Article pubs.acs.org/Organometallics
Thermal Dehydroboration: Experimental and Theoretical Studies of Olefin Elimination from Trialkylboranes and Its Relationship to Alkylborane Isomerization and Transalkylation Nandita M. Weliange,† David S. McGuinness,*,† and Jim Patel‡ †
School of Chemistry, University of Tasmania, Private Bag 75, Hobart 7000, Australia CSIRO Earth Science and Resource Engineering, 71 Normanby Road, Clayton North 3168, Australia
‡
S Supporting Information *
ABSTRACT: The thermal elimination of α-olefin from tri-n-alkylborane (dehydroboration) has been studied both experimentally and theoretically. High-temperature 1H NMR spectroscopy and temperature quench experiments revealed no evidence for practically significant concentrations of free olefin at temperatures up to 200 °C. Furthermore, attempts to remove 1-octene from tri-noctylborane by prolonged heating under vacuum were largely unsuccessful. The experimental findings were supported by high-level theoretical calculations, which predicted a very minor equilibrium concentration of dehydroboration products at temperatures less than 200 °C. The results revealed that a putative continuous dehydroboration process will most likely be a slow process and will be difficult to achieve selectively. At the same time, reversible dehydroboration was found to be the most likely route for alkylborane isomerization and transalkylation reactions, and theoretical findings were used to rationalize previous experimental findings. A range of alternative mechanistic proposals for isomerization and transalkylation were also evaluated and were found to be uncompetitive with dehydroboration.
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recently is the alkylboron-based cycle shown in Scheme 1.14 The cycle is based around the classical hydroboration work of
INTRODUCTION Linear α-olefins (LAOs) may be considered one of the most important classes of building blocks of the modern petrochemical industry. With unsaturation at the terminal position of the chain, LAOs are suited to straightforward functionalization and are used in the manufacture of surfactants, lubricants, and polymers.1 The typical route to produce LAOs involves metal-catalyzed oligomerization of ethylene.2 Traditional approaches lead to a mathematical distribution of αolefin chain lengths (Schulz−Flory or Poisson),3−5 while more recently the development of catalysts for selective oligomerization to the comonomer range olefins (1-butene to 1-octene) has been the main driver in this area.6−12 An attractive alternative to ethylene oligomerization is the conversion of abundant linear alkanes to LAOs. Such a process effectively also represents an alternative to the much sought after direct functionalization of alkanes (alkane activation), with some advantages: the alkane to α-olefin approach would maintain the benefit of utilizing low-cost alkanes, while yielding a feedstock that could be used directly in a great number of existing industrial processes. The dehydrogenation of alkanes to mono-olefins is already practiced on an industrial scale; perhaps the best known example is the UOP Pacol Process (paraffin conversion to olefin), which utilizes a heterogeneous Pt catalyst.13 However, this process leads to a mixture of predominately internal olefins. The major challenge to be solved in the development of an overall alkane to α-olefin process is, therefore, the conversion of internal olefins into LAOs, which is a contrathermodynamic process. One conceptual possibility for contrathermodynamic isomerization of internal olefins which we have been investigating © 2014 American Chemical Society
Scheme 1. Proposed Organoborane-Based Cycle for Contrathermodynamic Isomerization of Olefins
Brown15 and involves three steps. The first of these is insertion of an internal olefin into a B−H bond (hydroboration) and is normally a very rapid reaction which goes to completion. The second step, isomerization of a secondary alkylboron intermediate to a primary alkylboron, is a well-known aspect of hydroboration chemistry, although, as discussed herein, the mechanism of this reaction remains uncertain. Efficient transition-metal-catalyzed hydroboration−isomerization has Received: May 19, 2014 Published: August 12, 2014 4251
