Intermolecular reactions dienes

High levels of asymmetric induction have been achieved in the hydroboration of 1,3-, 1,4-, and 1,5-dienes with thexylborane (482,483,489,490). The first chiral center is formed by an intermolecular reaction. In the second step, the organoborane intermediate undergoes an intramolecular hydroboration, creating the second chiral center with high diastereoselectivity. 

Acyclic diene molecules are capable of undergoing intramolecular and intermolec-ular reactions in the presence of certain transition metal catalysts molybdenum alkylidene and ruthenium carbene complexes, for example [50, 51]. The intramolecular reaction, called ring-closing olefin metathesis (RCM), affords cyclic compounds, while the intermolecular reaction, called acyclic diene metathesis (ADMET) polymerization, provides oligomers and polymers. Alteration of the dilution of the reaction mixture can to some extent control the intrinsic competition between RCM and ADMET. 

Dienes can react intermolecularly or intramolecularly. Intermolecular reactions generate rings, usually alkenes or dienes. Alkenes methasis can be used to form very large rings, including 21-membered lactone rings. Metathesis with vinyl-cyclo-... 

Triple bonds can also participate in the metathesis reaction. Intramolecular reactions give vinylcycloalkenes, whereas intermolecular reactions provide conjugated dienes.301 The mechanism is similar to that for a, to-diene metathesis, but in contrast Reactions involving to diene cyclization, no carbon atoms are lost.302... 

The reaction has been applied in several synthetic contexts. The intermolecular reaction has been used to construct the conjugated diene side chain of mycothiazole, an antibiotic isolated from a sponge. 

A rationale for the cz s-selective cyclization for the intramolecular homoal-lylation of oo-dienyl aldehyde 64 is illustrated in Scheme 16. The scenario is essentially the same as the one proposed for the intermolecular reaction, and a Ni(0) species undergoes oxidative addition upon the diene and the aldehyde moieties through a conformation placing the aldehyde substituent and the diene anti to each other. An intermediate 66 undergoes (>-II elimination and czs-reductive elimination of the thus-formed Ni - H complex to produce 65. 

Murakami and coworkers [267] described a combination of an intermolecular ene-yne metathesis followed by a disrotatory 6ir-electrocyclic reaction to give six-membered cyclic dienes. Thus, reaction of 6/3-98 and of styrene (6/3-99) in the presence of a ruthenium catalyst at 100 °C led to the condensed cyclohexadienes 6/3-100, in good yield (Scheme 6/3.29). 

In Section 9.2, intermolecular reactions of titanium—acetylene complexes with acetylenes, allenes, alkenes, and allylic compounds were discussed. This section describes the intramolecular coupling of bis-unsaturated compounds, including dienes, enynes, and diynes, as formulated in Eq. 9.49. As the titanium alkoxide is very inexpensive, the reactions in Eq. 9.49 represent one of the most economical methods for accomplishing the formation of metallacycles of this type [1,2]. Moreover, the titanium alkoxide based method enables several new synthetic transformations that are not viable by conventional metallocene-mediated methods. 

Dailey and colleagues109 employed a domino Diels-Alder reaction to synthesize the complex hexacycle 146. The intermolecular reaction of tetracycle 143 with maleic anhydride 144 afforded a single adduct (145) which immediately underwent an intramolecular Diels-Alder reaction to give 146 (equation 42). This reaction is similar to a reaction performed previously by Prinzbach and colleagues110. Prinzbach observed that when alkynes were used as dienophiles, either domino or pincer Diels-Alder reactions occurred. In the latter type, the triple bond reacts with both diene units. 

The most widely exploited photochemical cycloadditions involve irradiation of dienes in which the two double bonds are fairly close and result in formation of polycyclic cage compounds. Some examples are given in Scheme 6.7. Copper(I) triflate facilitates these intramolecular additions, as was the case for intermolecular reactions. 

