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Synlett 2018; 29(11): 1421-1429
DOI: 10.1055/s-0037-1609682
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© Georg Thieme Verlag Stuttgart · New York

Cobalt-Catalyzed Hydroboration and Borylation of Alkenes and Alkynes

Ziqing Zuo
State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, P. R. of China   Email: huangzh@sioc.ac.cn
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Huanan Wen
State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, P. R. of China   Email: huangzh@sioc.ac.cn
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Guixia Liu
State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, P. R. of China   Email: huangzh@sioc.ac.cn
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Zheng Huang*
State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, P. R. of China   Email: huangzh@sioc.ac.cn
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We gratefully acknowledge the financial support from the National Natural Science Foundation of China (21432011, 21572255, 21732006)
Further Information

Publication History

Received: 21 March 2018

Accepted after revision: 21 March 2018

Publication Date:
23 April 2018 (online)

 
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Abstract

Incorporation of the boryl moiety across a carbon–carbon multiple bond is a powerful method for the synthesis of organoboron compounds. This kind of transformation could be realized with high chemo-, regio-, and stereoselectivity by using an appropriate transition-metal catalyst. This account summarizes the latest advances from our group in the area of cobalt-catalyzed hydroboration and borylation of alkenes and alkynes, which lead to the formation of a variety of organoboron compounds, including alkylboronates, 1,1,1-tris(boronates), 1,1-diborylalkenes, and 1,1-diboronates.

1 Introduction

2 Cobalt-Catalyzed Hydroboration of Alkenes

3 Cobalt-Catalyzed Dehydrogenative Borylations-Hydroboration

4 Cobalt-Catalyzed Double Dehydrogenative Borylations of 1-Alkenes

5 Cobalt-Catalyzed Hydroboration of Terminal Alkynes

6 Summary and Outlook


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Key words

alkenes - alkynes - borylation - cobalt - organoboronates

Biographical Sketches

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Ziqing Zuo was born in Hubei Province, China, in 1988. He received his B.Sc. degree from Wuhan University in 2012. In the same year, he joined Prof. Zheng Huang’s group at Shanghai Institute of Organic Chemistry and obtained his Ph.D degree in 2017 with research on cobalt- and iron-catalyzed hydrofunctionalizations of alkenes and alkynes. He then moved to the University of Delaware, and now works as a postdoctoral researcher with Prof. Donald A. Watson on developing novel transformations of nitroalkanes.

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Huanan Wen received his B.Sc. degree from Central China Normal University in 2014. He is currently pursuing his Ph.D. degree at the Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences under the guidance of Professor Zheng Huang. His research interests are in the areas of asymmetric catalysis, ligand design, and organometallic chemistry.

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Guixia Liu received her B.Sc. from NanKai University in 2002. She earned her Ph.D. degree in 2007 under the supervision of Prof. Xiyan Lu at the Shanghai Institute of Organic Chemistry. She carried out her postdoctoral studies with Professor Wei Wang at University of New Mexico (2008–2009) and with Prof. Isaac J. Krauss at Brandies University (2009–2011). She is currently working as an associate professor at the Shanghai Institute of Organic Chemistry. Her current research interests are focused on the development of novel transformations catalyzed by base-metals and polymer degradations.

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Zheng Huang received his B.Sc. and Master degrees from Nankai University under the direction of Prof. Xianhe Bu. In 2009, he earned his Ph.D. from the University of North Carolina at Chapel Hill under the guidance of Prof. Maurice Brookhart. Upon completion of his postdoctoral research with Prof. John F. Hartwig at the University of Illinois, Urbana-Champaign, in 2012, he started his independent career as a professor at the Shanghai Institute of Organic Chemistry. His current research interests include: i) the development of new catalytic methods for conversion of low-value light alkanes into value-added chemicals and polyolefin degradation, ii) development of low-cost, earth-abundant catalysts for new organic transformations, and iii) development of late-transition metal catalysts for synthesis of polyolefins with novel structures and new functionalities.

