Di- and Polyboron Compounds: Preparation and Chemoselective

Chapter 13

Di- and Polyboron Compounds: Preparation and Chemoselective Transformations Downloaded by PURDUE UNIV on December 5, 2016 | http://pubs.acs.org Publication Date (Web): November 30, 2016 | doi: 10.1021/bk-2016-1236.ch013

Liang Xu,1,2 Shuai Zhang,1 and Pengfei Li1,* 1Center

for Organic Chemistry, Frontier Institute of Science and Technology, Xi’an Jiaotong University, 99 Yanxiang Road, Xi’an, Shaanxi, 710054 China 2School of Chemistry and Chemical Engineering/Key Laboratory for Green Processing of Xinjiang Bingtuan, Shihezi University, Shihezi, Xinjiang, 832003 China *E-mail: [email protected]

Boron-selective chemical transformations of di- and polyboron compounds have emerged as a useful strategy for modular assembly of complex molecules. There has been great progress in this type of approach in the last decade based on the development and discriminate utilization of various boron species. These advances have encouraged modular construction of molecular diversity and complexity from easily-prepared or commercially available building blocks, as well as led to simplifying synthetic design and experimental operations. For this reason, this chapter will survey and present an overview of representative advances in this emerging field.

Introduction The biosynthesis of complex molecules, such as polypeptides, generally involves modular assembly processes based on simple di- and/or polyfunctionalized building blocks, such as amino acids. With nature as the model, modular synthesis of complex molecules from simple building blocks might provide an opportunity to simplify some daunting organic synthesis. Thus, increasing attention has been paid to the development and utilization of effective building blocks and the corresponding assembly methodologies. Modular synthesis generally involves consecutive chemoselective transformations, which are critical to the precise construction of molecular © 2016 American Chemical Society Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

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diversity and complexity. Among them, boron-selective transformations (1) based on functionally different boronyl groups are of particular importance since boronyl species are generally easily-prepared, shelf-stable, environment-friendly and compatible with diverse functional groups. Generally, boron-selective reactions refer to the chemical transformations in which the reactivity of two or more boronyl groups can be discriminated. Under certain reaction conditions, certain boronyl groups can be distinguished by promoting the conversion of the more reactive boronyl group and leaving the relatively inert boronyl group intact. The remaining boronyl group can usually be utilized and converted to other functional groups under different reaction conditions, therefore providing flexible and consecutive synthetic approaches to complex molecules. The key to realizing a boron-selective reaction is to differentiate the boronyl groups under the same reaction conditions. The different reactivity of boronyl groups sometimes derives from the different steric and/or electronic environment of the carbon atoms bearing the boron groups. In other cases, a proper combination of different masking groups on boron atoms plays a critical role. In 2007, Suginome (2) and Burke (3) independently realized the masking group-based boron-selective Suzuki–Miyaura coupling (SMC) reactions. These achievements supply a general solution for the difficulties in the preparation of functionalized organoboron reagents and inspired the following exploration in this field. This strategy has been developed rapidly and significant progress has been made in the last decade not only in SMC reactions but also in some other (catalyzed) transformations, contributing greatly to the prosperity of boron-selective transformations and providing efficient modular and even automated routes to synthesize natural products and organic materials (4). Boron-selective reactions may be classified into two categories based on the involved organoboron starting materials: one involves the utilization of two competitive organoboron reagents (Scheme 1, A), and the other utilizes di- or polyboron reagents (Scheme 1, B). This review will focus on the latter type of transformations and introduce the representative advances in this field, including the preparation (5) and chemoselective transformations of di- and polyboron compounds. Through multiple functionalizations of the C–B bonds, the boronyl groups on di- or polyboron building blocks are differentiated in successive reactions, and diverse structures will be achieved by making full use of each boronyl group. The following section will discuss the boron-selective reactions where boronyl groups of di- or polyboron reagents are reacted chemoselectively. Detailed discussion will be categorized according to the reaction type and starting materials used. The development and utilization of the former type of reactions (Scheme 1, A), specifically innovative examples in iterative coupling reactions (6), will not be discussed here but can be found in another separate chapter (by Dr. Watson). There is also a specific chapter (by Drs. Churches and Hutton) discussing the preparation and deprotection of protected boronic acid species. We would like to refer readers to these discussions when necessary.

