Structure and Reactivities of Pentacoordinated Phosphorus–Boron Bonded Compounds

Exploration of New Biomass‐Derived Solvents: Application to Carboxylation Reactions

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Selective hydroboration and synthetic utility of organoboranes thus obtained

Cited by 44 publications

References 220 publications

Group 5 Metallocene Complexes as Models for Metal-Mediated Hydroboration: Synthesis of a Reactive Borane Adduct, endo-Cp*2Nb(H2BO2C6H4), via Hydroboration of Coordinated Olefins

Group 5 Metallocene Complexes as Models for Metal-Mediated Hydroboration: Synthesis of a Reactive Borane Adduct, endo-Cp*2Nb(H2BO2C6H4), via Hydroboration of Coordinated Olefins

Synthesis, Structure and Reactivities of Pentacoordinated Phosphorus–Boron Bonded Compounds

Exploration of New Biomass‐Derived Solvents: Application to Carboxylation Reactions

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Group 5 Metallocene Complexes as Models for Metal-Mediated Hydroboration: Synthesis of a Reactive Borane Adduct, endo-Cp*₂Nb(H₂BO₂C₆H₄), via Hydroboration of Coordinated Olefins

Group 5 Metallocene Complexes as Models for Metal-Mediated Hydroboration: Synthesis of a Reactive Borane Adduct, endo-Cp*₂Nb(H₂BO₂C₆H₄), via Hydroboration of Coordinated Olefins