A domino reaction,in this case consisting of an inter- and an intramolecular Diels-Alder reaction, is a key step in the synthesis of the hydrocarbon pago-dane 30, reported by Prinzbach et al When the bA-diene 27 is treated with maleic anhydride 4, an initial intermolecular reaction leads to the intermediate product 28, which cannot be isolated, but rather reacts intramolecularly to give the pagodane precursor 29 ... 

Furo[2,3- ]pyridines are generated in a cyano [4-1-2] reaction that uses tungsten alkynols and aldehydes which cyclize to 1,3-dienes prior to reaction with nitriles (Scheme 9) <1998JA4520>. The reaction, which is activated by photolysis or with Me3N0-H20, works with unactivated nitriles in both intramolecular and intermolecular reactions to give moderate to good yields of product. 

Cyclopropane formation occurs from reactions between diazo compounds and alkenes, catalyzed by a wide variety of transition-metal compounds [7-9], that involve the addition of a carbene entity to a C-C double bond. This transformation is stereospecific and generally occurs with electron-rich alkenes, including substituted olefins, dienes, and vinyl ethers, but not a,(J-unsaturated carbonyl compounds or nitriles [23,24], Relative reactivities portray a highly electrophilic intermediate and an early transition state for cyclopropanation reactions [15,25], accounting in part for the relative difficulty in controlling selectivity. For intermolecular reactions, the formation of geometrical isomers, regioisomers from reactions with dienes, and enantiomers must all be taken into account. 

In their pioneering work on the catalytic carbopalladation reaction of 1,2-heptadiene with phenyl iodide in the presence of a suitable base, Shimizu and Tsuji observed the formation of the corresponding substituted 1,3-dienes 62 via a / -hydride elimination from the 7z>allyl intermediate 61 [61]. Based on these observations, a three-component Heck-Diels-Alder cascade process has been developed by Grigg and co-workers [73]. A wide variety of aryl and heteroaryl iodides were used for the intermolecular reaction with dimethylallene to afford the corresponding 1,3-dienes. These subsequently react in situ with N-methylmaleimide to give the bicyclic adducts 63 (Scheme 8.30). 

Oxidative cyclization is another type of oxidative addition without bond cleavage. Two molecules of ethylene undergo transition metal-catalysed addition. The intermolecular reaction is initiated by 7i-complexation of the two double bonds, followed by cyclization to form the metallacyclopentane 12. This is called oxidative cyclization. The oxidative cyclization of the a,co-diene 13 affords the metallacyclopentane 14, which undergoes further transformations. Similarly, the oxidative cyclization of the a,co-enyne 15 affords the metallacyclopentene 16. Formation of the five-membered ring 18 occurs stepwise (12, 14 and 16 likewise) and can be understood by the formation of the metallacyclopropene or metallacyclopropane 17. Then the insertion of alkyne or alkene to the three-membered ring 17 produces the metallacyclopentadiene or metallacyclopentane 18. 

Ni alkoxide as a stoichiometric reaction. However, in this reaction the silylnickel alkoxide 76 is formed, and its reductive elimination affords silyl ethers 77 and 78. At the same time, Ni(0) is regenerated to make the reaction catalytic. The formal total synthesis of elaeokanine C (79) was carried out by this reaction [25], Homoallylic alcohol 81 is obtained by the intermolecular reaction of benzaldehyde with the diene 80 and hydrosilane in high regio- and stereoselective manner [26],... 

This intramolecular reaction results in the formation of a cyclic system, and therefore it is called ring-closing metathesis (RCM). In this process a diene 36 is treated with a metal alkylidene 37. Two competing pathways are available via the intermediate metal alkylidene 38 A) RCM will occur to afford cyclic adducts 39 and B) intermolecular reaction can occur to form polymeric structures 40 (acyclic diene metathesis polymerization (ADMET)). The reaction is also complicated because of the possibility of ring-opening metathesis (ROM), the retro reaction of path A, and ring opening metathesis polymerization (ROMP) (path C).13... 