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


Scheme 1. Boron-Selective Chemical Transformations.

Boron-Selective SMC Reactions Since the inception of Suzuki-Miyaura coupling (SMC) reaction in 1979, it has become one of the most powerful and reliable methods for C–C bond construction (Scheme 2, A) (7). Along with this development, in order to synthesize more complicated molecules, synthetic chemists have also made great efforts to realize successive SMC reactions using multifunctionalized reagents. Electrophile-selective SMC reactions (Scheme 2, B), based on the discrimination of various electrophilic sites with leaving groups such as (pseudo)halogens, have been well developed to achieve this goal. In contrast, nucleophile-selective SMC reactions (Scheme 2, C) based on the discrimination of different boron sites on the same reagent have been far less explored, largely due to the difficulties in the preparation of di- or polyboron reagents.

Scheme 2. SMC Reactions and Strategies towards Successive SMC Reactions.


In the last decade, thanks to the discovery of feasible (catalytic) methods for preparing di- and polyboron reagents and utilization of several powerful boron-masking groups, boron-selective SMC reactions have also gained great development (8). We would like to highlight the representative examples in which boronyl groups of di- or polyboron reagents are well differentiated.

1,1-Diborylalkanes


1,1-Diborylalkanes with Two Identical Boronyl Groups In 2009, Shibata and coworkers reported a Rh-catalyzed method towards the preparation of 1,1-diborylalkanes via sequential regioselective hydroboration of terminal alkynes at room temperature (Scheme 3, A) (9).

Scheme 3. Chemoselective SMC Reactions of 1,1-Diborylalkanes.

Later, the same group realized the chemoselective SMC of 1,1-diborylalkanes for the first time (10). Generally, the palladium-catalyzed SMC reactions of alkyl boronates suffer from slow transmetalation, hence harsh conditions and/or excess alkyl boronates are usually necessary to achieve high yield for such transformations. However, Shibata’s group demonstrated that gem-bis-B(pin)-substituted alkanes were competent in chemoselective SMC 418 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


reactions and two pinacol boronates on the same C(sp3) atom could be well differentiated at room temperature (Scheme 3, B). At elevated temperature, side reactions such as protodeboronation occurred whereas it could be prevented at room temperature. Aryl iodides and bromides were both suitable electrophiles in these SMC reactions and extensive screening revealed that the utilization of strong bases such as LiOH, NaOH or KOH was necessary for successful transformations. A gem-boryl-assisted transmetalation process via a monoborate intermediate was proposed to be the key to the success of this chemoselective coupling reaction. In contrast, this type of transmetalation could not be achieved with common C(sp3)–B(pin) groups under mild conditions. After the first catalytic coupling, a common C(sp3)–B(pin) group without an adjacent B(pin) moiety was formed, which was reluctant to become a borate intermediate hence would not react further in SMC reactions under the same reaction conditions. This reactivity is seen with the competing reaction between reagents 1 and 2 (Scheme 3, C). As shown in Table 1, further exploration towards the chemoselective SMC reactions of diborylmethane revealed that an equimolar amount of KOH with diborylmethane, which ensured the predominant generation of monoborate intermediates and prevented the generation of detrimental diborate intermediates, was necessary for efficient conversion (11). This method could be used in preparing various benzylboronate derivatives.

Table 1. Chemoselective SMC Reactions of Diborylmethane.

Shibata’s group also disclosed a one-pot synthetic method towards various symmetrical and unsymmetrical diarylmethanes from diborylmethane via sequential SMC reactions (12). By adjusting the reaction temperature and the amount of base used, the reaction could be well controlled to achieve high yields of unsymmetrical diarylmethanes (Scheme 4, A). 419 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 4. Sequential Chemoselective SMC Reactions of 1,1-Diborylalkanes.

In 2014, a new and convenient synthetic approach towards 1,1-diborylalkanes which proceeded via gem-diborylation of N-tosylhydrazones under transitionmetal-free conditions, was developed by the Wang group (13). They also explored the stepwise couplings of 1,1-diborylalkanes with two aryl halides, which afforded diarylalkane derivatives in moderate yields (Scheme 4, B). The same group also developed an efficient synthetic method for preparing 9H-fluorenederivatives via sequential SMC reactions of 1,1-diborylalkanes with and 2,2′-dibromobiphenyls (Scheme 4, C) (14).



