Convergent and stereodivergent synthesis of complex 1-aza-7-oxabicyclo[2.2.1]heptanes

A convergent and stereodivergent pathway to highly substituted 1-aza-7-oxabicyclo[2.2.1]heptanes is described. It begins with a coupling reaction involving allylic alcohol, aldehyde, and LiHMDS to produce stereodefined primary homoallylic amines. Subsequent N-oxidation and condensation with formaldehyde or glyoxylate defines a convenient entry to densely functionalized homoallylic nitrones whose intramolecular annulation can be controlled to deliver one of two distinct heterocyclic skeletons, each with ≥20:1 stereoselection. Control of the stereochemistry in these reactions results from both control of the nitrone geometry and selective partitioning of the reaction pathway between direct [3 + 2] cycloaddition and tandem [3,3] rearrangement/[3 + 2] cycloaddition.     

Synthesis of homoallylic amines by hydrozirconation-imine addition of allenes

Hydrozirconation of allenes followed by in situ transmetalation to dialkylzinc leads to the formation of an allylic zinc species that, upon addition of aldimines to the reaction mixture, provides homoallylic amines in 64-85% yield. [reaction: see text]     

Palladium-catalyzed coupling of allyl acetates with aldehyde and imine electrophiles in the presence of Bis(pinacolato)diboron

[reaction: see text] An efficient one-pot procedure was developed for palladium-catalyzed electrophilic substitution of allyl acetates (2a-h) in the presence of bis(pinacolato)diboron (1). These reactions proceed with an excellent regioselectivity and with a remarkably high stereoselectivity. The catalytic transformations take place via palladium-catalyzed formation of allyl boronates, which subsequently react with aldehyde (3) and sulfon-imine (4) electrophiles to afford homoallylic alcohols (5a-h) and amines (6a-d), respectively. A particularly interesting mechanistic feature is that the allylic substitution of the transient allyl boronate with sulfon-imine requires palladium catalysis. This finding indicates that the formation of the homoallylic amine derivatives (6a-d) involves bis-allylpalladium intermediates.     

Thermal and catalyzed [3,3]-phosphorimidate rearrangements

[3,3]-Sigmatropic rearrangements have been widely utilized for the synthesis of structurally complex organic molecules because of the ease with which carbon-carbon bonds are formed in a regio- and stereocontrolled manner. However, there are far fewer [3,3]-rearrangements available for the selective formation of carbon-nitrogen bonds despite the enormous potential of such reactions for the preparation of stereodefined allylic amines. We describe here the scope and mechanism of a [3,3]-rearrangement of allylic phosphorimidates that provides access to stereodefined allylic amines of diverse structure. The reactive intermediate in the reaction, an allylic phosphorimidate, is produced in situ through the combination of readily available starting materials (allylic alcohols, chlorophosphites, and organic azides), rendering the reaction an efficient three-component process. Analogous to other [3,3]-rearrangements, the stereochemistry in an allylic alcohol starting material is transferred with fidelity to the allylic amine product and, further, allylic amines are produced as single olefin isomers. In addition, a crossover experiment indicates that the rearrangement is an intramolecular process. Finally, activation of the allylic moiety either through incorporation of electron-deficient functional groups or through the use of a transition-metal catalyst significantly facilitates the reaction and consequently the preparation of a wider range of substitution patterns.     

Rhodium‐Catalyzed Allylation of Aldehydes with Homoallylic Alcohols by Retroallylation and Isomerization to Saturated Ketones with Conventional or Microwave Heating

The treatment of an aldehyde with a tertiary homoallylic alcohol at 100–250 °C in the presence of cesium carbonate and a rhodium catalyst leads to allyl transfer from the homoallylic alcohol to the aldehyde. The process includes Rh‐mediated retroallylation to form an allyl rhodium species as the key intermediate. The homoallylic alcohol formed initially through allyl transfer is converted under the reaction conditions into the corresponding saturated ketone when bulky ligands are used. Microwave heating at 250 °C accelerates the reaction significantly.     

