Synthesis of Enantioenriched Tertiary Boronic Esters by the Lithiation/Borylation of Secondary Alkyl Benzoates

A 1,3-boron shift reaction of homoallenylboronates to synthesise 2-boryl-1,3-dienes

We describe a 1,3-boron shift-type reaction of homoallenylboronates at the center (sp) carbon in allenes to afford 2-boryl-1,3-dienes with a variety of substituents. Notably, this reaction occurs in situ with allenylboronates in the presence of carbamate and a small excess of sec-BuLi, and it is not necessary to isolate the unstable homoallenylboronates.

Synthesis of 3-borylated cyclobutanols from epihalohydrins or epoxy alcohol derivatives

There is an increasing interest in cyclobutanes within the medicinal chemistry community. Therefore, methods to prepare cyclobutanes that contain synthetic handles for further elaboration are of interest. Herein, we report a new approach for the synthesis of 3-borylated cyclobutanols via a formal [3 + 1]-cycloaddition using readily accessible 1,1-diborylalkanes and epihalohydrins or epoxy alcohol derivatives. 1-Substituted epibromohydrin starting materials provide access to borylated cyclobutanols containing substituents at three of the four positions on the cyclobutane core, and enantioenriched epibromohydrins lead to enantioenriched cyclobutanols with high levels of enantiospecificity (>98%). Finally, derivatization studies demonstrate the synthetic utility of both the OH and Bpin handles.

Homoallylboration of Aldehydes: Stereoselective Synthesis of Allylic-Substituted Alkenes and E-Alkenes

Cyclopropanated allylboration reagents participate in the homoallylation of aliphatic and aromatic aldehydes, generating substituted alkenes that are difficult to produce via other methods. In this study, we explored the scope and reactivity of homoallylation with cyclopropylcarbinylboronates bearing various aliphatic and aromatic α- and γ-substituents. α-Alkyl substituted boronates afforded E-disubstituted alkenyl secondary alcohols in high enantiomeric ratios, while aryl substituents promoted rearrangement. γ-Alkyl substituents all resulted in diastereoselective homoallylation, while aryl substitution changed the outcome to cyclopropylcarbinylation.

Stereodivergent Synthesis of 1,3-Dienes via Protodeboronation of Homoallenylboronic Esters

Vinylboronic esters and allylboronic esters are well known to afford olefins by protodeboronation, and therefore homoallenylboronic esters should be similarly available as precursors for 1,3-dienes, but this strategy has not been well explored due to the limited availability of homoallenylboronic esters. Here, we describe a versatile synthesis of homoallenylboronic esters via lithiation-borylation and subsequent 1,2-rearrangement. The resulting homoallenylboronic esters were successfully converted into Z- and E-1,3-dienes by protodeboronation using Bu₄NF and B(C₆F₅)₃/PhOH, respectively.

Transition Metal Catalyst Free Synthesis of Olefins from Organoboron Derivatives**

Stereoselective preparation of highly substituted olefins is still a severe challenge that requires well defined elimination precursors. Organoboron chemistry is particularly suited for the preparation of molecules with adjacent stereocenters. As organo boron substrates with leaving groups in β‐position can undergo stereospecific syn‐ or anti‐elimination, this chemistry harbors great potential for the synthesis of complex olefins. In recent years three main strategies emerged, which differ in their approach to the β‐functionalized organoboron elimination precursor. (i) Stereoselective preparation of such elimination precursor can be achieved by addition of a boron‐stabilized anion (d¹) to an aldehyde or ketone (a¹) or diastereoselective 1,3‐rearrangement of suitable boron‐ate‐complexes. Stereospecific methods rely either on (ii) diastereospecific 1,2‐metalate rearrangement of boron‐ate‐complexes that involve opening of appropriate heterocycles or (iii) addition of chiral carbenoids (d¹*) to chiral boronates (a¹*) with a leaving group in α‐position.