When vinyl bromides were treated with 1,1-diborylalkanes, boron-selective SMC reactions were also feasible, affording allylboron intermediates which could be isolated in moderate yield when 2,2-disubstituted vinyl bromides were used as the electrophiles. However, when 2-monosubstituted vinyl bromides and 1,1-dibromoalkenes were used as starting materials, 1,4-dienes and allenes were ultimately generated respectively via allylboron intermediates (Scheme 5) (15).

Scheme 5. SMC Reactions of 1,1-Diborylalkanes with Vinyl Bromides.

Recently, taking advantage of a suitable combination of chiral ligands and palladium precursors, asymmetric boron-selective SMC reactions of 1,1-diborylalkanes with aryl/vinyl halides have been developed by Morken and coworkers, enabling the efficient construction of nonracemic chiralbenzylboronates (16) from aryl iodides or bromides and γ,γ-disubstituted allylboronates (17) from 2,2-disubstituted vinyl bromides. For the construction of C(sp3)–C(sp3) bonds, Shibata’s group discovered that the palladium-catalyzed coupling reactions between diborylmethane and allyl halides or benzyl halides proceeded efficiently at room temperature, providing various homoallylboronates and alkylboronates with excellent chemoselectivity (Scheme 6, A) (18).



Scheme 6. Three Types of Reactions for C(sp3)–C(sp3) Bond Construction from 1,1-Diborylalkanes

Nonactivated primary alkyl electrophiles, which had not been utilized in palladium-catalyzed SMC reactions with 1,1-diborylalkanes, were found to be competent in similar copper-catalyzed/promoted transformations (19). By virtue of the chemoselective construction of C(sp3)–C(sp3) bonds, this method provided a new strategy for the preparation of alkylboronates. Generally, when more hindered 1,1-diborylalkaneswere utilized in such transformations, more copper salt is necessary to achieve efficient conversion and high yield (Scheme 6, B). In 2014, Morken’s group discovered that alkoxide-promoted selective deborylative alkylation of 1,1-diborylalkanes could be achieved under catalyst-free conditions (Scheme 6, C) (20). A boron-stabilized carbanion could be generated via alkoxide-induced deborylation, which was then trapped by alkyl halides, affording a simple and reliable access to alkylboronates. They conducted the deborylative alkylation reaction of 1,1-diborylethane with benzyl chloride under nitrogen, affording > 6g of product in 87% yield. In the three types of reactions mentioned above, when allyl(pseudo)halides were treated with diborylalkanes, SN2-selective alkylation products were always obtained, affording linear alkylboronates. More recently, Cho’s group realized copper-catalyzed SN2′-selective alkylation reactions of diborylalkanes with allylchlorides, affording branched alkylboronates (21). For examples, under the catalysis of CuI, diborylmethane could couple efficiently with cinnamyl phosphate in an SN2-selective version to obtain linear alkylboronate 3 (Scheme 7, A). However, when catalyst and electrophile were changed to Cu(IMes)Cl and cinnamyl chloride, respectively, the branched product 4 was formed in 78% yield under proper conditions (Scheme 7, B).



Scheme 7. SN2-Selective vs SN2′-Selective Alkylation of Diborylmethane.

1,1-Diborylalkanes with Two Different Boronyl Groups By virtue of proper combinations of copper precursors and chiral ligands, enantiomerically enriched gem-B(pin)/B(dan) compounds could be prepared via regio- and enantioselective hydroboration of B(dan)-substituted alkenes with B2(pin)2, as reported by Hall’s (22) and Yun’s (23) groups, respectively (dan = 1,8-diaminonaphthalene). The newly-incorporated B(pin) moiety could be converted to corresponding BF3K group, which was then applied in chemoselective and stereoselective SMC reactions with various organic electrophiles, affording enantioenriched benzylic or allylic boronates. The reserved B(dan) moiety could be deprotected under acidic conditions and then utilized in the following SMC reactions (Scheme 8).