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also been reported.16 The third step involves elimination of an α-olefin from trialkylboron (dehydroboration or dealkylation), and it is this step where there exists considerable uncertainty as to its feasibility. We reasoned that if dehydroboration can occur, and if done under conditions of kinetic control (essentially rapid removal of the liberated olefin), the cycle shown in Scheme 1 could lead to α-olefin selectively. While each step of the cycle is stoichiometric, the ability to recycle the boron hydride reagent could render the cycle catalytic. The existing literature is conflicting as to the extent of dehydroboration of trialkylboranes at elevated temperature. Brown reported on multiple occasions that trialkylboranes could be vacuum-distilled at temperatures up to ca. 200 °C without decomposition.17,18 A report from Rosenblum from around the same time suggests that heating tri-n-butylborane under vacuum results in slow formation of butene and n Bu2B2H4 (2−10 days, 90−130 °C),19 while tri-n-decylborane was reported to yield 1-decene and nDec4B2H2 during distillation at 180 °C and 10−3 Torr, although no time frame was reported.20 More recently Jaganyi21 has reported that tri-noctylborane (BOct3) forms appreciable equilibrium concentrations of free 1-octene when it is heated in a closed NMR tube, even at 50 °C, where 6.8% 1-octene relative to BOct3 was reported (up to 13.9% at 200 °C). On the basis of this work, a boron-based internal olefin to α-olefin isomerization process was proposed, as shown in Scheme 1, although this was not actually demonstrated experimentally.14 It occurred to us that these newer results do not seem to sit comfortably with what is generally known from classical studies of this chemistry: that hydroboration of linear olefins is a highly exothermic reaction which goes to practical completion and that the majority of these earlier studies seem to suggest that olefin liberation is slow to absent. While dehydroboration of hindered alkylboranes might be expected to be more favorable (although even tri-tert-butyl- and tri-sec-butylboranes can be distilled without decomposition22), the release of linear olefins should be less so. It is clear that, despite the textbook status of hydroboration chemistry, some longstanding uncertainties remain. For instance, a number of groups have recently revisited the factors controlling the regioselectivity of alkene insertion, where conventional transition state theory fails to account for experimental observations.23−25 Herein we have studied dehydroboration and related reactions both experimentally and theoretically with a number of aims in mind. First, the propensity for α-olefin liberation from tri-n-alkylboranes has been evaluated and the extent (equilibrium position) of this reaction investigated. The aim here was to establish if alkylboranes can be efficiently converted into α-olefins and [R2BH]2. Second, the role of dehydroboration in several alkylborane rearrangements (isomerization and olefin displacement) has been studied, with the aim of providing some clarity with regard to the mechanism of these processes. This involved studying a broader range of alkylboranes, as well as a range of mechanistic alternatives which have been proposed.
region were observed. It is possible, however, that rapid exchange reactions of free olefin at elevated temperature could mask its presence, and for this reason we have also investigated low-temperature quenching following heating of the BOct3. Temperature quenching was considered preferable to chemical quenching, which could potentially lead to conflicting side reactions. It was first necessary to establish if low temperatures are effective in slowing the reinsertion of olefin into R2BH, such that the free olefin can be observed. This was studied by adding 1-octene to an NMR solution of Oct2BH (which exists as the dimer, [Oct2B(μ-H)]2) at low temperature. At −30 °C the 1octene double-bond signals are clearly visible, as well as the methylene signals α to the double bond. Practically no hydroboration of 1-octene is observed at −30 °C, but the slow disappearance of octene is observable above −20 °C. A representative time series of spectra are shown in Figure 1 (T =
Figure 1. Time series of 1H NMR spectra showing the disappearance of 1-octene due to hydroboration with Oct2BH (T = 15 °C).
