Synthesis of Pinacolboronates via Hydroboration - ACS Symposium

Chapter 7

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Synthesis of Pinacolboronates via Hydroboration Andrew G. Karatjas,* Heidi A. McBriarty, Stephan I. Braye, and David Piscitelli Science Department, 8 Abbott Park Place, Johnson and Wales University, Providence, Rhode Island 02903 *E-mail: [email protected]

The synthesis of pinacolboronates is important due to their use in the Suzuki coupling reaction. Discussed here are methods for their synthesis including a new methodology that allows for the use of a traditional hydroboration for the synthesis of pinacolboronate derivatives.

Introduction The Suzuki (or Suzuki-Miyaura) coupling of organic molecules is one of the most important methods for the formation of carbon-carbon bonds (1). This importance was highlighted with the awarding of the 2010 Nobel Prize in Chemistry to Akira Suzuki for his pioneering work. The reaction which occurs between an organoboronic acid or organoboronic ester such as pinacolboronate 2, and most commonly an aryl or vinyl halide 1, forms a product (3) with a new carbon-carbon bond (Eq 1).

Due to its utility the Suzuki coupling reaction has found wide use in natural product synthesis and commercially. The Suzuki coupling is a key step in the synthesis of Valsartan by Novartis, a drug used to treat high blood pressure and © 2016 American Chemical Society Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


congestive heart failure with more than 22 million users worldwide (2). BASF has also found the Suzuki coupling to be valuable for an efficient synthesis of boscalid, a fungicide for high end specialty crops. The Suzuki coupling has also seen use in syntheses of natural products such as palytoxin and an early step in the synthesis of vancomycin (3). Numerous methods have been developed to synthesize the desired precursors (2) using transition metal catalysts which work well and include palladium (4), rhodium (5), and zirconium (6, 7) based compounds that are often highly air sensitive. In addition, these transition metal catalysts are expensive and require the use of pinacolborane (or B2pin2), a compound that can cost more than $3 per gram.

Synthesis of Pinacolboronates Miyaura et al. (8) first published a method for the synthesis of aryl pinacolboronates (5) which utilized palladium catalysis with aryl or vinyl halides coupling with bis(pinacolato)diboron (B2pin2) to form pinacolboronates (Eq 2). This procedure worked well for a variety of aryl halides ranging in yield from 60 to 98%. This methodology was an improvement over classical arylboronate synthesis which required the use of lithium-halogen exchange reactions involving alkyllithiums (9–11) or Grignard reactions.

As noted above, Grignard type reactions have also been used for the synthesis of aryl boronic acid and aryl pinacolboronic esters (12, 13) One of the best examples of the use of a Grignard type reaction is work done by Leermann et al. were able to synthesize a series of aryl boronic acids through Grignard formation from the corresponding aryl halide. Aryl boronic acids with various substitutions were synthesized in high yields. Clary et al. were able to synthesize the pinacol boronates directly using Grignard reagents. With this methodology (Eq 3), they were able to use aliphatic, aromatic, or vinylic halides to synthesize a large variety of pinacolboronate derivatives.

Billingsley and Buchwald (14) have shown success using palladium catalysis achieving yields over 90% for the synthesis of aryl pinacolboronates for a variety of aryl halides (Eq 4). These reactions use 0.1 – 4 mol% of a palladium catalyst for the conversion of aryl and heteroaryl halides to the corresponding pinacolborane ester. 228 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Murata et al. have found that the use of Pd(dba)2 with a variety of aryl halides successfully affords aryl pinacolboronates with yields ranging from 70 – 100% (15). Their methodology showed success with substituents located ortho, meta, or para to the halide substituent. In addition, they found that both electron donating and electron withdrawing substituents gave high yields for the methodology. Other work has also found good success with the use of palladium catalysts for the synthesis of aryl pinacolboronates. Wolan and Zaidlewicz (16) used a similar system as Miyaura but with the use of ionic liquids to achieve yields of 60-87% with a PdCl2(dppf) catalyst. Zhu and Ma explored the use of copper iodide as a catalyst for the conversion of aryl iodides to aryl pinacolboronates (17). Aryl iodides worked well for a variety of functionalized aromatics, but aryl bromides worked poorly (Eq 5).

