Substituted 1,3-Dienes

Vinylboranes are interesting dienophiles in the Diels-Alder reaction. Alkenylboronic esters show moderate reactivity and give mixtures of exo and endo adducts with cyclopentadiene and 1,3-cyclohexadiene (441). Dichloroalkenylboranes are more reactive and dialkylalkenylboranes react even at room temperature (442—444). Dialkylalkenylboranes are omniphilic dienophiles insensitive to diene substitution (444). In situ formation of vinyl-boranes by transmetaHation of bromodialkylboranes with vinyl tri alkyl tin compounds makes possible a one-pot reaction, avoiding isolation of the intermediate vinylboranes (443). Other cycloadditions of alkenyl- and alkynylboranes are known (445). 

The capto-dative effect has also been demonstrated by studying the bond dissociation process in a series of 1,5-dienes substituted at C-1, C-3, C-4, and C-6. 

A great advantage of catalyst 24b compared with other chiral Lewis acids is that it tolerates the presence of ester, amine, and thioether functionalities. Dienes substituted at the 1-position by alkyl, aryl, oxygen, nitrogen, or sulfur all participate effectively in the present asymmetric Diels-Alder reaction, giving adducts in over 90% ee. The reaction of l-acetoxy-3-methylbutadiene and acryloyloxazolidinone catalyzed by copper reagent 24b, affords the cycloadduct in 98% ee. The first total synthesis of ewt-J -tetrahydrocannabinol was achieved using the functionalized cycloadduct obtained [23, 33e] (Scheme 1.39). 

Cyclopolymerizations of other 1,6-dienes afford varying ratios of five- and six-membered ring products depending on the substitution pattern of the starting diene. Substitution of the olefinic methine hydrogen (e.g. 11, R- CH3) causes a shift from five- to six-membered ring formation. More bulky R substituents can prevent efficient cvclization and cross-linked polymers may result. 

Other commercially relevant monomers have also been modeled in this study, including acrylates, styrene, and vinyl chloride.55 Symmetrical a,pendant functional group are polymerized via ADMET and utilized to model ethylene-styrene, ethylene-vinyl chloride, and ethylene-methyl acrylate copolymers. Since these models have perfect microstructure repeat units, they are a useful tool to study the effects of the functionality on the physical properties of these industrially important materials. The polymers produced have molecular weights in the range of 20,000-60,000, well within the range necessary to possess similar properties to commercial high-molecular-weight material. 

Examples of Dienes and Dienophiles. The synthetic value of D-A reactions can be enhanced in various ways. In addition to hydrocarbon dienes, substituted dienes can be used to introduce functional groups into the products. One example that illustrates the versatility of such reagents is l-methoxy-3-trimethylsiloxy-l,3-butadiene... 

Various dienes substituted with heteroatoms such as 1-oxabuta-l,3-dienes have been used in organic synthesis, as shown in Eq. 8.613 and Eq. 8.7.14... 

Using esters instead of acids reduces the rate of formation of lactones and gives rise to trapping by solvent as well as the formation of overall diene substitution products. Oxidation of amidomalonic ester 57, for example, yields as major products the acetic acid trapping product 58 and the diene substitution product 59, but only 5% of lactone 60 (equation 26). The oxidation of the initially formed amidomalonic ester radical, of increased importance in this case due to the amide substituent, could be largely reduced through addition of sodium acetate or trifluoroacetic acid, which are known to reduce the oxidation potential of the Mn(III) acetate. 

The effects of the allylic substituent, the alkene geometry, and the diene substitution as well as the influence of resident stereogenic centers incorporated in the tether connecting the 1,3-diene and the alkene subunits were totally investigated. This process has been applied to the reactions of more elaborated systems including heterocyclic structures,367 and the total synthesis of (—)-gibboside.368... 

Many of the studies concerning ring-opening metathesis by well-characterized metathesis catalysts have employed substituted norbornenes or norborna-dienes. Substituted norbornenes and norbornadienes are readily available in wide variety, and they usually react irreversibly with an alkylidene. Norbornene itself is the most reactive, and the resulting polynorbornene probably is the most susceptible to secondary metathesis. Formation of polynorbornene often is used as the test reaction for ROMP activity. ROMP by well-defined species has been reviewed relatively recently [30], so only highlights and selected background material will be covered here. 

It is well known that the anodic oxidation of 1,3-dienes in nucelophilic solvents such as methanol and acetic acid gives mainly 1,4-addition products together with a small amount of 1,2-addition products [31]. If the 1,3-dienes substituted... 

Typical rubbers are ungrafted low gel diene rubbers, or mixtures of diene rubbers. Such rubbers include copolymers and block copolymers of conjugated 1,3-dienes, substituted styrene monomers and other monomers as shown in Table 8.3. 

Additions to l,3-dienes6 (12, 367-368 14, 249-250 15, 245). This reaction can be used to effect intramolecular cyclization of cyclic 1,3-dienes substituted by a suitable nitrogen nucleophile. Thus reaction of the amido diene 1 with lithium acetate catalyzed by Pd(OAc)2 (with benzoquinone as reoxidant) provides the ds-fused heterocycle cis-2, in which the acetoxy group is cis to the ring fusion, formed by an overall trans-1,4-oxyamidation of the diene system. Addition of a trace of LiCl improves the yield and results in an overall cis- 1,4-oxyamidation (equation I). Acetamides and carbamates can also be used in place of amides. 1,4-Chloroamidation can also be effected by use of 2 equiv. of LiCl. 

A significant factor leading to the somewhat complex photochemistry of 1,3-butadiene 3, is its s-trans s-cis conformer ratio, substantially disfavoring the s-cis conformation that is needed for [4+2] and [4+4] cycloaddition reactions. The position of this equilibrium is dependent, of course, on the diene substitution. 

