Double bonds substitution

Slee and LeGoff performed further investigations on the reaction of dimethyl acetylenedicarboxylate 4-20 with an excess of furan 4-21, as first described by Diels and Alder (Scheme 4.5) [la]. At 100 °C, 4-24 and 4-25 were not produced (as proposed), but rather 4-22 and 4-23, since at elevated temperature an equilibrium takes place and the primarily formed 4-24 and 4-25 isomerize to give a 6 1-mixture of the exo-endo and the exo-exo products 4-22 and 4-23, respectively. However, at lower temperature, in the primarily formed [4+2] cycloadduct the double bond substituted with the two carbomethoxy group acts as the dienophile to give the two products 4-24 and 4-25 in a 3 1 ratio with 96% yield within five weeks, as has been shown by Diels and Olsen [la,lc]. For a differentiation of these two types of adducts, Paquette and coworkers [7] used a domino and pincer product . The Cram group [8] described one of the first examples of a reaction of a tethered bisfuran 4-26 with dimethyl acetylenedicarboxylate 4-20a to give 4-27. 

Further studies demonstrated the influence of the double-bond substitution on both the reactivity and the stereoselectivity of the reaction [78-81]. Tamaru and co-workers reported then that using the same PdCl2/CuCl2/MeOH system on butenol derivatives, with the double bond in either the terminal or an internal position, furnished selectively y-butyrol-actones. This dicarbonylation process most probably includes (i) a lactoniza-tion step and (ii) a methoxycarbonylation step, as displayed in Scheme 11 in which we clarify some intermediate steps on a representative example [82, 83]. The use of propylene oxide as an additive promotes this Pd-catalyzed dicarbonylation by playing the role of an HC1 quencher to maintain neutral conditions. 

Chlorination of natural rubber (NR) is carried out with chlorine in carbon tetrachloride solution at 60-90°C to yield a chlorinated rubber containing about 65% chlorine, which corresponds to 3.5 chlorine atoms per repeat unit. The process is complex and includes chlorine addition to the double bond, substitution at allylic positions, and cyclization. Chlorinated rubber has high moisture resistance and is resistant to most aqueous reagents (including mineral acids and bases). It is used in chemical- and corrosion-resistant paints, printing inks, and textile coatings. Bromination of butyl rubber is also practiced [Parent et al., 2002]. 

Xmax is shifted to 410 nm in 44 (c and g annelation) it is shifted to 422 nm. The explanation of this effect is quite straight forward. Benzo-annelation across a double bond substitutes a bond with half double bond character for a double bond. In terms of the exciton model this decreases both the length of the interacting system and the strength of the interaction. 

Uncatalyzed addition reactions of Grignard reagents with nonconjugated alkenes and aUcynes are of limited use in synthesis. However, carbon-carbon double bonds substituted by a leaving group, such as an acetate, are susceptible to be displaced by organomagnesium compounds presumably by an addition-elimination pathway. A few examples have been reported (equation 64). 

H-Aziridines. In the presence of BF3 etherate, this reagent adds to double bonds substituted with three cyano or carboalkoxy groups to give an adduct that is converted into a I //-aziridine by treatment with triethylamine. 

The Dewar thiophene (2), possessing a strained double bond substituted with highly electronegative groups, undergoes cycloaddition with dienes (77JCS(P1)2355) and azides... 

Potassium hydroxide, 258 Trimethylsilyl chlorochromate, 327 of carbon-carbon double bonds substituted by heteroatoms m-Chloroperbenzoic acid, 76 of oximes to carbonyl compounds Lithium aluminum hydride-Hexa-methylphosphoric triamide, 159 Titanium(III) chloride-Diisobutylalu-minum hydride, 303 Trimethylsilyl chlorochromate, 327 of protected alcohols Chlorodimethylthexylsilane, 74 Formic acid, 137 p-Methoxyphenol, 181 of thioacetals and -ketals Methoxy(phenylthio)trimethyl-silylmethane, 182... 

The first and, possibly, the most intuitively satisfying among these concerns the nature of the lone pair and the fact that its participation into the w system bears unmistakable dependence on its mobility as controlled by the effective electronegativity of its heteroatomic host, the unit s suitability as a double-bond substitute decreasing in the order N > NH NAlkyl > S > NAcyl > O. It is further noted that this heteroatom-electronegativity effect is most spectacularly manifested in the unrestricted members of the (4n + 2 n series such as the nine-, thirteen-, and seventeen-membered rings where a change between the... 

