Alkenyl transfer

Taylor and colleagues have shown the sequential carbolithiation/[l,4]-Wittig rearrangement of styryl ethers 105 (e.g., 105a) which provide appropriately functionalized 2-alkylphenol 106 (e.g., 106a) in good yield . In this reaction, the alkenyl transfer occurs with retention of configuration (equations 59 and 60). 

A novel intermolecular alkenyl transfer of enamines has been developed and utilized in the preparation of cycloalk-enylaminomethyleneoxazolones, which could subsequently be cyclized to fused pyridones (Scheme 23) <19%TL8871>. 

Irrespective of the stereochemistry of the ot,p-enones, lithium alkenyltrialkylalanates, e.g. (28) and (29), obtainable by treatment of alkenylalanes with methyllithium (or n-butyllithium), are also excellent 1,4-conjugate alkenyl transfer reagents. The cornerstone of a general prostaglandin synthesis, as exemplified in Scheme 6, employs the alanate conjugate addition process exclusively.13... 

Acylation of alkenylsilanes with acid anhydrides is efficiently catalyzed by [RhCl(CO)2]2-61 Based on the fact that stoichiometric reaction of an alkenylsilane with the Rh complex gives the corresponding alkenylrhodium species, the proposed mechanism involves alkenyl transfer from Si to Rh as the initial step. 

Molander reported a variant of this sequential process that results in alkenyl transfer.20 For example, treatment of bromide 36 with SmI2-HMPA resulted in the formation of cyclopentanone 37. Subsequent intramolecular ketyl radical addition to the enol ether and collapse of the resultant organosamarium intermediate 38 gave cyclopentanol 39 in good overall yield (Scheme 6.15).20... 

The role of copper(I) as an additive is complex and has been discussed by Farina et al. although it can catalyse reactions in which L is PPha by a factor of more than 100, this is not the case when AsPhj is used. Transmetallation between copper and tin is slow, even in A-methylpyrrolidine as solvent [205]. Copper also affects product distribution [3] thus in tlie reaction between aryl triflates and 4-te/t-butylcyclohexen-l-yltributyltin there is competition between butyl transfer and the required alkenyl transfer. 

The C—F bond activation with aluminum reagents has been extensively employed for the synthesis of C-glycosides (8). Benzyl protected 2-fluorogycoside (7) was methylated and cyanated with high a-preference under mild conditions [3]. Likewise, alkenyl transfer was... 

A commonly proposed mechanism of the standard NHK protocol involves reduction of Ni(II) to Ni(0) by CrC, then oxidative addition of Ni(0) to the alkenyl iodide to generate an alkenyl Ni(II) species (Scheme 3-69). Transmetallation to CrCls (from the initial nickel reduction step) produces a Cr(III) alkenyl species, which undergoes direct addition to the aldehyde to generate product. Alternatively, alkenyl transfer from a vinyl nickel(II) species to CrCb may instead occur with release of the Cr(III) alkenyl species and a Ni(I) species. 

Indirect evidence for an activation step is found in the ability of pre-synthesized, stable, pentacoordinate silanes to transfer an organyl group onto palladium. Two examples are the use of catecholsilanes for alkenyl transfer and the use of TBAT as a phenylating reagent (Figure 7.1) [16, 74]. Yet there are also indications from earlier work with fluoride-activated fluorosilanes of how pentacoordinate silanes could play a role in the reaction. 

With Zn Lewis acids, only single a-insertion of alkynes (—> 42) is observed, while with AlMes double alkenylation (— 43) dominates. It is proposed that oxidative Ni(0) insertion to the 2-CH bond, hydronickelation of the triple bond, alkenyl transfer to C-2 of the pyridine, and reductive Ni-elimination are the decisive steps in the catalytic cycle, (d) The pyridyl residue may serve as a directing group in C-H insertion reactions of phenyl substituents at pyridine mediated by transition metals like Cu and Pd. For instance, 2-phenylpyridine can be regioselectively halogenated, acetoxylated, and cyanided (- products 44, 45, and 47) in the presence of Cu(OAc)2 [92] or amidated — 46) in the presence of Pd(OAc)2 [93] ... 

