Aryl Unit

The electronic nature of the substituent on the aryl unit actually brings about influences to the rate and the direction of the rearrangement, but it is a minor effect Other experimental conditions (solvent, reaction temperature, and reaction time) are more important for determining the rate and the isomer ratio ortho vs. para) of the rearranged product In some cases these observations are interpreted in terms of possible polar mechanisms for the aromatic Claisen rearrangement [15]. 

A number of Claisen rearrangements of hetero-aromatic compounds have been reported, which are also an important means for the preparation of substituted heterocycles [18]. Under thermal conditions, the reaction of 2-allyloxypyridine 13 gave a mixture of N-allylated and C-allylated products 14,15 [19]. A transition metal catalyst is effective for the selective formation of N-allylated product 14 under mild conditions [20]. 

In general, the substituent on the aUyl moiety exerts a slight effect on the reaction rate [21]. Goering et al. reported the relative rates for the ortho rearrangement of allyl and methylated allyl phenyl ethers [15b] in diphenyl ether. Except for the a-methylated substrate 16, which markedly enhances the rate, the relative rates of and y-methy-lated substrates 17,18 are similar to that of the parent allyl compound 1. 

The effect of geometry of the allyl group on the rate of Claisen rearrangement was examined by the use of both E and Z isomers of crotyl and drmamyl derivatives [22]. The reaction was carried out around at 200 °C in diphenyl ether. The results indicated that the E isomer is sUghtly more reactive than the Z isomer. 

Propargyl aryl ether is also an attractive substrate of the Claisen rearrangement for the construction of heterocycles such as chromene derivatives. 

---

The MH-type reaction of silanols and organotin compounds with olefins via a Pd(II)-mediated pathway has been reported by Hiyama and co-workers. Based on this pathway, a plausible MH-type reaction mechanism with arylboronic acids was presented in Fig. 26. According to this mechanism, the aryl unit migrated to... 

The concept of intramolecular coordination by the use of C, Y-chelating ligands of aryl units has been well established. However, the analogous chemistry of its cousin, the three-dimensional carborane, remains relatively unexplored. Recently, o-carborane has attracted much interest due to its ease of preparation and derivati-zation, thermal stability, and steric bulk. The easy preparation of o-carborane can be utilized for preparing useful compounds similar to the C, Y-chelating ligands of... 

The main method used to synthesise polysilanes 10.7 involves the thermally induced Wurtz coupling reaction of organodichlorosilanes with alkali metals [eqn (10.7)]. Although improvements in this process have been reported e.g. by the use of ultrasound), the harsh conditions for this reaction tend to limit the side groups that can be successfully introduced to non-functionalised alkyl and aryl units and makes scale-up unattractive. 

Further use of the Suzuki polymerization has yielded polymeric precursors to planar PPP derivatives 27 [105, 106] (Tour 93 and Lamba). The strategy, shown above in Scheme 32, utilized a postpolymerization cyclocondensation reaction to afford imine-bridged aryl units. Optical absorption data showed that the planarized polymers exhibited large bathochromic shifts relative to the un-cyclized parent polymers. 

Chan has reported that substitution of the 1,3-propa-dioxy ether linkage with a (25,35)-buta-dioxy ether linkage, as in 114, resulted in complete atropdiastereoselective coupling of the aryl units (Scheme 12.42). Reduction of the phosphine oxide (/ )-115 resulted in (R)-116 in 96% yield. This procedure eliminates a resolution step.140... 

In this section, we propose a new switching unit, in which two radicals are placed in the same aryl unit and Jt-conjugated chain is extended from 2- and 5-positions of the thiophene ring in one aryl unit of the diarylethene [70]. The photochromic reactivity and magnetic switching of the new diarylethene derivatives will be discussed. 

Group-selective cyclizations to aryl units (306c >307c) indicated that the initial intramolecular amidocupration leads to equatorially oriented radicals, which cyclized to... 

Taylor and colleagues discovered recently that IV-aryl malonamides or cyanoacetamides 312 cyclized in 53-92% yield to oxindole-3-carboxylates 313 in the presence of 5 mol% of Cu(OAc)2 using air as the stoichiometric reoxidant for the catalyst (Fig. 83) [412]. The reactions occur likely by initial formation of a copper enolate of 312. SET oxidation with elimination of CuOAc gives radical 312A, which undergoes a 5-exo cyclization to the aryl unit. A final oxidative... 

The oxidative coupling of two aryl units 1 leads formally to a loss of two electrons and two protons (Scheme 1). 

Generally, when two different aryl units are involved in a coupling reaction, the oxidation potentials of the two units will be different. In another mechanistic variant, the more easily oxidized phenol 14 can react with the other unperturbed aromatic unit 15 to furnish a biaryl radical 16, which then oxidizes further, deprotonates, and finally tautomerizes to give the product biaryl (Scheme 4). 

It is difficult to make generalizations about the oxidative coupling of arenes, given the number of possible mechanisms. Factors that influence the reaction course include the oxidation potential of each aryl unit, the substitution pattern, the solvent system, and the oxidation reagents themselves. Optimization then becomes a case-by-case effort in order to obtain the best selectivities and the best yields. Nevertheless, results over the last decade have... 

Very recently, Node and co-workers improved the efficiency of this particular oxidative phenolic coupling in the context of a synthesis of (+)-galanthamine (51a) [50]. By using a trialkoxyarene as one of the aryl units, they were able to obtain yields of 56b of up to 90 % when the nitrogen was protected with a formyl group and the donor aryl s oxygen atoms were capped by benzyl moieties (Table 11). These authors were even able to isolate an interesting narwedine-type product 57a in low yield. 

Scheme 39. Strategies for the asymmetric oxidative coupling of aryl units.

These examples illustrate one important point whereas oxidative arylic coupling is the natural (biosynthetic) pathway for linking two aryl units, it cannot be assumed that reproducing the reaction in vitro will furnish the same stereochemical outcome. This same issue of atrop-selectivity upon biaryl coupling was addressed by Evans and co-workers in an attempted biomimetic synthesis of 197, the M(5-7) unit of vancomycin [134, 135]. The more stable, natural isomer 197b is obtained only as a minor component in this kinetically controlled Ar-Ar coupling reaction (Scheme 48). However, subsequent thermal equilibration leads to a preponderance of the desired compound. 

Some natural compounds offer a chiral structural backbone that biases the outcome of the oxidative coupling of appended aryls (e.g., the ellagitannins). It was plausible to suppose, therefore, that two aryl units could be linked by a non-natural chiral tether to induce atrop-selective coupling upon exposure to an appropriate oxidant. In one of their attempts to realize the total synthesis of calphostin D (200) [136], Merlic and co-workers showed that, in the presence of dioxygen in trifluoroacetic acid (TFA), the precursor 198 affords the coupled compound 199 as a single diastereoisomer. Unfortunately, the relative configuration was incorrect for the calphostin target (Scheme 49). 

The vast majority of the currently known heteronuclear cluster compounds containing M(PR3) (M = Cu, Ag, or Au, R = alkyl or aryl) units were synthesized by treating an anionic mono-, di-, or polynuclear precursor with the complexes [MX(PR3)j (X = Cl, Br, or I). The first example of... 

R = Alkyl or Aryl) Units, with No Close Contact between the Group IB Metals... 

A highly stereoselective intramolecular couplings of 196, where two aryl units are connected through a chiral 1,4-dioxy moiety, afforded the chiral 197 (Equation 42) <1997TL1087, 1998CC2713, 2004PNA5815>. 

---