Saturated Boronic Acids, Boronates, and Trifluoroborates: An Update on Their Synthetic and Medicinal Chemistry

This review discusses recent advances in the chemistry of saturated boronic acids, boronates, and trifluoroborates. Applications of the title compounds in the design of boron-containing drugs are surveyed, with special emphasis on α-amino boronic derivatives. A general overview of saturated boronic compounds as modern tools to construct C(sp³ )-C and C(sp³ )-heteroatom bonds is given, including recent developments in the Suzuki-Miyaura and Chan-Lam cross-couplings, single-electron-transfer processes including metallo- and organocatalytic photoredox reactions, and transformations of boron "ate" complexes. Finally, an attempt to summarize the current state of the art in the synthesis of saturated boronic acids, boronates, and trifluoroborates is made, with a brief mention of the "classical" methods (transmetallation of organolithium/magnesium reagents with boron species, anti-Markovnikov hydroboration of alkenes, and the modification of alkenyl boron compounds) and a special focus on recent methodologies (boronation of alkyl (pseudo)halides, derivatives of carboxylic acids, alcohols, and primary amines, boronative C-H activation, novel approaches to alkene hydroboration, and 1,2-metallate-type rearrangements).

Enantioselective catalytic 1,2-boronate rearrangements

[Figure: see text].

Total Synthesis and Bioactivity Mapping of Geodiamolide H

The first total synthesis of the actin-stabilizing marine natural product geodiamolide H was achieved. Solid-phase based peptide assembly paired with scalable stereoselective syntheses of polyketide building blocks and an optimized esterification set the stage for investigating the key ring-closing metathesis. Geodiamolide H and synthetic analogues were characterized for their toxicity and for antiproliferative effects in cellulo, by characterising actin polymerization induction in vitro, and by docking on the F-actin target and property computation in silico, for a better understanding of structure-activity relationships (SAR). A non-natural analogue of geodiamolide H was discovered to be most potent in the series, suggesting significant potential for tool compound design.

Enantioselective synthesis of tertiary boronic esters through catalytic asymmetric reversed hydroboration

Chiral tertiary boronic esters are important precursors to bioactive compounds and versatile synthetic intermediates to molecules containing quaternary stereocenters. The development of conjugate boryl addition to α,β-unsaturated amide has been hampered by the intrinsic low electrophilicity of the amide group. Here we show the catalytic asymmetric synthesis of enantioenriched tertiary boronic esters through hydroboration of β,β-disubstituted α,β-unsaturated amides. The Rh-catalyzed hydroboration occurs with previously unattainable selectivity to provide tertiary boronic esters in high enantioselectivity. This strategy opens a door for the hydroboration of inert Michael acceptors with high stereocontrol and may provide future applications in the synthesis of biologically active molecules.

Construction of Azacycles by Intramolecular Amination of Organoboronates and Organobis(boronates)

Intramolecular amination of organoboronates occurs with a 1,2-metalate shift of an aminoboron "ate" complex to form azetidines, pyrrolidines, and piperidines. Bis(boronates) undergo site-selective amination to form boronate-containing azacycles. Enantiomerically enriched azacycles are formed with high stereospecificity.

Copper(I)-Catalyzed Asymmetric Conjugate 1,6-, 1,8-, and 1,10-Borylation

Catalytic asymmetric remote conjugate borylation is challenging as the control of regioselectivity is not trivial, the electrophilicity of remote sites is extenuated, and the remote asymmetric induction away from the carbonyl group is difficult. Herein, catalytic asymmetric conjugate 1,6-, 1,8- and 1,10-borylation was developed with excellent regioselectivity, which delivered α-chiral boronates in moderate to high yields with high enantioselectivity. The produced chiral boronate smoothly underwent oxidation, cross-coupling, and one-carbon homologation to give synthetically versatile chiral compounds in moderate yields with excellent stereoretention. Furthermore, a stereomechanistic analysis was conducted using DFT calculations, which provides insights into the origins of the regioselectivity. Finally, the present 1,6-borylation was successfully applied in an efficient one-pot asymmetric synthesis of (-)-7,8-dihydrokavain.