Yang et al. expanded the use of copper iodide as a catalyst for the synthesis of alkyl pinacolboronates (Eq 6) (18). They were able to achieve high yields for a large number of alkyl halides and alkyl tosylates. These ranged from very simple alkyl groups such as R = hexyl to more complicated R groups including ones containing esters, nitriles, ketones, alkenes, alcohols, ethers, amides, and substituted aromatics.

This type of conversion was expanded by Murphy et al. to include the use of iridium as a possible metal for catalysis for the synthesis of aryl pinacolboronates (Eq 7) (19, 20). Unlike previous methods, this methodology took advantage of C-H activation chemistry and did not require the use of a halogen or tosylate group for replacement by the boron group. A wide variety of functional groups were tolerated (yields ranged from 66 – 93%) including electron donating and electron withdrawing groups with the boron group being placed meta to the 229 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


original directing substituent. This methodology also easily allowed the product to be converted to the boronic acid or to the trifluoroborate salt.

With many of the previous methods requiring heat to carry out the synthesis of the pinacolboronates, the use of microwave conditions was explored. Harrisson et al. probed the use of microwave heating with iridium catalysis (21). They showed the use of 15 different substrates containing a variety of functional groups gave similar or higher yields to conventional heating methods. This was also an improvement in reaction times as most traditional procedures require 8 or more hours of heating. The mew microwave heated procedure was generally done after just five minutes. In addition, they demonstrated that a one pot borylation/Suzuki coupling could be carried out in 10 minutes of microwave heating using an iridium catalyst (3 mol%) in MTBE (methyl-tert-butyl ether) leading to a wide range of biaryl compounds in short reaction times (Scheme 1). Additionally, as with the previous iridium catalyzed work, no halogen was required on the aromatic ring.

Scheme 1. Microwave assisted borylation/Suzuki coupling.

Ghaffari et al. were also able to use iridium catalysts with bidentate ligands containing either nitrogen or phosphorus donating groups to activate the position ortho to electron withdrawing groups. The methodology was able to tolerate a large number of functional groups including esters, halogens, and ethers in high yield (22). 230 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Zhao et al. developed a “green” method for the synthesis of aryl pinacolboronates (23). Their methodology used a Sandmeyer reaction to form the initial aryldiazonium salt. The resulting diazonium salt underwent an SN2Ar borylation reaction yielding aryl boronic acids and esters in high yields. This methodology was found to tolerate numerous aromatic substituents (ortho, meta, and para) including halides, esters, amines, alcohols, and nitro groups. While many of these methods also work for the synthesis of alkenyl pinacolboronates, a number of additional methods that focus specifically on alkene products have also been developed. Takagi et al. used a palladium (3 mol%) cross-coupling reaction to convert vinyl triflates (16) into pinacolboronates (17) (24). The substrates were generally α,β-unsaturated carbonyls (esters, amides, and ketones). High yields were obtained for a variety of substrates in toluene or dioxane (Eq 8).

Gunathan et al. was able to extend the synthesis of vinyl pinacolboronates to the use of ruthenium catalyzed additions (25). A ruthenium complex was used to catalyze the anti-Markovnikov addition of pinacolborane directly to terminal alkynes. Interestingly, and unlike most other additions of borane groups to alkynes, this reactivity lead directly to the Z-isomer. Other metals have also been used successfully in the synthesis of vinyl pinacolboronates. Rawat and Sreedhar (26) used an iron catalyst (FeCl3) to achieve E-vinyl boronates in high yield with high selectivity. Yosheda et al. was also able to achieve similarly high yields and selectivity for E-alkenes using silver based catalysts. Under the conditions of the silver catalyzed transformation, alcohols were also tolerated (27). Gao and Hoveyda (28) used a nickel catalyzed hydroalumination reaction in the presence of DIBAL-H to affect the synthesis of highly substituted vinyl pinacolboronates. Suginome et al. also used a nickel catalyst to provide vinyl pinacol boronates (29). Shirakawa et al. were able to synthesize E-1-alkyenylpinacolboronate esters using catalytic dicyclohexylborane at room temperature (30). The reaction proceeded in THF using pinacolborane and 5 mol% dicyclohexylborane (Eq 9). A similar transformation was effected by Ho et al. using various carboxylic acids as catalysts for the borylation reaction (31).