The Cp2ZrCl2-catalyzed cycloalumination of functionally substituted olefins with triethylaluminum

Reactions of functionally substituted olefins (allylamines, sulfides and ethers, homoallylic alcohols and amines, as well as vinyl ethers) with Et₃Al in the presence of Cp₂ZrCl₂ as a catalyst were studied. Cycloalumination of allylamines occurs with high regioselectivity to furnish after subsequent deuterolysis 4-deutero-2-(deuteromethyl)butyl-substituted amines. Cycloalumination of alkyl allyl sulfide is accompanied by a side process of the C-S bond cleavage. In the case of allyl and vinyl ethers, no cycloalumination products are formed under the reaction conditions. However, the reactions with homoallylic alcohol and amine after deuterolysis gave the corresponding dideutero-containing compounds in good yields.     

A general nickel-catalyzed hydroamination of 1,3-dienes by alkylamines: catalyst selection, scope, and mechanism

A simple colorimetric assay of various transition-metal catalysts showed that the combination of DPPF, Ni(COD)(2), and acid is a highly active catalyst system for the hydroamination of dienes by alkylamines to form allylic amines. The scope of the reaction is broad; various primary and secondary alkylamines react with 1,3-dienes in the presence of these catalysts. Detailed mechanistic studies revealed the individual steps involved in the catalytic process. These studies uncovered unexpected thermodynamics for the addition of amines to pi-allyl nickel complexes: instead of the thermodynamics favoring the reaction of a nickel allyl with an amine to form an allylic amine, the thermodynamics favored reaction of a nickel(0) complex with allylic amine in the presence of acid to form a Ni(II) allyl. The realization of these thermodynamics led us to the discovery that nickel and some palladium complexes in the presence or absence of acid catalyze the exchange of the amino groups of allylic amines with free amines. This exchange process was used to reveal the relative thermodynamic stabilities of various allylic amines. In addition, this exchange reaction leads to racemization of allylic amines. Therefore, the relative rate for C-N bond formation and cleavage influences the enantioselectivity of diene hydroaminations.     

烯丙胺与C_(60)的热引发[3+2]环加成反应

烯丙胺类化合物与C_(60)在热引发下发生新型的[3+2]环加成反应。反应有较 好的立体选择性。该反应可能经过了热引发的烯丙胺的单电子转移过程，产生烯丙 基自由基，并进一步与C_(60)加成及环化。文中C_(60)衍生物的结构均经过谱学方 法确证。     

Synthetic and mechanistic studies of indium-mediated allylation of imines in ionic liquids

Aldimines derived from aryl and non-enolizable aliphatic aldehydes were allylated with allyl bromide mediated by indium powder in [bpy][BF4] (bpy = N-butylpyridine) to give good yields of the corresponding homoallylic amines. Selective formation of monoallylated amines can be achieved by varying the amount of bromide ion additive in the form of [bpy][Br]. The transient organoindium intermediates, allylindium(I) and allylindium(III) dibromide formed in the reaction, were studied by NMR spectroscopy to explain the selectivity.     

Regio- and enantioselective control in the reactions of alpha-(N-carbamoyl)alkylcuprates with allylic phosphates

Alpha-(N-carbamoyl)alkylcuprates (RCuCNLi or R2CuLi) react with allylic phosphates to afford homoallylic amines in good chemical yields. Regioselectivity is governed by steric factors in both the cuprate reagent and phosphate substrate and systems can be designed to give either the S(N)2' or S(N)2 substitution product cleanly. Excellent enantioselectivities can be achieved with either a scalemic alpha-di[(N-carbamoyl)alkyl]cuprate and an achiral phosphate or with a scalemic allylic phosphate and an achiral cuprate reagent. [reaction: see text]     

Amine-directed hydroboration: scope and limitations

Iodine activation induces intramolecular hydroboration of homoallylic and bis-homoallylic amine boranes with good to excellent control of regiochemistry compared to control experiments using excess THF*BH 3. Deuterium labeling and other evidence confirm that the iodine-induced hydroboration reaction of homoallylic amine boranes occurs via an intramolecular mechanism equivalent to the classical 4-center process and without competing retro-hydroboration. Longer carbon chain tethers result in lower regioselectivity, whereas the shorter tether in allylic amines results in a switch to dominant intermolecular hydroboration. Regioselectivity in THF*BH 3 control experiments is higher for the allylic amine boranes compared to the iodine activation experiments, whereas the reverse is true for homoallylic amine borane activation.     