1

Introduction

Organoboronates have found widespread applications in organic synthesis, material science as well as biochemistry.[1] Especially, transition-metal-catalyzed cross-coupling reactions of organoboron compounds have emerged as one of the most powerful protocols for carbon–carbon and carbon–heteroatom bond formation.[2] One unique advantage of organoboronic acid derivatives over other commonly used organometallic nucleophiles is their stability; indeed, most of them can be isolated, purified, and stored in air. In addition to cross-coupling reactions, they are versatile intermediates in various organic transformations,[3] [4] [5] such as additions to carbonyl compounds, imines and imnium ions,[3] oxidative hydroxylations,[4a] and aminations.[4b]

The classic methods for the synthesis of organoboronates involve the reactions of Grignard or lithium reagents with suitable boron compounds.[6] Although very useful, these methods suffer from poor functional-group tolerance, multistep synthesis, and the formation of waste inorganic by-products. As a consequence, the development of transition-metal catalysis for the synthesis of highly functionalized organoboronic acid derivatives has attracted increasing interest. Three important classes of catalytic approaches have been accomplished: (i) catalytic hydroboration and borylation of C–C multiple bonds; (ii) catalytic borylations of aryl or alkyl halides; and (iii) catalytic C–H bond borylations (Scheme [1]). The catalytic approaches are generally more step- and atom-efficient than the traditional noncatalytic procedures, and usually occur under mild conditions with good functional-group compatibility.

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Scheme 1 Common methods for synthesis of organoboronates

The catalytic hydroboration and borylation of C–C multiple bonds allow for the introduction of the boryl moiety to alkenes or alkynes, which provide efficient approaches to synthetically useful organoboronates with high chemo-, regio-, and stereoselectivity from readily available starting materials. Remarkable progress in this field has been achieved by catalysis based on precious metals.[7] To date, noble catalyst systems such as Rh[8] and Ir[9] complexes remain the most commonly employed catalysts for such transformations. Although the precious-metal catalytic systems have reached good generality and synthetic utility in certain instances, their high cost, low abundance, and environmental issues has inspired extensive investigations on the search for earth-abundant and sustainable base-metal catalysts, such as Fe, Co, Ni,[10] and Cu.[11] More significantly, the distinct electronic structures and unique redox ability of Fe and Co provide new opportunities for catalyst development, which may lead to unusual reactivity and selectivity.[12] In the past decade, a set of highly active well-defined iron catalysts for alkene hydroborations have been discovered.[13] Pioneering work was reported by Ritter, who disclosed iminopyridine iron catalysts for highly regio- and stereoselective 1,4-hydroboration of 2-substituted 1,3-dienes to generate primary (E)-allylboronates.[14] Our group later reported iron-catalyzed 1,4-hydroboration of 1-aryl-substituted 1,3-dienes for selective synthesis of secondary (Z)-allylboronates.[15a] In 2013, our group developed an electron-rich iron complex ligated by a PNN ligand for the first example of hydroboration of α-olefins.[15b] Notably, this iron catalytic system shows superior activity and selectivity to the Rh and Ir catalysts. Chirik[16a] and Thomas[16b] independently reported that bis(imino)pyridine iron complexes are effective for anti-Markovnikov hydroborations of internal alkenes and internal alkynes, respectively. Iron-catalyzed dehydroborylations of vinylarenes has also been disclosed by Ge to afford E-vinyl boronate esters selectively.[17] In addition, several hydroborations of alkynes to alkenylboranes were achieved by catalysis based on well-defined or in-situ generated low-valent iron catalysts.[18] Furthermore, the versatility of iron catalysis could be further expanded to diboration and carboboration of alkynes, as demonstrated by the work of Nakamura et al.[19]

As also being a member of the first-row late transition metals, cobalt is similar to iron in some respects. Some common features include low cost and readily availability. In addition, they generally involve distinct electronic structures and high density of states when employed in catalytic transformations. Recently, iron and cobalt have been shown to exhibit comparable and unique efficiency in a range of catalytic reactions.[20] This account mainly describes our endeavors on the development of cobalt-catalyzed hydroboration and borylation of alkenes and alkynes in the past several years.


