Stabilized borata-alkene formation: structural features, reactions and the role of the counter cation

Abstract

Dimethylbenzofulvene adds Piers' borane [HB(C6F5)2] at the indene double bond to give a mixture of regioisomeric boranes 8a,b. Subsequent isomerization under equilibrium conditions gives the isopropyl substituted 1H and 3H borylindenes 10a,b. Their treatment with the bulky LiTMP base under frustrated Lewis pair conditions resulted in a clean deprotonation reaction to give the borata-alkene 14. Its X-ray crystal structure analysis indicated a pronounced B[double bond, length as m-dash]C double bond character and thus a borata-benzofulvene description. The borata-alkene underwent (probably stepwise) [4 + 2] cycloaddition reactions with chalcone derivatives and a formal [6 + 2] cycloaddition with phenylmethylketene. Many products and derivatives were characterized by X-ray diffraction.

The 9‐Borataphenanthrene Anion

“…The 1 HNMR spectrum of 1 lacks the aliphatic CÀHsignal at 3.88 ppm for the proton at the 10-position and features ad iagnostic singlet at À0.01 ppm for the trimethyl-silyl group,s hifted downfield from À0.30 ppm for D. 11 BNMR spectroscopy revealed an upfield shift from 65.8 ppm for D to 40.4 ppm for 1,consistent with boratabenzene and borataalkene species. [57][58][59][70][71][72][73][74][75][76] Theoretical calculations examining the electronic structure of 1 yield ageometry (as afree anion) in agreement with the X-ray diffraction structures (Table 1). Thec alculated HOMO is p-symmetric with the largest coefficients on the…”

“…In their respective proton NMR spectra, broad quartets were observed at 2.62 and 2.69 ppm, with matching coupling constants to the doublets in the 11 BNMR spectrum (J = 88.5 Hz and 87.5 Hz) confirming the presence of ab oron-bound hydride.T his was further supported by a 1 H{ 11 B} NMR experiment in which the quartets became singlets and a 11 B{ 1 H} NMR experiment in which the doublets became singlets.This differs from the limited known B=Ch ydroboration reactions in that the hydride is not bridged between the two boron centers and the anti B=C hydroboration product is observed. [74,[77][78][79] Allowing the reaction to proceed for longer periods of time showed ad ecrease in the intensity of the doublet at À11.1 ppm accompanied by an increase in the intensity of the doublet at À12.1 ppm in the in situ 11 BNMR spectra. Conducting variable-temperature 1 HNMR studies with a1 :1 mixture of syn-3 and anti-3 did not result in any interconversion of the two diastereomers;instead at 70 8 8C, the intensity of the trimethylsilyl singlet for syn-3 decreased with the emergence of at rimethylsilyl singlet at À0.01 ppm that corresponds to 1.Inaddition, the Bpin methyl singlets for syn-3 decreased in intensity and asinglet at 1.28 ppm corresponding to pinacolborane emerged.…”

The 9‐Borataphenanthrene Anion

“…This was further supported by a 1 H{ 11 B} NMR experiment in which the quartets became singlets and a 11 B{ 1 H} NMR experiment in which the doublets became singlets. This differs from the limited known B=C hydroboration reactions in that the hydride is not bridged between the two boron centers and the anti B=C hydroboration product is observed …”

“…The 1 H NMR spectrum of 1 lacks the aliphatic C−H signal at 3.88 ppm for the proton at the 10‐position and features a diagnostic singlet at −0.01 ppm for the trimethylsilyl group, shifted downfield from −0.30 ppm for D . 11 B NMR spectroscopy revealed an upfield shift from 65.8 ppm for D to 40.4 ppm for 1 , consistent with boratabenzene and borataalkene species …”

“…with matching coupling constants to the doublets in the 11 BNMR spectrum (J = 88.5 Hz and 87.5 Hz) confirming the presence of ab oron-bound hydride.T his was further supported by a 1 H{ 11 B} NMR experiment in which the quartets became singlets and a 11 B{ 1 H} NMR experiment in which the doublets became singlets.This differs from the limited known B=Ch ydroboration reactions in that the hydride is not bridged between the two boron centers and the anti B=C hydroboration product is observed. [74,[77][78][79] Allowing the reaction to proceed for longer periods of time showed ad ecrease in the intensity of the doublet at À11.1 ppm accompanied by an increase in the intensity of the doublet at À12.1 ppm in the in situ 11 BNMR spectra. Conducting variable-temperature 1 HNMR studies with a1 :1 mixture of syn-3 and anti-3 did not result in any interconversion of the two diastereomers;instead at 70 8 8C, the intensity of the trimethylsilyl singlet for syn-3 decreased with the emergence of at rimethylsilyl singlet at À0.01 ppm that corresponds to 1.Inaddition, the Bpin methyl singlets for syn-3 decreased in intensity and asinglet at 1.28 ppm corresponding to pinacolborane emerged.…”