A Combined DFT/IM-MS Study on the Reaction Mechanism of Cationic Ru(II)-Catalyzed Hydroboration of Alkynes

Abstract

Recently, the Furstner group reported the first general trans-hydroboration of internal alkynes by using a cationic ruthenium(II) complex, [Cp*Ru(MeCN) 3 ]PF 6 , as the catalyst. Density functional theory (DFT) calculations have been carried out to elucidate the reaction mechanism and the origin of stereoselectivity. The reaction mechanism was suggested to initiate with the rate-determining oxidative hydrogen migration to stereoselectively form a metallacyclopropene intermediate (that determines the trans selectivity), followed by a reductive boryl migration to form the unusual trans-addition alkenyl-borane product. A combined ion-mobility mass spectrometry (IM-MS) and DFT study has also been employed to investigate the unsuccessful reaction with terminal alkynes. Key oxidative-coupling intermediates have been identified. Our results suggest that [2 + 2 + 2] cycloaddition of terminal alkynes to form a very stable arene compound could be the reason for the unsuccessful hydroboration of the terminal alkynes. Moreover, unreactive catecholborane reagent attributes the strong coordination of its arene part with the catalyst. Our proposed nonclassical mechanism also accounted for the other related Ru(II)-catalyzed reactions (such as hydrogenation and hydrogermylation). Our combined computational and experimental study provides in-depth mechanistic understanding and insights on the unusual trans-addition catalyzed by the cationic ruthenium(II) complexes and could help design the other trans-addition reactions.

Mechanism and Origin of the Stereoselectivity in the Palladium‐Catalyzed trans Hydroboration of Internal 1,3‐Enynes with an Azaborine‐Based Phosphine Ligand

“…[28] The former process proceeds via ac oncerted transition state (such as I-TS7 or I-TS8 in Scheme 6), which were reported as favorable by Wu and co-workers for their Ru system. [13] However, according to our calculations, this process involving aP dc atalyst( C!I-TS7 or I-TS8)r equires high energy barriers (over 40 kcal mol À1 ), and thus is kinetically prohibited. The highb arriers observed in this case were ascribed to the steric hindrance between Bcata nd the bulky ligand (see the Supporting Information).…”

“…It is noteworthy that C= Cd ouble-bond rotationi sn ormally caused by as teric effect, and can be facilitated by the formation of an M=Cd ouble bond (such as in Y,w hich is favored in the Ru catalytic system). [13] However,t hese factors do not exist in the current system.T herefore, only the cis product could be generated through Path I-3, which contradicts the experimental observa-tions. Subsequente fforts were made to find alternative pathways to rationalize the trans selectivity.…”

“…Considering that Paths I-1 and I-2 are both impractical, our focus was diverted to the reported alkyne-assisted oxidative addition [13] (Scheme 6, Path I-3). The stable alkene-Pd complex D first isomerizes to the alkyne-Pd complex C before further transformation.S tarting from C,t he Bcat moiety in HBcatc an either approacht he Pd centero rf orm aC ÀBb ond with the alkyne.…”

“…Third, alkyne-assisted oxidative addition of the HÀBb ond was reported by the Wu group in their recents tudy of the Ru-catalyzed trans hydroboration of internal alkynes. [13] Therefore, Path I-3 (Scheme 2) was also taken into consideration. Based on above proposals( Paths I-1, I-2, and I-3), an intermediate like X can be generated( Scheme 2).…”

“…If X is followed by an ormalr eductive elimination, only the cis product is formed. According to Wu's investigationo fF ürstner's ruthenium catalytic system, [13] X-like intermediates might undergo isomerization to form am etal carbene (i.e., X!Y), in which the free rotation of the C=C double bond is now possible. In this way,t he trans product could be generated.…”