Studies on the Lithiation, Borylation, and 1,2-Metalate Rearrangement of O-Cycloalkyl 2,4,6-Triisopropylbenzoates

A broad range of acyclic primary and secondary 2,4,6‐triisopropylbenzoate (TIB) esters have been used in lithiation‐borylation reactions, but cyclic TIB esters have not. We have studied the use of cyclic TIB esters in lithiation‐borylation reactions and looked at the effect of ring size (3‐ → 6‐membered rings) on the three key steps of the lithiation‐borylation protocol: deprotonation, borylation and 1,2‐metalate rearrangement. Although all rings sizes could be deprotonated, the cyclohexyl case was impractically slow, and the cyclopentyl example underwent α‐elimination faster than deprotonation at −78 °C and so could not be used. Both cyclobutyl and cyclopropyl cases underwent rapid borylation, but only the cyclobutyl substrate underwent 1,2‐metalate rearrangement. Thus, the cyclobutyl TIB ester occupies a “Goldilocks zone,” being small enough for deprotonation and large enough to enable 1,2‐migration. The generality of the reaction was explored with a broad range of boronic esters.

Stereospecific 1,2-Migrations of Boronate Complexes Induced by Electrophiles

The stereospecific 1,2-migration of boronate complexes is one of the most representative reactions in boron chemistry. This process has been used extensively to develop powerful methods for asymmetric synthesis, with applications spanning from pharmaceuticals to natural products. Typically, 1,2-migration of boronate complexes is driven by displacement of an α-leaving group, oxidation of an α-boryl radical, or electrophilic activation of an alkenyl boronate complex. The aim of this article is to summarize the recent advances in the rapidly expanding field of electrophile-induced stereospecific 1,2-migration of groups from boron to sp2 and sp3 carbon centers. It will be shown that three different conceptual approaches can be utilized to enable the 1,2-migration of boronate complexes: stereospecific Zweifel-type reactions, catalytic conjunctive coupling reactions, and transition metal-free sp2 -sp3 couplings. A discussion of the reaction scope, mechanistic insights, and synthetic applications of the work described is also presented.

Ring-Opening Lithiation-Borylation of 2-Trifluoromethyl Oxirane: A Route to Versatile Tertiary Trifluoromethyl Boronic Esters

Stereogenic trifluoromethyl-substituted carbon centers are highly sought-after moieties in pharmaceutical and agrochemical discovery. Here, we show that lithiation-borylation reactions of 2-trifluoromethyl oxirane give densely functionalized and highly versatile trifluoromethyl-substituted α-tertiary boronic esters. The intermediate boronate complexes undergo the desired 1,2-rearrangement of the carbon-based group with complete retentive stereospecificity, a process that was only observed in non-polar solvents in the presence of TESOTf. Although the trifluoromethyl group adversely affects subsequent transformations of the α-boryl group, Zweifel olefinations provide trifluoromethyl-bearing quaternary stereocenters substituted with alkenes, alkynes and ketones.

How to make C–N bonds using boronic acids and their derivatives without transition metals

No need for transition-metal catalysis in amination, amidation, nitration or nitrosation reactions with boron derivatives as reagents.

Transition metal-free synthesis of alkyl pinacol boronates

This review systematically outlined the research in the area of transition metal free synthesis of alkyl pinacol boronates, which are versatile and important scaffolds to construct diverse organic compounds.

Redox-neutral organocatalytic Mitsunobu reactions

Nucleophilic substitution reactions of alcohols are among the most fundamental and strategically important transformations in organic chemistry. For over half a century, these reactions have been achieved by using stoichiometric, and often hazardous, reagents to activate the otherwise unreactive alcohols. Here, we demonstrate that a specially designed phosphine oxide promotes nucleophilic substitution reactions of primary and secondary alcohols in a redox-neutral catalysis manifold that produces water as the sole by-product. The scope of the catalytic coupling process encompasses a range of acidic pronucleophiles that allow stereospecific construction of carbon-oxygen and carbon-nitrogen bonds.

