Current Developments in the Catalyzed ... - ACS Publications

Chapter 6

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Current Developments in the Catalyzed Hydroboration Reaction Stephen J. Geier, Christopher M. Vogels, and Stephen A. Westcott* Mount Allison University, Department of Chemistry and Biochemistry, Sackville, New Brunswick E4L 1G8, Canada *E-mail: [email protected]

The hydroboration reaction involves the addition of B-H bonds to sites of unsaturation and provides an elegant route to reduce alkenes, alkynes, ketones, aldehydes, aldimines, etc. The use of metals to catalyze this reaction has been known for several decades. This chapter discusses examples of the most recent advancements in this area including the use of earth-abundant metals, heterogeneous catalysts, reactions involving more challenging substrates such as CO2, and diboron ‘hydroboration’ sources.

Introduction The hydroboration of unsaturated organic fragments, such as C=C, C=O, and C=N double bonds, as well as C≡C triple bonds, is a powerful methodology in organic synthesis developed by H.C. Brown, who was awarded the Nobel prize in 1979, along with George Wittig, for his outstanding work in boron chemistry. In the case of hydrocarbon substrates, the reaction is believed to proceed via initial coordination of the electron-rich double (or triple bond) to the electron-deficient boron atom using its empty p-type orbital (Scheme 1). A four-centered transition state subsequently forms where the hydride preferentially binds to the carbon atom best able to stabilize a carbocation type intermediate (most substituted carbon atom) and the boryl (BR′2) group adds to the less-substituted carbon atom in a cis-fashion to give the corresponding organoborane product (1).

© 2016 American Chemical Society Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 1. The Proposed Mechanism for the Conventional Uncatalyzed Hydroboration Reaction (1).

Organoboranes are remarkably valuable precursors and have all but usurped organostannanes owing to their reduced toxicities compared to their tin analogs. Commonplace in organic synthesis and pharmaceutical and agricultural chemistry, organoboranes can be readily transformed into a wide range of organic compounds containing diverse functional groups, a few of which are illustrated in Scheme 2. Several excellent reviews have recently appeared detailing the extent with which organoboranes can be used in organic synthesis and you are encouraged to read these excellent publications in order to gain a further understanding of the importance of these simple molecules in organic synthesis (2–5). Recent studies have also shown that many simple organoboron compounds can display a variety of potent bioactivites (6).

Scheme 2. Transformation of Organoboranes (1–4).

210 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Although the hydroboration reaction currently plays an integral role in organic synthesis (7–14) and chemical education (15), one of the challenges associated with this addition reaction involves generating products with high selectivity. For instance, reactions with derivatives of H3B.X (X = Lewis base) often give products where the small boryl fragment (BH2) has added to the more substituted carbon atom of the substrate. Of the many hydridoborane reagents (compounds containing a single B-H bond) developed over the years to circumvent this deficiency, catecholborane (HBcat, cat = 1,2-O2C6H4) and pinacolborane (HBpin, pin = 1,2-O2C2Me4) have emerged as a unique class of dioxaborolanes owing to their relative stability, safe handling procedures, ease of synthesis and high selectivities. Unfortunately, elevated temperatures are frequently required to affect these transformations and decomposition of the starting borane, into ‘BH3’ species, results in several organoborane products (16). Following the remarkable discovery that rhodium complexes activate the B-H bond in HBcat to form isolable hydridorhodium boryl species (17), Männig and Nöth subsequently demonstrated that these same metal complexes could be used to catalyze the hydroboration of alkenes and alkynes at room (or even lower) temperature (18). In this initial study, Männig and Nöth demonstrated that addition of HBcat to 5-hexen-2-one at room temperature resulted in the expected borate product where the borane has added to the more reactive carbonyl group (Scheme 3a). However, when this reaction was carried out at lower temperatures, and in the presence of a catalytic amount of [RhCl(PPh3)3], addition of the borane proceeded at the normally less reactive alkene group to give the corresponding organoboronate ester derivative. This seminal discovery resulted in a tremendous amount of research focused on discovering and exploiting the advantages this metal catalyzed variant had over conventional hydroboration reactions. Most of the early work involved the catalyzed hydroboration of alkenes and alkynes. One of the most remarkable findings to come out of this early work was the discovery that late transition metal complexes, predominantly rhodium and iridium species, could be employed to generate the unexpected branched products (Scheme 3b) in the hydroboration of vinyl arenes. The impact of this unusual finding was further enhanced when chiral catalysts were able to provide products with high enantioselectivities. It has been postulated that these reactions proceed by activation of the B-H bond via oxidative addition at the metal center to give a hydridoboryl metal intermediate. Coordination of the alkene (or alkyne) to the metal center, followed by insertion of the organic into the M-H or the M-B bond, with a subsequent reductive elimination step would generate the corresponding organoboronate ester product (Scheme 4). The unique regioselectivity involved in hydroborations of vinyl arenes was purported to be a result of these late metals preferring a benzylic-type intermediate after insertion of the alkene into the M-H bond. Several reviews on the early stages of the metal-catalyzed hydroboration have been published discussing the scope and limitations of this remarkable reaction (19–21). The aim of this report is not to provide a comprehensive account of the area, but to briefly highlight some of the recent developments in the past years that have revitalized the use of this reaction in organic synthesis.



