Direct α‐Vinylidenation of Aldehydes and Subsequent Cascade: Gold and Amine Catalysts Work Synergistically

Carbonyl‐substituted allenes are highly important synthetic intermediates for a number of heterocycles and strained‐ring systems. However, chemistry of allenyl aldehydes has not been explored as extensively as their ketone, ester, or amide analogues because of a lack of general synthetic methods. Described herein is the first direct α‐vinylidenation of aldehydes and an α‐vinylidenation/γ‐functionalization cascade to access tri‐ and tetrasubstituted allenyl aldehydes using a combination of a gold catalyst and an secondary amine. The reactive enamine intermediate of an aldehyde reacts with the gold‐activated hypervalent silylethynyl benziodoxolone to selectively generate the corresponding trisubstituted allenyl aldehyde. The allenyl aldehyde can further react with another equivalent of the alkynylation reagent or other electrophiles to afford tetrasubstituted allenes bearing an aldehyde group, an acetylene, and a halogen functionality. This method enables rapid access to polysubstituted furans from aldehydes.     

Regiocontrolled, palladium-catalyzed bisfunctionalization of allenyl esters. Multicomponent coupling approaches to highly substituted alpha,beta-unsaturated delta-lactones

A palladium-catalyzed regioselective bisfunctionalization of allenyl esters with boronic acids (nucleophiles) and aldehydes (electrophiles) was demonstrated. The three-component coupling afforded alpha,beta-unsaturated delta-lactones under mild conditions and with excellent chemo-, regio-, and diastereoselectivity. Aromatic, heteroaromatic and vinylic boronic acids (R1B(OH)2) reacted with ethyl 2,3-butadienoate and benzaldehyde to afford the corresponding 4-R(1),6-Ph-disubstituted alpha,beta-unsaturated delta-lactones in 62-78% yields. Lactones derived from aromatic, heteroaromatic, and vinylic aldehydes were isolated in 51-58% yields, while aliphatic aldehydes were less reactive. The regiochemistry of bisfunctionalization of allenyl ester homologues remained controlled by the ester substituent, and the reactions afforded cis-4,5,6-trisubstituted alpha,beta-unsaturated delta-lactones and esters of (Z)-syn-3,4,5-trisubstituted-5-hydroxy-2-pentenoic acids in combined 47-65% yields. The superior performance of a pi-allylpalladium(II) dimer catalyst featuring an auxiliary allyl ligand derived from beta-pinene, among diverse palladium(II) catalysts, was demonstrated. A catalytic cycle involving an unsymmetrical bis-pi-allylpalladium complex as the key intermediate was proposed, and the communication highlights the synthetic potential of such intermediates. However, the efficiency of asymmetry transfer remained low (<20%).     

Nickel-catalyzed couplings and cyclizations involving allenes, aldehydes, and organozincs

Cyclizations of allenyl aldehydes and intermolecular couplings of allenes and aldehydes provide a direct procedure for the preparation of homoallylic alcohols. The couplings and cyclizations involve the use of diorganozinc reagents as reducing agents and Ni(COD)₂ as the catalyst.     

Copper(I)-catalyzed substitution of propargylic carbonates with diboron: selective synthesis of multisubstituted allenylboronates

A versatile synthetic method for the preparation of the allenylboronates, Cu(I)-catalyzed reaction of propargylic carbonates with a diboron, is described. A Cu(O-t-Bu)-Xantphos catalyst system was effective for the preparation of various allenylboron compounds, allenylboronates with different substitution patterns, those with functional groups, and an axially chiral one. The Lewis acid promoted stereoselective addition to aldehydes leading to homopropargylic alcohols showed the usefulness of the new allenylboronates.     

A Brønsted acid mediated cascade enone synthesis from aldehydes containing a tethered propargylsilane

MeSO3H effects the intramolecular allenylation of a series of aldehydes 1 to provide allenyl alcohol product 3 as a single diastereoisomer. Cyclization proceeds rapidly at -78 degrees C. However, when the reaction is performed at room temperature, aldehyde 1 provides enone product 7 instead. A mechanism for the formation of this product is proposed in which the initially formed allenyl alcohol 3 undergoes dehydration to provide an allyl carbocation, which is trapped with water, thereby installing the enone.     

CuCl‐Catalyzed Aerobic Oxidation of Allylic and Propargylic Alcohols to Aldehydes or Ketones with 1:1 Combination of Phenanthroline and Bipyridine as the Ligands

We developed a modified protocol for the oxidation of 2,3‐allenyl alcohols using CuCl with 1:1 combination of phenanthroline and bipyridine as the catalyst. To further investigate the applicability of this system, other types of alcohols such as allylic and propargylic alcohols have been tested: we found that both allylic and propargylic alcohols may be oxidized to the corresponding aldehydes or ketones using molecular oxygen in air as the oxidant with moderate to excellent yields.     

