Integrating Allyl Electrophiles into Nickel‐Catalyzed Conjunctive Cross‐Coupling

Allylation and conjunctive cross‐coupling represent two useful, yet largely distinct, reactivity paradigms in catalysis. The union of these two processes would offer exciting possibilities in organic synthesis but remains largely unknown. Herein, we report the use of allyl electrophiles in nickel‐catalyzed conjunctive cross‐coupling with a non‐conjugated alkene and dimethylzinc. The transformation is enabled by weakly coordinating, monodentate aza‐heterocycle directing groups that are useful building blocks in synthesis, including saccharin, pyridones, pyrazoles, and triazoles. The reaction occurs under mild conditions and is compatible with a wide range of allyl electrophiles. High chemoselectivity through substrate directivity is demonstrated by the facile reactivity of the β‐γ alkene of the starting material, whereas the ?‐ζ alkene of the product is preserved. The generality of this approach is further illustrated through the development of an analogous method with alkyne substrates. Mechanistic studies reveal the importance of the dissociation of the weakly coordinating directing group to allow the allyl moiety to bind and facilitate C(sp³)?C(sp³) reductive elimination.     

Palladium‐Catalyzed [2+2+1] Spiroannulation via Alkyne‐Directed Remote C−H Arylation and Subsequent Arene Dearomatization

Palladium‐catalyzed alkene‐directed cross‐coupling of aryl iodide with another aryl halide through C?H arylation opens a unique avenue for unsymmetrical biaryl‐derived molecules. However, homo‐coupling of aryl iodides often erodes the overall synthetic efficiency. Reported herein is a highly chemoselective Pd⁰‐catalyzed alkyne‐directed cross‐coupling of aryl iodides with bromophenols, which was subsequently followed by phenol dearomatization to furnish a very attractive [2+2+1] spiroannulation. Notably, possible homo‐coupling of aryl iodides was not observed at all. Mechanistic studies indicated that a five‐membered aryl/vinyl palladacycle most likely accounts for promoting the key step of biaryl cross‐coupling.     

Visible Light‐Induced Room‐Temperature Heck Reaction of Functionalized Alkyl Halides with Vinyl Arenes/Heteroarenes

The first visible light‐induced Pd‐catalyzed Heck reaction of α‐heteroatom substituted alkyl iodides and ‐bromides with vinyl arenes/heteroarenes has been developed. This transformation efficiently proceeds at room temperature and enables synthesis of valuable functionalized allylic systems, such as allylic silanes, boronates, germanes, stannanes, pivalates, phosphonates, phthalimides, and tosylates from the corresponding α‐substituted methyl iodides. Notably, synthesis of the latter substrates failed under existing thermally induced Pd‐catalyzed conditions, which highlights the importance of visible light for this transformation.     

Solid‐Phase Organic Synthesis of 1,4‐N‐Vinyl‐ and 1,4‐N‐Allyl‐Triazoles with the Sulfonate Linker

Polymer‐supported 2‐azidoethyl sulfonate and 3‐azidopropyl sulfonate reagents have been developed and applied to the solid‐phase organic synthesis of 1‐vinyl‐ and 1‐allyl‐1,2,3‐triazoles, respectively, by CuI‐mediated azide‐alkyne cycloadditions and subsequent cleavage from the polymer support through elimination reaction promoted by DBU. The advantages of this new synthetic method include simple operation and high yield of the products, as well as good stability of the reagents.     

Boron-directed regio- and stereoselective enyne cross metathesis: efficient synthesis of vinyl boronate containing 1,3-dienes

[reaction: see text] Regio- and stereoselective enyne cross metathesis reactions between borylated alkynes and terminal alkenes were realized to provide a variety of functionalized vinyl boronates. High chemical yield and regioselectivity was achieved irrespective of substituents on the alkyne and alkene counterparts, whereas Z/E-selectivity was found to be dependent upon the substituents both on the alkyne and alkene.     