Scheme 3. Chemo- and Regioselective Hydroborations Using Transition Metals.

Scheme 4. Plausible Mechanism for the Late Transition Metal Catalyzed Hydroboration of Vinyl Arenes (19).

Earth-Abundant Metals As chemists continue to search for greener and more economically-viable methodologies in organic synthesis, the use of first row transition metals continues to be examined in detail for a number of different catalytic reactions. Indeed, considerable effort has focused recently on replacing expensive and air-sensitive commonly used rhodium- and iridium-based hydroboration catalyst systems with readily available earth-abundant metals. Ritter and McNeill have recently published an excellent account of their work detailing the use of low-valent iron complexes containing nitrogen-based ligands for the 1,4-functionalization of 1,3-dienes (22). For instance, the authors have been able to generate a variety of allylic alcohols, upon oxidation, using iminopyridine-derived ligands coordinated to FeCl2 (Scheme 5). The metal is initially reduced to Fe(0) using magnesium and, once again, the reaction is believed to proceed via initial oxidative addition of HBpin to the metal center. Rauchfuss and co-workers have extended this work 212 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


by using tridentate phosphine-iminopyridine ligands and FeBr2, and employing NaBEt3H to reduce the metal and activate the catalyst (23).

Scheme 5. The Iron Catalyzed Hydroboration of tert-Butyl 4-methylenehex-5-enoate.

Complexes of cobalt have also received a considerable amount of attention recently for their use in hydroboration reactions. Chirik and co-workers have been instrumental in developing this chemistry and have designed an elegant and simple catalyst system based on the air-stable precursor [CoCl2(PPh3)2] that effectively catalyses the isomerization-hydroboration of alkenes (24). Unlike other metal complexes which catalyse this tandem reaction to give products where the boryl fragment is incorporated at the terminal position, this unique cobalt system favors boron incorporation adjacent to π-systems (Scheme 6). The activated catalytic species in these reactions is [CoH(N2)(PPh3)3], which is generated by the addition of NaBEt3H to [CoCl2(PPh3)2] and PPh3 under an inert-atmosphere. Interestingly, this catalyst system can also be used to generate 1,1-diboron compounds from α,ω-dienes using consecutive hydroboration steps. Chirik’s group has also shown that a series of bis(imino)pyridine cobalt complexes can be used to selectively catalyze the hydroboration of terminal alkynes in high yields and preference for the (Z)-isomer (25). Catalyzed hydroborations of alkynes to give products with (Z)-selectivity have been reported previously but were generally thought to proceed via a metal vinylidene intermediate. In this present study, the authors hypothesize that the reaction with cobalt occurs via a mechanism that involves insertion of an alkynylboronate ester into a Co-H bond. Fine tuning of the imine substituents alters the relative rates of catalyst activation and is responsible for the stereochemical outcome of the reaction, allowing the authors to generate either the (Z)- or (E)-product selectively. 213 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 6. The Cobalt Catalyzed Hydroboration of Alkenes. In a complementary study, Sydora, Turculet, Stradiotto and co-workers have designed a three coordinate (N-phosphinoamidinate)cobalt(amido) precatalyst for the isomerization-hydroboration of alkenes with HBpin to selectively generate products where the boron group has been incorporated at the terminal position (26). Another notable recent example was reported by Lu and co-workers for the enantioselective hydroboration of aryl ketones using HBpin and a catalytic amount of a cobalt complex containing iminopyridine oxazoline ligands (27). The reaction was tolerant of a wide variety of functional groups including halides, amines, ethers, sulfides, esters and amides. High enantioselectivites were also achieved in reactions of substituted diaryl ketones. While it might not come as a surprise that other transition metal complexes could be used to facilitate the hydroboration reaction, a stunning breakthrough came when it was reported that main group compounds could also be used to accelerate this addition reaction (28). Since this seminal discovery, several other main group species have been found to be active catalysts for the hydroboration reaction. Hill and co-workers have been redefining the chemistry of organometallic β-diketiminato magnesium complexes and have recently reported on the use of a n-butyl precatalyst to affect the hydroboration and dearomatization of pyridine derivatives (Scheme 7) (29). The reaction is thought to proceed by formation of a reactive Mg-H intermediate, which then coordinates the pyridine group and via a subsequent 1,2-insertion step into the C=N double bond generates the reduced product, where the boryl group has added to the more electron-rich nitrogen atom. It should be noted that the 1,4-addition product is also observed to some extent in these reactions. Yang, Parameswaran, Roesky and co-workers have found that related β-diketiminato aluminum dihydrides also function like 214 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


