Conjugated diene reaction with

Another reaction unique to conjugated dienes is the Diels-Alder cycloaddition. Conjugated dienes react with electron-poor aikenes (dienophiles) in a single step through a cyclic transition slate to yield a cyclohexene product. The reaction is stereospecific, meaning that only a single product stereoisomer is formed, and can occur only if the diene is able to adopt an s-cis conformation. 

The use of chiral bis(oxazoline) copper catalysts has also been often reported as an efficient and economic way to perform asymmetric hetero-Diels-Alder reactions of carbonyl compounds and imines with conjugated dienes [81], with the main focus on the application of this methodology towards the preparation of biologically valuable synthons [82]. Only some representative examples are listed below. For example, the copper complex 54 (Scheme 26) has been successfully involved in the catalytic hetero Diels-Alder reaction of a substituted cyclohexadiene with ethyl glyoxylate [83], a key step in the total synthesis of (i )-dihydroactinidiolide (Scheme 30). 

The mode of reaction of titanacydobutenes with carbonyl compounds is largely dependent on steric factors (Scheme 14.31) [72]. Ketones and aldehydes tend to insert into the titanium—alkyl bond of 2,3-diphenyltitanacydobutene, and homoallylic alcohols 70 are obtained by hydrolysis of the adducts 71 [65a,73]. On the contrary, when dialkyl-substi-tuted titanacydobutenes are employed, the reaction with aldehydes preferentially proceeds through insertion into the titanium—vinyl bond. Thermal decomposition of the adducts 72 affords conjugated dienes 73 with E-stereoselectivity as a result of a concerted retro [4+2] cycloaddition [72]. 

The axially chiral (allenylmethyl) silanes 110 were also prepared in optically active form using chiral Pd catalysts [98]. For the asymmetric synthesis of 110, a Pd/(R)-segphos system was much better in terms of enantioselectivity than the Pd/(R)-binap catalyst. Under the optimized conditions, 110m and llOt were obtained in 79% ee (57% yield) and 87% ee (63% yield), respectively (Scheme 3.56). The enantio-merically enriched (allenylmethyl) silanes 110 served for Lewis acid-promoted SE reaction with tBuCH(OMe)2 to give conjugated dienes 111 with a newly formed chiral carbon center (Scheme 3.56). During the SE reaction, the allenic axial chirality was transferred to the carbon central chirality with up to 88% transfer efficiency. 

Rees and co-workers in their study of the reactions of trithiazyl trichloride in the preparations of heterocyclic compounds have noted that the isothiazolo[5,4-, isothiazole compound 140 was produced in low yield on reaction with conjugated dienes, along with the other heterocyclic systems 142-145 in much higher yields (Equation 28). Since it is known that trithiazyl trichloride is in thermal equilibrium with its monomer NSCl (Equation 29), the authors propose the so-called criss-cross cycloaddition reaction (Equation 30) which has been reported for azabu-tadienes, but this represents the first example of such a criss-cross cycloaddition to an all-carbon diene <1998CC1207>. 

In the Diels-Alder reaction, a conjugated diene reacts with an a,P-unsaturated carbonyl compound, generally called a dienophile. A die-nophile is a reactant that loves a diene. The most reactive dienophiles usually have a carbonyl group, but it may also have another electron-withdrawing group, e.g. a cyano, nitro, haloalkene or sulphone group conjugated with a carbon-carbon double bond. 

Conjugated dienes react with oxygen under the influence of light to give cyclic peroxides 76.77B The reaction has mostly779 been applied to cyclic dienes.780 The scope extends to certain aromatic compounds,781 e.g.,... 

Both stoichiometric and catalytic reactions involving 7r-allylpalladium complexes are known. Reactions involving 7r-allylpalladium complexes become stoichiometric or catalytic depending on the preparative methods of the 7r-allylpalladium complex. Preparation of the 7r-allylpalladium complexes 6 by the oxidative addition of various allylic compounds 5, mainly esters to Pd(0), and their reactions with nucleophiles are catalytic. This is because Pd(0) is regenerated after the reaction with the nucleophile, and the Pd(0) reacts again with allylic compounds to form the complex 6. These catalytic reactions are treated in Section 4.3. However, the preparation of 7r-allyl complexes 6 from alkenes 7 requires Pd(II) salts. Subsequent reaction with nucleophiles generates Pd(0). As a whole, Pd(II) is consumed, and the reaction ends as the stoichiometric process, because in situ reoxidation of Pd(0) to Pd(II) is not attainable in this case. Also, 7i-allylpalladium complex 9 is formed by the reaction of conjugated dienes 8 with Pd(II), and the reaction of 9 with nucleophiles is stoichiometric. 

The nucleophilic 7i-allyltitanium complex 67 is prepared by the reaction of the conjugated diene 65 with titanocene hydride 66, generated in situ by the treatment of titanocene dichloride with 2 moles of z-PrMgCl [21]. The complex is nucleophilic and reacts with aldehydes regio- and stereoselectively to give homoallylic alcohols [22]. 

The reaction is somewhat limited in scope because cyclobutadimerization is a potentially competing reaction for substrates which are ionizable but not sterically hindered, and conjugated dienes react with oxygen under these same conditions to give 1,4-endoperoxides (vide infra). The reaction is best carried out using the more potent, hexabromo salt at -78 °C, and has been found to be incompletely stereospecific. Consequently, a distonic cation radical intermediate has been postulated to play a role in the mechanism (Scheme 14). 

The second reaction unique to conjugated dienes is Diels-AIder cyclo-addition. Conjugated dienes react with electron-poor alkenes (dienophiles)... 

Reactions of the second type are carried out with palladium compounds or complexes of either bivalent or zero-valent states. Since these reactions proceed catalyti-cally without using reoxidants they are more useful than the stoichiometric processes. Telomerization of conjugated dienes, reactions of allylic and alkenyl esters and ethers, and various organic halides belong to this type. 

In the Diels-Alder reaction, a conjugated diene reacts with a dienophile. Tlie dienophile is named for its affinity to react witli the diene. 

Nickel-cataly/.ed hydromagncsialion of conjugated diene s with Grignard reagents seems less fruitful from Ihe s nihenc point of view. Attempted reactions of butadiene and isoptene resulted in a formal hydromagiiesiation of the corresponding... 

In a Diels-Alder reaction, a conjugated diene reacts with a dienophile to form a cyclic compound in this concerted [4 + 2] cycloaddition reaction, two new a bonds are formed at the expense of two tt bonds. The conjugated diene must be in an s-cis conformation. The reactivity of the dienophile is increased by electron-withdrawing groups attached to the sp carbons. The HOMO of one reactant and the LUMO of the other are used to show the transfer of electrons between... 

The Diels-Alder Reaction essentially consists in the direct combination of a compound possessing a conjugated diene system with a reagent that contains either a double-bond or a triple-bond, usually activated by conjugation with additional multiply-bonded systems, such as cyano, carbonyl, nitro, phenyl functions. It ultimately adds on to the 1, 4-positions of a conjugated diene system e.g., buta-1, 3-diene) with the formation of a 6-membered ring. Importantly, the ethylenic (double-bond) or acetylenic (tripple-bond) compormd is normally termed as the dienophile, the second reactant as the diene and the final desired product as the adduct. A few typical examples of such reagents are, namely maleic anhydride, para-benzoquinone, acetaldehyde and acetylene dicarboxylic esters. 

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