Triflates, vinyl reaction

In subsequent studies, methyl vinyl ketone (2.0 mmole) was chosen as the dienophile so as to determine the combined effect of the ionic liquid (2 mL) and the Lewis acids (0.2 and 0.5 wt%) upon the yield and selectivity. Without the Lewis acid catalyst, this system demonstrated a 52% conversion of the cyclopentadiene (2.2 mmol) in 1 h with the endojexo selectivity being 85/15. The cerium triflate-catalyzed reaction was quantitative in 5 min and the endo. exo selectivity was very good for this experiment as well (94 6, endo. exo). Also with the scandium or yttrium salts tested, reactions came to completion in a short time with high stereo-selection. Cerium, scandium and yttrium triflates are strong Lewis acids known to be quite effective catalysts in the cycloadditions of cyclopentadiene with acyclic aldehydes, ketones, quinones and cycloalkenones. These compounds are expected to act as strong Lewis acids because of their hard character and the electron-withdrawing triflate group. On the other hand, reaction times of 1 hour were required for... 

Palladium-catalyzed arylation of the electron-rich olefin bntyl vinyl ether has been accomplished in the ionic liquid l-butyl-3-methylimidazolium tetrafluoroborate using as the arylating agents aryl iodides and bromides instead of the commonly used, but commercially unavailable and expensive, aryl triflates. The reaction proceeds with high efficiency and remarkable regioselectivity, leading almost exclnsively to substitution by various aryl groups at the olefinic carbon a to the heteroatom of butyl vinyl ether (Xu et ak, 2001). 

Organozinc reagents, including the Reformatsky reagents, are extensively used in transition metal catalysed coupling reactions with aryl halides or triflates, vinyl halides, and allylic halides or acetates, as reviewed by Erdik156. Nickel and palladium complexes are... 

Muratake et al. reported the intramolecular a-arylation of ketones [55,56]. Thus, polycyclic compounds are readily obtained from aromatic keto-bromides and keto-triflates (Eq. 16). Bromo-amides can give the corresponding cycliza-tion products (Eq. 17) [52]. Related intramolecular vinylation reactions to give aliphatic polycyclic compounds have also been reported (Eq. 18) [57,581. The intramolecular cyclization of aromatic halo-ketones under carbon monoxide, which proceeds by mechanism C, gives the corresponding a-acylated products (Eq.l9) [321. 

This hindered, non-nucleophilic base has been used in the alkylation of arenes with vinyl triflates. This reaction proceeds via vinyl cations. ... 

Enol triflates have emerged as attraetive alternatives to vinyl halides in the Stille coupling partner due to their ease of preparation from readily available carbonyl compounds. The addition of LiCI has been found to be beneficial to Stille enol triflate coupling reactions. Thus, it was not surprising that coupling 5-tributylstannylpyrimidine to enol triflate 134 proceeded in good yield in the presence of LiCI. In this case, the addition of Cul as cocatalysts was found to also be beneficial to the reaction outcome [57]. 

Carbonylative Carbon-Carbon Bond Formation. A general, mild (50 °C), and high yielding conversion of halides and triflates into aldehydes via Pd(Ph3P)4-catalyzed carbonylation (1-3 atm CO) in the presence of Tributylstannane has been described (eq 21). The range of usable substrates is extensive and includes Arl, benzyl and allyl halides, and vinyl iodides and triflates. The reaction has been extended to include ArBr by carrying out the carbonylation at 80 °C under pressure (50 atm CO), using poly(methylhydrosiloxane) (PMHS) instead of tin hydride. ... 

The palladium-catalysed Mizoroki-Heck reaction is the most efficient route for the vinyla-tion of aryl/vinyl halides or triflates. This reaction, in which a C—C bond is formed, proceeds in the presence of a base (Scheme 1.1) [1, 2], Nonconjugated alkenes are formed in reactions involving cyclic alkenes (Scheme 1.2) [le, 2a,c,e,g] or in intramolecular reactions (Scheme 1.3) [2b,d-g] with creation of stereogenic centres. Asymmetric Mizoroki-Heck reactions may be performed in the presence of a chiral ligand [2], The Mizoroki-Heck reaction has been intensively developed from a synthetic and mechanistic point of view, as expressed by the impressive number of reviews and book chapters [1,2]. 

Use of sodium bicarbonate or sodium carbonate in the presence of a phase transfer catalyst permits especially mild conditions to be used for many systems. Pretreatment with nickel bromide causes normally unreactive aryl chlorides to undergo Pd-catalyzed substitution. Solid phase catalysts in which the palladium is com-plexed by polymer-bound phosphine groups have also been developed. Aryl triflates have also been found to be excellent substrates for Pd-catalyzed vinylations. Scheme 8.6 illustrates the vinylation reaction. 

Two papers by the same authors have appeared on the alkylation of monosub-stituted benzene derivatives with vinyl triflates (e.g. R2C=CPh03SCF3, where R = Ph, Me, and 1-cyclohexenyl to cyclo-octenyl triflates). The reactions are carried out in the presence of 2,6-di-t-butyl-4-methylpyridine (a non-nucleo-philic base, to neutralise the triflic acid formed during reaction) and a large excess of the aromatic substrate. The mechanism is thought to involve a vinyl cation intermediate. 

In contrast, the Stille reaction was one of the first cross-coupling reactions performed on solid support. For the palladium-catalyzed coupling of various aryl, vinyl, or alkynyl stannanes with aryl or alkenyl bromides, iodides, or triflates, the reaction conditions employed were chosen in analogy to the liquid phase procedures and often feature an arsine or trifurylphosphine as an added ligand. Due to the tedious separation of tin and organo tin reagents or by-products in solution phase, this carbon-carbon bondforming reaction is particularly suitable for solid-phase synthesis. ... 

Vinyl acetate reacts with the alkenyl triflate 65 at the /3-carbon to give the 1-acetoxy-1,3-diene 66[68]. However, the reaction of vinyl acetate with 5-iodo-pyrimidine affords 5-vinylpyrimidine with elimination of the acetoxy group[69]. Also stilbene (67) was obtained by the reaction of an excess of vinyl acetate with iodobenzene when interlamellar montmorillonite ethylsilyl-diphenylphosphine (L) palladium chloride was used as an active catalyst[70]. Commonly used PdCl2(Ph3P)2 does not give stilbene. 

Vinylation can also be done by Pd-catalysed cross-coupling in which one component is used as a halide or triflate and the other as a stannane (Stille reaction) or boronic acid (Suzuki reaction). Entry 9, Table 11.3, is an example of the use of a vinylstannane with a haloindole. lndole-3-boronic acids, which can be prepared by mcrcuration/boration, undergo coupling with vinyl triflates (Entry 10). 

Indoles with carbocyclic halogen or triflate substituents are potential starting materials for vinylation, arylation and acylation via palladium-catalysed pro-cesses[l]. Indolylstannanes. indolylzinc halides and indolylboronic acids are also potential reactants. The principal type of substitution which is excluded from such coupling reactions is alkylation, since saturated alkyl groups tend to give elimination products in Pd-catalysed processes. 

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