transition metal catalysts and can be employed in the hydroboration of aldehydes (30), ketones (30), and terminal alkynes (31). Likewise, a 1,3,2-diazaphospholene has been shown to effectively promote the hydroboration of both aliphatic and aromatic carbonyl groups using HBpin (32).

Scheme 7. The Magnesium Catalyzed Hydroboration of Pyridine with HBpin. Interestingly, bulky two-coordinate germanium(II) and tin(II) hydrides have been prepared for the first time and used as catalysts in organic synthesis. These unusual low-valent p-block metal hydrides were found to be active catalysts for the hydroboration of a wide range of unactivated aldehydes and ketones using HBpin (33). Quantitative conversions were achieved using these main group hydrides with catalyst loadings as low as 0.05% and with turnover frequencies in excess of 13,300 h-1 in specific cases. The main group hydride is generated in situ by addition of HBpin to the catalyst precursor LMOtBu (M = Ge, Sn), which then is thought to add in a 1,2-fashion to the carbonyl compound via a four-centered transition state. A subsequent metathesis step involving addition of HBpin to the main group alkoxide, once again involving a four-centered transition state, would then generate the corresponding reduced product (Scheme 8). The activities in this study rival those of well-known transition metal catalysts and highlight the potential and promise of using main group compounds for this important reaction.

Scheme 8. Proposed Catalytic Cycle for the Hydroboration of Carbonyl Groups Using Low Valent Ge and Sn Complexes. 215 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Heterogeneous Catalysis Although much less studied than its homogeneous counterpart, the potential to translate catalyzed hydroboration technologies into a heterogeneous environment has vast implications in industrial chemistry. Pruski, Sadow and co-workers have recently shown that the hydroboration of aldehydes and ketones can also be effectively carried out using a silica-supported zirconium catalyst prepared by the reaction of [Zr(NMe2)4] and mesoporous silica nanoparticles (MSN) (34). The active catalytic species, characterized as [Zr(NMe2)n@(MSN)], is reported to have a surface structure primarily consisting of [≡SiOZr(NMe2)3] groups and reacts with HBpin to form Me2NBpin and a material that contains surface bound zirconium-hydride and surface bonded borane [≡SiOBpin] in approximately 1:1 ratios. The resulting material can catalyze the selective hydroboration of a wide range of carbonyl groups with HBpin in the presence of functional groups that are often reduced under typical homogeneous catalytic conditions. The air-exposed catalytic material can be recycled without any significant loss of activity. Late transition metal nanocrystals prepared in onium salts using supercritical carbon dioxide have been shown to be active catalysts for the hydroboration of phenylacetylene using HBpin (35). In a related study, iron oxide nanoparticles supported on magnesia (MgO) were found to catalyze the ‘hydroboration’ of both terminal and internal alkynes using the diboron source (vide infra) B2pin2 (36). In this same study, the authors also reported that platinum supported on MgO gave the complementary diboration products.