Regio‐ and Enantioselective Synthesis of Trifluoromethyl‐Substituted Homoallylic α‐Tertiary NH2‐Amines by Reactions Facilitated by a Threonine‐Based Boron‐Containing Catalyst

A method for catalytic regio‐ and enantioselective synthesis of trifluoromethyl‐substituted and aryl‐, heteroaryl‐, alkenyl‐, and alkynyl‐substituted homoallylic α‐tertiary NH₂‐amines is introduced. Easy‐to‐synthesize and robust N‐silyl ketimines are converted to NH‐ketimines in situ, which then react with a Z‐allyl boronate. Transformations are promoted by a readily accessible l ‐threonine‐derived aminophenol‐based boryl catalyst, affording the desired products in up to 91 % yield, >98:2 α:γ selectivity, >98:2 Z:E selectivity, and >99:1 enantiomeric ratio. A commercially available aminophenol may be used, and allyl boronates, which may contain an alkyl‐, a chloro‐, or a bromo‐substituted Z‐alkene, can either be purchased or prepared by catalytic stereoretentive cross‐metathesis. What is more, Z‐trisubstituted allyl boronates may be used. Various chemo‐, regio‐, and diastereoselective transformations of the α‐tertiary homoallylic NH₂‐amine products highlight the utility of the approach; this includes diastereo‐ and regioselective epoxide formation/trichloroacetic acid cleavage to generate differentiated diol derivatives.     

Site‐Selective 1,2‐Dicarbofunctionalization of Vinyl Boronates through Dual Catalysis

A modular, site‐selective 1,2‐dicarbofunctionalization of vinyl boronates with organic halides through dual catalysis is described. This reaction proceeds under mild conditions and is characterized by excellent chemo‐ and regioselectivity. It thus represents a complementary new technique for preparing densely functionalized alkyl boron architectures from simple and accessible precursors.     

Vinyl Triflimides—A Case of Assisted Vinyl Cation Formation

A new concept for selectivity control in carbocation‐driven reactions has been identified which allows for the chemo‐, regio‐, and stereoselective addition of nucleophiles to alkynes—assisted vinyl cation formation—enabled by a Li⁺‐based supramolecular framework. Mechanistic analysis of a model complex (Li₂NTf₂⁺?3 H₂O) confirms that solely the formation of a complex between the incoming nucleophile and the transition state of the alkyne protonation is responsible for the resulting selective N addition to the vinyl cation. Into the bargain, a general, operationally simple synthetic procedure to previously inaccessible vinyl triflimides is provided.     

Cross‐Coupling Reaction of Saccharide‐Based Alkenyl Boronic Acids with Aryl Halides: The Synthesis of Bergenin

A convenient synthetic pathway enabling D ‐glucal and D ‐galactal pinacol boronates to be prepared in good isolated yields was achieved. Both pinacol boronates were tested in a series of cross‐coupling reactions under Suzuki–Miyaura cross‐coupling conditions to obtain the corresponding aryl, heteroaryl, and alkenyl derivatives in high isolated yields. This methodology was applied to the formal synthesis of the glucopyranoside moiety of papulacandin D and the first total synthesis of bergenin.     

Visible‐Light‐Enabled Stereodivergent Synthesis of E‐ and Z‐Configured 1,4‐Dienes by Photoredox/Nickel Dual Catalysis

A stereodivergent reductive coupling reaction between allylic carbonates and vinyl triflates to furnish both E‐ and Z‐configured 1,4‐dienes has been achieved by visible‐light‐induced photoredox/nickel dual catalysis. The mild reaction conditions allow good compatibility of both vinyl triflates and allylic carbonates. Notably, the stereoselectivity of this synergistic cross‐electrophile coupling can be tuned by an appropriate photocatalyst with a suitable triplet‐state energy, providing a practical and stereodivergent means to alkene synthesis. Preliminary mechanistic studies shed some light on the coupling step as well as the control of the stereoselectivity step.     

Organoaluminum‐Mediated Direct Cross‐Coupling Reactions

We present a direct cross‐coupling reaction between arylaluminum compounds (ArAlMe₂?LiCl) and organic halides RX (R=aryl, alkenyl, alkynyl; X=I, Br, and Cl) without any external catalyst. The reaction takes place smoothly, simply upon heating, thereby enabling the efficient and chemo‐/stereoselective formation of biaryl, alkene, and alkyne coupling products with broad functional group compatibility.     