In the ring-closing metathesis reaction, intramolecular metathesis closes a ring to form a small cyclic molecule with concurrent loss of a small molecule (ethylene). Conversely, in the case of the acyclic diene metathesis reaction, macromolecules are formed by successive intermolecular condensation of two olefinic molecules [1],... 

Sensitized intramolecular reaction of two 1,3-dienes (86) (Sch. 17) yields predominantly the [2+2] adduct 87, with small amounts of [4+4] adduct 89 and little, if any, [4+2] product 88 [58,59], consistent with Hammond results for intermolecular reactions of acyclic dienes (Sch. 4). Benzophenone-sensitized reaction yields a mixture of two isomers of 87. Heating this mixture to 200 °C converts both isomers of 87 to cyclo-octadiene 89 [58]. Unsensitized photoreaction of 86 in the presence of copper(I) triflate gives a significant amount of [4+2] adduct 88. Extended irradiation time converts much of 87 and 89 into 88, as well as producing secondary products [59]. Copper triflate-mediated photocycloaddition of a related tethered diene-monoalkene, gave only the [2+2] adduct [59]. 

The protic acid and Lewis acid-catalyzed [4 + 2] cycloaddition reactions of electron-rich alkenes with imines derived from anilines and aryl aldehydes constitute an extensively explored class of 2-azadienes capable of providing the products of a formaJ Diels-Alder reaction (equation 9).i5.27.i77 a useful extension of these studies and in efforts to increase the rate of the Att participation of simple 2-aza-1,3-buta-dienes in [4 + 2] cycloaddition reactions, Mariano and coworkers have examined the Lewis acid-catalyzed intermolecular reactions of (l ,3 )-l-phenyl-2-aza-l,3-pentadiene with electron-rich dienophiles, including enol ethers. Reductive work-up of the cycloaddition reactions provided the pro-... 

Ri r3 In an alkene metathesis two alkenes react with an appropriate catalyst to form two new alkenes. There are different types of alkene pj2 r4 metathesis reactions The intermolecular reaction is called cross metathesis (CM), whereas intramolecular metathesis is divided into ring-closing metathesis (RCM) and ring-opening metathesis (ROM). Also two polymerization versions of alkene metathesis exist metathesis polymerization of acyclic dienes and ring-opening metathesis polymerization (ROMP). 

The metathesis of acyclic alkadienes and polyenes may follow an inter- or intramolecular pathway. The intramolecular metathesis of an a,tfi-diene yields ethylene and a cyclic alkene, while the intermolecular reaction results primarily in the formation of ethylene and a symmetric triene (eq. (2)). The loss of a small molecule like ethylene serves to drive the equilibrium to the product side. 

Cyclisation of dienes, tandem cyclisation, combination of intramolecular and intermolecular reactions... 

Isolated dienes typically show no "special" stability. Their two re-bonds interact independent of one another and they compete as intermolecular reaction sites depending on steric, kinetic, and thermodynamic factors. Often, tliese types of dienes can be treated merely as larger, more complex alkenes, because the two bonds cannot be said to have communication (proximal electron delocalization due to location). 

Intermolecular reactions of species (69) with simple alkenes have received little attention. Recently, a study of the reaction of 5-ethoxy-2-pyrrolidinone with several 1,3-dienes in the presence of acid was published. When a mixture of ethoxylactam and 2,3-dimethylbutadiene is stirred in neat formic acid, the formates (73) and (74) are isolated as the main products in 43% yield. The bicyclic product (75) is obtained in only 16% (equation 38). If the reaction is carried out in benzene with p-toluenesulfonic acid as catalyst (75) is formed in 19% yield. Other dienes show similar behavior, producing bicyclic compounds as byproducts in low to moderate yields except for one or two cases, as illustrated with com-... 

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