15 °C). The kinetics of hydroboration have been investigated in detail previously;26−33 thus, we have not done so again here. It is sufficient to point out that the half-life of hydroboration under our conditions is around 35 min at 15 °C. This reaction rate provides ample opportunity to monitor the reaction and shows that any significant amount of free olefin at high temperatures should be observable if the sample is rapidly quenched in a low-temperature bath. Tri-n-octylborane in toluene-d8 was heated in a sealed NMR tube at 155 °C for 5 h. The sample was then rapidly immersed in a ca. −80 °C cold bath and transferred to the precooled (−50 °C) NMR probe. In no instance was any evidence for free olefin or Oct2BH witnessed; only clean signals for BOct3 were observed. While this experiment seems to rule out significant concentrations of dehydroboration products, we were interested to see if any unobservable low levels of olefin could be removed by heating under vacuum. Samples of BOct3 were heated under vacuum (ca. 10−2 mm) under conditions of reflux (oil bath temperatures of 95−200 °C were tested) and any escaping volatiles collected in a liquid nitrogen cold trap. In all cases only trace volatile compounds were collected in the cold trap. Gas chromatographic analysis revealed this material to be composed of variable proportions of 1-butanol, isomeric noctenes, and n-octane. The butanol was subsequently found to originate from the BH3·THF solution which was initially
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RESULTS AND DISCUSSION Experimental Studies of Dehydroboration. The dehydroboration reaction was first studied by 1H NMR spectroscopy using tri-n-octylborane. We reasoned that the formation of free octene at a level of several percent should be observable by 1H NMR. Variable-temperature monitoring of a BOct3 solution at temperatures up to 150 °C revealed no evidence for dehydroboration: in particular, no signals in the double-bond 4252
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employed to prepare the BOct3. 1H NMR analysis of the BH3· THF reagent showed signals characteristic of boron−butoxide groups. These impurities persisted in the BOct3 then prepared from BH3·THF in variable amounts, depending upon the batch or age of starting material used. We subsequently learned that the reaction between B2H6 and THF to yield B(OC4H9)3 has been reported.34 The formation of butanol at high temperature evidently results from protonation of the butoxide impurities by an unknown mechanism, perhaps just trace hydrolysis, given the minute amounts formed. There was no clear stoichiometric relationship between the amount of butanol and octenes formed; thus, the two may be unrelated. Nonetheless, it was considered that this side reaction may lead to misleading results, and as such we subsequently synthesized BOct3 from BH3·SMe2, which afforded a pure product for further studies. When pure BOct3 was heated under vacuum (160 °C, 2 h), the results were broadly similar, except that no butanol was formed. Once again only traces of volatiles were collected in the cold trap. This liquid was composed of ca. 95% n-octenes and 5% n-octane, of which 10% was 1-octene. The extremely limited extent of reaction prevented analysis to ascertain the fate of any reacted BOct3. It seems possible that the trace octenes collected resulted from dehydroboration. At the same time, the observation of predominately nonterminal octenes is more consistent with past reports of other alkylborane pyrolysis reactions. When it was heated to high temperature, BOct3 was reported to form the boron heterocycle shown in reaction 1,
Table 1. Calculated Gibbs Free Energies of Activation and Reaction for Dehydroboration Reactions ΔG⧧ (kJ mol‑1)
method
ΔGreact (kJ mol‑1)
Me2BEt → /2[Me2BH]2 + Ethylene M06/6-311+G(2d,p) 158 CBS-QB3 158 W1BD 160 Me2BnBu → 1/2[Me2BH]2 + 1-Butene M06/6-311+G(2d,p) 142 CBS-QB3 142 Bu3B → 1/2[Bu2BH]2 + 1-Butene M06/6-311+G(2d,p) 137 CBS-QB3 133 Hex3B → 1/2[Hex2BH]2 + 1-Hexene M06/6-311+G(2d,p) 134 1
81 67 69 64 54 68 53 64
have studied elimination from tri-n-hexylborane with the M06 method (Table 1, bottom). The good agreement between energies for BBu3 and BHex3 elimination reactions at the same level of theory provides confidence that the smaller BBu3 model is representative of higher alkylboranes. The free energy surface for the dehydroboration reaction for Me2B−nBu and nBu3B is shown in Figure 2 (CBS-QB3