# 2

Cobalt-Catalyzed Hydroboration of Alkenes

In 1997, the Zaidlewicz group reported the first cobalt-catalyzed hydroboration of α-olefins and conjugated dienes using bis(phosphine)-ligated cobalt dichloride complexes as the precatalysts and catecholborane (HBcat) as the borane reagent.[21] Although the yield and regioselectivity still needs to be improved, this pioneering work demonstrated the possibility of employing cobalt-based catalysts for hydroboration of alkenes or alkynes.[22] [23] [24] [25] [26] [27] [28] [29] [30] [31]

Our previous work showed that a PNN iron complex is highly efficient for hydroboration of α-olefins with pinacolborane (HBPin).[15] We intuitively expected a similar performance for the analogous PNN cobalt catalysts in such transformations. In 2014, our group reported the preparation of PNN cobalt complexes and their applications in alkene ­hydroboration reactions (Scheme [2]).[22] The cobalt complex (iPrPNN)CoCl2 (1) with a tridentate electron-donating phosphine ligand is remarkably efficient for catalytic anti-Markovnikov hydroboration of α-olefins with HBPin. Most reactions occur smoothly at room temperature with only 0.005–0.05% catalyst loading, giving the anti-Markovnikov products in high isolated yields with excellent regio- and chemoselectivity as well as broad functional group tolerance. In addition to this work, a few cobalt catalysts of tridentate ligands have been developed for selective hydro­boration of terminal alkenes (Scheme [2]).[23]

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Scheme 2 PNN Co-catalyzed alkene hydroboration with pinacolborane

The PNN cobalt-based hydroboration process can be carried out on large scale, giving a turnover number (TON) of 19800, which represents the highest turnover number observed for the known metal-catalyzed alkene hydroborations (Scheme [3]). The synthetic utility of this protocol was further demonstrated by a one-pot, two-step cobalt-catalyzed alkene hydroboration and palladium-catalyzed Suzuki cross-coupling of alkylboronates with aryl chlorides to afford the alkyl-aryl coupling compounds.

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Scheme 3 PNN cobalt-catalyzed styrene hydroboration on large scale

The PNN cobalt catalyst exhibits much higher catalytic activity than the related PNN iron catalyst in alkene hydroborations. In addition, styrene derivatives are relatively challenging for the PNN iron catalytic system because of the formation a dehydrogenative borylation by-product. In contrast, both vinylarenes and aliphatic α-olefins undergo selective hydroboration reactions with high yields by using the PNN cobalt system. Notably, both the PNN Co and Fe systems are limited to hydroboration of monosubstituted 1-alkenes, while internal or gem-disubstituted alkenes[24] are inactive under the reaction conditions.

The internal alkenes are challenging substrates for catalytic hydroboration process, probably due to their steric hindrance. In 2013, the group of Chirik[23a] disclosed that bis(imino)pyridine cobalt methyl complexes could catalyze the tandem isomerization-hydroboration of internal alkenes, leading to boron incorporation at the terminal position of an alkyl chain (Scheme [4a]).[25a] [b] [c] The method even works efficiently for tri- and tetrasubstituted alkenes to generate the primary alkylboronates regioselectively. Interestingly, different regioselectivity could be realized by changing the cobalt catalyst.[25d] With (PPh3)3CoH(N2) as the catalyst, isomerization-hydroboration of alkenes selectively installs the boron substituent at the position adjacent to the π-system, even when the substrate contains a terminal alkene (Scheme [4b]).

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Scheme 4 Cobalt-catalyzed isomerization-hydroboration of alkenes

Transition-metal-catalyzed enantioselective hydroboration reaction provides an efficient approach to optically active boronate compounds, which constitute vital synthetic intermediates in organic synthesis. After our report on the PNN cobalt-catalyzed hydroboration of α-olefins, we wondered whether we could realize the enantioselective process by using a suitable chiral cobalt catalyst. To this end, in 2014 we designed and synthesized a novel iminopyridine-oxazoline (IPO) ligand and the corresponding cobalt methyl complex 2. The (IPO)CoMe complex 2 exhibits high activity and unprecedented level of enantioselectivity in asymmetric hydroboration of 1,1-disubstituted alkenes (Scheme [5]).[26] In fact, 1,1-disubstituted alkenes are regarded as a challenging class of substrates in asymmetric transformations due to the difficulty in discriminating the enantiotopic faces of such substrates.[27] The IPO cobalt-catalyzed asymmetric hydroboration of 1,1-disubstituted aryl alkene with HBpin afforded the chiral boronate esters with enantioselectivity up to 99.5% ee under mild conditions with a low catalyst loading. In addition to diverse 1,1-disubstituted aryl alkenes, this protocol is also active for 1,2-substituted internal olefins. However, there remain some limitations for the reactions of substrates such as 1,1-dialkylethenes and 1,1-diarylethenes, which resulted in low enantioselectivities. This method was successfully applied for the facile synthesis of Naproxen in high yield with excellent enantioselectivity (98% ee).

