Thermal transformation of organoboranes : applicability of ¹¹B NMR spectroscopy and supporting molecular modelling.

INTRODUCTION

Demand for LAOs was estimated at 3.5 million tons at the end of 2003, while production reached only 2.5 million tons (a market worth $2.5 billion at 2003 prices).[1] In particular, the production of 1-octene has not kept pace with demand. Palladium Chemistry – Rand African University and University of Cape Town Rhodium Chemistry – Rand African University.

Figure 1.2 - World capacity for linear-α α α α-olefin production in 2003 by producer [1]

General Aspects of Hydroboration

Aryl substituents in the 2-position (eg, styrenes) show increased influence of the electronic effect, causing increased addition of boron to the non-terminal position [22]. The hydroboration reaction is a controlled cis addition of the borohydride bond to an alkene.

Hydroborating Agents

A study of directive effects in the hydroboration reaction revealed that terminal (straight chain) alkenes react by adding 94% of the boron reactants to the terminal position in a terminal alkene. A very important development in the hydroboration chemistry of NaBH4 is the adaptation of the reaction to liquid-liquid phase transfer catalysis conditions. The best results for this study were obtained by stirring a n-butyl bromide olefin solution with a mixture of a saturated aqueous solution of NaBH4 and quaternary onium salt (Q+X-) at room temperature under nitrogen.

Table 1.2 - Catalyst Effects on the Hydroboration-Oxidation of Styrene with the Cp 2 TiCl 2 / 18-Crown- 18-Crown-6/NaBH 4 System [55]

Kinetic Aspects of the Hydroboration Reaction

Rate-determining dissociation of the borane-solvent complex is followed by a rapid reaction of the free borane and the alkene. The induction effect observed in this case supported the pre-rate-determining dissociation of the borane-solvent complex.

Table 1.3 - First Order Rate Constants (k 1 ) for the Hydroboration of 1-Hexene by 9-BBN in Various Solvents at 25 o C [78]

Thermal Transformations of Organoboranes

The synthetic applicability of the isomerization reaction of trialkylboranes has been reviewed in the literature.[109]. The synthetic utility of the isomerization-displacement reaction of trialkylboranes was recognized early in the open and patent literature.

Table 1.5 - Hydroboration Mediated Isomerisation of 3-Hexene at 125 o C [19]

Study Objectives

The scope of 11B NMR spectroscopy as a technique can also be assessed by examining the association of trialkylboranes with the compounds shown in Figure 1.9.

EXPERIMENTAL

• Apparatus
• Purification of Reagents
• Preparation of Samples for GC Analysis
• Hydroboration of Alkenes
• Thermal Dealkylation Studies
• Computational Modelling
• Kinetic Studies

Data for determining the response factor for 1-octene are listed in table 2.5 and the chromatogram for identification shown in figure 2.4. Transfer of the borane from the borane solution to the 1-octene solution was achieved within 30 seconds.

Table 2.1 - Details for drying of reagents and solvents

RESULTS AND DISCUSSION: DISPLACEMENT STUDIES

Thermal Dealkylation of Tri- n -Alkylboranes

The data in Table 3.1 shows that there is no change in the internal pressure of the reactor after the first hour. The results are shown in Table 3.5 as relative percentages of the product distribution at the temperatures examined. This can be seen from the relatively constant value of the sum of the decanol isomers.

The results obtained from this study are shown in Table 3.6 as relative percentages of product distribution at the temperatures examined. In the case of the thermal dealkylation reactions of both tri-n-decylborane and tri-n-octylborane, two unexpected product types were observed.

Table 3.1 - Product Distribution (Percentage) Results for the Time Effect Study of B(decyl) 3 at 150 ºC Product Distribution (%) at Time Interval (hours)

Thermal Dealkylation of Tri- n -octylborane after Removal of Solvent

The dealkylation of tri-n-octylborane was investigated at 150 ºC (equation 3.17) to monitor the effect of a selected series of Lewis bases on the dealkylation process in the absence of the other solvent. Such a reduction in the octanol balance is also observed for DMF, HMPA and (MeO)3PO, which also causes back isomerization of the alkyl chain. Such elimination is not conceivable in the case of the present study where only 10 mol% of Lewis base was used.

The dealkylation of B(4-octyl)3 was also investigated at 150 ºC to monitor the effect of a selected series of Lewis bases on the dealkylation process in the absence of the other solvent. An investigation of the liberated olefin provides some supporting evidence for the absence of the isomerizing effect of the Lewis bases in this case (Table 3.14 and Figure 3.13).

Table 3.10 - Temperature Dependence of the Liberation of 1-Octene from Thermal Dealkylation of B(octyl) 3

Displacement Studies on Trialkylboranes

This was chosen to ensure that most of the 1-octene (boiling point = 122-3 ºC) is in the liquid phase. The results of the study are shown in Table 3.16 and shown graphically in Figure 3.15 and Figure 3.16. In terms of the displacement levels achieved (summation of the octanol isomers formed by rehydroboration), the results presented in Table 3.16 are in stark contrast.

Furthermore, rehydroboration upon dealkylation of the first butane molecule from tri-n-butylborane results in the formation of B(n-Bu)2(4-octyl). The increased levels of dealkylation result in the existence of boron hydrogen bonds, which would catalyze the isomerization of the 4-octyl chain.

Table 3.15 - Effect of selected Lewis bases of the displacement reaction of tri-n-butylborane with 1- 1-octene at 120 ºC

Conclusions

This goes against the goal of moving the boron atom away from an internal position in the carbon chain. This also requires making a wise choice of the Lewis catalyst, as the Lewis bases that catalyze the back-isomerization are among those that have a clear influence on the displacement rate, as reported by Rutkowski et al. [137] . The study of the dealkylation of tri-4-octylborane shows that the Lewis bases are benign in alkyl chain migration.

