Allyl phenyl carbonate

By 1984, the palladium-catalyzed aUyhc alkylation reaction had been extensively studied as a method for carbon-carbon bond formation, whereas the synthetic utility of other metal catalysts was largely unexplored [1, 2]. Hence, prior to this period rhodium s abihty to catalyze this transformation was cited in only a single reference, which described it as being poor by comparison with the analogous palladium-catalyzed version [6]. Nonetheless, Yamamoto and Tsuji independently described the first rhodium-catalyzed decarboxylation of allylic phenyl carbonates and the intramolecular decarboxylative aUylation of aUyl y9-keto carboxylates respectively [7, 8]. These findings undoubtedly laid the groundwork for Tsuji s seminal work on the regiospecific rho-... 

Operation of the insertion-elimination mechanism has been demonstrated in the reaction of rhodium hydride complex, RhHL4 (L=PPh3), with two isomeric allyl phenyl carbonates [56]. Unbranched 2-butenyl phenyl carbonate was found to give branched allylic phenyl ether exclusively, whereas the decarboxylation of the branched l-methyl-2-propenyl phenyl carbonate afforded unbranched 2-butenyl phenyl ether. These results can be accounted for by assuming a precata-lytic and catalytic insertion-elimination process as shown in Scheme 7. 

Evidence for this mechanism comes from the observation that the rearrangement takes place with an inversion of the allyl group. That is, allyl phenyl ether containing a 14C label on the allyl ether carbon atom yields o-allylphenol in which the label is on the terminal vinylic carbon (green in Figure 18.1). It would be very difficult to explain this result by any mechanism other than a pericyclic one. We ll look at the reaction in more detail in Section 30.8. 

Generation and Reaction of Allyltitanium Reagents (Section 9.3) 2-(4-Bromophenyl)-l-phenyl-3-buten-l-ol [42] To a solution of l-(4-bromophenyl)allyl ethyl carbonate (285 mg, 1.0 mmol) and Ti(OiPr)4 (0.296 mL, 1.0 mmol) in diethyl ether (5 mL) was added iPrMgBr (1.20 m in diethyl ether, 2.0 mmol) at 50 °C. The resulting yellow solution was stirred at —50 to —40 °C for 1.5 h, in the course of which it became brown. Benzaldehyde (74.3 mg, 0.70 mmol) was then added at —40 °C and the mixture was allowed to warm to 0 °C over a period of 30 min. After the addition of aqueous 1 n HC1 (5 mL) at this temperature, the mixture was allowed to warm to ambient temperature. The organic layer was separated and the aqueous layer was extracted with diethyl ether (10 mL). The combined organic layers were washed with saturated aqueous NaHC03 solution (5 mL), dried over... 

Some allyl phenyl ethers with an alkyl substituent on the end carbon of the allyl group rearrange to give the normal ortho-Claisen product together with another isomeric O-allyl phenol. The latter, formed by the rearrangement of the normal product, has been established. This is called abnormal Claisen rearrangement, is illustrated by the following example. 

Place 47 g (0.5 mol) of phenol, 60.5 g (0.5 mol) of allyl bromide (Expt 5.54), 69.1 g (0.5 mol) of anhydrous potassium carbonate and 100 ml of acetone in a 250-ml, two-necked round-bottomed flask fitted with a reflux condenser and sealed stirrer unit, and boil on a steam bath for 8 hours with stirring. Pour the reaction mixture into 500 ml of water, separate the organic layer and extract the aqueous layer with three 20 ml portions of ether. Wash the combined organic layer and ether extracts with 2 m sodium hydroxide solution, and dry over anhydrous potassium carbonate. Remove the ether with a rotary evaporator and distil the residue under reduced pressure. Collect the allyl phenyl ether, b.p. 85°C/19mmHg the yield is 57 g (85%). 

Palladium-mediated cydization based on the reactivity of o-alkynyl or alkenyl-phenyl isonitriles have been developed [105]. On the basis of their earlier studies on the three-component synthesis of allyl aryl cyanamides [106], Yamamoto and co-workers reported a palladium-catalyzed three-component coupling reaction of 2-alkynylisocyanobenzenes 122 with allyl methyl carbonate and trimethylsilylazide leading to N-cyanoindoles 125 [107]. One of the key steps of the proposed mechanism is the formation of 7i-allylpalladium carbodiimide 123 and its isomerization to rc-allylpalladium cyanamide complex 124 (Scheme 8.50). 

Alcohols are protected as allyl ethers, which are difficult to cleave with the Pd catalyst and deprotected by other methods [149]. Alcohols are conveniently converted to allyl carbonates 334 by treatment with allyl chloroformate (333). The allyl carbonates are deprotected using HCO2H [150], and HSnBu3 [151]. This method is called the AOC (allyloxycarbonyl) method. Phenols are protected as allyl phenyl ethers, which can be cleaved with HSnBu3 [152]. 

Add a little bromine in carbon tetrachloride to each ether. The allyl phenyl ether, being unsaturated, quickly decolorizes the bromine, but the ethyl phenyl ether does not. 

