Reduction pericyclic reaction

Interest in the aqueous medium spread quickly and many, sometimes surprising, discoveries were made [3]. Today pericyclic [4], condensation [5], oxidation [6] and reduction [7] reactions are routinely carried out in aqueous medium. The recent discovery of water-tolerant Lewis acids such as lanthanide triflates, Bi(OTf)j, Sc(OTf)j and Y(OTf)j has revolutionized organometallic chemistry [5a, 7]. 

The proposed mechanism involves either path a in which initially formed ruthenium vinylidene undergoes nonpolar pericyclic reaction or path b in which a polar transition state was formed (Scheme 6.9). According to Merlic s mechanism, the cyclization is followed by aromatization of the ruthenium cyclohexadienylidene intermediate, and reductive elimination of phenylruthenium hydride to form the arene derivatives (path c). A direct transformation of ruthenium cyclohexadienylidene into benzene product (path d) is more likely to occnir through a 1,2-hydride shift of a ruthenium alkylidene intermediate. A similar catalytic transformation was later reported by Iwasawa using W(CO)5THF catalyst [14]. 

Cycloadditions are characterized by two components coming together to form two new o-bonds, at the ends of both components, joining them together to form a ring, with a reduction in the length of the conjugated system of orbitals in each component. Cycloadditions are by far the most abundant, featureful, and useful of all pericyclic reactions. 

Photochromic compounds functioning by an oxidation-reduction mechanism (electron transfer), especially a photoreduction mechanism, are known in inorganic materials such as silver halides, which are utilized in eyewear lenses. Although the number of organic photochromic compounds operating via electron transfer is fewer than those by isomerization, heterolytic (or homolytic) cleavage, and pericyclic reactions, several classes of compounds have been reported, such as thiazines,1 viologens,2 and polycyclic quinones.3... 

Dewar, M. J. S., McKee, M. L. Ground states of molecules. 50. MNDO study of hydroboration and borohydride reduction. Implications concerning cyclic conjugation and pericyclic reactions. J. Am. Chem. Soc. 1978,100, 7499-7505. 

The triester (38) was needed for an investigation into intramolecular pericyclic reactions between electron-rich (a) and electron-poor (b) double bonds.A Wittig disconnection on double bond (b) (nearer the centre of the molecule) demands an a-dicarbonyl compound whichever way we write it. The keto diester (39) can easily be made from a malonate ester by a-oxidation, so this route is preferred. Further disconnection of phosphonium salt (40) suggests allylic alcohol (41) as intermediate and hence regioselective reduction (Chapter 14) of a,/3-unsaturated aldehyde (42) (Chapter 19). 

In addition to the classical reactions, this book covers many techniques and reactions that have more recently gained wide use among practicing chemists. Molecular-orbital theory is introduced early and used to explain electronic effects in conjugated and aromatic systems, pericyclic reactions, and ultraviolet spectroscopy. Carbon-13 NMR spectroscopy is treated as the routine tool it has become in most research laboratories, and the DEPT technique is introduced in this edition. Many of the newer synthetic techniques are also included, such as asymmetric hydrogenation and epoxidation, use of sodium triacetoxyborohydride, Birch reduction, Swern oxidations, alkylation of 1,3-dithianes, and oxidations using pyridinium chlorochromate. 

In our previous book on domino reactions [4h], we have classified domino reactions according to the mechanism of the different steps. This organizing principal will also be used in this book, and you will find chapters about transition metal catalysis including carbonylation, metathesis and CH-activation, nucleophiHc substitutions, radical reactions, pericyclic reactions, Michael reactions, aldol reactions, oxidations, and reductions. 

Reduction-triggered pericyclic reaction has been used for the biomimetic total synthesis of endiandric acids A-G [72]. Endiandric acids are polycychc natural products isolated from the Australian plant Endiandra introrsa and exist naturally as a racemic mixture which is found to have very high antibacterial effect [73]. In 1980, Black and coworkers [74] proposed a hypothesis that these polycyclic systems are formed in nature by a series of electrocycUzation reactions. On the basis of this hypothesis, Nicolaou et al. [72c] in 1982 reported an excellent domino reduction/electrocyclization process for the synthesis of endiandric acid A 127 (Scheme 9.25). Selective hydrogenation using a Lindlar catalyst of diacetylenic diol 123 afforded the bicyclic diol 126. The reaction presumably proceeds via polyene 124, which then undergoes a spontaneous 8 [i-electrocyclization to give 125. It follows a second 6n-electrocyclization to afford bicyclic 126, which is further converted to endiandric acid A 127. 

Pericyclic reactions provide some of the most elegant examples of the importance of orbital symmetry in chemical reactions. Unlike in organic chemistry, however, pericyclic reactions are not of great importance in inorganic chemistry. That said, we will encounter a few significant examples in this book, including the reduction of carbon-carbon double bonds by diimide (Section 5.7a) and certain selenium dioxide oxidations (Section 6.16). 

The mechanism of olefin metathesis does not involve the classic reactions we have covered—namely, oxidative addition, reductive elimination, (3-hydride elimination, etc. Instead, it simply involves a [2+2] cycloaddition and a [2+2] retrocycloaddition. The [2+2] terminology derives from pericyclic reaction theory, and we will analyze this theory and the orbitals involved in this reaction in Chapter 15. In an organometallic [2+2] cycloaddition, a metal alkylidene (M=CR2) and an olefin react to create a metal lacyclobutane. The metalla-cyclobutane then splits apart in a reverse of the first step, but in a manner that places the alkylidene carbon into the newly formed olefin (Eq. 12.83). Depending upon the organometallic system used, either the alkylidene or the metallacycle can be the resting state of the... 

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