B−B Bond Nucleophilicity in a Tetraaryl μ‐Hydridodiborane(4) Anion

Abstract

The tetraaryl μ‐hydridodiborane(4) anion [2H]− possesses nucleophilic B−B and B−H bonds. Treatment of K[2H] with the electrophilic 9‐H‐9‐borafluorene (HBFlu) furnishes the B3 cluster K[3], with a triangular boron core linked through two BHB two‐electron, three‐center bonds and one electron‐precise B−B bond, reminiscent of the prominent [B3H8]− anion. Upon heating or prolonged stirring at room temperature, K[3] rearranges to a slightly more stable isomer K[3 a]. The reaction of M[2H] (M+=Li+, K+) with MeI or Me3SiCl leads to equimolar amounts of 9‐R‐9‐borafluorene and HBFlu (R=Me or Me3Si). Thus, [2H]− behaves as a masked [:BFlu]− nucleophile. The HBFlu by‐product was used in situ to establish a tandem substitution‐hydroboration reaction: a 1:1 mixture of M[2H] and allyl bromide gave the 1,3‐propylene‐linked ditopic 9‐borafluorene 5 as sole product. M[2H] also participates in unprecedented [4+1] cycloadditions with dienes to furnish dialkyl diaryl spiroborates, M[R2BFlu].

Anionic Organoboranes: Delicate Flowers Worth Caring for

“…While these rearrangements were initially perceived as a nuisance, it soon turned out that they rather are the prerequisite for an intriguing follow-up chemistry providing access to synthetically useful "masked 9borafluorenyl anions", [:BFlu] -. [37,48,64] DFT calculations on [10] revealed the existence of four isomeric and energetically comparable minimum structures. Especially the second most favorable one, [10′′] -, contributes decisively to the understanding of the reactivity of [10] -(Scheme 11).…”

“…While protonation [38] leads back to the diborane(6) 9, the addition of the 9H-9-borafluorene adduct 15•thf provides the B 3 cluster [19] -, an organyl derivative of the prominent [B 3 H 8 ]anion (Scheme 11). [64] The kinetic product [19] has a tendency to isomerize to the thermodynamic product [19′]via hydride and phenyl shifts. Such reversible Wagner-Meerwein-type rearrangements are commonly encountered in the chemistry of 9 and its reduction products (compare e.g., [9] 2vs.…”

“…Especially the second most favorable one, [10′′] -, contributes decisively to the understanding of the reactivity of [10] -(Scheme 11). [64] As a B(sp 2 )-B(sp 3 ) isomer, [10′′]can be regarded as an adduct between [:BFlu] -(as Lewis base) and HBFlu (as Lewis acid; 15) and is therefore reminiscent of the adducts [pinB-Bpin(OR)]of bis(pinacolato)diboron which are widely used as sources of [:Bpin]nucleophiles. [65] In agreement with this view, THF solutions of [10] react with MeI or Me 3 SiCl to give 9-Me- (16) or 9-SiMe 3 -9-borafluorene (20) and the by-product 15•thf (Scheme 11).…”

“…[65] In agreement with this view, THF solutions of [10] react with MeI or Me 3 SiCl to give 9-Me- (16) or 9-SiMe 3 -9-borafluorene (20) and the by-product 15•thf (Scheme 11). [64] When the unsatu-rated electrophile allyl bromide is used, the product of the nucleophilic substitution reaction, 21, traps the in situ generated 15•thf by a hydroboration reaction at its terminal olefin to generate the 1,3-bis(9-borafluorenyl)propane 22. The chimeric nature of the [:BFlu]fragment as a nucleophilic or carbenoid species has been proven by treating [10] with the well-established carbene-trapping reagent 2,3-dimethyl-1,3-butadiene (DMB) so that the spiroborate [23] is formed via a [4+1]-electrocyclization; additional DMB is consumed in a double hydroboration reaction with the by-product 15•thf to give 24 (Scheme 11).…”

“…The chimeric nature of the [:BFlu]fragment as a nucleophilic or carbenoid species has been proven by treating [10] with the well-established carbene-trapping reagent 2,3-dimethyl-1,3-butadiene (DMB) so that the spiroborate [23] is formed via a [4+1]-electrocyclization; additional DMB is consumed in a double hydroboration reaction with the by-product 15•thf to give 24 (Scheme 11). [64] Also the reactivity pattern of the diborane(4) dianion [11] 2can be explained similarly as in the case of [10] -. For example, treating Li 2 [11] with excess MeI yields 2 equiv.…”