Organometallic Chemistry: Volume 41 / Edition 1 available in Hardcover, eBook
Organometallic Chemistry: Volume 41 / Edition 1
- ISBN-10:
- 1782624163
- ISBN-13:
- 9781782624165
- Pub. Date:
- 07/31/2017
- Publisher:
- RSC
Organometallic Chemistry: Volume 41 / Edition 1
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CHAPTER 1
Recent developments and applications of Lewis acidic boron reagents
James R. Lawson and Rebecca L. Melen
DOI: 10.1039/9781782626923-00001
One field of organometallic chemistry that has seen great advancements over the last 20 years is that of main-group chemistry, in particular boron chemistry, that has led to a wealth of new discoveries. In this review, we will focus on modern advancements in this growing field, such as interesting uses of firmly established reagents, such as tris(pentafluorophenyl) borane, B(C6F5)3, which has demonstrated extensive applications in a wide variety of chemistry. In addition to this, a number of novel Lewis acidic boranes and borocations have been recently synthesised, which are often structurally tailored for a specific role such as borylation reactions or use in main-group catalysis. The reactions of these compounds are broad in scope, inclusive of borylation substitution reactions, addition of B–E across p-bonds and applications in pharmaceuticals and materials science. In addition, boron reagents often constitute the Lewis acid moiety of frustrated Lewis pairs (FLPs), an area of main-group chemistry that has also expanded rapidly, with numerous applications notably in main-group catalysis. Newly discovered Lewis acidic boron reagents and their implementations are proving to be an appealing and exciting applications-based field as more advances are discovered.
1 Introduction to Lewis acidic boron compounds
Boron reagents are often employed as Lewis acids due to their strongly electrophilic nature granted by a vacant p-orbital into which electrons can be received. Many neutral boranes have been synthesised and utilised, such as trialkyl-, triaryl- and trihalo-boranes, although as the field of boron chemistry has grown, more complex and structurally diverse boron reagents have been reported. One of the key features of neutral borane species is that the Lewis acidity can be attenuated by variation of the three substituents bound to boron. An example of this is tris(pentafluorophenyl) borane [B(C6F5)3], a powerful Lewis acid due to the electron withdrawing effects of the three perfluorinated aryl rings, which was first synthesised in the 1960s. Since this discovery, other strongly Lewis acidic boranes have been reported, select examples of which are included herein.
A useful tool when considering Lewis acidic boranes is the ability to determine experimentally their Lewis acidity, allowing them to be placed on a scale, such as in Fig. 1. The most well-known procedures for this use NMR spectroscopic analysis. The Gutmann–Beckett method involves the coordination of triethylphosphine oxide (Et3PO) to a Lewis acid and recording the chemical shift in the 31P NMR spectrum. The Lewis basic oxygen atom of Et3PO can form an adduct with boron reagents, causing deshielding of the adjacent phosphorus atom, the degree of which can be measured to ascertain the Lewis acidity of the boron reagent. Childs method instead uses crotonaldehyde and 1H NMR spectroscopy.
The chemistry of boranes is well documented, with numerous examples of borylation reactions which involve the formation of new B–E bonds with a π-nucleophile, such as arenes, alkenes and alkynes. The first reported examples of borylation reactions were dependent upon transition-metal catalysts, which can be problematic due to potentially high catalyst cost and more difficult purification of products. More recently, new methodologies have sought to increase the reactivity of boranes, in order to avoid the necessity of metal catalysts. One potential answer to this has been the study of borocations, which have seen recent advancements in synthesis and applications. These compounds have the capacity to have very high Lewis acidity, due to the cationic nature of the boron centre.
Boranes such as B(C6F5)3 are commonly used as the Lewis acid component of frustrated Lewis pairs (FLPs), with a few recent exceptions involving silylium, phosphonium, aluminium and carbon Lewis acids reported. FLPs are composed of sterically encumbered Lewis acids and bases that, due to the high levels of steric obstruction, are unable to form classical adducts, but have been observed to undergo unique reactivity with various other reagents. For example, the steric bulk and electron-withdrawing nature of the three C6F5 groups of B(C6F5)3 are the reason it is a commonly used Lewis acid in FLP chemistry. Applications of FLPs include small molecule activation, such as H2, as shown in Scheme 1, along with many others.
This review aims to cover recent reports of emergent boron chemistry. This will begin by examining new methodologies for the synthesis of novel Lewis acidic boranes and borocations, to demonstrate how this field has evolved with these developments. This review will then demonstrate the utilisation of boron reagents by discussing a series of reactions that use a variety of boron-based compounds. These range in scope from relatively simple borylation reactions, including additions and substitutions, to more intricate methodologies, such as boron induced cyclisations, to create a range of complex cyclic products. Modern developments in FLP chemistry and boron-based catalysts, will also be probed, covering advancements in catalytic chemistry. The scope of this review will focus on the most recent examples from the last 5–10 years, although some historical examples are included for context.
