Organoboron compounds. XIX. Kinetics of the thermal isomerization of .alpha.-branched trialkylboranes

Rossi, Frances M.; McCusker, Patrick A.; Hennion, G. F.

doi:10.1021/jo01288a041

Cited by 14 publications

(9 citation statements)

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“…The rearrangement of t -Bu into i -Bu substituent at boron is rare but not unprecedented. It has been observed during the preparation or distillation of sterically hindered t -butyl boranes . The selective t -Bu → i -Bu isomerization observed upon addition of HOTf to 1 parallels, to some extent, the reverse regioselectivity noticed upon additions of HCl and HOTf to 3 , and thereby further supports the hypothesis that a t -Bu group at boron prevents sterically the addition of the trifluoromethane sulfonate group (that would have led to 6′ ).…”

Section: Resultssupporting

confidence: 63%

Ionic-Type Reactivity of 1,3-Dibora-2,4-diphosphoniocyclobutane-1,3-diyls: Regio- and Stereoselective Addition of Hydracids

et al. 2009

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Hydrogen chloride and trifluoromethane sulfonic acid readily add to the symmetrically substituted 1,3-dibora-2,4-diphosphoniocyclobutane-1,3-diyl (1) and unsymmetrically substituted 1,3-dibora-2,4-diphosphoniobicyclo[1.1.0]butane (3) under mild conditions, substantiating some formal analogies between the PBPB diradicaloids D and alkenes. X-ray diffraction analyses carried out on all the resulting 1,3-diborata-2,4-diphosphoniocyclobutanes (2, 4-6) revealed that the reactions proceed with complete stereo- and regio-selectivity, and that an unusual t-Bu --> i-Bu isomerization can occur at boron. DFT calculations shed more light on the factors controlling the regioselectivity of the additions, its concerted versus stepwise character, and support an original two-step mechanism for the t-Bu --> i-Bu isomerization, involving a carbocationic sigma-borane adduct as a key intermediate.

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Section: Resultssupporting

confidence: 63%

Ionic-Type Reactivity of 1,3-Dibora-2,4-diphosphoniocyclobutane-1,3-diyls: Regio- and Stereoselective Addition of Hydracids

et al. 2009

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“…Here again we note accordance with early experimental studies. Rossi and co-workers 60 studied the kinetics of isomerization of t Bu-, sec Bu-, and i Pr-substituted boranes and, on the basis of entropies of activation, concluded that intermediate degrees of olefin restraint or freedom were present, depending upon the borane.…”

Section: Scheme 2 Alternate Alkylborane Isomerization Mechanismsmentioning

confidence: 99%

Thermal Dehydroboration: Experimental and Theoretical Studies of Olefin Elimination from Trialkylboranes and Its Relationship to Alkylborane Isomerization and Transalkylation

2014

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The free energies of reaction calculated in Gaussian (1 atm standard state concentration) were converted to 1 M standard state values in order to calculate the equilibrium data in Figure 3. The concentration at 1 atm, [1 atm], at each temperature is given by PV/RT, from which the entropy correction, S corr , is given by -Rln(1/[1 atm]). Applying this correction to each of the reactants leads to the corrected ∆G react values given in Table S1. Supporting InformationRadkiewicz-Poutsma and co-workers 4 , raised the concern that B3LYP geometry optimization, as implemented in the CBS-QB3 scheme, may lead to a poor geometry for the borane-olefin π-complex, and as such the energetics from CBS-QB3 may be incorrect for the weak R 2 BH-olefin complexes studied herein. This could also have an influence on the free energy barrier for olefin dissociation in these cases. In addressing this, we first discuss the findings of Gilbert and Radkiewicz-Poutsma, and then evaluate the effect of different methods on our results.Gilbert 3 evaluated various DFT methods as well as MP2 for predicting both the geometry and dissociation energy of amineborane dative bonds. It was found that B3LYP predicts B-N bond dissociation energies (BDEs) that are lower than found experimentally, and also predicts B-N bond distances that are too long. Radkiewicz-Poutsma and co-workers 4 extended the study of amine-borane dative bonds. They found that the poor performance of B3LYP in calculating the B-N BDE is due to how this method calculates the energy of the dative bond, rather than being due to the geometry resulting from optimization with the B3LYP method. Bond energies resulting from high level single point calculations on B3LYP geometries were found to be reliable, and CCSD(T) calculations were relatively insensitive to whether an MP2 or B3LYP geometry was used, despite MP2 leading to a tighter B-N bond. This is consistent with the PES corresponding to the dative bond B-N distance being very flat. Both authors found that the MP2 method leads to more reliable B-N bond distances, and MP2 BDEs were closer to experimental or CCSD(T) values. We note that our work has not employed B3LYP energies (apart from thermal corrections to Gibbs free energy); in all cases higher level composite energies on B3LYP geometries have been obtained. We also note that while validating theoretical studies of propene hydroboration (more directly analogous to our work), Singleton 1 found B3LYP to be reliable for energy calculations.We have investigated the effect of MP2 geometry optimization by studying elimination and re-addition of propene from B i Pr 3 , as shown in Figure 5(a) of the main article. CBS-QB3 optimisation in this case led to the loosest post-elimination borane-olefin complex encountered in this work, presumably for steric reasons. The B-C α distance in the i Pr 2 BH••••propene complex is 4.5 Å in this case. The transition structures (elimination and re-addition), π-complex and separated products (propene and i Pr 2 BH) have been re-optimized at the MP2/6-31+G(...