Tandem zirconocene homologation-aldimine allylation

[reaction: see text]. Hydrozirconation of internal and terminal alkynes followed by in situ transmetalation to dimethylzinc and treatment with diiodomethane leads to chain extended allylic organometallics. Addition to N-phosphinoyl or N-sulfonyl aldimines provides homoallylic amines in 48-87% yield and 3:2 to >20:1 diastereomeric ratios favoring anti-products.     

Development of a highly alpha-regioselective metal-mediated allylation reaction in aqueous media: new mechanistic proposal for the origin of alpha-homoallylic alcohols

This paper described a general method to obtain alpha-adduct homoallylic alcohols using indium, zinc, and tin in water. A new mechanism was proposed to account for the formation of these synthetically difficult-to-obtain molecules. Generally, this method can be performed with a wide range of aldehydes and allylic halides with just 6 equiv of water added, giving the alpha-adduct in high selectivities. To account for the origin of the alpha-homoallylic alcohol, the reaction mechanism was carefully studied using (1)H NMR, a crossover experiment, and the inversion stereochemical studies of 22 beta gamma-adduct homoallylic sterol to the 22 alpha alpha-adduct homoallylic sterol. From the results of mechanism studies, it is possible that two mechanism pathways coexisted in the metal-mediated alpha-regioselective allylation. The metal salts formed from the metal-mediated allylation can catalyze the gamma-adduct to undergo a bond cleavage to generate the parent aldehyde in situ followed by a concerted rearrangement, perhaps a retroene reaction followed by a 2-oxonia[3,3]-sigmatropic rearrangement to furnish the alpha-adduct. The alpha-adduct can also be synthesized via the formation of an oxonium ion intermediate between the gamma-adduct and the unreacted aldehyde. The proposed mechanisms were further supported by experimental findings from the addition of InBr(3) to gamma-adduct under similar conditions.     

Palladium pincer-complex catalyzed allylation of tosylimines by potassium trifluoro(allyl)borates

Palladium pincer complex 1 catalyzes the reaction of trifluoro(allyl)borate 2 with a wide range of tosylimines (3) under mild and neutral reaction conditions. This catalytic transformation affords homoallylic amines (4) in good to excellent yield. Mechanistic studies suggest that a transmetalation reaction between complex 1 and the borate salt 2 provides an eta(1)-allylpalladium complex, which subsequently reacts with the imine substrate. [Reaction: see text]     

Copper‐Catalyzed Enantioselective Allyl–Allyl Coupling between Allylic Boronates and Phosphates with a Phenol/N‐Heterocyclic Carbene Chiral Ligand

Copper‐catalyzed enantioselective allyl–allyl coupling between allylboronates and either Z‐acyclic or cyclic allylic phosphates using a new chiral N‐heterocyclic carbene ligand, bearing a phenolic hydroxy, is reported. This reaction occurs with exceptional S_N2′‐type regioselectivities and high enantioselectivities to deliver chiral 1,5‐diene derivatives with a tertiary stereogenic center at the allylic/homoallylic position.     