2 Synthesis of Lewis acidic boron reagents
The development of novel boranes is an area that has shown considerable growth, often with an applications-based methodology. Many techniques are focused on increasing the Lewis acidity of the boron centre, historically achieved with fluorinated boranes such as the aforementioned B(C6F5)3. The borane tris[3,5-bis(trifluoromethyl)phenyl]borane (BArF3) (1, Fig. 2) was synthesised and found to be a more powerful Lewis acid than B(C6F5)3 via the Gutmann–Beckett method. Studies also indicated that BArF3 can form FLPs with select Lewis bases, and is capable of activating molecular hydrogen.
A different approach was to synthesise boranes featuring cationic substituents, which can cause a strong negative inductive effect, thus increasing the Lewis acidity of the boron centre as a result. A recent example of this involved the synthesis of cationic analogues of trimesitylborane (2–4, Fig. 3). These boranes were air and moisture stable, postulated to be a result of the steric protection of the boron centre afforded by six ortho-methyl groups. Cyclic voltammetry was used to measure the reduction potential of these boranes, where it was found that there was almost a linear trend in their reduction potential, as determined by the number of ammonium substituents present. This strategy provides an approach that allows control of the redox properties of the boranes.
Wagner et al. synthesised a trio of novel mono-haloboranes featuring 3,5-bis(trifluoromethyl)phenyl-groups. These electron-withdrawing groups were designed to prevent steric crowding at the boron centre by forgoing ortho-substituents on the phenyl rings, potentially increasing achievable reactivity. Beginning with (3,5-(CF3)2C6H3)Li and BH3 · SMe2, sequential reactions afford 5, the precursor to the desired haloboranes. Reactions of 5 with KHF2/Me3SiCl, BCl3, and BBr3, generate fluoro, chloro, and bromo haloboranes 6, respectively. Each of these species was isolated and fully characterised (Scheme 2).
Wildgoose et al. have designed a novel triaryl borane (7, Fig. 4), the first structurally characterised 1:1:1 hetero-tri(aryl)borane to be reported. This compound acts as the Lewis acid component in FLPs to cleave H2 heterolytically, and has been demonstrated to be compatible with a number of Lewis bases, namely P(tBu)3, 2,6-lutidine and 2,2,6,6-tetramethylpiperidine (TMP). It was observed that the degree of conversion to the cleaved product over time was dependant on the Lewis base, with P(tBu)3, and 2,6-lutidine providing higher conversions than TMP. This synthetic methodology, namely step-wise addition of groups to a borane, represents a remarkably useful way to access this family of boranes, allowing greater modification of future triarylboranes in order to suit the desired reactivity.
In addition to neutral boranes, considerable interest has been directed at borocations, compounds that were defined over 30 years ago, but more recently have developed into an interesting new field of boron chemistry. These compounds often possess high Lewis acidity due to the formal positive charge on the boron centre. Borocations are often defined and characterised by their coordination number, which corresponds to a general trend in relative Lewis acidity, as shown in Fig. 5. The 2-coordinate boriniums are generally the most reactive, but are often highly unstable precluding widespread application. The 4-coordinate boroniums, on the other hand, exhibit high levels of structural stability, but suffer poor reactivity due to the fully occupied coordination sphere at boron. Indeed, it is often the 3-coordinate borenium cations that are most reported in the literature, as they offer a compromise between the other borocations; they possess the added stability of an L-type donor ligand which allows much easier manipulation (under an inert atmosphere) whilst reactivity is enhanced due to the unsaturated coordination sphere.
The synthesis of borenium cations typically follows the same general pathway. Beginning with a Lewis basic neutral borane, a donor ligand is coordinated, often an amine, to generate an adduct. This is followed by either halide or hydride abstraction from the boron adduct, forming the positively charged borocation. In order to facilitate borenium formation, the donor ligand must bind strongly enough to boron that it does not dissociate during the halide/hydride abstraction step. Additionally, the B–X bond (X1/4hydride or halide) should be weakened by ligand binding so as to induce the abstraction. This approach allows for great diversity in the structure of borocations, and hence their reactivity through variation of the substituents (R-groups) at boron and the donor ligand. For example, electron-donating groups can be used to stabilise borenium cations, whereas the Lewis acidity can be increased with electron-withdrawing groups. More recently, borocations that feature N-heterocyclic carbenes (NHCs) have been synthesised. One such example featured an NHC-stabilised borenium 8 which could be isolated and structurally characterised (Scheme 3).
Following this, Robinson et al. synthesised a novel borenium stabilised by an N-heterocyclic olefin (NHO). The NHO was reacted with the strong Lewis acid BBr3, forming an isolable adduct 9. It was observed that in the presence of THF, 9 is capable of cleaving the C–O bond of the solvent, ring-opening two equivalents whilst simultaneously delivering a bromine atom, generating borenium species 10 with Br- as the counter ion (Scheme 4).