Hydrogen Atom Transfer Induced Boron Retaining Coupling of Organoboronic Esters and Organolithium Reagents

α-Functionalization of alkyl boronic esters and homologation of aryl boronic esters by regioselective radical C(sp³)–H activation in boron-ate complexes is reported. Reaction of commercial or readily accessed aryl boronic acid pinacol esters with alkyl lithium reagents provides boron-ate complexes. Selective α-C–H abstraction by in situ generated trifluoromethyl radicals leads to radical anions that undergo electron transfer oxidation followed by 1,2-aryl/alkyl migration from boron to carbon to give the α-arylated/alkylated alkyl boronic esters. The valuable boronic ester functionality remains in the products and the cheap trifluoromethyl iodide acts as the oxidant in these C–C couplings. The 1,2-alkyl migration from boron to carbon is highly stereospecific allowing access to stereoisomerically pure boronic esters.

Investigation of the Deprotonative Generation and Borylation of Diamine-Ligated α-Lithiated Carbamates and Benzoates by in Situ IR spectroscopy

Diamine-mediated α-deprotonation of O-alkyl carbamates or benzoates with alkyllithium reagents, trapping of the carbanion with organoboron compounds, and 1,2-metalate rearrangement of the resulting boronate complex are the primary steps by which organoboron compounds can be stereoselectively homologated. Although the final step can be easily monitored by ¹¹B NMR spectroscopy, the first two steps, which are typically carried out at cryogenic temperatures, are less well understood owing to the requirement for specialized analytical techniques. Investigation of these steps by in situ IR spectroscopy has provided invaluable data for optimizing the homologation reactions of organoboron compounds. Although the deprotonation of benzoates in noncoordinating solvents is faster than that in ethereal solvents, the deprotonation of carbamates shows the opposite trend, a difference that has its origin in the propensity of carbamates to form inactive parasitic complexes with the diamine-ligated alkyllithium reagent. Borylation of bulky diamine-ligated lithiated species in toluene is extremely slow, owing to the requirement for initial complexation of the oxygen atoms of the diol ligand on boron with the lithium ion prior to boron-lithium exchange. However, ethereal solvent, or very small amounts of THF, facilitate precomplexation through initial displacement of the bulky diamines coordinated to the lithium ion. Comparison of the carbonyl stretching frequencies of boronates derived from pinacol boronic esters with those derived from trialkylboranes suggests that the displaced lithium ion is residing on the pinacol oxygen atoms and the benzoate/carbamate carbonyl group, respectively, explaining, at least in part, the faster 1,2-metalate rearrangements of boronates derived from the trialkylboranes.

Total Synthesis of Aplysiasecosterol A

Aplysiasecosterol A (1) is a structurally unusual 9,11-secosteroid isolated from the sea hare Aplysia kurodai. We have accomplished the first and asymmetric total synthesis of 1 in a convergent fashion. The left-hand segment bearing three adjacent stereocenters was constructed through desymmetrizing reduction, ketalization, and radical cyclization. A strategy of asymmetric 2-bromoallylation followed by spontaneous desymmetrizing lactolization enabled a more expeditious access to this segment. The right-hand segment was prepared through two different approaches: one featuring Myers alkylation and Suzuki-Miyaura coupling and the other relying upon Aggarwal lithiation-borylation and Zweifel-Evans olefination. The two fragments were coupled by a Reformatsky type reaction. The three consecutive stereocenters embedded in the central domain of 1 were generated by an iron-mediated, hydrogen atom transfer based radical cyclization reaction.

A Modular Approach to the Synthesis of gem-Disubstituted Cyclopropanes

A diastereoselective, Pd-catalyzed Suzuki-Miyaura coupling reaction of geminal bis(boryl)cyclopropanes has been developed. The reaction offers a highly modular approach to the synthesis of tertiary cyclopropylboronic esters. The resulting boronic esters may be further functionalized to afford a range of gem-disubstituted cyclopropanes, which represent an important structural motif in the pharmaceutical industry. Sequential Suzuki-Miyaura cross-coupling reactions of gem-bis(boryl)cyclopropanes are also reported. The coupling protocols are compatible with a broad range of functionalized aryl and heteroaryl bromides.

