Double-functionalized arenes

Softer nucleophiles N, for example, H or PBub, add directly (reversibly in the latter case) to the exo face of the arene to give the [Re(j7 -C6R6N-6-exo)(CO)3] (142) complexes without any participation of the metal center. Attempted extension to Re complexes of the well-established methodology for double functionalization of arenes earlier developed for the Mn analogs on the basis of [Mn(j -dienyl)(CO)2(NO)]+ (see Manganese Organometallic Chemistry) is thwarted by the limitations foimd in the preparation of the cations [Re()]5 -dienyl)(CO)2 (NO)]+. 

Nanostructured Carbon Materials for Catalysis 24 Heterocycles from Double-Functionalized Arenes Transition Metal Catalyzed Coupling Reactions 25 Asymmetric Functionalization of C—H Bonds... 

In this book, we are going to discuss the application of transition metal catalysts in the synthesis of heterocycles by using double-functionalized arenes as substrates. The chapters are organized by the size of the rings formed and sub-divided by the substrates applied. In order to make this book more applicable and readable, the preparation of the double-functionalized arenes applied will be mentioned first. 

For the double-functionalized arenes applied, in general, they can be divided into three main analogues (Table l.l). One is 1,2-dihaloarenes, also called 1,2-di-electrophilic arenes (DBA) the next is orf/ro-halogen activated arenes, also called 1,2-electrophilic-nulceophilic coexisted arenes (ENA) the third is called 1,2-di-nulceophilic arenes (DNA). 

Heterocycles from Double-Functionalized Arenes Transition Metal Catalyzed Coupling Reactions... 

Table 1.1 List of frequently used double-functionalized arenes. ...

Here, we can conclude that halogenation can effectively activate the parent molecules of double-functionalized arenes. 

Allenes as an interesting class of unsaturated compounds have been explored in cyclizations with 2-iodoanilines and 2-iodophenols as well. In 1991, Larock and co-workers reported the cyclization of allenes with double functionalized arenes. Various five- and six-membered heterocycles were formed in high yields (Scheme 2.85). But the main problem is the long reaction time one or two days are usually needed. 

In summary, the main applications of double-functionalized arenes in the synthesis of five-membered heterocycles have been discussed. Various types of five-membered heterocycles were produced by the combination of different functional groups. In the following chapter, the preparation of six-membered heterocycles will be discussed. 

In summary, the main achievements in the transition-metal-catalyzed synthesis of heterocycles from double-functionalized arenes have been summarized and discussed. 

Macroheterocycles have many special properties and have been reported with applications in various areas. Procedures for preparing macroheterocycles have been developed as well among them, the procedures based on using double-functionalized arenes as substrates will be summarized and discussed in this chapter. 

In summary, the main achievements in the synthesis of macrocyclic heterocycles from double functionalized arenes have been summarized and discussed. Compared with five- and six-membered heterocycles, macrocycles have been explored less. With the special reactivities and properties of macroheterocycles, especially the seven- and eight-membered cases, it will be interesting and necessary to work on this topic. 

The preparation of heterocyclic compounds is one of the main branches of organic chemistiy due to the prevalence of heterotycles in various areas. More than four out of five of the best-selling drugs in the USA in 2013 have contained at least one heterocycle in their structure. Hence, efforts to develop efficient procedures to synthesize heterocycles are in demand. Nowadays, it is clear that current research is focused on mono-functionalized arenes, which are employed in C-H activation-functionalization reactions. But most of the developed C-H activation methodologies have met with problems such as harsh reaction conditions, high catalyst loading etc. As the topic of heterocycle synthesis has become more related to fine chemical synthesis and pharmaceuticals, there is more of a focus on the reaction efficiency, functional group tolerance and related parameters. All these points lead to one conclusion a book on heterocycles produced from double functionalized arenes is necessary. 

The main contributions to the synthesis of heterocycles based on double-functionalized arenes as substrates have been summarized and discussed. The contents are catalogued by the size of the ring formed and sub-divided by the substrates applied. The reactions included here usually have high efficiency and selectivity. However, the main disadvantage for most of the procedures mentioned is the requirement of pre-activation of the starting materials, which sometimes means that the substrate scope is narrow and limited. In the future, the development of highly efficient methodologies without pre-activation of substrates, such as C-H activation, will be in demand. 

Highly diasteroselective and chemoselective reductions may be performed on the hydroxy functions of (r/6-arene)-tricarbonylchromium complexes. Treatment of the chromium-complexed benzylic alcohol 29 with triethylsilane and boron trifluoride etherate in dichloromethane at —78° to 0° gives only diastereomer 30 in 75% yield (Eq. 40).181 In a similar fashion, treatment of the complexed exo-allyl-endo-benzylic alcohol 31 with an excess of Et3SiH/TFA in dichloromethane at room temperature under nitrogen produces only the endo-aflyl product 32 in 92% yield after 1.5 hours (Eq. 41). It is noteworthy that no reduction of the isolated double bond occurs.182... 

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