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


Synthesis of Pinacolboronates Using Iodo Pyridine Borane While many of these methods are highly successful, most of these methods rely on expensive transition metal catalysts and pinacolborane. Prior work had shown success in the synthesis of alkylpinacolboronates using various alkenes (32). These procedures have the potential to be significantly cheaper than many traditional procedures. By making the pinacolboronate as part of the synthesis, the use of costly pinacolborane could be avoided. Traditionally, synthesis of alkylpinacolboronates via the use of traditional hydroboration reactions has been avoided. In many cases, these reactions are known to lead to a mixture of mono-, di-, and trialkylboranes. For hydroboration methodology involving oxidative workup, this distinction is not noteworthy, but for the work discussed here where incorporation of the boron is part of the product structure; selectivity is critical. This issue becomes even a greater challenge when considering the fact that the intermediate monoalkylborane derivative is thought to be more reactive than the starting borane reagent (33). It was thought that we could synthesize vinyl pinacolboronates using traditional hydroboration methods. This would avoid the need for the costly pinacolborane reagent by building the pinacolboronate group as part of the methodology. Based on work by Clay and Vedejs (34) it was proposed that the use of an activated pyridine borane could be useful for the synthesis of alkylborane derivatives. Their work showed that through the use of pyridine borane, a preference was seen for the formation of the monoalkyl borane. However, there was still a significant amount of dialkylborane recovered (35). Karatjas and Vedejs (32) expanded this work to include the synthesis of pinacol boronate esters using activated pyridine borane (PyBH2I). This work showed two clear trends: (1) The use of more hindered alkenes improved the yield without raising the equivalents of pyridine borane (Table 1, entries 6, 10, 12), although longer reaction times were required (e.g. the use of 1-phenylcyclohexene gave a purified yield of 67% after stirring for 18 hours, but only 2% yield with 86% recovered starting material were found after 2 hours (which was the standard reaction time for mono- and disubstituted alkenes). For more hindered alkenes, the use of additional equivalents of pyridine borane did not result in a signficant increase in yield (Table 1, entry 10, an increase to six equivalents only yielded 74% product.). (2) The use of higher equivalances of pyridine borane increased the yield for less hindered alkenes (Table 1, entries 7, 8, Table 2, entries 1, 7, 8, 9). When using α-methylstyrene as the starting alkene, at low equivalences of pyridine borane, an approximately 1:1 ratio of the desired product and the dialkyl borinic acid 23 were recovered. However, as the equivalances of pyridine borane were increased, the amount of dialkyl borinic acid 23 decreased until six equivalents were used. At this point, the amount of the byproduct (23) was below the detection limit of the method of analysis (1H NMR). Similar results were found with alkenes such as norbornene (Table 1, entries 7 & 8). Based on this previous work, we proposed a method to synthesize alkenyl pinacolboronates for direct use in the Suzuki coupling reaction. It was thought that the hydroboration methodology used for alkenes with iodopyridine borane, which worked well with highly substituted alkenes to form alkyl pinacolboronates, could 232 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


be applied to alkynes to form alkenyl pinacolboronates. If successful, this route could provide a route to the Suzuki precursors using traditional hydroboration methods. Work commenced using diphenylacetylene with varying amounts of PyBH2I (Eq 10). These attempts were successful as the use of 1.5 equivalents of PyBH2I led to a purified yield of 79%. Additional equivalents of reagent did not lead to increased yields. This was consistent with past work that showed high yields for more substituted alkenes.

Table 1. Conversion of Alkenes to Pinacolboronates.



Table 2. Conversion of α-methylstyrene.