Transition-Metal-Free Suzuki-Type Cross-Coupling Reaction of Benzyl Halides and Boronic Acids via 1,2-Metalate Shift

Cross-coupling of organoboron compounds with electrophiles (Suzuki-Miyaura reaction) has greatly advanced C-C bond formation and has been well received in medicinal chemistry. During the past 50 years, transition metals have played a central role throughout the catalytic cycle of this important transformation. In this process, chemoselectivity among multiple carbon-halogen bonds is a common challenge. In particular, selective oxidative addition of transition metals to alkyl halides rather than aryl halides is difficult due to unfavorable transition states and bond strengths. We describe a new approach that uses a single organic sulfide catalyst to activate both C(sp³) halides and arylboronic acids via a zwitterionic boron "ate" intermediate. This "ate" species undergoes a 1,2-metalate shift to afford Suzuki coupling products using benzyl chlorides and arylboronic acids. Various diaryl methane analogues can be prepared, including those with complex and biologically active motifs. The reactions proceed under transition-metal-free conditions, and C(sp²) halides, including aryl bromides and iodides, are unaffected. The orthogonal chemoselectivity is demonstrated in the streamlined synthesis of highly functionalized diaryl methane scaffolds using multi-halogenated substrates. Preliminary mechanistic experiments suggest both the sulfonium salt and the sulfur ylide are involved in the reaction, with the formation of sulfonium salt being the slowest step in the overall catalytic cycle.

Formation of Olefins by Eliminative Dimerization and Eliminative Cross-Coupling of Carbenoids: A Stereochemical Exercise

Two carbenoids combine to generate an olefin by a mechanism involving formation of an ate complex, 1,2-metalate rearrangement, and β-elimination. As each stage of this eliminative coupling is stereospecific, the overall stereochemical outcome can be understood and, in principle fully controlled, providing that the absolute stereochemical configurations of the reacting carbenoid species are defined. In contrast to traditional alkene syntheses, the eliminative cross-coupling of carbenoids offers a connective approach to olefins capable of precisely targeting a given isomer regardless of the nature of the features distinguishing the isomers. The formation of olefins by the eliminative dimerization and eliminative cross-coupling of carbenoids is reviewed with a range of illustrative examples, including the reactions of α-lithiated haloalkanes, epoxides, and carbamates. An emphasis is placed on stereochemical analysis and methods to generate sp³ -hybridized carbenoids in stereodefined form are surveyed.

Asymmetric Synthesis of Tertiary Alcohols and Thiols via Nonstabilized Tertiary α-Oxy- and α-Thio-Substituted Organolithium Species

Nonstabilized α-O-substituted tertiary organolithium species are difficult to generate, and the α-S-substituted analogues are configurationally unstable. We now report that they can both be generated easily and trapped with a range of electrophiles with high enantioselectivity, providing ready access to a range of enantioenriched tertiary alcohols and thiols. The configurational stability of the α-S-organolithium species was enhanced by using a less coordinating solvent and short reaction times.

α-Sulfinyl Benzoates as Precursors to Li and Mg Carbenoids for the Stereoselective Iterative Homologation of Boronic Esters