along with internal octenes and hydrogen.35 These reactions become more rapid above 200 °C. While the occurrence of trace dehydroboration cannot be ruled out, certainly the result of this experiment is again inconsistent with a significant equilibrium concentration of free alkene at elevated temperature. In the next section, we attempt to quantify this by way of theoretical modeling. Theoretical Evaluation of Dehydroboration. A number of model dehydroboration reactions were studied with the aim of obtaining accurate thermochemical data for this reaction. Tri-n-butylborane was judged the most suitable model, as it should be representative of the higher alkylboranes yet is small enough to be treated at a level of theory capable of chemical accuracy. Elimination of propene and particularly ethylene may not be representative of loss of the higher LAOs, as substitution leads to stabilization of the liberated alkene, and some of this effect will also be felt at the transition state.36 Nonetheless, the elimination of ethylene from Me2B−Et was first studied for benchmarking purposes, as the size of this model allowed us to compute energies with the W1BD37−39 method, which is expected to lead to kJ mol−1 accuracy. The results are summarized in Table 1, along with the other theoretical methods evaluated (M0640−42 and CBS-QB343,44). Looking first at the elimination reaction from Me2B−Et at the top of Table 1, the agreement between W1BD and CBSQB3 for both the activation barrier and the free energy of reaction is satisfying. We can therefore be reasonably confident in the accuracy of the CBS-QB3 calculated energies in larger trialkylboranes. The M06 density functional appears to lead to accurate activation barriers but slightly overestimates the endergonicity of the reaction. This appears to be a general trend for the higher alkylboranes (Table 1). Despite this, we
Figure 2. Gibbs free energy surface for elimination of butene from R2B−nBu (CBS-QB3 energies).
energies). In each of these cases the B3LYP geometry optimization within CBS-QB3 leads to a loosely bound olefin π complex (B···C 3.5−3.8 Å) following the transition structure. The M06 DFT method leads to a similar result, whereas geometry optimization with the MP2 method leads to tighter π complexes (B···C ca. 2.9 Å). In most cases, the olefin π complex is higher in energy than the free olefin plus dialkylborane, and in fact the tighter π complexes from MP2 optimization are also higher in free energy than the looser B3LYP geometries. A very low free energy barrier for dissociation is also predicted, which leads us to conclude that the olefin π complex will in most instances not be a persistent species but rather will rapidly dissociate to free olefin and R2BH. Exceptions to this conclusion are discussed below, while the effect of the theoretical method employed on π-complex binding is discussed in detail in the Supporting Information. Dialkylboranes are known to form hydride-bridged dimers,15 and consistent with this, the dimer is the lowest energy species after olefin elimination. The energy change to this point is 4253
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not achieve this, but more elaborate methods of distillation or separation might. The question is whether this could be achieved at a useful rate. This possibility may help explain the few older reports of slow dehydroboration under conditions of vacuum distillation. A further limitation will be the occurrence of other undesired thermal decomposition reactions at high temperature.35 While a high equilibrium concentration of free olefin at mild temperatures would suggest a relatively simple dehydroboration reaction is possible,14,21 this is clearly not the case. In the following sections, we investigate the involvement of intermediates from dehydroboration in alkylborane rearrangements. Alkylborane Isomerization. Early on in the development of hydroboration chemistry, it was discovered that alkylboranes can rearrange upon heating to thermodynamically more stable products. This typically takes the form of secondary or tertiary alkyboranes rearranging to primary alkylboranes, such that boron is attached at the least sterically hindered position of the alkyl chain. Brown proposed a mechanism of reversible dehydroboration/hydroboration steps;45,46 the same process of olefin elimination/addition is now well established in transition-metal catalysis, where it is referred to as chain walking (reaction 3).47−49 The chain-walking mechanism for
therefore taken as the free energy of reaction. The energies of reaction for both alkylboranes shown in Figure 2 are practically the same, and we consider the value for nBu3B, ΔGreact = 53 kJ mol−1, to be an accurate estimate of the Gibbs reaction energy for trialkylboranes with butyl substitution or higher (reaction 2, where n = 4+). B(CnH 2n + 1)3 → CnH 2n + 1 2 [(CnH 2n + 1)2 BH]2
(2)
In order to estimate a condensed-phase equilibrium constant, the calculated free energy (1 atm standard state concentration) has been converted to a 1 M standard state, giving a corrected ΔGreact value of 57 kJ mol−1 at 298 K (see the Supporting Information for details). The equilibrium constant calculated for reaction 2 at 298 K is therefore 9.7 × 10−11 M−1/2, which for neat tri-n-octylborane equates to a free 1-octene concentration of ca. 5 × 10−7 M, or around 2 × 10−5 % 1-octene relative to Oct3B. This clearly indicates that the amount of dealkylation is extremely limited and will not be practically observed under these conditions. The theoretical equilibrium position as a function of temperature was evaluated by calculating the free energy of reaction at temperatures up to 200 °C. The results are illustrated in Figure 3, again for tri-n-octylborane. While the