1,2-Diborylalkanes Formal asymmetric carbohydroxylation of alkenes was realized by Morken’s group by virtue of a tandem one-pot asymmetric diboration/boron-selective SMC/ oxidation sequence in 2004 (24). In this process, Rh-catalyzed enantioselective diboration of terminal aliphatic alkenes with B2(cat)2 afforded chiral 1,2-diborylalkane intermediates, which could in situ participate in boron-selective coupling with aryl halides and aryl triflates. More accessible primary C–B(cat) bonds reacted faster, leaving the secondary C–B(cat) bonds intact that were then oxidized to hydroxyl groups (Scheme 9). This one-pot transformation provided a concise method for preparing versatile optically active intermediates from simple alkenes.



Scheme 8. Synthesis and Application of gem-B(pin)/B(dan) Compounds.

Scheme 9. Formal Asymmetric Carbohydroxylation of Alkenes.

Later, the same group described a tandem one-pot diboration/hydroboration/ boron-selective SMC/oxidation sequence, facilitating the enantioselective synthesis of chiral aromatic and alkenyl diols (25). Pd-catalyzed enantioselective diboration of prochiral allenes afforded chiral allyl vinyl boronates, which went through hydroboration with 9-BBN to form triboron intermediates. Then, by simply adding aryl/vinylhalides and Cs2CO3, boron-selective SMC reactions occurred efficiently and diastereoselectively without an additional palladium catalyst or ligand. The least hindered C–B bonds reacted selectively and the two inert boronyl groups were oxidized to hydroxyl groups to generate diols products (Scheme 10).



Scheme 10. The Synthesis and Application of Triboron Intermediates.

In 2008, Molander’s group described that organotrifluoroborates containing alkene moiety could undergo hydroboration with 9-BBN to prepare diverse diboron products (26), including the 1,2-diborylethane (27). Chemoselective SMC reactions between the resulting trialkylboranes and aryl/vinyl halides could be realized, affording organotrifluoroborates which could be isolated or exposed to the following coupling reactions in a one-pot version (Scheme 11). The 1,2-diborylethane thus could function as a 1,2-dianion equivalent to link two different electrophiles via palladium-catalyzed sequential SMC reactions.

Scheme 11. The Synthesis and Application of 1,2-Diborylethane.

Taking advantage of a chiral imidazolinium salt, Hoveyda’s group realized a Cu-catalyzed enantioselective tandem double-hydroboration of terminal alkynes, (28), furnishing enantiomerically enriched 1,2-diborylalkanes such as product 5 (Scheme 12). A following boron-selective SMC reaction was also feasible with the terminal primary C–B bond reacting with β-bromoenone 6 chemoselectively.



Scheme 12. Cu-Catalyzed Double Hydroboration of Alkynes and SMC Reaction.

In 2014, Morken’s group realized Pt-catalyzed asymmetric diboration of terminal alkenes with B2(pin)2 (29). The obtained 1,2-diborylalkanes were also compatible with boron-selective SMC reactions. They also demonstrated that the presence of a β-B(pin) could greatly accelerate the transmetalation of the less hindered primary B(pin) group. Since the boronyl groups could function as versatile functional group precursors, the previously unreacted secondary B(pin) groups were readily transformed to afford a broad array of chiral compounds. Thus, the enantioselective diboration/selective cross-coupling (DCC) strategy, combined with further conversion of boronyl groups, provided a reliable and flexible platform that could convert readily available terminal alkenes into complex chiral compounds. As shown in Scheme 13, for the synthesis of the pharmaceutical agent Lyrica (pregabalin), by virtue of the DCC strategy, chiral secondary boronic ester 7 was prepared conveniently from two simple alkenyl substrates in excellent yield. Subsequently, the stereospecific boronates homologation, amination and protection sequence afforded intermediate 8, which was then oxidized and deprotected to complete the synthesis, affording the aimed product in an overall 36% yield from 7. In the above-mentioned examples, when 1,2-diborylalkanes were exposed to SMC reactions, the less hindered C–B bonds usually reacted preferentially. Very recently, Morken’s group developed a novel β-hydroxyl-directed regioselective coupling reactions of 1,2-diborylalkanes to inverse such selectivity, enabling boron-selective SMC reactions of the inherently less reactive secondary C–B bonds preferentially (Scheme 14) (30).



Scheme 13. DCC Strategy and Its Application.