The pyrrolizidine natural products can be obtained by exploitation of the Petasis MCR for combining aldehydes, amines, and vinylboronic acids, as shown by Pyne and coworkers in 2008 [35]. Application of aldopentoses yields polyhydroxylated allyl amines, that provides opportunity for further derivatization to, for example, uniflorines. The MCR commences by boronate-assisted imine formation and protonation (111). Subsequent alkenyl transfer from the borate to the protonated imine delivers the secondary allyl amine 116 in excellent diastereoselectivity (Scheme 14.14). 

Readily synthesised and stable diaryliodine(III) and alkynyl(aryl)iodine(III) reagents react under mild conditions with representative d square-planar complexes to form a range of new classes of d octahedral complexes. For these sterically and electronically unsaturated d substrates, the reactions most likely occur via polar mechanism, consistent with the known reactivity of these nucleophiles toward other electrophiles. Transfer of alkenyl groups from [I(alkenyl)Ph] to d ° and d reagents appear to proceed similarly. However, the range of transition metals studied to date is limited Pd(II) and Pt(II) for aryl transfer Pt(0), Pd(II), Pt(II), Rh(I) and Ir(I) for alkynyl transfer and Pt(0), Rh(I) and Ir(I) for alkenyl transfer. 

Finally, cross-coupling reactions involving aluminum reagents have primarily been used for alkenyl transfers from the carbo- and hydroalumination products of terminal alkynes. A number of interesting results were published in recent years and will surely broaden the application of alanes and aluminum sesquihalides to these C,C-bond forming reactions. 

Aryl sulfides are prepared by the reaction of aryl halides with thiols and thiophenol in DMSO[675,676] or by the use of phase-transfer catalysis[677]. The alkenyl sulfide 803 is obtained by the reaction of lithium phenyl sulfide (802) with an alkenyl bromide[678]. 

Step 4 Proton transfer from ammonia converts the alkenyl anion to an alkene ... 

The mechanism includes two single electron transfers (steps 1 and 3) and two proton transfers (steps 2 and 4) Experimental evidence indicates that step 2 is rate determining and it is believed that the observed trans stereochemistry reflects the dis tribution of the two stereoisomeric alkenyl radical intermediates formed in this step... 

Mercuration. Mercury(II) salts react with alkyl-, alkenyl-, and arylboranes to yield organomercurials, which are usehil synthetic intermediates (263). For example, dialkyhnercury and alkyhnercury acetates can be prepared from primary trialkylboranes by treatment with mercury(II) chloride in the presence of sodium hydroxide or with mercury(II) acetate in tetrahydrofuran (3,264). Mercuration of 3 -alkylboranes is sluggish and requires prolonged heating. Alkenyl groups are transferred from boron to mercury with retention of configuration (243,265). 

Mixed-Metal Systems. Mixed-metal systems, where a zirconium alkyl is formed and the alkyl group transferred to another metal, are a new apphcation of the hydrozirconation reaction. These systems offer the advantages of the easy formation of the Zr—alkyl as well as the versatiUty of alkyl—metal reagents. For example, Cp2ZrRCl (R = alkyl or alkenyl) reacts with AICI3 to give an Al—alkyl species which may then be acylated with... 

Tomoi and coworkers adopted a somewhat more direct approach in their synthesis of 16-crown-5 derivatives bearing a single alkenyl residue. They hoped to obtain precursors to polymers which could be used as phase transfer catalysts. In this approach I,I-bis-chloromethylethylene (a-chloromethallyl chloride) was allowed to react with the dianion of tetraethylene glycol (NaH/THF). By this method, methylene-16-crown-5 could be isolated in 66% yield after vacuum distillation. Ozonolysis led, in almost quantitative yield, to the formation of oxo-16-crown-5 as shown in Eq. (3.38). These authors prepared a number of other, closely related species by similar methods. 

The ability of Fischer carbene complexes to transfer their carbene ligand to an electron-deficient olefin was discovered by Fischer and Dotz in 1970 [5]. Further studies have demonstrated the generality of this thermal process, which occurs between (alkyl)-, (aryl)-, and (alkenyl)(alkoxy)carbene complexes and different electron-withdrawing substituted alkenes [6] (Scheme 1). For certain substrates, a common side reaction in these processes is the insertion of the carbene ligand into an olefinic C-H bond [6, 7]. In addition, it has been ob-... 

Alkenyl alcohols, (E) disubstituted, 55, 66 ALKYL HALIDES, alkenes from, 55, 103 C-ALKYLATION, phase transfer catalysis... 

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