alkylborane rearrangement is certainly the simplest explanation; however, a number of observations on the nature of isomerization have led to alternate proposals. First, Brown observed that the isomerization reaction proceeds at a much greater rate when excess diborane is present, such that there exists a proportion of boron hydride containing species in solution. In contrast, the isomerization of pure trialkylboranes requires more forcing conditions.50 Second, a number of studies have shown that the rearrangement reaction proceeds with a high degree of stereospecificity, which is different from that obtained by direct hydroboration of the supposed intermediate olefin under the same conditions.51−54 An intermediate olefin π complex, as shown in reaction 3, which never dissociates, can explain this stereospecificity, but a number of other mechanisms can account for it as well. The alternate isomerization mechanisms which have been studied herein are shown in Scheme 2. The first mechanism (a) was proposed by Williams55 and can effectively be thought of as a β-hydride elimination with synchronous intramolecular insertion into a second B−H bond. This mechanism can account for both the hydride effect and the sterospecificity of the isomerization reaction. The related 1,2-hydride shift mechanism (b) has also been investigated, although this mechanism does not explain the hydride effect (and might be expected to be a high-energy process). Mechanism (c) was proposed by Rutkowski and co-workers22 and involves a sixmembered transition state formed from two trialkylborane units. Again, this mechanism can explain the observed stereospecificity of the reaction but does not obviously account for the hydride effect. A final possibility we have considered, not previously proposed to our knowledge, is that shown in (d). This proposal involves β-hydride elimination with
Figure 3. Calculated (CBS-QB3) equilibrium percentage of free 1octene in tri-n-octylborane as a function of temperature.
amount of free 1-octene clearly increases, as anticipated, even at 200 °C there is only ca. 0.02% free octene relative to Oct3B expected (corresponding to a concentration of ca. 5 × 10−4 M in neat alkylborane). These high-level theoretical calculations therefore support our experimental findings and explain the failure to observe free olefin under all conditions. It can be concluded with some certainty that, for n-alkylboranes, the equilibrium concentration of free olefin will be extremely low under practical conditions. These findings agree with classical studies, which show that olefin hydroboration is a highly favorable reaction which goes to completion. Notwithstanding the above conclusions, it is worth pointing out that the dehydroboration reaction might be feasible under conditions of kinetic control, as suggested in the Introduction. The activation barrier is not so high as to prevent some rate of conversion at high temperature, provided the liberated olefin could be removed efficiently enough to compete with the reverse reaction back to reactant. Clearly our rather simple experiment of refluxing tri-n-octylborane under vacuum does 4254
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propene to form the n-propylborane is somewhat lower in energy. We were unable to find a transition structure corresponding to the isomerization mechanism (a) shown in Scheme 2. Potential energy surface scans show that such an intermediate is higher in energy (by approximately 35 kJ mol−1) than the simple case of β-hydride elimination with no involvement of the second hydride. As such, all optimizations led to a standard olefin elimination transition structure; there is no stabilization of this transition structure by forcing the second hydride to interact with the olefinic group. Likewise, we could not find a transition structure corresponding to mechanism (d), for essentially the same reason. Intermediate structures such as shown in (d) appear to be significantly higher in energy than the standard elimination transition structure, and optimization always led downhill to this more stable stationary point, with a distant second Me2BH unit. Again, there is no stabilization of the transition structure by forcing a second B−H moiety to interact with the olefinic group. A transition structure corresponding to mechanism (b) was successfully located, but with a ΔG⧧ barrier of 420 kJ mol−1; it is clearly not competitive with the elimination/addition mechanism. Finally, possibility (c) was investigated with an ethyl group on each boron (R′ = H in mechanism (c) in Scheme 2). A first-order saddle point along this pathway does not appear to exist, but rather a second-order saddle point was located 432 kJ mol−1 above the reactants. As illustrated in Scheme 3, one imaginary vibrational mode (1405i cm−1)