A novel enantioselective (2Z)-alk-2-enylation of aldehydes via an allyl-transfer reaction from chiral allyl donors prepared from (+)-isomenthone

A highly enantioselective (2Z)-alk-2-enylation of aldehydes was successfully achieved by an allyl-transfer reaction from a chiral allyl donor, which was easily obtained by separation of a diastereomeric mixture of the corresponding homoallylic alcohol gamma-adducts derived from (+)-isomenthone with alk-2-enylmagnesium chloride. [reaction: see text]     

Rearrangements of the carbanions derived from allyl benzyl thioether

The metalation of allyl benzyl thioether involves the benzylic or the allylic hydrogens. The benzylic carbanion undergoes a rapid[2,3] sigmatropic shift whereas the allylic carbanion gives rise to various rearrangements, among them migration of the allylic unit to the para position with allylic inversion. The temperature dependence of the ratio of products arising from the benzylic carbanion vs those from the allylic carbanion shows that the allylic-to-benzylic carbanion transformation occurs only under special conditions: (a) with slow addition of the base; (b) with thioether in excess relative to the base, and (c) on raising the temperature of the reaction medium from ?78° to ?15°. In the last instance, the proton transfer is intramolecular as shown with labeled thioethers. The extent of the different rearrangements depends on the temperature and solvent. A choice of mechanism cannot be made at this time for the para migration 5→9a. A leaving group effect on the reaction regioselectivity of the carbanion from allyl methyl thioether with benzyl halides has been noticed. The presence of dibenzyl indicates that, in addition to S_N2 reactions, some electron transfer process is occurring.     

Catalyst-free allylation of 2-aminophenol–derived aldimines with allyltrichlorosilane under thermal conditions

Allylation of 2-aminophenol-derived aldimines using allyltrichlorosilane under catalyst free conditions has been developed. This reaction afforded the corresponding homoallylic amines in good to excellent yields (68–94%). The salicylaldehyde-derived aldimines as well as benzoylhydrazone also found to react with allyltrichlorosilane smoothly under the same conditions, to furnish the corresponding homoallylic amine derivatives. This study suggests that the phenolic –OH group acts as an anchoring group for the transfer of allyl group from allyl silane reagent.     

Preparation of stereodefined homoallylic amines from the reductive cross-coupling of allylic alcohols with imines

Regio-, diastereo-, and enantioselective coupling reactions between imines and allylic alcohols have been developed. These coupling reactions deliver complex homoallylic amine products through a convergent C-C bond forming process that does not proceed through intermediate allylic organometallic reagents. In general, convergent coupling, by exposure of an allylic alkoxide to a preformed Ti-imine complex, occurs with allylic transposition in a predictable and stereocontrolled manner. While simple diastereoselection in these reactions is high, delivering anti-products with ≥20:1 selectivity, the organometallic transformation described is compatible with a diverse range of functionality and substrates (including aliphatic and aromatic imines, allylic silanes, trisubstituted alkenes, vinyl- and aryl halides, trifluoromethyl groups, thioethers, and aromatic heterocycles). Alkene geometry of the products is a complex function of the allylic alcohol structure and is consistent with a mechanistic proposal based on syn-carbometalation followed by syn-elimination by way of a boat-like transition state geometry. Single asymmetric coupling reactions provide a means to translate the stereochemical information of the allylic alcohol to the homoallylic amine or to control diastereoselection in the coupling reactions of achiral allylic alcohols with chiral imines. Double asymmetric coupling reactions are also described that afford a unique means to control stereoselection in these complex convergent coupling processes. Finally, empirical models are proposed that are consistent with the observed stereochemical course of these coupling reactions en route to chiral homoallylic amines possessing di- or trisubstituted alkenes and anti- or syn- relative stereochemistry at the allylic and homoallylic positions.     

Concise syntheses of nonracemic gamma-fluoroalkylated allylic alcohols and amines via an enantiospecific palladium-catalyzed allylic substitution reaction

Alpha-fluoroalkylated allyl mesylates reacted with various carboxylates and amines in the presence of tetrakis(triphenylphosphine)palladium(0) catalyst to give the corresponding gamma-fluoroalkylated (E)-allylic alcohol derivatives and amines, respectively, in excellent yields. In almost all cases, no other regio- and stereoisomers were produced. Application of this palladium-catalyzed allylic substitution reaction to various nonracemic mesylates afforded chiral gamma-fluoroalkylated allylic alcohol derivatives and amines without any loss of enantiomeric excess through the reaction.