Challenging Substrates As mentioned previously, the majority of the early work with catalyzed hydroborations involved the addition of hydridoboranes to alkenes and alkynes. Reactions of alkynes were particularly troublesome giving rise to isomeric mixtures of products and even suffering from over-reduction of the triple bond, where two equivalents of the hydridoborane added to the initial substrate. However, Pereira and Srebnik revealed that Schwartz’s reagent, [Cp2Zr(H)Cl], could be used to efficiently catalyze the addition of one equivalent of HBpin to both terminal and internal alkynes (37). This important study not only demonstrated that early metals could be used to facilitate this addition reaction, but with the judicious choice of metal complex and hydridoborane, more challenging substrates could also be reduced effectively. The catalyzed hydroboration of carbonyl groups has received increased attention over the past few years. Most recently, [RuCl2(p-cymene)]2 (38) and the readily-available copper carbene complex [(IPr)CuOtBu] (39) (IPr = N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) have been shown to catalyze the hydroboration of aldehydes and ketones using HBpin. Moreover, Sadow and co-workers have also shown that organometallic magnesium complexes can be used in the hydroboration of esters (40). Even more remarkable is the use of this 216 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


chemistry in the reduction of the greenhouse gas carbon dioxide (41–47). Many transition metal complexes and main group compounds have been developed in recent years that actively catalyze the hydroboration of CO2 to generate methanol as the ultimate product. Of singular interest are a family of ambiphilic phosphine boranes that catalyze the hydroboration of CO2 using either HBcat or H3B.SMe2 (Scheme 9) (47). Interestingly, addition of HBcat to CO2 in the presence of these phosphine boranes resulted in formaldehyde adducts, which were postulated as the catalytically-active species and not just the resting states. Turnover frequencies for reductions using H3B.SMe2 were reported to be 228 h-1 at room temperature and 873 h-1 at 70°C. Elevated termperatures were required in reactions with H3B.SMe2 to activate the correspondingly generated R3P.BH3 adducts.

Scheme 9. Hydroboration of Carbon Dioxide Catalyzed by Phosphine Boranes.

In light of the importance of amines in pharmaceutical, agricultural and food chemistry, it is somewhat surprising that the catalyzed hydroboration of C-N double and triple bonds has received so little attention since the inception of this remarkable reaction as it provides a gentle and efficient route to these valuable compounds. Indeed, only a single report has appeared in recent years that describes the use of an organometallic magnesium complex that was used to catalyze the hydroboration of a series of aldimines and ketimines (48). The same group used this magnesium complex in the first reported catalyzed hydroboration of organic isonitriles RNC (49). Reactions proceeded under mild conditions when R = alkyl and afforded the corresponding 1,2-diborylated amine products (Scheme 10). Conversely, Geri and Szymczak concurrently found that ruthenium complexes containing bifunctional pincer ligands were excellent catalyst precursors for the addition of HBpin to a wide range of nitriles R′CN where R′ = aryl (50). Remarkably, reactions using these ruthenium complexes gave 1,1-diborylated amine products R′CH2N(Bpin)2 in moderate to high yields (Scheme 10). More recently, Gouverneur and co-workers found that copper complexes could be used as catalysts for the insertion of B-H bonds into trifluorodiazoalkanes using the unusual boron source of Ph3P.BH3 (51). 217 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 10. Catalyzed Hydroboration of Isonitriles and Nitriles.

Arguably one of the most elegant and synthetically relevant studies involving substrates containing C-N multiple bonds does not actually involve reduction of these groups but utilizes the electronics of oximes to direct the hydroboration to a proximal site of unsaturation. Takacs and co-workers have been redefining the area of carbonyl-directed catalytic asymmetric hydroborations (CAHB) recently and have reported a CAHB variant using oximes as a route to chiral tertiary boronic esters and their corresponding alcohols, upon oxidative workup (Scheme 11) (52). The authors have found that hydroboration of alkyl-substituted methylidene and trisubstituted alkenes using HBpin and a rhodium catalyst precursor containing chiral phosphite ligands affords tertiary boronic esters with yields up to 87% and enantiomeric ratios up to 96:4 e.r. Reactions of aryl-substituted oximes gave terminal alcohols in an analogous manner as that observed in carbonyl-directed CAHB reactions but only in low yields. Deuterium labelling studies revealed that reactions with these aryl oximes (R = Ph) were further complicated by an unexpected C-H activation step. Under these conditions, a mixture of products were generated arising from an ortho-borylation step involving one of the oxime phenyl rings (which is subsequently oxidized to the alcohol) along with concomitant formation of H2 which, in the presence of the metal catalyst, hydrogenates both the new product and some of the unreacted substrate (53).



Scheme 11. Oxime-Directed CAHB Reactions.