Preparation of Allyl Phenyl Ether, f A mixture of 188 g. of phenol, 242 g. of allyl bromide, 280 g. of finely ground calcined potassium carbonate, and 300 g. of acetone is refluxed on the steam bath for eight hours. A heavy predpitate of potassium bromide begins to form soon after the refluxing is started. After cooling, water is added the product is taken up in ether and washed twice with 10% aqueous sodium hydroxide solution. The ether solution is dried over potassium carbonate, and, after removal of the ether, the residue is distilled under diminished pressure. The yield is 230 g. (86%), b.p. 85°/19 mm., dll 0.9845. The residue is so small (6 g.) that the distillation might be omitted unless a very pure product is desired. About 1% of allyl 2-allylphenyl ether (a product of C-alkylation) is formed by this procedure. 

The experimental KIEs were determined for the aliphatic Claisen rearrangement in p-cymene at 120°C and for the aromatic Claisen rearrangement either neat at 170°C or in diphenyl ether at 220°C. Changes in 2H, 13C or 170 composition were determined for unreacted substrates. For carbon analysis of allyl vinyl ether the C5 carbon was used as an internal standard. The C4 atom and rneta aryl protons were used as references in analysis of allyl phenyl ether. The 170 analysis was based on a new methodology. The results are summarized in Table 1, along with predicted isotope effects calculated for experimental temperatures by means of different computational methods. The absolute values of predicted isotope effects for C4 and C5 atoms varied with theoretical level and all isotope effects were rescaled to get reference effects equal to 1.000. 

Ethylene reacted with iodonium salts in the presence of a palladium catalyst and a base to afford directly 1,2-bis arylated products (stilbenes). Styrene underwent arylation under similar conditions [44], Allylic cyclic carbonates were efficiently phenylated by diphenyliodonium tetrafluoroborate because of the mild conditions, no ring opening occurred, as was the case when iodobenzene was used. 

We also observed similar phenomena in the reaction of silyl enol ethers with cation radicals derived from allylic sulfides. For example, oxidation of allyl phenyl sulfide (3) with ammonium hexanitratocerate (CAN) in the presence of silyl enol ether 4 gave a-phenylthio-Y,5-un-saturated ketone 5. In this reaction, silyl enol ether 4 reacts with cation radical of allyl phenyl sulfide CR3 to give sulfonium intermediate C3, and successive deprotonation and [2,3]-Wittig rearrangement affords a-phenylthio-Y,6-unsaturated ketone 5 (Scheme 2). Direct carbon-carbon bond formation is so difficult that nucleophiles attack the heteroatom of the cation radicals. 

Transfer of organic groups from tin to carbon electrophiles, e.g. alkyl halides and acyl hahdes, can occur in the presence of a transition metal (e.g. Pd) catalyst (equation 40). Reactivity sequences for elecfrophihc carbon-tin bond cleavages are generally allyl > phenyl > benzyl > vinyl > methyl > higher alkyl. The precise sequence is somewhat dependent on the solvent and electrophile. 

Allyl phenyl ethers undergo an intramolecular [3,3]-sigmatropic rearrangement (the Claisen rearrangement) to form the C-alkyl derivative (Scheme 4.18). A consequence of the electrocyclic mechanisms is that the y-carbon atom of the allyl ether becomes attached to the aromatic ring. 

A mixture of 188 g. (2.0 moles) of phenol, 242 g. (2.0 moles) of allyl bromide [Org. Syntheses Coll. Vol. 1, 27 (1941)], 280 g. of potassium carbonate, and 300 g. of acetone is heated on the water bath under reflux for 8 hours. Excess water is added, and the mixture is extracted with ether. The ethereal solution is washed with dilute aqueous sodium hydroxide solution, then with water, and the ether is removed by distillation. Distillation of the residual oil gives 230 g. (86%) of allyl phenyl ether. The compound boils at 191-192°/760 mm. and 85°/19 mm. 

In preliminary reports, the y-carbon of the carbanion of allyl phenyl sulfoxide has been shown to attack cyclopentenone and cyclohexenone by 1,4-addition to deliver vinyl sulfoxides. " The lithiated carbanion (75) of l-(phenylsulfinyl)-2-octene ( Z = 85 15) adds to 4-(-butoxycyclopent-2-en-l-one (74) to give jy -( )-vinylic sulfoxide (76) and anti-( )-vinylic sulfoxide (77) in the ratio of 79 21. It has been suggested that (76) arises almost exclusively from the ( )-(75), and (77) derives from the (Z)-(75). Both the products have the same geometry about the double bond, but differ in configuration at the allyiic carbon atom (equation 21), ... 

In the precatalytic process the rhodium hydride precursor undergoes insertion into the butenyl carbonate to form an alkylrhodium complex. (3-Elimina-tion yields 1-butene and phenylcarbonatorhodium complex. Upon decarboxylation a phenoxorhodium complex is produced that undergoes the SN2 type reaction with 2-butenyl phenyl carbonate to liberate the branched allylic ether, 1-... 

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