Synthetic Reactions Using Low‐valent Titanium Reagents Derived from Ti(OR)4 or CpTiX3 (X = O‐i‐Pr or Cl) in the Presence of Me3SiCl and Mg

In the presence of Me₃SiCl, Ti(OR)₄ or CpTiX₃ (X = O‐i‐Pr or Cl) is reduced by Mg powder in THF to gradually generate a specific low‐valent titanium (LVT) species that mediates several synthetic reactions. The LVT‐catalyzed C–O bond‐cleaving reactions of allyl and propargyl ethers and esters generate parent alcohols and carboxylic acids, respectively. O‐allyl and propargyl carbamates are also readily deprotected by the LVT to afford parent amines. In addition, the respective reductive N–S or O–S bond cleavage of sulfonamides or sulfonyl esters mediated by the LVT was developed as a novel facile deprotection method. The reagent catalyzes intra‐ and intermolecular alkyne or alkyne/nitrile cycloaddition to produce substituted benzenes and pyridines, while epoxides and oxetanes are reduced to alcohols via an LVT‐mediated homolytic ring opening. The McMurry coupling of aryl aldehydes and ketones proceeds with the LVT under homogeneous and mild reaction conditions and is effective for the polymerization of aromatic dialdehydes, generating conjugated polymers. Finally, imino‐pinacol coupling of imines is mediated by the LVT to provide 1,2‐diamines.     

Electrochemical Synthesis of gem-Difluoro- and γ-Fluoro-Allyl Boronates and Silanes

The electrochemical synthesis of fluorinated allyl silanes and boronates was disclosed. The addition of electrogenerated boryl or silyl radicals onto many α-trifluoromethyl or α-difluoromethylstyrenes in an undivided cell allowed the formation of a large panel of synthetically useful gem-difluoro and γ-fluoroallyl boronates and silanes (64 examples, from 31 % to 95 % yield). In addition, a scale up of the reactions under continuous flow was showcased using an electrochemical reactor with promising volumetric productivity (688 g.L⁻¹.h⁻¹ and 496 g.L⁻¹.h⁻¹). Moreover, the synthetic utility of these building blocks was highlighted through versatile transformations. Finally, plausible reaction mechanisms were suggested to explain the formation of the products.     

On the Gold‐Catalyzed Generation of Vinyl Cations from 1,5‐Diynes

Conjugated 1,5‐diynes bearing two aromatic units at the alkyne termini were converted in the presence of a gold catalyst. Under mild conditions, aryl‐substituted dibenzopentalenes were generated. Calculations predict that aurated vinyl cations are key intermediates of the reaction. A bidirectional approach provided selective access to the angular annulated product in high yield, which was explained by calculations.     

Nickel-catalyzed coupling of allyl chlorides and enynes

The Ni-catalyzed coupling of allyl chlorides and enynes has been developed; the cyclization of enynes was triggered by the addition of pi-allylnickel species to the alkyne part, followed by the incorporation of the alkene part.     

1‐Silacyclopent‐2‐enes and 1‐silacyclohex‐2‐enes bearing functionally substituted silyl groups in 2‐positions. Novel electron‐deficient Si?H?B bridges

The reactions of alkyn‐1‐yl(vinyl)silanes R₂Si[C?C‐Si(H)Me₂]CH?CH₂ [R = Me (1a), Ph (1b)], Me₂Si[C?C‐Si(Br)Me₂]CH?CH₂ (2a), and of alkyn‐1‐yl(allyl)silanes R₂Si[C?C‐Si(H)Me₂]CH₂CH?CH₂ (R = Me (3a), R = Ph (3b)] with 9‐borabicyclo[3.3.1]nonane in a 1:1 ratio afford in high yield the 1‐silacyclopent‐2‐ene derivatives 4a, b and 5a, and the 1‐silacyclohex‐2‐ene derivatives 6a, b, respectively, all of which bear a functionally substituted silyl group in 2‐position and the boryl group in 3‐position. This is the result of selective intermolecular 1,2‐hydroboration of the vinyl or allyl group, followed by intramolecular 1,1‐organoboration of the alkynyl group. In the cases of 4a, b, potential electron‐deficient Si? H? B bridges are absent or extremely weak, whereas in 6a,b the existence of Si? H? B bridges is evident from the NMR spectroscopic data (¹H, ¹¹B, ¹³C and ²⁹Si NMR). The molecular structure of 4b was determined by X‐ray analysis. Copyright © 2005 John Wiley & Sons, Ltd.     

