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

Abstract

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.

Stretched σ-borane complexes of rhodium: A theoretical study

“…Garcia and coworker have recently reported the r-borane complexes of nickel(0) [10]. Synthesis and bonding of the transition metal complexes containing residual BÁ Á ÁH interaction have been reported [11][12][13][14][15][16][17][18]. Mechanistic studies have revealed that r-borane complexes can be intermediates in catalytic hydroboration and borylation reactions [13,[19][20][21].…”

Metallaheteroborane clusters of group 5 transition metals derived from dichalcogenide ligands

“…The average bridging NbeSe distance (2.6992 Å) in 2 is about 0.06 Å longer than that observed in [Nb 2 Se 4 (Et 2 NCS 2 ) 4 ] (2.6392 Å) [24]. The average NbeB distance of 2.386 Å lies in the range of those reported for the niobium boron complexes [25]. The angle between the wings of the Nb 2 B 2 "butterfly" of 163.74 is comparable to the related cluster [(Cp * Mo) 2 B 4 H 8 (m-Seph)] [13].…”

Hydroboration, Diboration, Silylboration, and Stannylboration

“…The proposed mechanism for the metallocene complexes of Group 5 metals and lanthanides involves the coordination of alkene to the metal hydride (51), followed [67]. The product will be then provided via the direct σ-bond metathesis between HBcat and 53 or an oxidative addition/reductive elimination sequence (53 → 54 → 51).…”

“…The product will be then provided via the direct σ-bond metathesis between HBcat and 53 or an oxidative addition/reductive elimination sequence (53 → 54 → 51). These processes are studied in detail for the Group 5 metals such as Nb and Ta [67][68][69]; however, an ab initio calculation [70] for the hydroboration of C 2 H 4 with HB(OH) 2 supports an analogous cycle for the Cp 2 Sm catalyst. Due to the high oxophilicity of the samarium atom, the last σ-bond metathesis (53 → 51) yields an acid/base complex between EtB(OH) 2 and Cp 2 SmH.…”

“…The rhodium-catalyzed hydroboration-oxidation of allylic alcohol 64 [81] yields the syndiol, whereas the uncatalyzed reaction of 9-BBN preferentially produces the antidiol. The origin of diastereofacial selectivity arises from differences in the mechanism of π-complexation between the transition metals (65) and the main metals (67), together with the steric effect of the substituents (66) [81]. The π-complexation of transition metals can be stabilized by the increased back-donation from a filled dorbital of the metal to a π*-orbital of alkene, which is enhanced when the best σ-acceptor adopts the anti position (65) and the smallest group (S) is in the inside crowded position (66).…”