Carbonates 2-aryl allylic

When allylic alcohols are used as an alkene component in the reaction with aryl halides, elimination of /3-hydrogen takes place from the oxygen-bearing carbon, and aldehydes or ketones are obtained, rather than y-arylated allylic alcohoIs[87,88]. The reaction of allyl alcohol with bromobenzene affords dihydrocinnamaldehyde. The reaction of methallyl alcohol (96) with aryl halides is a good synthetic method for dihydro-2-methylcinnamaldehyde (97). 

The nickel-catalysed reaction of aryl, allyl or vinyl sulfides with Grignard reagents has been found to be an effective way for the conversion of carbon-sulfur bonds into carbon-carbon bonds [318]. However, simple aliphatic substrates are inert. 

Similar to the formation of allylmagnesium chloride (25), the oxidative addition of allyl halides to transition metal complexes generates allylmetal complexes 26. However, in the latter case, a 7i-bond is formed by the donation of 7i-electrons of the double bond, and resonance of the n-allvl and 7i-allyl bonds in 26 generates the 7i-allyl complex 27 or (/ -allyl complex. The carbon-carbon bond in the 7i-allyl complexes has the same distance as that in benzene. Allyl Grignard reagent 25 is prepared by the reaction of allyl halide with Mg metal. However, the 7i-allyl complexes of transition metals are prepared by the oxidative addition of not only allylic halides, but also esters of allylic alcohols (carboxylates, carbonates, phosphates), allyl aryl ethers and allyl nitro compounds. Typically, the 7i-allylpalladium complex 28 is formed by the oxidative addition of allyl acetate to Pd(0) complex. 

In the reaction of allylic alcohols, elimination of /1-hydrogen from an OH-bearing carbon takes place to give saturated carbonyl compounds, rather than arylated allylic alcohols [25,26]. The reaction of methallyl alcohol (43) with bromobenzene affords a-... 

Scheme8.5. Palladium-catalyzed cross-coupling reactions of stannanes and other carbon nucleophiles with aryl, allyl, and vinyl bromides [56, 69-72],...

Coupling of Arl and unsaturated epoxides. Palladium-catalyzed coupling of CftHsI with unsaturated epoxides in which the two groups are separated by one or more carbon atoms can provide arylated allylic alcohols as the major product. Highest yields obtain with Pd(OAc)2 as catalyst, 1-3 equiv. of an alkali metal formate as... 

There are very few reports on the aryl-allyl or aryl-benzyl coupling reaction, but these combinations are very popular in other organometalHcs (Scheme 38). Arylation of allyl acetates or carbonates was catalyzed by NiCl2(PPh3)2 or NiCl2(dppf) with net inversion of the stereochemistry of the C-0 bond for... 

Unfortunately, attempts to perform this substitution reaction on cyclohexenol and geraniol led to the exclusive formation of the corresponding silyl ethers. It thus would seem that one requirement for effective carbon-carbon bond formation is that allylic alcohols be secondary and have possess y,y-disubstitution. Pearson, however, discovered a method with less restriction on the natiue of the substrate he used allylic acetates with y-mono-substitution or primary alcohols [96]. Not only ketene silyl acetals but also a diverse set of nucleophiles including aUyl silane, indoles, MOM vinyl ether, trimethylsilyl azide, trimethylsilyl cyanide, and propargyl silane participate in the substitution of y-aryl allylic alcohol 90 to give allylated 91 (Sch. 45). Further experimental evidence suggests that these reactions proceed via ionization to allylic carboca-tions—alcohols 90 and 92 both afforded the identical product 93. 

Allylindoles are generated from 2-C allyl transfer process which accompi corresponding aryl allyl ethers and froo allyl" and propargyl carbonates. 

Unlike all-carbon aryl halide substrates, regioselective Stille coupling of 2,3-dihalofu-rans has been observed. Taking advantage of C(2) activation, allyl furan 130 is available upon coupling of 2,3-dibromofuran 46 with allyl tributyltin [56, 57]. Likewise, dibro-mobenzofurans react exclusively in the 2-position coupling of benzo furan 131 with aryl-stannane 132 furnishes 2-arylbenzofuran 133 in excellent yield [120]. 

The mechanistic details of the mnlti-step Fischer sequence are best represented by the sequence shown below. Labelling stndies proved the loss of the non-aromatic nitrogen as ammonia, and in some cases intermediates have been detected by and NMR spectroscopy. The most important step - the one in which a carbon-carbon bond is made, marked A - is electrocyclic in character and analogous to the Claisen rearrangement of aryl allyl ethers. 

Asymmetric ene reaction of N-sulfinylcarbamatesf The ability of Lewis acids to promote ene reactions (11,413,414 12,389) is useful for asymmetric reactions. Thus the SnCU-promoted reaction of chiral N-sulfinylcarbamates (1) with alkenes results in thermally unstable adducts (2) in 65-91% yield. Use of trans-2-phenylcyclohexanol (13,244) or 8-phenylmenthol as the source of chirality results in high diastereoselective induction in generation of the new carbon to sulfur bond (usually >95 5). This reaction is applicable to both (E)- and (Z)-alkenes, but the former react more readily. These ene adducts can be transformed into optically active allylic alcohols (4) by N-alkylation and conversion to an aryl allylic sulfoxide (3), which undergoes rearrangement in the presence of a thiophile (piperidine) to 4, with retention of configuration at carbon imparted in the ene reaction. The overall process effects enantioselective allylic oxidation of an alkene with retention of the original position of the double bond. 

The most common preparative method to prepare the aryl allyl ether is the Williamson s ether synthesis [la,b]. Typically, aryl allyl ethers can be obtained from phenol derivatives and allylic halide under basic conditions (KjCOj) in refluxing acetone. This method is convenient for the preparation of simple allyl aryl ethers. However, some side reactions such as a competitive C-allylation (Sn2 type reaction) often accompany the formation of undesired byproducts. Mitsunobu reaction of phenol derivatives with allylic alcohols instead of allylic halides can be used under mild conditions [13]. In particular, when the allyl halide is unstable, this procedure is effective instead of the Williamson s ether synthesis. This method is also useful for the preparation of chiral allyl aryl ether from chiral allylic alcohol with inversion at the chiral center. Palladium catalyzed O-allylation of phenols is also applicable, but sometimes a lack of site-selectivity with unsymmetrical allylic carbonate [14] may be a problematic issue. 

The M6L4 coordination cage was also used as a protective group for control of the regioselectivity of nucleophilic substitution of aryl-substituted allylic chlorides [35]. Normally, both a and -y carbons of allylic chlorides can be the sites of reaction and attacked by nucleophile (Fig. 9.22). Therefore, two regioisomeric products are expected in this reaction. 

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