锡存在下取代的炔丙基溴与醛反应的选择性

报道三甲基硅基和苯基取代的炔丙基溴在锡存在下与醛的反应, 并讨论了反应的选择性。三甲基硅基炔丙基溴与醛的反应产率约为71-84%。产物中丙二烯醇与高炔丙基醇之比在44 :56到25 : 75之间, 苯基炔丙基溴的反应得到了产物丙二烯醇。     

Enantioselective arylative cyclization of allenyl aldehydes with arylboronic acids under Pd(II)-diphosphine catalysis

A Pd(OAc)2-SEGPHOS combination catalyzes the first enantioselective arylative cyclization of allenyl aldehydes with arylboronic acids to provide cis-fused five- and six-membered cyclic homoallylic alcohols. The excellent diastereo- and enantioselectivity and the fact that the reaction proceeds at room temperature in the absence of any additives make the process highly practical.     

A facile route to acyclic substituted alpha,beta-unsaturated aldehydes: the allene Claisen rearrangement

[reaction: see text] Sigmatropic rearrangement of allenyl ethers furnishes alpha,beta-unsaturated aldehydes in good yield.     

Synthesis of α-allenic aldehydes and allenyl trimethylsilyl ketones via (trimethylsilyloxy)butatrienes

α-Lithiated (trimethylsilyloxy)butatrienes, generated by 1,4-elimination of methanol from 1-trimethylsilyloxy-4-methoxy-2-alkynes with tert-butyllithium, give mixtures of α-allenic aldehydes and allenyl trimethylsilyl ketones which can be separated by distillation. The ratio aldehyde/ketone is solvent dependent.     

Preparation of homoallylic alcohols by nickel-catalyzed cyclizations of allenyl aldehydes

The direct cyclization of allenyl aldehydes with organozincs in the presence of Ni(COD)(2) provides synthetically versatile homoallylic alcohols. Both monosubstituted and 1,3-disubstituted allenes participate in the process, with the latter allowing preparation of stereochemically defined trisubstituted alkenes. [reaction: see text]     

Stereoselective synthesis of Baylis-Hillman-type adducts via allenolates generated by acyl migration

[reaction: see text] The rearrangement of the carbamoyl group in allenyl carbamate 1 in the presence of n-BuLi in situ generates the allenolate 3, which is subsequently intercepted with aromatic aldehydes furnishing (Z)- or (E)-configured Baylis-Hillman-type adducts 4 or 5. The double bond isomers can be interconverted by a retro aldol-type reaction.     

Rhodium-catalyzed enantioselective alkynylative cyclization of allenyl aldehydes with terminal alkynes

Asymmetric addition of terminal alkynes to allenyl aldehydes took place in the presence of an acetylacetonate complex [Rh(acac)L1] (L1 = binap or segphos) as a catalyst to give indanol derivatives in high yields with high regio- and enantioselectivity. The acetylacetonate (acac) ligand on the rhodium is important for the selective formation of the indanol derivatives.     

Selenylated dienes: synthesis, stereochemical studies by Se NMR, and transformation into functionalized allenes

2-Phenylselanyl-1,3-dienes 3-8 were prepared by a Wittig or Wittig-Horner-Emmons procedure starting from α-phenylselanyl α,β-unsaturated aldehydes. Ratio and configuration of each diene isomers were determined by ⁷⁷Se and ¹H NMR. These dienes were then oxidized into selenoxides, which could be isolated in some cases. In THF, [2,3]-sigmatropic rearrangement of allylic selenoxides, selenimides, and dihalo-selenuranes occurred, yielding allenyl alcohols 12-15, allenyl carbamates 16c-19c, and 1-haloalkyl allenes 20c-22c, respectively.     

Unprecedented Spectroscopic and Computational Evidence for Allenyl and Propargyl Titanocene(IV) Complexes: Electrophilic Quenching of Their Metallotropic Equilibrium

The synthesis and structural characterization of allenyl titanocene(IV) [TiClCp₂(CH=C=CH₂)] 3 and propargyl titanocene(IV) [TiClCp₂(CH₂?C≡C?(CH₂)₄CH₃)] 9 have been described for the first time. Advanced NMR methods including diffusion NMR methods (diffusion pulsed field gradient stimulated spin echo (PFG‐STE) and DOSY) have been applied and established that these organometallics are monomers in THF solution with hydrodynamic radii (from the Stokes–Einstein equation) of 3.5 and 4.1 Å for 3 and 9 , respectively. Full ¹H, ¹³C, Δ¹H, and Δ¹³C NMR data are given, and through the analysis of the Ramsey equation, the first electronic insights into these derivatives are provided. In solution, they are involved in their respective metallotropic allenyl–propargyl equilibria that, after quenching experiments with aromatic and aliphatic aldehydes, ketones, and protonating agents, always give the propargyl products P (when carbonyls are employed), or allenyl products A (when a proton source is added) as the major isomers. In all the cases assayed, the ratio of products suggests that the metallotropic equilibrium should be faster than the reactions of 3 and 9 with electrophiles. Indeed, DFT calculations predict lower Gibbs energy barriers for the metallotropic equilibrium, thus confirming dynamic kinetic resolution.     