However, given the differences in sterics and geometry of alkynes compared to alkenes, there were concerns that yields might not be as high as those found for alkenes, even with higher equivalences of borane. The next system explored was 1-hexyne (Table 3, entries 1-2). It was expected that a terminal alkyne would result in lower yields due to competition with dialkylborane formation, similar to results seen with systems such as dodecene (Table 1, entry 2). However, this system displayed an additional problem during purification. TLC analysis of the crude product mixture indicated partial decomposition of the product. A workup procedure giving pure material without the need for chromatography was developed to avoid this decomposition. This method led to similar levels of purity as the pure material recovered from column chromatography but without the decomposition seen with silica gel. While disappointing that 1-hexyne gave such low yields, it was not unexpected and further screening was done to ascertain the scope of this reaction.



Table 3. Conversion of Alkynes to Pinacolboronates.

Increasing the size of the R group on the terminal alkyne did result in an increase in the yield for the reaction (Table 3, entries 5, 11). This was not unexpected as the use of 1-hexyne was postulated to have a second problem. The product, 27a, has a relatively low boiling point and it would not be surprising for product to have been lost during purification and isolation (36). Longer chain alkynes did lead to slightly increased yields (entries 5 and 11), similar to those yields seen for monosubstituted alkenes (although at higher equivalences of pyridine borane). Internal alkynes have shown higher levels of success and conversion to alkenyl pinacolboronates in high yield without the need for a large excess of reagent (Table 3, entries 6, 7). However, the use of an ethyl in place of a methyl group (Table 3, entry 8) did not give a satisfactory yield. For the internal alkynes tested, the addition of an excess of PyBH2I did not lead to a significant increase in the amount of product formed. As it was clear that the use of terminal alkynes was problematic, two alternative methods were explored as possible solutions to the problem. Since it is expected that the lack of steric bulk is the primary issue, increased sterics on the alkyne as well as the reagents used were explored. The use of 2,6-lutidine borane instead of pyridine borane led to a decreased yield in all trials.


With this unfortunate result, it was postulated that the use of a silyl-protected alkyne could provide the necessary steric bulk needed to raise the yield. Initial attempts made using phenylacetylene and a silyl-protected derivative were explored. As expected, hydroboration of phenylacetylene led to a low yield of the desired product (Table 4, entries 1-2). However, using commercially available 1-phenyl-2-trimethylsilylacetylene, 29, (Table 4, entries 3-4) showed a significant increase in yield. Additionally, under the basic conditions where the pinacol functionality is formed, the TMS group is cleaved yielding the same product as from phenylacetylene.


Table 4. Hydroboration of Phenylacetylenes.

With this successful result in hand, the exploration of other substrates was explored (along with different silyl groups). Synthesis of the silyl protected alkynes proceeded in high yield (Table 5).

Table 5. Synthesis of Silyl-Protected Alkynes


With this set of silyl protected alkynes in hand, attempts at the hydroboration were made. After the success with the TMS group, the results obtained were disappointing but may be useful for future work. In all cases (Table 6), decreased yields were recovered. However, it was seen that with larger R groups, a change in regiochemistry was seen. Both the TES and TIPS groups showed at least a partial reversal of regiochemistry. Larger silyl groups proved to be sterically too large as no identifiable products were recovered.


Table 6. Hydroboration of Silyl-Protected Alkynes.

Conclusion Much work has been done in the synthesis of pinacolboronates. These valuable Suzuki-Miyaura coupling precursors are valuable for organic synthesis and a wide variety of synthetic methods have been designed to allow for their synthesis. A new method using a traditional hydroboration reaction has been presented here that has met with limited success. However, there is promise for success in the future as much work remains to be done. The synthesis of alkenyl pinacolboronates under these conditions is a useful endeavor, and the potential to reverse the regiochemistry with the simple switching of protecting groups is highly promising. However, the reaction still suffer from the traditional issues seen with hydroboration reactions based on sterics, selectivity, and the increased reactivity of the intermediates synthesized. This new method works via a traditional hydroboration of alkynes and has seen some initial success for internal alkynes. Additionally, it has been seen in some cases that the use of a silyl-protected alkyne can be useful for increasing the yield of the hydroboration of terminal alkynes. Future work may include exploration to expand the scope of functional groups that can be tolerated as well as continuing to expand the usefulness of different silyl protecting groups as a method for conversion of terminal alkynes. 237 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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Synthesis of Pinacolboronates via Hydroboration - ACS Symposium

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