Scheme 2. Alternate Alkylborane Isomerization Mechanisms
synchronous insertion into a second B−H unit and is effectively an intermolecular version of mechanism (a). It was reasoned such a mechanism might avoid the energetic cost of olefin formation (as occurs in stepwise elimination/reinsertion), and it can also account for both the hydride effect and the stereospecificity of the reaction. The addition/elimination mechanism was modeled by evaluating the isomerization of Me2BiPr to Me2BnPr, as shown in Figure 4. In principle a hydride-bridged borane
Scheme 3. Representation of Second-Order Saddle Point Corresponding to the Putative Binuclear Isomerization Mechanism
corresponds to the process of interest, while the other (1183i cm−1) corresponds to a σ-bond metathesis reaction producing ethane and a 1,2-diborylethane. The Me2BEt reactants and diborylethane are linked by a more relevant first-order saddle point (reaction 4), with a ΔG⧧ barrier of 222 kJ mol−1. While
Figure 4. Gibbs free energy surface for isomerization of Me2BiPr to Me2BnPr via the elimination/addition mechanism (CBS-QB3 energies).
dimer (such as that shown in Figure 2) could also be an intermediate, but this possibility does not change the overall activation energetics. As shown in Figure 4, the controlling barrier for isomerization is that for elimination of propene from Me2BiPr (ΔG⧧ = 148 kJ mol−1), whereas the reinsertion of 4255
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such reactions might be significant at very high temperatures,35 the results presented here show that this transformation is not competitive with the elimination/addition mechanism and is unlikely to be relevant to isomerization. We conclude that the stepwise elimination/addition mechanism (reaction 3) is the most likely pathway to alkylborane isomerization. An explanation for the hydride effect and stereospecificity of the reaction is still required, however. It occurred to us that the hydride effect may be steric in nature, whereby substitution of a bulky sec- or tert-alkyl group by hydride might simply leave more room at boron for the βhydride elimination reaction. It is known from transition-metal chemistry that steric congestion at either the metal or alkyl group can hinder β-hydride elimination, by preventing the necessary M−C−C−H coplanar arrangement in the transition state. This possibility has been investigated by comparing the energetics of isomerization from BiPr3, iPr2BH, and iPrBH2, as illustrated in Figure 5. The free energy surface in Figure 5a shows the isomerization of BiPr3, and as before, the highest barrier (144 kJ mol−1) is that for elimination of propene. As can be seen in Figure 5b,c, the barriers to elimination from iPr2BH (111 kJ mol−1) and iPrBH2 (85 kJ mol−1) are considerably lower than that for BiPr3. These findings suggest that even a low concentration of R2BH or RBH2 present in the system could provide an effective catalyst for the isomerization reaction. It is also noted that the intermediate π complexes located in Figure 5b,c are much tighter (B−Colefin = 1.86−2.05 Å) than those observed thus far for trialkylboranes. These more strongly bound complexes are the immediate postelimination products and are followed by modest barriers for readdition of propene. In early work by Rickborn51,52 and Field,53 such intermediate π complexes were considered to offer an explanation for the stereospecific nature of the isomerization reaction. For this to be the case, readdition of olefin must occur faster than dissociation, which would be the case if the barrier to loss/ addition of olefin from/to BH3 is greater than the barrier to insertion; in other words, π-complex formation is the ratedetermining step during hydroboration. Rickborn considered this unlikely (a view supported by most theoretical studies of the time56−59) and concluded that the intermediate π complex involved during isomerization is in some way different from that involved in hydroboration (or that no π complex is involved in hydroboration).52 Recent high-level theoretical work, however, reveals that π-complex formation can be the rate -limiting step in hydroboration. Singleton23 located a free energy saddle point for coordination/dissociation of propene with BH3 which is estimated to be 26 kJ mol−1 above the propene−BH3 π-complex intermediate (CCSD(T)/aug-ccpvtz//B3LYP/6-31G(d); see the Supporting Information for details). This barrier (shown in blue in Figure 5c) is significantly greater than that for reinsertion of bound propene, and therefore our results, coupled with Singleton’s work, provide an explanation for the observed stereospecificity of alkylborane isomerization in some cases, such as that observed by Rickborn and Field. We note that all cases (to our knowledge) where stereospecific isomerization has been observed51−54 involve isomerization of RBH2 species. In each of these, the formation of discrete olefin π-complex intermediates, which reinsert faster than they dissociate, can reasonably be expected on the basis of the results presented in Figure 5. Both Rickborn and Field also found that at higher temperatures, or over prolonged reaction times, a non-