Diels-Alder reactions. The laboratories of Breslow and of Grieco have reported that water can enhance the rate of Diels-Alder reactions of dienes that possess carboxylic acid or similar hydrophilic groups (12, 314). Liotta et al. have examined solvent effects on cycloaddition reactions of benzoquinones with dienes substituted with a relatively hydrophobic group, and report significant rate enhancement in ethylene glycol relative to benzene (26 1) or even to reactions in the absence of a solvent. They attribute the solvent effect to aggregation of the diene and the quinone. 

Formyluracils and 4-formylpyrazoles undergo smooth olefinations in THF with indium metal (0.8equiv.), BF3 OEt2 (1 equiv.), and allyl bromide (1 equiv.) providing the respective diene-substituted heterocycles in a single step (Equation (51)).239... 

In aromatic hydrocarbons, some substituted alkenes, dienes, substituted acetylenes and ketones, one half of the n orbitals are empty and an electron can easily be placed in these antibonding orbitals. The capture of an electron by the acceptor molecule is an exothermic process because the energy of the antibonding orbitals lies below the level of the ionization potential of the acceptor radical anion. Many radical anions formed from unsaturated molecules are themselves stable they do not decompose and may exist indefinitely under suitable experimental conditions [182a],On the other hand, they react easily with other molecules. 

A surprising steric sensitivity is frequently observed for these radical cation reactions. As was shown previously during the discussion of chemoselectivity, the variable positioning of bulky substituents has effects on periselectivity as well. DFT calculations on the influence of diene substitutions for the neutral reaction have demonstrated a behavior similar to that of the radical cation reaction. Although... 

Related to the clusters with coordinated alkenes are the diene-substituted complexes Os3(CO)10(cis-C4H6), Os3(CO)10(frons-C4H6) (409), and Ru6C(CO)15(MeCH=CH—CH=CHMe) (60). In these clusters the mode of bonding of the individual olefinic C-C bonds is similar to that of a simple alkene, although in the two latter cases the ligand spans two adjacent metal centers. 

Cope rearrangement (10, 31). Acyclic 1,5-dienes substituted at C, by an electron-withdrawing group undergo Pd(II)-catalyzed Cope rearrangement at 40°. 

Reaction of 1 and related silyl anions with 1,3-dienes gives 1,4-disilyl-2-butenes. The (E)-isomer is the exclusive or major product. HMPT or 1,3-dimethyl-2-imidazolidinone (DMI) is essential. 1,3-Dienes substituted at C, and/or C4 do not undergo this reductive disilylation. 

The efficient trapping of the cyclohexadienyl anionic intermediates with protons raises the possibility of qnenching with carbon electrophiles. The process is not as general as the proton quench. However, when the nucleophile adds essentially irreversibly, quenching with a limited set of carbon electrophiles is successful. For example, addition of 2-lithio-l,3-dithiane to benzene-Cr(CO)2T, followed by addition of ethyl iodide and then oxidation or addition of a donor ligand (CO, PhsP), produces a cyclohexa-l,3-diene substituted by both acetyl (Me + CO) and the nucleophile (Scheme 47).134,209 insertion of CO occurs, without... 

The general trend for acyclic 1,4-dienes substituted with functional groups possessing electron donor or acceptor characteristics is highlighted by the examples shown in equation (46). The polarity of the intermediates plays a major role in determining the regiochemistry. From an empirical viewpoint, the more electron-rich double bond in the substrate remains as the double bond in the product. 

Asymmetric Diels-Alder reactions of dienes substituted widi a removable chiral moiety with prochiral dienophiles have been much less extensively studied. Hie few successful examples involve ester or ether derivatives of 1,3-dienols. 

The TTyir singlet states of the cyclopropenes (101) are reactive and lead to re2irrangement products by way of a carbene (102) mechanism as outlined above. Typical examples of the reaction are shown in the Scheme 8, where it can be seen that the rearrangement affords the isomeric substituted furans (103) and (104) from (101, X = 0) and pyrroles (105) and (106) from (101, X =NMe). This latter reaction also yields the diene-substituted pyrrole (107), which is good evidence for the proposed carbene mechanisms. The thiophene derivative (101, X = S) yields a single product (108) on photoexcitation. 

Common error alert The cis or trans configurations of the double bonds of a 1,3-diene substituted at the terminal C atoms should not be confused with the s-cis and s-trans conformations of the diene. 

Dienes substituted with RO and R2N groups (e.g., Danishefsky s diene, 1-methoxy-3-trimethylsilyloxy-1,3-butadiene) are particularly good substrates for Diels-Alder reactions, but alkyl-substituted dienes and even butadiene itself are common substrates. Benzene rings are very poor dienes in Diels-Alder reactions,... 

Dienes substituted with heteroatoms, X or Y in 145, give allyl (Y in 146) or vinyl (X in 146) derivatives by the Diels-Alder reaction. The heteroatom(s) not only provide latent functionality in the product but also control the regiochemistry of the Diels-Alder reaction itself86. In Danishefsky s vemolepin synthesis87, a compound of type 146 was converted into an enone 147 by hydrolysis since 146 is a 1,3-disub-stituted allyl compounds like the enone precursors in the last section. 

Diene)tricarbonyliron complexes have found use as synthons for the preparation of functionalized dienes. Substituted 4-vinylcyclohexene derivatives are isomerized by pen-tacarbonyliron into a mixture of conjugated cyclohexadiene tricarbonyl iron complexes . When the 4-vinyl cyclohexene 90 was refluxed with 1.2 equivalents of fclCOjs in di-n-butyl ether, a 3 1 mixture of cyclohexadiene isomers 91 and 92 was acquired in 75% overall yield (equation 48). 

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