Olefinic double bonds substituted with one or more electron-withdrawing groups show significant dipolarophilic activity in cycloaddition reactions with organic azides,43,276-278 similar to the electron-rich double bonds of enamines and enol ethers the reactivity is less pronounced in azide additions compared to that observed in diazomethane reactions.7 The first triazolines reported resulted by the action of aryl azides on benzoquinones.1,279-281 As a rule, stereospecific cis additions occur,32 which are usually unidirectional except in the case of methacrylic derivatives67 and certain alkenes bearing... 

Position of Double Bond Substitution at C-5 5 -Uradl Derivative... 

Photoisomerization of E-C=C double bonds substituted with heteroaromatic groups that allow intramolecular hydrogen bonding were found to afford one-way E - Z... 

It is well established that 5-exo cyclizations of hexenyl-type radicals are generally favored over their 6-endo counterparts due to stereoelectronic reasons <85T3925 87JOC959>. In fact, in most cases the 6-endo regioselectivity is only observed in those systems in which the exo pathway is decelerated by the extent of the double bond substitution or by certain electronic situations <06JA7712>. 

A number of mercury-containing 3-pyrazolidinones have been prepared by Schrauth and Bauerschmidt.1275 Treatment of l-aryl-3-pyrazolin-5-ones having no substituent at C-4 with mercuric acetate in methanol causes addition of acetoxymercuri and methoxy groups at the 3,4-double bond, substitution of acetoxymercuri at C-4 and mercuration in the aryl rings (eq. 248). 2-Pyrazolin-5-ones also undergo... 

The reaction of olefins with lead tetraacetate has not been a useful method in organic synthesis, because reactions such as addition of an oxygen functional group to the double bond, substitution of hydrogen at the allylic position, and C-C bond cleavage can occur to give complex mixtures of products. With some specific alkenes, however, reaction with lead tetraacetate can afford synthetically important compounds cleanly. For instance, reaction of the diacid with 6 equiv. lead tetraacetate in acetonitrile gave the dilactone in excellent yield (Scheme 13.36) [59]. 

Because a-cyanoacrylates contain a double bond substituted by two electron-withdrawing substituents, they are highly susceptible to anionic initiation, and water is basic enough to initiate very rapid polymerization. Therefore, because moisture can initiate polymerization and because the formed polymer is able to firmly adhere to moist surfaces, it has evoked considerable medical interest as a tissue adhesive (19. 20). 

In certain instances, however, Lewis-acid-mediated Michael additions show a slight dependence on the geometry of both the donor and acceptor (vide supra). Hence, the first-order analysis must be modified to include the differential effects induced by the double-bond substitution patterns. By consideration of these effects and by minimization of the adverse gauche-type interactions, trends in Lewis-acid-mediated additions where the conjugate addition is likely to be the actual product-determining step can be rationalized. 

Hydrogenation reactions that are efficiently controlled by chiral Rh-phosphine catalysts involve those carbon-carbon double bonds substituted with carboxylic and amide groups. They are precursors of a-amino acids, compounds that are very important protein building blocks. The best substrate is (Z)-a-acetamidocinnamic acid 10 ... 

As with the parent substrate, 2-substituted methylenecyclopropanes react with dialkyl fu-marates exclusively by distal ring opening, forming a mixture of regioisomeric ring and double bond substituted methylenecyclopentanes. The product distribution is dependent on the conditions employed as well as on the nature of the substituent32. 

Sometimes, however, the conjugation effect is overruled by double-bond substitution, as shown by examples 12701 and 13999. 

The hydrogenation of steroidal enol ether 3 show s that a double bond substituted with an alkoxy group is deactivated. The A5 double bond is hydrogenated in a completely diastereose-lective manner yielding preferentially the isomerized product 4, as well as some nonisomerized product24,... 

Effect of allylic and double bond substitution on product configuration. Scheme 6.7 illustrates the influence of allylic strain between alkyl substituents on the double bond and allylic positions, uncomplicated by substitution at the carbanionic carbon. As shown in Scheme 6.7a, tin-lithium exchange affords an anion that rearranges (cf Figure 6.5a, Ri = n-Bu, R2 = Me, = Z = H) to give a near quantitative yield of alkene with 96-97% diastereoselectivity [32]. In this example, A1-2 strain is relieved when the butyl group adopts the pseudoaxial orientation. 

These relationships are a result of the stabilizing effect of branching and double-bond substitution and hold quite generally, except when branching or substitution results in van der Waals repulsions (see Section 2.3), which have a destabilizing effect. 

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