Figure 5. Gibbs free energy surface for isomerization of R2BiPr to R2BnPr (R = H, iPr) via the elimination/addition mechanism (CBSQB3 energies). The free energy barrier to propene dissociation shown in blue in (c) is evaluated at the CCSD(T)/aug-cc-pvtz//B3LYP/631G(d) level; see the text and the Supporting Information.
stereospecific isomerization process becomes significant and proposed this may occur via “leakage” over a higher-barrier dissociative pathway. This notion is supported by the present study. The results also suggest that the mechanism of isomerization prevailing, whether dissociative intermolecular or intramolecular (or a combination), will depend upon both the degree of boron substitution and the identity of that substitution. Here again we note accordance with early experimental studies. Rossi and co-workers60 studied the kinetics of isomerization of tBu-, secBu-, and iPr-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. Olefin Displacement (Transalkylation). Like the isomerization reaction, displacement of an alkyl group from boranes with another olefin was discovered early in the development of this chemistry.45,61−63 The most straightforward mechanistic 4256
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proposal for this transformation is that shown in reaction 5, whereby olefin elimination occurs in a first step, followed by
addition of a displacing olefin (normally at a higher concentration) in its place. This, however, represents a further aspect of hydroboration chemistry where some mechanistic uncertainty still exists. Mikhailov and co-workers64 found that the rate of the displacement reaction is dependent upon the concentration of the displacing olefin, which they considered inconsistent with the stepwise reaction, since the initial elimination (which does not involve the displacing olefin) should be rate determining. Likewise, the fact that the rate of displacement depends also on the structure of the displacing olefin was taken as evidence against the stepwise mechanism. The same authors also studied the regioselectivity of styrene addition during displacement versus that occurring through hydroboration of styrene with the putative R2BH intermediate. Different regioselectivities in each case were again taken as evidence against the stepwise mechanism.65 We note, however, that displacement and hydroboration were carried out at different temperatures in this work (displacement 130 °C versus hydroboration 0 °C). As such, some caution is warranted in drawing conclusions from this aspect of the work. Mikhailov proposed an alternative mechanism involving concerted βhydride transfer to the displacing olefin (reaction 6). This process is now well established in transition-metal chemistry; we have herein compared the two alternative mechanisms for displacement at boranes. The stepwise versus concerted displacement reactions are compared in Figure 6, using displacement of 1-butene from Me2BnBu, by 1-butene, as a model. We consider the effect of different displacing olefins below. The free energy of activation of the concerted process is considerably higher than the stepwise route and as such seems unlikely to be competitive. As the first elimination step is an endergonic process, in the absence of a displacing olefin the elimination products will simply return back to reactant. In the presence of a displacing olefin, the competition between the forward and reverse reactions of R2BH and olefin will be influenced by the relative concentrations of each olefin. Considered in this light, it can be seen how the concentration of displacing olefin may influence the overall rate of displacement, even via the stepwise mechanism. Only when the barrier to addition of the displacing olefin is sufficiently lower than the barrier to readdition of the released olefin could there be expected a zero-order dependence on displacing olefin concentration. Some representative cases involving substituted α-olefins and internal displacing olefins are represented in Figure 7. With internal or sterically encumbered displacing olefins, the barrier to the second addition step is increased somewhat in comparison to that in linear α-olefins. It is again evident
Figure 6. Gibbs free energy surface for olefin displacement via stepwise (black) and concerted (blue) pathways (CBS-QB3 energies).