Melen et al. recently reported a new methodology for synthesising borenium compounds, using diimines and dichlorophenylborane. When select diaryldiimines were reacted with dichlorophenylborane (PhBCl2), it was observed that 11 was formed, with the by-product HCl being formed also. Subsequent addition of aluminium trichloride abstracted the chloride from HCl causing addition of the proton to compound 11 forming the borenium compound 12 (Scheme 5). Through attempts with a range of diamine precursors, it was found that the steric properties of the diimine were important to achieve borenium ion formation, as an absence of substitution in the ortho-position of the aryl rings resulted in more complex, unclean reactivity. Conversely, too much steric bulk in this position precluded reaction all together. This work represents a new methodology for the synthesis of borocations.
3 Applications of novel boranes and borocations
A large proportion of novel boranes and borocations are utilised in borylation reactions, such as dehydroborylation, hydroboration, carboboration and haloboration. Due to the different requirements of each of these reactions (most notably the nature of the groups bound to the boron reagent), as well as the variety of substrates, the ability to functionalise the structure and attenuate the reactivity of the boron reagent is of vital importance. The products of these reactions can be thought of as intermediates en route to more complex, and more valuable products, as the addition of a boron species into the molecular structure allows greater functionalisation through subsequent cross-coupling reactions.
The dehydroborylation reaction allows for the direct insertion of boron into a molecule by transforming a C–H bond into a C–B bond. Historically, Brown et al. reported that trialkoxyboranes were suitable borylation reagents when combined with lithiated organic species, including alkynes, generating alkynyl boronates 13, as seen in Scheme 6. The lithiated reagents were synthesised from alkyne precursors, hence requiring a multi-step reaction to acquire the borylated products.
Modern approaches avoid the necessity of metallation by using (commonly) an amine base to deprotonate the substrate, which can either be added separately or, as has been reported with certain borocations, incorporated into the structure of the reagent. Recent advances have shown that certain borenium species undergo selective dehydroborylation of arenes and heteroarenes. Ingleson et al. have probed this area considerably, and have shown that highly reactive dihalo-boreniums of the general formula [Cl2B(L)][AlCl4] trigger dehydroborylation of a range of arenes, as shown in Scheme 7, such as N-TIPS-pyrrole 14. The enhanced electrophilicity of the borenium cations was generated by the use of two covalently bound halide atoms, which allowed the reactions to proceed quickly at ambient temperature. In addition, modulation of the Lewis base ligand allowed access to several diborylated heteroaryl species 15 (Scheme 8). Isolation of these compounds as air-stable pinacol boronate esters increases the usefulness of the products, as they can then be employed as critical scaffolds to more complex molecules via cross-coupling reactions, allowing the products to be used as synthetic 'building blocks' towards desirable synthetic targets.
Recently, Fontaine et al. demonstrated a metal-free catalytic approach to arene borylation, utilising borane (1-TMP-2-BH2-C6H4)2 (TMP = 2,2,6,6-tetramethylpiperidine) 16 as a main-group catalyst.29 As depicted in Scheme 9, borane 16 initially reacts with the arene, in this case N-methyl pyrrole, activating the C–H bond in the 2-position, generating H2. The intermediate is subsequently reacted with H–BPin, causing catalyst regeneration and producing the desired borylated arene as the pinacol boronate ester 17. The substrate scope was expanded beyond N-substituted pyrroles, to include indoles, furans and electron rich thiophenes, often producing yields in excess of 80%. A catalyst loading of 2.5 mol% of 16 achieved optimal results.
The dehydroborylation of arenes, heteroarenes and alkenes was reported by Repo et al. who demonstrated that 2-aminophenylboranes can be used to generate the borylated products (18, Scheme 10). This reaction was reported to proceed via a C–H insertion in an FLP-type fashion, wherein the boron and amine heterolytically split the C–H bond. This reactivity is promoted by the close structural proximity of the Lewis acid and base moieties, resulting in a relatively low kinetic barrier to reaction (ΔG = 21.0 kcal mol-1 for thiophene).
In addition to substitution reactions such as dehydroborylations, boranes and borocations can be used in addition reactions, such as hydro-, carbo-, and halo-boration. These elementoborations feature the addition of boron and another group to a p-system. Discoveries of 1,n-addition (n = 1, 2 or 3) have been reported of boron reagents to alkynes, allowing the incorporation of a boron unit whilst generating a vinyl species. Likewise, vinyl species themselves can be targeted, in order to generate borylated alkyl-compounds. Again, many historical examples require a metal catalyst. More recent examples have harnessed main-group reagents such as borocations to access these borylated alkenes in a metal-free fashion. These products are desirable as they have been shown to be useful building blocks to more complex compounds, often utilising cross-coupling reactions through the boron unit.
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