The stereoselective reagent-controlled homologation of boronic esters is one of a small number of iteratable synthetic transformations that if automated could form the basis of a veritable molecule-making machine. Recently, α-stannyl triisopropylbenzoates and α-sulfinyl chlorides have emerged as useful building blocks for the iterative homologation of boronic esters. However, α-stannyl benzoates need to be prepared using stoichiometric amounts of the (+)- or (-)-enantiomer of the scarcely available and expensive diamine sparteine; also, these building blocks, together with the byproducts that are generated during homologation, are perceived as being toxic. On the other hand, α-sulfinyl chlorides are difficult to prepare with high levels of enantiopurity and are prone to undergo deleterious acid-base side-reactions under the reaction conditions for homologation, leading to low stereospecificity. Here, we show that the use of a hybrid of these two building blocks, namely, α-sulfinyl triisopropylbenzoates, largely overcomes the above drawbacks. Through either the sulfinylation of α-magnesiated benzoates with either enantiomer of Andersen's readily available menthol-derived sulfinate or the α-alkylation of enantiopure S-chiral α-sulfinyl benzoates, we have prepared a range of highly enantiopure mono- and disubstituted α-sulfinyl benzoates, some bearing sensitive functional groups. Barbier-type reaction conditions have been developed that allow these building blocks to be converted into lithium (t-BuLi) and magnesium (i-PrMgCl·LiCl) carbenoids in the presence of boronic esters, thus allowing efficient and highly stereospecific homologation. The use of magnesium carbenoids allows carbon chains to be grown with the incorporation of sensitive functional groups, such as alkyl/aryl halides, azides, and esters. The use of lithium carbenoids, which are less sensitive to steric hindrance, allows sterically encumbered carbon-carbon bonds to be forged. We have also shown that these building blocks can be used consecutively in three- and four-step iterative homologation processes, without intervening column chromatography, to give contiguously substituted carbon chains with very high levels of enantio- and diastereoselectivity.

Alkynyl Moiety for Triggering 1,2-Metallate Shifts: Enantiospecific sp2 -sp3 Coupling of Boronic Esters with p-Arylacetylenes

The enantiospecific coupling of secondary and tertiary boronic esters to aromatics has been investigated. Using p‐lithiated phenylacetylenes and a range of boronic esters coupling has been achieved by the addition of N‐bromosuccinimide (NBS). The alkyne functionality of the intermediate boronate complex reacts with NBS triggering the 1,2‐migration of the group on boron to carbon giving a dearomatized bromoallene intermediate. At this point elimination and rearomatization occurs with neopentyl boronic esters, giving the coupled products. However, using pinacol boronic esters, the boron moiety migrates to the adjacent carbon resulting in formation of ortho boron‐incorporated coupled products. The synthetic utility of the boron incorporated product has been demonstrated by orthogonal transformation of both the alkyne and boronic ester functionalities.

Enantioselective palladium-catalyzed diboration of 1,1-disubstituted allenes

A practical and enantioselective palladium-catalyzed diboration of 1,1-disubstituted allenes is developed by employing a P-chiral monophosphorus ligand, BI-DIME. A series of diboronic esters containing a chiral tertiary boronic ester moiety are formed in excellent yields and ee's with the palladium loading as low as 0.2 mol%. DFT calculations revealed a concerted mechanism of oxidative addition of bis(pinacolato)diboron and allene insertion, as well as a critical dispersion effect on the origins of the enantioselectivity. The method is successfully applied to the concise and enantioselective synthesis of brassinazole.

Synthesis of Functionalized Alkenes by a Transition-Metal-Free Zweifel Coupling

The Zweifel reaction is a powerful method for the synthesis of alkenes, serving as a transition-metal-free alternative to the Suzuki-Miyaura reaction. To date, the scope of the Zweifel coupling has been rather narrow and has focused mainly on the coupling of vinyllithium reagents to synthesize simple aryl- and alkyl-substituted olefins. Herein, the development of a general transition-metal-free coupling process enabling the coupling of Grignard reagents or organolithiums is described. This method enables the enantiospecific synthesis of a wide variety of functionalized acyclic and cyclic olefin products.

Synthesis of Chiral Tertiary Boronic Esters: Phosphonate-Directed Catalytic Asymmetric Hydroboration of Trisubstituted Alkenes

Highly enantioselective rhodium-catalyzed hydroboration of allylic phosphonates by pinacolborane affords chiral tertiary boronic esters. The β-borylated phosphonates are readily converted to chiral β- and γ-hydroxyphosphonates and aminophosphonates and to phosphonates bearing a quaternary carbon stereocenter. The utility of the latter is illustrated by the synthesis of (S)-(+)-bakuchiol methyl ether.