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Scheme 5 IPO cobalt-catalyzed asymmetric hydroboration of 1,1-disubstituted alkenes
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Scheme 6 OAP cobalt-catalyzed asymmetric hydroboration of α-substituted styrenes

The group of Lu also independently developed the same IPO cobalt system[28] and the related iron catalyst system[29] for asymmetric hydroboration of 1,1-disubstituted aryl alkenes. Recently, Lu developed a chiral oxazoline aminoisopropylpyridine (OAP) ligand that exhibits high activity and enantioselectivity for cobalt-catalyzed hydroboration of sterically hindered styrenes (Scheme [6]).[30]

The control of regioselectivity in hydroboration of alkenes is important. While hydroboration of 1,1-disubstituted aryl alkyl alkenes under IPO Co catalysis gave the anti-Markovnikov products exclusively (Scheme [5]), α-vinylsilanes reacted with HBPin to afford Markovnikov products with excellent regioselectivity using (PDI)Co 3 as the catalyst (Scheme [7]).[31] Experiments indicate that the inversion of the site selectivity arises from the substitution of an alkyl group with a silyl moiety in the alkenes rather than the choice of different Co catalyst. Various α-vinylsilanes could undergo hydroborations smoothly to furnish geminal borosilanes selectively, which, upon further oxidation, will lead to the formation of α-silyl-substituted tertiary alcohols.

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Scheme 7 IPO cobalt-catalyzed asymmetric hydroboration of 1,1-disubstituted alkenes

# 3

Cobalt-Catalyzed Dehydrogenative Borylations-Hydroboration

During our studies on PNN iron-catalyzed hydroboration of styrenes with HBPin, we found that dehydrogenative borylation products represented non-negligible by-products.[15] We envisioned that replacing HBPin with a diboron reagent (B2Pin2) might favor the dehydrogenative borylation process. Screening of various PNN iron and cobalt complexes revealed that PNN cobalt complex 4, bearing phosphino tert-butyl groups, is highly active and selective for triborylation of styrenes (Scheme [8]).[32] Under optimized conditions, a variety of vinylarenes could react with two equivalent of bis(pinacolato)diboron (B2Pin2) to construct 1,1,1-tris(boronates) with excellent selectivity, high yields and good functional group tolerance. Since methods for selective synthesis of 1,1,1-tris(boronates) are quite limited,[33] this cobalt-catalyzed triborylation of styrenes constitutes a worthy alternative in this field. The synthetic utility of 1,1,1-tris(boronates)[34] is demonstrated by NaOMe-promoted deborylative alkylation reactions, giving the internal geminal bis(boronates).

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Scheme 8 PNN cobalt-catalyzed synthesis of 1,1,1-tris(boronates)

Deuterium labeling studies and stoichiometric experiments were conducted to elucidate the mechanism of this cobalt-catalyzed triborylation process. Preliminary data suggest that the cobalt-catalyzed styrene triborylation reaction undergoes a tandem double dehydrogenative borylation–hydroboration pathway (Scheme [9]). The precatalyst cobalt dichloride is activated by NaBHEt3 to form the Co(I) hydride species, which is converted into Co(I) boryl species and HBPin upon transmetalation with B2Pin2. Subsequently, styrene insertion and β-H elimination takes place to yield monoborylalkene, which undergoes another step of de­hydrogenative borylation to form 1,1-diborylakene. Finally, hydroboration of 1,1-diborylakene with HBPin formed in situ gives the desired 1,1,1-tris(boronate) product.

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Scheme 9 Proposed mechanism for the formation of 1,1,1-tris(boronates)

# 4

Cobalt-Catalyzed Double Dehydrogenative Borylations of 1-Alkenes

In our studies on cobalt-catalyzed triborylation of vinylarenes, 1,1-diborylalkene was proposed as the key intermediate generated from double dehydrogenative borylations (DHBs) of alkenes. As an expansion of our work on developing base-metal-catalyzed borylations of C–C multiple bonds, we began to consider the possibility of selective synthesis of 1,1-diborylalkenes from readily available alkenes and B2Pin2.[35] The development of such a transformation is of interest because 1,1-diborylalkenes[34] are versatile building blocks for stereoselective synthesis of polysubstituted alkenes.