Scheme 14. Hydroxyl-Directed Selective SMC Reactions.

This hydroxyl-directed selective SMC reaction merged seamlessly with hydroxyl-directed metal-free diboration and thus allowed for rapid functionalization of homoallylic alcohol derivatives. Importantly, the reaction was compatible with various electrophiles such as (hetero)aryl and alkenyl halides/triflates. Not only terminal alkenes, but also internal olefins and trisubstituted alkenes were suitable starting material for such transformations. This achievement might inspire the development of other novel innovative strategies to overcome the inherent electronic and/or steric limitations in boron-selective SMC reactions.



Di- and Polyborylalkenes The selective SMC reactions of di- and polyborylalkenes which bear two similar boronyl groups have been well-documented, enabling the stereocontrolled preparation of polysubstituted olefins that are important structural motifs in natural products and functional materials. This strategy was explored as early as 1996 and revealed by Miyaura’s group (31). They realized the synthesis of (E)-1,2-bis(boryl)-1-hexene via the platinum-catalyzed diboration of 1-hexyne with B2(pin)2. Then regioselective SMC reaction occurred between this diborylalkene and aryl, vinyl, benzyl and allyl halides, providing the corresponding (E)-(1-butyl-1-alkenyl)boronic esters in good yields. The less hindered terminal C–B bond reacted preferentially (Scheme 15).

Scheme 15. Boron-Selective SMC Reactions of 1,2-Diborylalkenes. More recently, Sawamura’s group developed phosphine-catalyzed anti-selective vicinal diboration of the triple bond in alkynoates to prepare α,β-diborylacrylates (32) such as compound 9. The two boronyl groups of the diborylacrylates could be differentiated and transformed in sequential SMC reactions, affording a diversity of unsymmetrical tetrasubstituted alkenes. The first SMC reaction occurred selectively at the α-boron site to give alkenylboronate 10 without the formation of the diarylation product (Scheme 16). Such selectivity is probably due to the presence of the ester group whose electron-withdrawing resonance effect might render the β-carbon less nucleophilic.

Scheme 16. Boron-Selective SMC Reactions of α,β-Diboryl Acrylates. 428 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


In 2001, Hiyama and Shimizu developed a strategy to prepare 1,1diborylalkenes such as compound 12 via gem-diborylation of 1,1-dibromoalkenes with B2(pin)2 (33). They also demonstrated the feasibility of stepwise SMC reactions for such products. An allylation/phenylation sequence afforded double coupling product 13 in a high overall yield (Scheme 17).

Scheme 17. Preparation and Application of 1,1-Diborylalkenes.

Later, Hiyama and Shimizu realized the stereoselective SMC reactions of 1,1-diboryl-2-arylalkenes (34). Perfect discrimination of two geminal alkenyl boryl groups could be achieved and the coupling reaction with aryl/vinyl iodides took place exclusively with the boryl moiety trans to the aryl groups. In combination with the subsequent SMC reactions of previously inert boronyl groups, the whole stepwise SMC approach provided an efficient and stereocontrolled access to triarylated alkenes. Sequential SMC reactions could also be accomplished as a one-pot version (Scheme 18).

Scheme 18. Stereoselective Couplings of 1,1-Diborylalkenes with Electrophiles. 429 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


In 2013, Nishihara’s group reported that Pd(OAc)2/tOctNC catalyzed regio- and stereoselective silaboration of alkynylboronates could afford 1,1diborylalkenes (35). They also realized the preparation of 1,2-diborylalkenes via Pt-catalyzed diboration of alkynylsilanes (36). The obtained trimetalated products could also go through boron-selective SMC reactions with the boryl moiety trans to the aryl groups reacting preferentially. More recently, by virtue of an iridium complex supported by a SiNN pincer ligand, Ozerov’s group successfully converted terminal alkynes into triborylalkenes via sequential C–H borylation of alkynes and dehydrogenative diboration of alkynylboronates with HB(pin). The obtained triborylalkenes were demonstrated to be compatible with stereoselective SMC reactions, affording trans-1,2-diborylalkenes (37). As illustrated in examples below, among the three boronyl groups, the one trans to the aryl group coupled with electrophiles preferentially (Scheme 19, A).