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“…2 The synthetic utility of the isomerization-displacement reaction of trialkylboranes was recognized early in the open and patent literature. [3][4][5][6][7][8] Enhancement of the isomerization reaction may be achieved by the use of excess hydroborating agent, which in the temperature ranges of the isomerization results in back-isomerization of the alkene. Fortunately, this is not a serious consideration in the case of terminal alkenes where the use of excess displacing alkene converts the catalytic excess (10-20 mol %) of borane used at the start of the isomerization reaction into trialkylborane, thereby lowering the possibility of back-isomerization.…”

Section: Introductionmentioning

confidence: 99%

“…[12][13][14] Though most investigations point toward complete dissociation of the alkyl chain from the boron atom (large enthalpies and positive entropies for the dissociation reaction), 8,15 some compounds, especially, s-trialkylboranes, exhibit only partial dissociation (smaller enthalpies and negative entropies of dissociation). 8,15 There are also investigations that show that some alkylborane isomerization reactions take place via a mechanism that does not involve dehydroboration, but one that involves bridging. 12,14,16 Investigations by Rousseau 17 and Rutkowski 4 found activation energies of 122 and 118 kJ/mol for the displacement of decene from tridecylborane by 1-pentene and 1-octene, respectively.…”

Section: Introductionmentioning

confidence: 99%

“…Probably the best method of obtaining pure α-olefin after isomerization is setting up the isomerization reaction as a distillation-displacement reaction with continual removal of α-olefin. The mechanism of the isomerization reaction is thought to involve a rate-determining dehydroboration step. − Though most investigations point toward complete dissociation of the alkyl chain from the boron atom (large enthalpies and positive entropies for the dissociation reaction), , some compounds, especially, s -trialkylboranes, exhibit only partial dissociation (smaller enthalpies and negative entropies of dissociation). , There are also investigations that show that some alkylborane isomerization reactions take place via a mechanism that does not involve dehydroboration, but one that involves bridging. ,, …”

Section: Introductionmentioning

confidence: 99%

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Thermal Dealkylation of Tri-n-octylborane: Effect of Lewis Bases on Extent and Regioselectivity

Mzinyati

Jaganyi

2009

Ind. Eng. Chem. Res.

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The dealkylation of tri-n-octylborane in the absence of bulk solvent was investigated in the temperature range 50-200 °C. Results indicate a linear increase in liberation of olefin from the trialkylborane in this temperature range, though the extent of dealkylation is not significant. Investigation of the regioselectivity of the dealkylation reaction shows no thermal-induced back-isomerization on the addition of 10 mol % of Lewis base, though the addition of dimethylformamide, dimethyl sulfoxide, hexamethylphosphoric triamide, and trimethyl phosphate results in back-isomerization of the alkyl chain.

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Organoboron compounds. XIX. Kinetics of the thermal isomerization of .alpha.-branched trialkylboranes

Cited by 14 publications

References 0 publications

Ionic-Type Reactivity of 1,3-Dibora-2,4-diphosphoniocyclobutane-1,3-diyls: Regio- and Stereoselective Addition of Hydracids

Ionic-Type Reactivity of 1,3-Dibora-2,4-diphosphoniocyclobutane-1,3-diyls: Regio- and Stereoselective Addition of Hydracids

Thermal Dehydroboration: Experimental and Theoretical Studies of Olefin Elimination from Trialkylboranes and Its Relationship to Alkylborane Isomerization and Transalkylation

Thermal Dealkylation of Tri-n-octylborane: Effect of Lewis Bases on Extent and Regioselectivity

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