The selective double DHBs of terminal vinylarenes with B2Pin2 could be accomplished by the use of a (PNN)-Co catalyst in the presence of one equivalent of CsF in DMF, producing 1,1-diborylalkenes in high yields with wide functional group tolerance (Scheme [10a]). The double DHBs of more challenging alkyl 1-alkenes were also achieved, but with different regioselectivity, furnishing cis-1,2-diborylalkenes selectively (Scheme [10b]). Furthermore, slight modification of the reaction conditions by reducing the amount of CsF from 1 to 0.4 equivalent allowed for selective synthesis of trans-monoborylalkenes (Scheme [10c]).

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Scheme 10 PNN cobalt-catalyzed dehydrogenative borylations of 1-alkenes

While Co-catalyzed triborylation of alkenes relied on hydroboration of 1,1-diborylalkenes with HBPin generated in situ, the hydroboration process should be suppressed for the selective synthesis of 1,1-diborylalkenes. The choice of solvent and additive is critical to this end. With Co as the catalyst, DMF could react with HBPin to produce Me3N, and the addition of CsF has little influence on this reduction (Scheme [11a]). On the other hand, treatment of CsF with B2Pin2 formed a putative anionic fluoride diboron adduct X (Scheme [11b]). The reaction of this diboron adduct with alkene and B2Pin2 under the catalytic conditions produces the desired 1,1-diborylalkenes in high selectivity (Scheme [11c]), implying that the diboron adduct X is likely an active boryl species in the double DHBs process. All these results indicated that DMF can inhibit the undesired hydroboration by consuming HBPin generated in situ, and CsF has an important effect on promoting the DHBs.

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Scheme 11 Mechanistic studies on the role of DMF and CsF

Stepwise cross-couplings of 1,1-diborylalkenes allows for the synthesis of stereodefined triarylalkenes bearing three different substituents (Scheme [12]). The use of aryl ­iodides as the coupling partner under Pd catalysis enables the first aryl group (Ar2) to be installed exclusively at the position trans to the Ar1 group. The resulting trans-1,2-diaryl-1-borylalkenes coupled with aryl bromides to complete the synthesis of triarylalkenes. In addition, the conversion of the simple terminal vinylarenes into triarylalkenes can be realized in a one-pot, three-step procedure, obviating the need for isolation of the intermediates.

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Scheme 12 Stereoselective synthesis of triaryl alkenes from 1,1-diborylalkenes

It should be noted that the synthesis of 1,1-diborylalkenes could also be accomplished via a Co-catalyzed 1,1-diboration of terminal alkynes, as reported by the Chirik group (Scheme [13]).[34] Mechanism studies indicate this transformation is initiated by the formation of a cobalt acetylide intermediate, which reacts with B2Pin2 to yield a cobalt alkynylboronate. After insertion of the alkynylboronate into the Co–B bond, the resulting vinylcobalt could be protonated by terminal alkyne to produce the 1,1-diborylalkene and regenerate the cobalt acetylide species. In addition, a sequential cobalt-catalyzed 1,1-diboration–hydro­boration of terminal alkynes has been developed for the synthesis of 1,1,1-tris(boronates).

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Scheme 13 Cobalt-catalyzed 1,1-diboration of alkynes for the synthesis of 1,1-diborylalkenes

# 5

Cobalt-Catalyzed Hydroboration of Terminal Alkynes

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Scheme 14 IPO cobalt-catalyzed sequential hydroboration of terminal alkynes

After gaining some advances in cobalt-catalyzed alkene hydroborations, we turned our efforts to the exploration of alkyne hydroboration. In fact, our initial idea was to develop a cobalt catalyst for asymmetric synthesis of chiral 1,2-diboryl compounds from sequential hydroboration of alkynes. However, with the chiral IPO cobalt complex 2 as the precatalyst, the hydroboration of 1-hexyne did not produce 1,2-diboronate ester. Instead, 1,1-diboronate ester was obtained in high yield with exclusive regioselectivity (Scheme [14]).[36] Both the alkyl and aryl terminal alkynes underwent the selective dual hydroborations smoothly under mild conditions with a good tolerance of various functional groups. The synthetic utility of this alkyne hydroboration was demonstrated by the sequential coupling of the 1,1-diboryl compound with aryl bromides and aryl iodides to construct diarylmethane derivatives.