Scheme 19. Two Newly Developed Methods for the Synthesis of 1,1-Diborylalkenes. Sawamura’s group also developed a new synthetic method for the preparation of 1,1-diborylalkenes (38) via a Brønsted base catalyzed reaction between terminal alkynes and B2(pin)2. Various terminal alkynes which were conjugated with carbon-oxygen/nitrogen double bonds including propiolates, propiolamides, and 2-ethynylazoles were compatible with this transformation. The two geminal boronyl groups of the 1,1-diborylalkenes could be differentiated and reacted in 430 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


stepwise SMC reactions. The first coupling reaction occurred selectively at the boron site trans to the ester group (Scheme 19, B). In the above-mentioned cases, the starting materials contained two or more identical boronyl groups. However, some synthetic requirements could not be met using such substrates. For example, it was difficult to reverse the preferential reactivity of terminal and internal alkenyl boronyl groups without directing groups. Along with the development of boronyl-protecting groups (refer to the chapter of Drs. Churches and Hutton), di- and polyboron alkenes containing two or more different masked boronyl groups were prepared and utilized to solve such problems. Burke’s group (3) developed an efficient masking group for boronic acids, i.e., N-methyl iminodiacetic acid (MIDA) in 2007. Since then, they have prepared a library of building blocks based on the inert reactivity of MIDA boronates to build a synthetic platform for iterative and even automated synthesis of complex molecules (4) (refer to the chapter of Dr. Watson). Among such building blocks, the diborylalkenes can function as precursors for halo-B(MIDA) building blocks or linchpins to connect two different electrophiles by virtue of boron-selective reactions (39), which leave the B(MIDA) moiety intact (Scheme 20, A).

Scheme 20. Di- and Polyborylalkenes with Different Boronyl Functionalities. 431 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


An unsymmetrical diboron reagent, B(pin)–B(dan), could be applied in diboration of alkynes, affording 1,2-diborylalkenes, as disclosed by Suginome’s group. The B(dan) groups were incorporated to the terminal carbon atoms regioselectively. As a result of the inertness of B(dan) groups, selective SMC reactions were then realized with internal C–B bonds reacting preferentially (Scheme 20, B) (40). This was in sharp contrast to the above-mentioned reactivity of 1,2-di(pinacolatoboronic ester)alkenes. Thus, these two protocols are complementary to each other. By virtue of Pt-catalyzed diboration of alkynyl MIDA boronates, Nishihara’s group realized the synthesis of 1,1,2-triborylalkenes which contained two different boronyl functionalities in 2014. Three C–B bonds on the obtained products could be well discriminated in the following SMC reactions, consuming the internal B(pin) groups and affording 1,1-diborylalkenes selectively (Scheme 20, C) (41). Di- and Polyborylarenes Generally, when two or more identical boronyl groups are on the same building block, it is difficult to realize selective SMC reactions among these boronyl groups (42). However, via careful control of the reaction conditions, especially the ratio of nucleophiles and electrophiles, a few cases of selective SMC reactions have been disclosed, affording aim products in moderate yields (43). If such reactions commence with differently protected di- or polyborylarenes, they may become more convenient and efficient. In fact, differentiated diborylarenes have been used as double nucleophilic linkers to converge various electrophilic building blocks. In 2007, Suginome’s group developed an efficient masking group for boronic acids, i.e., 1,8-diaminonaphthalene (dan). The dan-protected boronyl group was found to be inert in most SMC reaction conditions, therefore they have applied haloaryl-B(dan) in iterative synthesis of polyarenes via sequential boron-selective SMC/deprotection iteration (refer to the chapter of Dr. Watson). To make best use of the inertness of the B(dan) moiety, they also prepared diborylarenes containing both B(dan) and B(pin) via Miyaura borylation of haloaryl-B(dan) compounds. The differently protected diborylarenes then underwent cross-coupling with aryl/ vinyl halides at the B(pin) moiety exclusively, leaving the B(dan) moiety intact (Scheme 21, A) (44). For the synthesis of differentiated diborylarenes, direct C–H borylation of the readily available aryl MIDA boronates would be a more convenient choice, as reported by Li’s group (45). They successfully obtained a broad range of di- and triborylarenes from simple monoboron compounds via a one-step transformation. The newly-incorporated B(pin) moiety could be selectively consumed in the following chemoselective SMC reactions. The thus obtained MIDA boronates could be utilized directly in the following SMC reactions via the slow-release strategy, providing multi-substituted arenes (Scheme 21, B). More recently, Li’s group also realized a boron-selective SMC reaction to differentiate aryl B(dan) and B(MIDA) with the B(dan) moiety remaining intact (Scheme 21, C), based on the different stability of these two groups in aqueous basic conditions (46). 432 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 21. Diborylarenes for Boron-Selective SMC Reactions.

