Allylic alkyne sources

C=C-Z Allylic sources Enolates C=C-0 Enamines C=C-NR2 Enol ethers C=C-OR The better donor on the pi bond is more reactive Substitutions Additions. Carbon vs. heteroatom decision Extended enolates. Allylic alkyne sources... 

Allylic Sources Ambident Nucleophiles Better Donors Increase Reactivity Vinylogous Allylic Sources Allyiic Alkyne Sources... 

Previous syntheses of terminal alkynes from aldehydes employed Wittig methodology with phosphonium ylides and phosphonates. 6 7 The DuPont procedure circumvents the use of phosphorus compounds by using lithiated dichloromethane as the source of the terminal carbon. The intermediate lithioalkyne 4 can be quenched with water to provide the terminal alkyne or with various electrophiles, as in the present case, to yield propargylic alcohols, alkynylsilanes, or internal alkynes. Enantioenriched terminal alkynylcarbinols can also be prepared from allylic alcohols by Sharpless epoxidation and subsequent basic elimination of the derived chloro- or bromomethyl epoxide (eq 5). A related method entails Sharpless asymmetric dihydroxylation of an allylic chloride and base treatment of the acetonide derivative.8 In these approaches the product and starting material contain the same number of carbons. 

Addition of an electrophile (see Electrophile) to metal-bound cyanides will often form an isocyanide ligand (see Electrophile), -CsN-R. For example, the compound [Fe P(OMe)3 (NO)2(j7 -C3H4R)], which is a source of the allyl cation ( -C3H4R)+, reacts with trans-[Mn(CN)(CO)(dppm)2] to alkylate the cyanide, giving an allyl isocyanide ligand (equation 8). The tungsten alkyne... 

Tetracarbonylnickel is an inexpensive (although toxic) source of Ni° and its use for the in situ generation of allylnickel intermediates dates back at least 25 years. In fact, the nickel allylation/methoxycarbo-nylation of alkynes with allyl chloride and an excess of Ni(CO)4 in methanol has b n reviewed (Scheme 15). Accordbgly, dienoates (68) (35-80%) were obtained in a regio- and stereo-selective manner corresponding to a metallo-ene cis addition of an allylnickel compound at the less-substituted termirud C-1 of (67), followed by a CO insertion into the resulting vinylnickel species. The dienyl esters (68) were accompanied by variable amounts of cyclopentenones, indicating the possibility of consecutive CO insertion processes. 

Additives (cosolvents) which serve as ligands have great influence on the reducing power of Smlj. Allylic and propargylic derivatives are reduced via it-allylpalladium species, and the proton source has important effects on the generation of allenes or alkynes. Chiral allenic esters are obtained when pantolactone delivers a proton to racemic organosamarium species derived from 4-phosphato-2-alkynoic esters. ... 

Chatgilialoglu and Curran have found that allyltris(triniethylsilyl)silanes react with a variety of alkyl halides to provide allylation products via an Sh2 process mediated by the tris(trimethylsilyl)silyl (TTMSS) radical (Scheme 10.199) [523]. In this system the allylsilanes work as radical-allylating agents and TTMSS radical sources. We have used the reactivity of allyltris(trimefhylsilyl)silanes for allylsilyla-tion of alkenes and alkynes via a radical chain mechanism (Scheme 10.199) [524]. 

Alkylations and silylations. Moderately stabilized allylic and propargylic anions are readily generated by treating substituted alkenes and alkynes with BuLi. l-(Benzotriazol-l-yl)propargyl ethyl ethers are a convenient source of alkynyl ketones by virtue of their facile alkylation. 

Figure 1 shows typical organophosphorus substrates, which usually act directly, or after activation at a metal center, as nucleophiles. Most include a reactive P-H bond, which, for P(V) substrates, is involved in an important tautomerization equilibrium which interconverts four- and three-coordinate compounds 1 and 2. The source of electrophilic carbon for C-P bond formation is usually unsaturated (alkynes, alkenes, aldehydes, etc), or contains a good leaving group (aryl and alkyl halides, allyl acetates). 

Pd-catalyzed hydrogenolysis of allylic compounds with formates is an efficient and mild method. The hydride generated from the palladium formate attacks the more substimted side of the allylic system to give less substituted olefins in contrast to the case with other hydride sources. Pd-catalyzed hydrogenolysis of propargylic compounds affords either aUenes or alkynes depending on the structure of the propargylic compounds. 

Takemoto et al reported that ln(OTf)3 catalyzed the tandem allylation and cyclization of O-alkynylarylaldimines to form 1,2-dihydroisoquinolines, with a proton source to accelerate the rate of the reactions [233] (Figure 8.110). The authors proposed two pathways for the reaction, with the ln(OTf)3 functions either to activate the imine for an initial allylation followed by cyclization, or to activate the alkyne for cyclization followed by allylation. The authors also reported that nucleophiles other than allyltributyl stannane, notably silyl enol ethers, could also be used. 

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