Conjugated diene, 1,2-addition stability

Conjugated dienes undergo several reactions not observed for nonconjugated dienes. One is the 1,4-addition of electrophiles. When a conjugated diene is treated with an electrophile such as HCl, 1,2- and 1,4-addition products are formed. Both are formed from the same resonance-stabilized allylic carbocation intermediate and are produced in varying amounts depending on the reaction conditions. The L,2 adduct is usually formed faster and is said to be the product of kinetic control. The 1,4 adduct is usually more stable and is said to be the product of thermodynamic control. 

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 intramolecular addition of carbon nucleophiles to alkenes has received comparatively little attention relative to heterocyclization reactions. The first examples of Pd-catalyzed oxidative carbocyclization reactions were described by Backvall and coworkers [164-166]. Conjugaled dienes with appended al-lyl silane and stabilized carbanion nucleophiles undergo 1,4-carbochlorination (Eq. 36) and carboacetoxylation (Eq. 37), respectively. The former reaction employs BQ as the stoichiometric oxidant, whereas the latter uses O2. The authors do not describe efforts to use molecular oxygen in the reaction with allyl silanes however, BQ was cited as being imsuccessful in the reaction with stabihzed car-banions. Benzoquinone is known to activate Ti-allyl-Pd intermediates toward nucleophilic attack (see below. Sect. 4.4). In the absence of BQ, -hydride eUm-ination occurs to form diene 43 in competition with attack of acetate on the intermediate jr-allyl-Pd" species to form the 1,4-addition product 44. 

The higher stability of primary anion 37 as compared to secondary anion 38 explains the predominant formation of branched isomers. The high reactivity of conjugated dienes and styrenes compared with that of monoolefins is accounted for by the formation of new resonance-stabilized anions (39 and 40). Base-catalyzed alkylation with conjugated dienes may be accompanied by telomerization. The reason for this is that the addition of a second molecule of diene to the 39 monoadduct anion competes with transmetallation, especially at lower... 

The reader will recall equations 9 and 10 are generally ca 5 and 42 kJ moT1 endothermic. However, we generally lack enthalpies of formation for the compounds with the same affixed X as the above with which to compare reactions 9,10 and 16. Possibly, though very unlikely, the reaction of two conjugated dienes to form a linearly conjugated tetraene will show considerable additional stabilization. We note one comparison of related 4 and 3 chromophore species. Consider the de-acetylenation reaction... 

Conjugated dienes have alternating single and double bonds. They may undergo 1,2- or 1,4-addition. Allylic carbocations, which are stabilized by resonance, are intermediates in both the 1,2- and 1,4-additions (Sec. 3.15a). Conjugated dienes also undergo cycloaddition reactions with alkenes (Diels-Alder reaction), a useful synthesis of six-membered rings (Sec. 3.15b). 

Free radical addition to conjugated dienes involves addition of a radical at position 1 of the diene, generating a resonance-stabilized radical A which will attack other molecules, e.g. X-Y via its 2- or 4-position, yielding 1,2- and 1,4-addition products B and C, respectively (Scheme 2.45). 

Addition of HX to a conjugated diene forms 1,2- and 1,4-products because of the resonance-stabilized allylic carbocation intermediate. 

In a reaction similar to those given in Eqs. (20) and (25), Grigg [40,41] also employed lithium acetate as an oxygen nucleophile in place of the amine and stabilized carbon nucleophile, respectively, shown in these equations. This led to a 1,4-addition of carbon and oxygen to the conjugated diene. 

The addition of a noii-stabilized carbon nucleophile and another nucleophile to a conjugated diene has similarities to the addition of H-Nu (cf. Section 8.2.1). The formation of RPdX from oxidative addition of RX and Pd(0) corresponds to the genei-ation of a palladium hydride species in the H-Nu addition (Scheme 8-3). 

The use of a stabilized carbanion as external nucleophile in the arylation or vinylation of conjugated dienes leads to a 1,4-addition of carbon atoms. This was first demonstrated by Dieck and co-workers [38] in 1983, who showed that l-bromo-2-methylpropene and sodium dimethyl malonate reacted with isoprene in the presence of a palladium catalyst to give 20 in moderate yield [Eq.(21)]. 

Thus the hybrid nature of the allyl cation governs both steps of electrophilic addition to conjugated dienes the hrst, through stabilization the second, by permitting attachment to either of two carbon atoms. 

How can we account for the unusual reactivity of conjugated dienes In our discussion of halogenation of the simple alkenes (Sec. 3.27), we found that not only orientation but also relative reactivity was related to the stability of the free radical formed in the first step. On this basis alone, we might expect addition to a conjugated diene, which yields a stable allyl free radical, to occur faster than addition to a simple alkene. 

The fact is that conjugated dienes are more reactive than simple alkenes. In the present case, then—and in most cases involving alkenes and free radicals, or alkenes and carbonium ions—the factors stabilizing the transition state are more important than the factors stabilizing the reactant. However, this. is not always true. (It does not seem to be true, for example, in electrophilic addition to conjugated dienes.)... 

The situation is exactly analogous to the one discussed for addition to conjugated dienes (Sec. 8.24). Both reactant and transition state arc stabilized by resonance whether reaction is faster or slower than for simple alkenes depends upon which is stabilized more (see Fig. 8.9, p. 275). 

Electrophilic addition to simple alkenes takes place in such a way as to form the most stable intermediate carbonium ion. Addition to a,j3-unsaturatec arbonyl compounds, too, is consistent with this principle to see that this is so, however, we must look at the conjugated system as a whole. As in the case of conjugated dienes (Sec. 8.20), addition to an end of the conjugated system is preferr jd, since this yields (step 1) a resonance-stabilized carbonium ion. Addition to the carbonyl oxygen end would yield carbonium ion 1 addition to the j3-catbon end would yield carbonium ion II. 

Dichlorocarbene, as a rule, undergoes 1,2-addition to the great majority of conjugated di- and polyenes. Investigation of the competitive cycloaddition of dichlorocarbene to alkenes showed that conjugated dienes are more reactive than alkenes due to stabilization of the transition state by the adjoining double bond. The same effect created by a suitable substituent at C2 in a 1,3-diene, directs the addition of dichlorocarbene to the 1,2-double bond, e.g. formation of 1 - and 2. ... 

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