Selective Allylboration Reaction of Di- and Polyboron Reagents The addition of allylic boronyl groups to polar unsaturated bonds, such as carbonyl groups in aldehydes and ketones, is known as allylboration reaction, which is able to deliver new functional groups and construct C–C bonds in a onepot version. Generally, the reaction is considered to occur via a closed chair-like six-membered transition state. The carbonyl groups are activated by the Lewis acidic boron atom. This transition state enables the transformation to occur in a diastereoselective fashion. By virtue of chiral boronyl groups or asymmetric catalysts, enantioenriched homoallylic alcohols may be reliably prepared (47). In the past decades, along with the development of the allylboration reaction, a series of diboron compounds have also been applied in such transformations. Generally, these compounds contain a reactive boronyl group at allyl position and a second temporarily inert boronyl group is connected at a different position. The former type of boronyl groups can be selectively used in the boron-selective allylboration reactions. Exploiting the versatile reactivity of the latter inert boronyl groups, a variety of complex functionalized intermediates become accessible. 433 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Single Selective Allylboration Reaction


This strategy was explored as early as 1995 and disclosed by Brown’s group (48). Optically active [(E)-γ-(boronic ester)allyl]diisopinocampheylborane 15 could be prepared via hydroboration of allenyl boronates with (dIpc)2BH. Then the allyl Ipc-borane unit of such diboron reagent could react preferentially with aldehydes, affording allylboration product 16 with a newly generated allyl boronate moiety. A following stereo-retentive oxidation afforded anti-1,2-diols in high diastereo- and enantioselective fashion (Scheme 22).

Scheme 22. The First Example of Boron-Selective Allylboration.

By virtue of the combination of Pd2(dba)3 and a chiral phosphoramidite ligand, Morken’s group realized a regioselective and enantioselective diboration reaction of allenes with B2(pin)2. The obtained diboration products contained an allyl and alkenyl B(pin) group, respectively. The allyl B(pin) groups were then utilized in allylboration of aldehydes (49) and imines (50), affording vinylboronate intermediates that were readily utilized in the following transformations, such as oxidation and SMC reactions (Scheme 23). The same group also realized the Pt-catalyzed enantioselective 1,4-diboration (51) and 1,2-diboration (52) of 1,3-dienes. In both cases, the obtained allylboronyl groups were transformed selectively in the allylboration step. After oxidation, synthetically useful chiral 2-buten-1,4-diols or 2-buten-1,5-diols were ultimately obtained (Scheme 24).



Scheme 23. Diboration of Allenes and the Following Allylboration Reactions.

Scheme 24. Diboration of 1,3-Dienes and the Following Allylboration Reactions.


Double Allylboration Reactions


In 2002, Roush’s group applied the allylic boronyl group of intermediate 16 which was formed after the initial allylboration step of the Brown strategy into a further allylboration reaction (53). by modification of the reaction temperature, this double allylboration reaction could be realized consecutively in a one-pot version with two different aldehydes, producing (E)-1,5-anti-diols in excellent stereoselectivity (Scheme 25, A). This type of products were formally linked by the carbon skeleton of the diboron compounds. Interestingly, when the allenylboronic ester contained a bulky diol unit, via the hydroboration/double allylboration sequence, (Z)-syn-1,5-diols could be obtained in high yield and high level of enantioselectivity (Scheme 25, B).