Diboron Sources Compounds containing direct B-B single bonds have found considerable utilization in the analogous catalyzed diboration reaction, where both boryl fragments add to the corresponding substrates (4, 5). However, one of the more unusual, yet creative, uses of these diboron compounds has been in the unconventional ‘hydroboration’ reaction where only one boryl group is delivered to the substrate and the hydrogen source comes from the solvent (54). This area has expanded tremendously in recent years and many exceptional studies, far too numerous to mention in this report, continue to be published. Although one boryl group is ultimately sacrificed in these ‘hydroboration’ reactions, a major advantage to using diboron sources, as compared to hydridoborane species, is their relative stability and ease of handling. Indeed, Yao, Deng and co-workers have recently reported the hydroboration of terminal alkynes in aqueous media using B2pin2 and a cyclodextrin-bispyridine copper complex (55). Reactions proceeded with good regiocontrol in favor of the branched isomer, albeit minor amounts of the linear product were also generated and reactions with aryl alkynes gave lower yields than those using alkyl alkynes (Equation 1). Nevertheless, this study represents a major advancement in this area as the more reactive B-H bonds in hydridoboranes prohibits traditional hydroboration reactions from being carried out in such green solvents.



While other transition metal complexes can be used to facilitate these remarkable reactions (56), Yao and Deng have shown that ‘hydroboration’ reactions of terminal alkynes using B2pin2 can be carried out in the presence of a strong base, such as LiOt-Bu, without the need of a transition metal (57). In this study, both aryl- and alkyl alkynes reacted efficiently with high levels of regiocontrol to give the corresponding (E)-alkenylboronic ester derivatives. Yang and Song have expanded the scope of this metal-free variant on a diverse range of terminal aryl alkynes using Cs2CO3 as the base (58). Interestingly, under these reaction conditions the corresponding saturated products were formed in high yields where the boron group has added to the unsubstituted carbon atom. These reactions are believed to proceed by successive ‘hydroboration’, diboration, and protodeboronation steps (Scheme 12). This methodology was extended to aryl alkenes and used in the synthesis of biologically-active scaffolds. In a related study, the ‘hydroboration of aryl alkenes with B2pin2 has been catalyzed by a ligand-free FeCl2 system (59).

Scheme 12. Postulated Mechanism for the Base-Catalyzed ‘Hydroboration’, Diboration and Protodeboronation of Aryl Alkenes and Aryl Alkynes. The potential of using this remarkable ‘hydroboration’ reaction in organic synthesis has been highlighted recently by several groups. Noteably, Tang and co-workers have examined the rhodium-catalyzed asymmetric ‘hydroboration’ of α-aryl enamides using B2pin2 as an effective route to generate a series of α-amino boronic esters in good yields and excellent enantioselectivities (up to 99% ee) (60). A P-chiral monodentate ligand, in conjunction with the rhodium starting precursor [Rh(nbd)2]BF4 (nbd = norbornadiene), was reported to be responsible for the unusual Marknovnikov selectivity to afford chiral tertiary boronic esters. Zhao and Montgomery have utilized this emerging hydroboration methodology in the 220 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


functionalization of aryl alkenes using a copper-catalyzed ‘hydroboration’/orthocyanation reaction (61). The products arising from this bis-functionalization were further transformed using a combined AgNO3/Selectfluor-mediated coupling of the Bpin and cyano groups in the annulation of new five membered rings (Scheme 13). Finally, a regioselective ‘hydroboration’ of dehydroalanine derivatives has been developed by Piersanti and co-workers in the synthesis of amino acids and peptides bearing unnatural side-chains (62). For a more comprehensive look into reactions of diboron compounds as hydroboration sources, you are encouraged to read the excellent review by Yoshida (63).

Scheme 13. Applications of Hydroboration Reactions Using Diboron Sources.

Conclusion The catalyzed hydroboration reaction is a remarkably powerful methodology for the functionalization of a wide range of substrates, affording organoboron products, compounds that are widely used in all aspects of synthetic chemistry. After the explosion of initial interest in this reaction, the area stagnated for several years and it was largely used as a diagnostic to examine the efficacy of new metal complexes for their potential activity in the hydroboration of vinyl arenes. However, there has been a significant rebirth in interest as researchers try to solve some of the earlier challenges associated with these catalyzed reactions as well as expand on its potential use in organic synthesis. This report was intended to hightlight some of the recent advances in this area within the past year. There has been remarkable progress involved in developing earth-abundant catalysts as opposed to the traditional Rh and Ir-based systems, developing heterogeneous catalysts, using this reaction in the reduction of more challenging substrates and the incredible discovery that diboron sources can be used in ‘hydroborations’ in green aqueous solvents. While tremendous advancements have been achieved recently, it is pretty clear that the future of this amazing reaction has not yet reached its full potential. 221 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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