Figure 7. Gibbs free energy surface for olefin displacement with different displacing olefins (CBS-QB3 energies).
from Figure 7 how the rate of the stepwise reaction can be dependent upon the displacing olefin. The reaction should be most rapid with 1-butene, followed by cis- and trans-2-butene and then 2,4,4-trimethyl-1-pentene. This sequence matches that observed by Brown for analogous olefins (1-octene > 2-octene > 2,4,4-trimethyl-1-pentene).63 This close accord between experiment and theory further supports a stepwise mechanism for the displacement reaction.
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SUMMARY AND CONCLUSION Herein we have studied the dehydroboration reaction of trialkylboranes with the aim of clarifying the ease and extent of this reaction under practical conditions. The presence of appreciable equilibrium amounts of free olefin would be likely to lead to a facile dehydroboration reaction,14 as the free olefin could be continuously removed by simple vacuum distillation. This is clearly not the case with primary alkylboranes, however. Both experiment and theory seemingly rule out a significant 4257
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Organometallics
Article
6H, CH3). 13C NMR (Tol-d8): δ 33.29, 32.53, 30.16, 29.97, 27.63, 23.24 (CH2), 16.45 (BCH2), 14.42 (CH3). Variable-Temperature 1H NMR Studies of BOct3. A toluene-d8 solution of BOct3 (50.0 μL, 0.1 mmol) was prepared (total volume 450.0 μL) in a J. Young NMR tube inside a glovebox. The sealed sample was monitored by VT 1H NMR up to 150 °C, with no evidence for free olefin observed. In the temperature quench experiments, the sealed tube with contents was heated to 155 °C for ∼5 h and thereafter immediately placed in a slurry of acetone/LN2 (T ≈ −80 °C). The tube was transferred to the precooled NMR probe (−50 °C) and analyzed by 1H NMR spectroscopy at −50, −30, and −20 °C. In all cases, only signals for BOct3 were observed. Low-Temperature Addition of 1-Octene to Oct2BH. A toluene-d8 solution of Oct2BH (50.0 μL, 0.17 mmol) was prepared (total volume 450.0 μL) inside a glovebox in a screw-capped NMR tube fitted with a septum. The tube was placed in an ice/salt bath (temperature −10 °C) and 1-octene (20.0 μL, 0.12 mmol) added via the septum, which was then further sealed with Parafilm. The sample was immediately taken to the precooled NMR probe (−30 °C) and analyzed by 1H NMR spectroscopy. Olefin Elimination Studies from BOct3. The general procedure that was followed is illustrated by the following example. In a flamedried two-armed Schlenk flask containing a stirring bar, BOct3 (1.0 mL, 2.26 mmol) was transferred. The flask was connected to a cold trap, and the complete apparatus was placed under vacuum at room temperature for a few minutes. The trap was then placed in a liquid N2 bath and the BOct3 was heated in an oil bath with stirring (T = 160 °C), with any volatiles being collected in the cold trap. Any highboiling distillate could collect in the second arm of the Schlenk flask (air cooled) prior to the cold trap. After 2 h, the flask was taken out from the oil bath and left under dynamic vacuum for an additional 1/2 h to ensure that all volatiles were collected. A trace amount of clear liquid (