Propargylsilanes as Reagents for Synergistic Gold(I)‐Catalyzed Propargylation of Carbonyl Compounds: Isolation and Characterization of σ‐Gold(I) Allenyl Intermediates

Reported herein is the isolation and characterization, for the first time, of a σ‐gold allenyl complex as an intermediate in gold catalysis. This intermediate was captured during the study of a novel gold(I)‐catalyzed propargylation of carbonyl compounds with propargylsilanes. Notably, the gold‐catalyzed propargylation reaction, which proceeds with aldehydes and ketones, can be driven to the formation of either homopropargyl silyl ethers or the in situ synthesis of corresponding 2‐silyl‐4,5‐dihydrofurans.     

Construction of Acyclic All-Carbon Quaternary Stereocenters and 1,3-Nonadjacent Stereoelements via Organo/Metal Dual Catalyzed Asymmetric Allenylic Substitution of Aldehydes

A method for the asymmetric construction of functionalized acyclic all-carbon quaternary stereocenters and 1,3-nonadjacent stereoelements has been developed via organo/metal dual catalyzed asymmetric allenylic substitution of branched and linear aldehydes, by developing an unknown acyclic secondary-secondary diamine as the enabling organocatalyst. Although it is believed that secondary-secondary diamines are difficult to be used as the organocatalysts in organo/metal dual catalysis, this study demonstrates that such diamines can be successfully combined with a metal catalyst in organo/metal dual catalysis. Our study enables the asymmetric construction of two important classes of motifs which were previously difficult to access, axially chiral allene-containing acyclic all-carbon quaternary stereocenters and 1,3-nonadjacent stereoelements bearing allenyl axial chirality and central chirality, in good yields with high enantio- and diastereoselectivity.     

NHC and visible light-mediated photoredox co-catalyzed 1,4-sulfonylacylation of 1,3-enynes for tetrasubstituted allenyl ketones

The modulation of selectivity of highly reactive carbon radical cross-coupling for the construction of C–C bonds represents a challenging task in organic chemistry. N-Heterocyclic carbene (NHC) catalyzed radical transformations have opened a new avenue for acyl radical cross-coupling chemistry. With this method, highly selective cross-coupling of an acyl radical with an alkyl radical for efficient construction of C–C bonds was successfully realized. However, the cross-coupling reaction of acyl radicals with vinyl radicals has been much less investigated. We herein describe NHC and visible light-mediated photoredox co-catalyzed radical 1,4-sulfonylacylation of 1,3-enynes, providing structurally diversified valuable tetrasubstituted allenyl ketones. Mechanistic studies indicated that ketyl radicals are formed from aroyl fluorides via the oxidative quenching of the photocatalyst excited state, allenyl radicals are generated from chemo-specific sulfonyl radical addition to the 1,3-enynes, and finally, the key allenyl and ketyl radical cross-coupling provides tetrasubstituted allenyl ketones.

Unprecedented NHC and photocatalysis co-catalyzed radical 1,4-sulfonylacylation of 1,3-enynes has been realized, providing structurally diversified tetrasubstituted allenyl ketones via allenyl and ketyl radical cross-coupling.     

Synthesis of optically active α-(allenyl)- and α-substituted-α-(allenyl)glycines

The synthesis of various types of optically active α-(allenylsilane-containing)glycines via a chirality-transferring ester-enolate Claisen rearrangement of α-acyloxy-α-alkynylsilanes is described. The conversion of the rearranged products into the optically active silicon-free α-(allenyl)- and α-substituted-α-(allenyl)glycines was achieved by the removal of the Me₂PhSi- or TMS group from the allene terminus.     

Synthesis of homopropargyl alcohols via insertion of allenyl carbenoids into acyclic organozirconium bonds

Insertion of allenyl carbenoids (3-tosyloxy-1-lithioalk-1-ynes) into organozirconium complexes gave allenyl-zirconocenes via a 1,2-zirconate rearrangement. Trapping of the allenyl-zirconium species with aldehydes and ketones gave, after hydrolysis, a series of homopropargyl alcohols. Enantioenriched products were prepared by insertion of the lithium carbenoid derived from (S)-but-3-yn-2-yl 4-toluenesulfonate into alkyl- and alkenyl-chlorozirconocenes.