This may be due in part to the decreased interaction of the Lewis base and the boron atom due to steric crowding around the boron atom. This mechanism involves the idea of ​​partial dealkylation, which is a more satisfactory explanation for the isomerization behavior of trialkylboranes at the milder temperatures studied.

Recommendations for future work

It was for these reasons that a kinetic 11B NMR spectroscopy study on the reduction of selected nitriles by the borane-dimethyl sulfide complex was carried out. Pseudo-first order rate constants, kobs, for the reduction of acrylonitrile by the borane-dimethyl sulfide complex (0.16 M) at different concentrations (3-8 M) and temperatures (15-30 °C) were obtained from 11B NMR arrayed experiments and subsequent integration of the borane and vinyliminoborane peaks. The dimethyl sulfide dissociated in the first step reattaches to the boron atom of the aldiminoborane, as shown in the second step.

The negative ∆S≠ value indicates that the overall mechanism of the reduction of acrylonitrile by BH3.SMe2 in the presence of excess dimethyl sulfide is an associative process. However, the gradient HSQC spectrum showed that the dimethyl sulfide does reattach to the boron atom after the formation of the aldiminoborane.

Figure 4.1 - Arrayed 11 B-{ 1 H} spectrum for reduction of acrylonitrile with borane dimethyl sulfide complex (0.16 M) in CH 2 Cl 2 at 25 °C and 11 B spectrum (inset) showing species identified during

In addition, the importance of the dissociation of the dimethyl sulfide from borane to the reduction process was also illustrated as shown by the dependence of the observed rate of the reaction on the concentration of dimethyl sulfide (Figure 4.4). The lack of reaction with propionitrile and benzonitrile at 25 ºC can be attributed to the lack of stability of their adducts with BH3 as demonstrated by the low equilibrium constants for the formation of their adducts with borane which are K and K for C6H5C≡N.BH3 and EtC . ∆complexationE = -75.1 kJ.mol-1), as illustrated in Figure 4.7 and Table 4.3. Furthermore, the data in Table 4.3 show that these compounds have higher activation barriers (∆activationE = 42.7 and 42.3 kJ.mol-1, respectively for propionitrile and benzonitrile) for reduction by BH3 compared to acrylonitrile (∆activationE = 34.7 kJ). mol-1).

The lack of significant catalytic activity of methyl iodide is not a major problem with respect to a boron-mediated catalytic cycle, since the hydroboration step is not the most crucial (rate-determining step) in the cycle. Furthermore, catalysis of the hydroboration reaction would only be necessary during the start-up period of the reactor, after which all further rehydroboration would involve dialkyl and monoalkylboranes during the displacement reaction.

The Gibbs free energy can also be expressed in terms of the binding enthalpy (∆HBIND ) and entropy (∆SBIND) as shown in equation 4.17. A typical 11B NMR spectrum of the tributylborane solution used in the binding studies is shown in Figure 4.13. The enthalpies and entropies of binding of the selected Lewis bases to tributylborane derived from the plots are summarized in Table 4.11.

Another factor to consider is the spontaneity of the process, which is reflected in The shift of the Lewis base-trialkylborane adduct at infinite dilution indicates the chemical environment of the boron atom.

Table 4.5 - CBS-4 calculated bond dissociation energies (298 K) for selected Lewis base-borane complex

Computational Rationalisation of Binding of Lewis Bases to Trialkylboranes

As mentioned earlier, there is a balance of absolute electronegativity in adduct formation, such that µA = µB = µAB. This is expected since the HOMO and LUMO energy levels of the two trialkylboranes are similar (Table 4.12). The effect of nitrogen atoms on the HOMO energy level is most strongly reflected in the absolute electronegativity (χº values), where HMPA, tributylamine, tripropylamine and HMPT have lower absolute electronegativity values ​​(χº < 2 eV).

Analysis of the absolute hardness values ​​(η) in Table 4.12 reveals an interesting trend within the oxygen donor atoms. However, the data in Table 4.12 show that the ∆N values ​​are also influenced by the χº values ​​of the boranes, which fall in the expected order: ∆N(LB→BMe3) >.

Figure 4.20 - Graph showing relationship between electronic energy (E) and number of electrons (N)

Conclusions

Plotting LD→A against ∆N generally groups the Lewis bases by donor atom type (Figure 4.25), with DMSO as an exception. Given the direct information on the environmental state of the boron core, the technique would be of great importance to those investigating the behavior of trialkylboranes with various Lewis bases and catalysts. Of these two, the concepts of chemical hardness is the more robust as it shows more direct correlation to the experimentally observed findings.

Rousseau, A Study of the Displacement Reaction in the Boron-Induced Isomerization of Alkenes (MSc Thesis), Potchefstroom University for Christian Education, 2002. Nöth and B Wrackmeyer, Nuclear Magnetic Resonance Spectroscopy of Boron Compounds, Spronger-Verlag, Berlin Heidelberg, New York, 1978; (c) G.

Appendix A: Dealkylation Studies

Thermal Dealkylation: Time Dependence Studies

Mass percentages of components in Tables and 6.7 were all divided by the mass percentage of the internal standard (n-octane) to obtain the corresponding relative mass percentage ratios in Tables and 6.8, respectively. Each relative mass percentage ratio was divided by the corresponding calculated sum and multiplied by one hundred to express it as a percentage of the sum, to obtain the data shown in Tables and 3.6.

Table 6.2 - Relative mass percentage ratios for the time dependence study on B(decyl) 3 at 150 o C Relative Mass Percentage Ratio

Thermal Dealkylation: Temperature Dependence Studies

THERMAL DEALKYLATION OF TRI- N -OCTYLBORANE AFTER REMOVAL OF SOLVENT