Monitoring of the cobalt-catalyzed hydroboration of 1-hexyne in situ revealed the formation of a substantial amount of trans-vinylboronate ester, along with a low yield of the 1,1-diboronate at the early stage. This trans-vinylboronate was gradually converted into 1,1-diboryl compound over the course of the reaction, and no other products were detected during the whole process. These data indicate that the hydroboration of alkynes produces the monohydroboration product trans-vinylboronate as the intermediate, which undergoes subsequent hydroboration to give 1,1-diboronate ester. The regioselectivity observed for the second hydroboration may arise from the site-selective insertion of vinylboronate into the Co–B bond with the Co center located at the less sterically hindered carbon, which is consistent with the regioselectivity observed in the cobalt-catalyzed hydroboration of 1,1-diborylalkenes.[32] [34]

Monohydroboration of terminal alkynes generally gives vinylboronate esters with (E)-stereoselectivity as a result of syn-addition of the B-H group to the C–C triple bonds with anti-Markovnikov selectivity.[8a] [37] As an exception, the Chirik group recently demonstrated that a cobalt methyl complex (CyAPDI)CoCH3 (5) ligated by bis(imino)pyridine ligand afforded high (Z)-stereoselectivity for catalytic hydroboration of terminal alkynes (Scheme [15]).[38] Isotopic labeling and stoichiometric studies established that the mechanism of this cobalt-catalyzed (Z)-selective hydroboration involves regioselective syn-hydrocobaltation of an alkynylboronate intermediates, which accounts for the unusual stereoselectivity.

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Scheme 15 Hydroboration of terminal alkynes with (Z)-selectivity using (CyAPDI)CoCH3 (5) as the catalyst

# 6

Summary and Outlook

The cobalt-catalyzed hydroboration and borylation of alkenes and alkynes have been extensively studied over the past five years. Well-defined cobalt catalysts of tridentate ligands developed by this group and other groups have showed high activity and excellent regio- and/or stereoselectivity in a variety of hydroboration and borylation reactions, which constitute worthy complements to noble-metal-catalyzed processes.

One notable limitation of the known cobalt catalysts is the lack of catalytic activity for hydroboration of multisubstituted internal alkenes. When using acyclic internal alkenes as the substrates, the existing cobalt catalysts in general effect olefin isomerization to form the more reactive terminal olefin, which undergoes the hydroboration reaction. Thus, the regio- and even enantioselective addition of a boryl group to multisubstituted internal C–C double bonds without isomerization is a reaction worthy of being pursued.


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Scheme 1 Common methods for synthesis of organoboronates
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Scheme 2 PNN Co-catalyzed alkene hydroboration with pinacolborane
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Scheme 3 PNN cobalt-catalyzed styrene hydroboration on large scale
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Scheme 4 Cobalt-catalyzed isomerization-hydroboration of alkenes
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Scheme 5 IPO cobalt-catalyzed asymmetric hydroboration of 1,1-disubstituted alkenes
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Scheme 6 OAP cobalt-catalyzed asymmetric hydroboration of α-substituted styrenes
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Scheme 7 IPO cobalt-catalyzed asymmetric hydroboration of 1,1-disubstituted alkenes
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Scheme 8 PNN cobalt-catalyzed synthesis of 1,1,1-tris(boronates)
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Scheme 9 Proposed mechanism for the formation of 1,1,1-tris(boronates)
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Scheme 10 PNN cobalt-catalyzed dehydrogenative borylations of 1-alkenes
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Scheme 11 Mechanistic studies on the role of DMF and CsF
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Scheme 12 Stereoselective synthesis of triaryl alkenes from 1,1-diborylalkenes
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Scheme 13 Cobalt-catalyzed 1,1-diboration of alkynes for the synthesis of 1,1-diborylalkenes
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Scheme 14 IPO cobalt-catalyzed sequential hydroboration of terminal alkynes
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Scheme 15 Hydroboration of terminal alkynes with (Z)-selectivity using (CyAPDI)CoCH3 (5) as the catalyst