Selective uni- and bidirectional homologation of diborylmethane

Diborylmethane can be homologated uni- and bidirectionally by using enantiomerically pure lithium-stabilized carbenoids to give 1,2- and 1,3-bis(boronic esters), respectively, in good yield and with excellent levels of enantio- and diastereoselectivity. The high sensitivity of the transformation to steric hindrance enables the exclusive operation of either manifold, effected through the judicious choice of the type of carbenoid, which can be a sparteine-ligated or a diamine-free lithiated benzoate/carbamate. The scope of the 1,2-bis(boronic esters) so generated is complementary to that encompassed by the asymmetric diboration of alkenes, in that primary-secondary and primary-tertiary 1,2-bis(boronic esters) can be prepared with equally high levels of selectivity and that functional groups, such as terminal alkynes and alkenes, are tolerated. Methods for forming C2-symmetric and non-symmetrical anti and syn 1,3-bis(boronic esters) are also described and represent a powerful route towards 1,3-functionalized synthetic intermediates.

Enantiospecific Trifluoromethyl-Radical-Induced Three-Component Coupling of Boronic Esters with Furans

In the presence of trifluoromethylsulfonium reagents, boronate complexes derived from 2-lithio furan and non-racemic secondary and tertiary alkyl or aryl boronic esters undergo deborylative three-component coupling to give the corresponding 2,5-disubstituted furans with excellent levels of enantiospecificity. The process proceeds via the reaction of boronate complexes with a trifluoromethyl radical, which triggers 1,2-metallate rearrangement upon single-electron oxidation. Alternative electrophiles can also be used in place of trifluoromethylsulfonium reagents to effect similar three-component coupling reactions.

Stereospecific functionalizations and transformations of secondary and tertiary boronic esters

This feature article discusses the range of stereospecific transformations available to enantioenriched boronic esters, and their applications in synthesis.

Development of Enantiospecific Coupling of Secondary and Tertiary Boronic Esters with Aromatic Compounds

The stereospecific cross-coupling of secondary boronic esters with sp² electrophiles (Suzuki–Miyaura reaction) is a long-standing problem in synthesis, but progress has been achieved in specific cases using palladium catalysis. However, related couplings with tertiary boronic esters are not currently achievable. To address this general problem, we have focused on an alternative method exploiting the reactivity of a boronate complex formed between an aryl lithium and a boronic ester. We reasoned that subsequent addition of an oxidant or an electrophile would remove an electron from the aromatic ring or react in a Friedel–Crafts-type manner, respectively, generating a cationic species, which would trigger 1,2-migration of the boron substituent, creating the new C–C bond. Elimination (preceded by further oxidation in the former case) would result in rearomatization giving the coupled product stereospecifically. Initial work was examined with 2-furyllithium. Although the oxidants tested were unsuccessful, electrophiles, particularly NBS, enabled the coupling reaction to occur in good yield with a broad range of secondary and tertiary boronic esters, bearing different steric demands and functional groups (esters, azides, nitriles, alcohols, and ethers). The reaction also worked well with other electron-rich heteroaromatics and 6-membered ring aromatics provided they had donor groups in the meta position. Conditions were also found under which the B(pin)- moiety could be retained in the product, ortho to the boron substituent. This protocol, which created a new C(sp²)–C(sp³) and an adjacent C–B bond, was again applicable to a range of secondary and tertiary boronic esters. In all cases, the coupling reaction occurred with complete stereospecificity. Computational studies verified the competing processes involved and were in close agreement with the experimental observations.

Modular, Catalytic Enantioselective Construction of Quaternary Carbon Stereocenters by Sequential Cross-Coupling Reactions

The catalytic Suzuki-Miyaura cross-coupling with chiral γ,γ-disubstituted allylboronates in the presence of RuPhos ligand occurs with high regioselectivity and enantiospecificity, furnishing nonracemic compounds with quaternary centers. Mechanistic experiments suggest that the reaction occurs by transmetalation with allyl migration, followed by rapid reductive elimination.

Full chirality transfer in the synthesis of hindered tertiary boronic esters under in situ lithiation-borylation conditions

Hindered tertiary neopentyl glycol boronic esters can be prepared by using in situ lithiation-borylation of enantiopure secondary benzylic carbamates at -20 °C with full chirality transfer.