(NHC)NiH‐Catalyzed Regiodivergent Cross‐Hydroalkenylation of Vinyl Ethers with α‐Olefins: Syntheses of 1,2‐ and 1,3‐Disubstituted Allyl Ethers

Cross‐hydroalkenylation of a vinyl ether ( 1 ) with an α‐olefin ( 2 ) was first achieved by a set of [NHC‐Ni(allyl)]BAr^F (NHC=N‐heterocyclic carbene) catalysts. Both 1,2‐ and 1,3‐disubstituted allyl ethers were obtained, highly selectively, by using NHCs of different sizes. In contrast, the chemoselectivity (i.e., 1 as acceptor and 2 as donor) was controlled mostly by electronic effects through the catalyst–substrate interaction. Sterically bulkier alkenes ( 2 ) were used as preferred donors compared to smaller alkenes. This electronic effect also served as a basis for the first tail‐to‐head cross‐hydroalkenylations of 1 with either a vinyl silane or boronic ester.     

Preparation of Vinyl Arenes by Nickel‐Catalyzed Reductive Coupling of Aryl Halides with Vinyl Bromides

This work emphasizes the synthesis of substituted vinyl arenes by reductive coupling of aryl halides with vinyl bromides under mild and easy‐to‐operate nickel‐catalyzed reaction conditions. A broad range of aryl halides, including heteroaromatics, and vinyl bromides were employed to yielding products in moderate to excellent yields with high functional‐group tolerance. The nickel‐catalytic system displays good chemoselectivity between the two C(sp²)‐halide coupling partners, thus demonstrating a mechanistic pathway distinct from other stepwise protocols.     

Synthesis and functionalization of dendron‐polymer conjugate based hydrogels via sequential thiol‐ene “click” reactions

Fabrication and functionalization of hydrogels from well‐defined dendron‐polymer‐dendron conjugates is accomplished using sequential radical thiol‐ene “click” reactions. The dendron‐polymer conjugates were synthesized using an azide‐alkyne “click” reaction of alkene‐containing polyester dendrons bearing an alkyne group at their focal point with linear poly(ethylene glycol)‐bisazides. Thiol‐ene “click” reaction was used for crosslinking these alkene functionalized dendron‐polymer conjugates using a tetrathiol‐based crosslinker to provide clear and transparent hydrogels. Hydrogels with residual alkene groups at crosslinking sites were obtained by tuning the alkene‐thiol stoichiometry. The residual alkene groups allow efficient postfunctionalization of these hydrogel matrices with thiol‐containing molecules via a subsequent radical thiol‐ene reaction. The photochemical nature of radical thiol‐ene reaction was exploited to fabricate micropatterned hydrogels. Tunability of functionalization of these hydrogels, by varying dendron generation and polymer chain length was demonstrated by conjugation of a thiol‐containing fluorescent dye. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 926–934     

Redox‐Triggered CC Coupling of Diols and Alkynes: Synthesis of β,γ‐Unsaturated α‐Hydroxyketones and Furans by Ruthenium‐Catalyzed Hydrohydroxyalkylation

Direct ruthenium‐catalyzed C? C coupling of alkynes and vicinal diols to form β,γ‐unsaturated ketones occurs with complete levels of regioselectivity and good to complete control over the alkene geometry. Exposure of the reaction products to substoichiometric quantities of p‐toluenesulfonic acid induces cyclodehydration to form tetrasubstituted furans. These alkyne‐diol hydrohydroxyalkylations contribute to a growing body of merged redox‐construction events that bypass the use of premetalated reagents and, hence, stoichiometric quantities of metallic by‐products.