1,3-Butadiene, 1,2-addition reactions stability

In degree 2 only reactivity degrees are treated vis- i-vis exothermic polymerization in particular and addition reactions on the double bond (ethylene, butadiene, styrene, propylene), easy peroxidation (isopropyl oxide, acetaldehyde), hydrolysis (acetic anhydride). Possibly only propionitrile and substances with code 0 have an actual NFPA stability code. Every time one has to deal with the NFPA code one has to interpret it after carefully reading the paragraphs in Part Two. 

Addition Reactions. All the metallaaromatic species kinetically stabilized by a Tbt group showed considerably high reactivity toward the reagents such as water, methanol, styrene, phenyl acetylene, mesitonitrile oxide, benzophenone, and 2,3-dimethyl-1,3-butadiene. Some of them were allowed to react with elemental chalcogen such as sulfur and selenium to give several kinds of unique cyclic chalcogenides as the cycloadducts. In Scheme 60 were summarized the reactions of the sila- and germabenzenes as the representative. 

Kinetic versus thermodynamic control. A plot of Gibbs free energy versus reaction coordinate for Step 2 in the electrophilic addition of HBr to 1,3-butadiene. The resonance-stabilized allylic carbocation intermediate reacts with bromide ion by way of the transition state on the left to give the 1,2-addition product. It reacts with bromide ion by way of the alternative transition state on the right to give the 1,4-addition product. 

N-Aminobenzoxazolin-2-one (4), which was readily prepared by animation of benzoxazolin-2-one with hydroxylamine-O-sulfonic acid, is also a useful nitrene precursor (Scheme 2.2). Oxidation of 4 with lead(iv) acetate in the presence of a conjugated diene resulted in exclusive 1,2-addition of nitrene 5, to yield vinylazir-idine (6) in 71 % yield [6]. The formation of vinylaziridines through 1,2-additions of methoxycarbonylnitrene (2) or amino nitrene 5 contrasts with the claimed 1,4-ad-dition of nitrene itself to butadiene [7]. Since the reaction proceeded stereospecif-ically even at high dilution, the nitrene 5 appears to be generated in a resonance-stabilized singlet state, which is probably the ground state [8]. 

The monomer addition scheme, shown at the top, requires an initiator which is capable of removing a hydrogen atom from the allylic position of the butadiene, resonance stabilization of the radical from AIBN does not permit this initiator to effect this reaction while benzoyl peroxide is capable of reaction to remove a hydrogen atom and initiate the reaction. On the other hand the polymeric radical addition scheme requires that homopolymerization of the monomer be initiated and this macroradical then attack the polymer and lead to the formation of the graft copolymer. Huang and Sundberg explain that the reactivity of the monomer... 

The data in Table I and in Figures 1 and 2 show that at very low pressures, such as are commonly used in electrical discharge techniques, it would be difficult to observe the stabilized addition products in the reaction of oxygen atoms with ethylene, propylene, and 1,3-butadiene. The other olefins are likely to be more favorable in this respect and indeed the addition products formed with butene-1 have been observed under such conditions, as will be discussed later. 

The formation of 1,2-dimethylcyclobutene (Formula 385) in the vapor phase irradiation of 2,3-dimethyl-l,3-butadiene (Formula 384) is not quenched by oxygen or nitric oxide (169). Addition of inert vapor (diethyl ether) increased the quantum efficiency in this reaction (169). The inert vapor presumably removes excess vibrational energy from the product cyclobutene thus stabilizing the product (169). Rate studies on the cis- and Jrans-isomers of 1,3-pentadiene in solution indicate that the iraras-isomer is the only source of 3-methylcyclobutene (169). The photoisomerization to 3-methylcyclobutene is faster than photoisomerization of trans- to m-l,3-pentadiene (169). 

As with the transition state of the [4+2]-addition of butadiene and ethene (Figure 15.8) both HOMO/LUMO interactions are stabilizing in the transition state of the [2+2]-addition of ketene to ethene (Figure 15.13). This explains why [2+2]-cycloadditions of ketenes to alkenes—and similarly to alkynes—can occur in one-step reactions while this is not so for the additions of alkenes to alkenes (Section 15.2.3). 

The addition of a C-2 (equation 1 R = H > alkyl, aryl > OMe NR2), C-3, or C-4 electron-donating substituent to a 1 -oxa-1,3-butadiene electronically decreases its rate of 4ir participation in a LUMOdiene-controlled Diels-Alder reaction (c/. Table 5). Nonetheless, a useful set of C-3 substituted l-oxa-l,3-buta-dienes have proven to be effective dienes ° and have been employed in the preparation of carbohydrates (Table 6). The productive use of such dienes may be attributed to the relative increased stability of the cisoid versus transoid diene conformation that in turn may be responsible for the Diels-Alder reactivity of the dienes. Clear demonstrations of the anticipated [4 + 2] cycloaddition rate deceleration of 1-oxa-1,3-butadienes bearing a C-4 electron-donating substituent have been detailed (Table 6 entry 4). >> "3 In selected instances, the addition of a strong electron-donating substituent (OR, NR2) to the C-4 position provides sufficient nucleophilic character to the 1-oxa-1,3-butadiene to permit the observation of [4 + 2] cycloaddition reactions with reactive, electrophilic alkenes including ketenes and sul-fenes, often in competition with [2 + 2] cycloaddition reactions. ... 

The mechanism of electrophilic addition of HX involves two steps addition of H (from HX) to form a resonance-stabilized carbocation, followed by nucleophilic attack of X at either electrophilic end of the carbocation to form two products. Mechanism 16.1 illustrates the reaction of 1,3-butadiene with HBr. 

The rhodium-catalyzed addition of ethylene to 1,3-butadiene to yield 1,4-hexadiene (5a, 151) proceeds via a similar mechanism (151) with the exception that, upon formation of the alkylrhodium(III) species, the hexadiene synthesis proceeds without further change in the oxidation state of the metal. In these reactions with butadiene the coordinated alkyl groups are either chelate or 7r-allyl structures which appear to stabilize Rh(III) (151). The addition of propylene to butadiene and isoprene to produce [Pg.297]

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