Scheme 25. Double Allylboration Sequence. Kinetically controlled hydroboration of monosubstituted allenes with 10-TMS-9-BBD-H (9-BBD = 9-borabicyclo[3.3.2]decane) afforded (Z)-γborylallylboranes, which were then utilized in the following double allylboration reactions, affording (Z)-anti-1,5-diols (Scheme 25, C) (54). In 2009, Soderquist and co-workers successfully prepared a novel double-allylating reagent with two different borane components. Hydroboration of allenyl 10-TMS-9-BBD with 10-Ph-9-BBD-H afforded two regioisomeric trans-1,3-diborylpropenes through a series of 1,3-borotropic shifts. Such mixtures could be utilized directly in the following selective asymmetric allylboration of 436 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


ketones and/or ketimines. It was noted that the 10-Ph-9-BBD moiety was more reactive than its 10-TMS counterparts. The first allylboration was followed by a sterically driven 1,3-borotropic rearrangement, affording new allyl boranes that could not react with ketones (or ketimines) further. The second allylboration was triggered by the addition of an aldehyde (or aldimine). Thus, the sequential addition of ketones (or ketimines) and aldehydes (or aldimines) would generate three stereogenic centers in a one-step procedure (Scheme 26) (55).

Scheme 26. Double Allylboration of Ketones and Aldehydes.

For the synthesis of 2-methyl-1,5-anti-pentenediols, Roush’s group recently disclosed an efficient synthetic route via kinetically controlled hydroboration and double allylboration sequence (56). Double allylboration of the kinetic product, (Z)-17, afforded (Z)-2-methyl-1,5-anti-pentenediols. In the second allylboration step, BF3•OEt2 catalysis was necessary to reach high enantioselectivity. (Z)-17 could isomerize to (E)-17 at 65 °C, which could go through double allylboration reactions to synthesize (E)-2-methyl-1,5-anti-pentenediols. In this case, good yield and high enantioselectivity could be achieved without the assistance of BF3•OEt2 (Scheme 27). 437 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 27. Kinetically Controlled Stereoselective Double Allylboration.

Scheme 28. The Synthesis of N-Acetyl Dihydrotetrafibricin Using Three Double Allylboration Reagents.

In view of the powerful capacity for stereospecific construction of diverse diols structure, the double allylboration also gained application in natural product synthesis. Morken’s group has applied this strategy in the construction of highly substituted and stereochemically complex cyclohexanols via cascade allylborations of diboron compounds with dicarbonyls (57). Recently, Roush and Nuhant completed the diastereoselective synthesis of N-acetyl dihydrotetrafibricin methyl ester, which involved three-fold double 438 Coca; Boron Reagents in Synthesis ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

allylboration processes to construct stereocenters and assemble synthetic fragments convergently (Scheme 28) (58). This work spectacularly exemplifies how chemoselective double allylboration reactions can be used in the modular synthesis of complex molecules.


Conclusion Based on the extensive research about the synthesis and reactivity of organoboron compounds, the boron-selective reactions have gained great advance and productive application in the last decade in particular. This selectivity has encouraged the preparation and utilization of a variety of bifunctional building blocks, such as organic halides containing masked boronyl groups and di- and polyboron compounds. These versatile molecules can provide modular and flexible platform to construct molecular diversity and complexity via consecutive boron-selective transformations. The synthetic value of reactivity-differentiated di- and polyboron compounds has been validated by the convergent assembly of various pre-functionalized fragments. The research towards their preparation and utilization has undergone significant advancement in recent years. However, compared with the diverse transformations of common boronyl groups, reaction types of reported boron-selective transformations are relatively limited to Suzuki–Miyaura coupling or the allylboration reactions although sporadic examples of other types of reactions have been disclosed. For examples, 1,1-diborylalkanes could react with carbonyl groups of ketones to generate tetrasubstituted alkenyl boronates with one boronyl group eliminated selectively (59); the terminal B(pin) moieties of (Z)-1,2-diborylalkenes would participate in Borono−Mannich reactions selectively to generate boronated amino esters (60). It appears that if yet more organoboron-involved reaction types could be realized in boron-selective version then a great number of diverse structures may be readily synthesized from the present building block library. Therefore, further exploration in this area, especially the development of more convenient synthetic methods towards di- and polyboron building blocks and methodologies to discriminate and make best use of the each boronyl group, will consolidate the existing synthetic platform for modular synthesis of complex molecules, thus benefiting the scientific community of organic synthesis, biochemistry, medical research and material science which are usually perplexed by tedious syntheses of their targeted products.

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Di- and Polyboron Compounds: Preparation and Chemoselective

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