Dienes, also

Reactions of trifluoroacetonitnle oxide with conjugated dienes also lead to regiospecific additions [36] (equation 36) Its addition to the strain-activated double... 

Conjugated dienes also undergo electrophilic addition reactions readily, but mixtures of products are invariably obtained. Addition of HBr to 1,3-butadiene, for instance, yields a mixture of two products (not counting cis-trans isomers). 3-Bromo-l-butene is the typical Markovnikov product of 1,2-addition to a double bond, but l-bromo-2-butene appears unusual. The double bond in this product has moved to a position between carbons 2 and 3, and HBr has added to carbons 1 and 4, a result described as 1,4-addition. 

Simple 1,3-dienes also undergo a thermal monocyclopropanation reaction with methoxy(alkyl)- and methoxy(aryl)carbene complexes of molybdenum and chromium [27]. The most complete study was carried out by Harvey and Lund and they showed that this process occurs with high levels of both regio-and diastereoselectivity. The chemical yield is significantly higher with molybdenum complexes [27a] (Scheme 7). Tri- and tetrasubstituted 1,3-dienes and 3-methylenecyclohexene (diene locked in an s-trans conformation) fail to react [28]. The monocyclopropanation of electronically neutral 1,3-dienes with non-heteroatom-stabilised carbene complexes has also been described [29]. 

Most of the time, the addition is predominantly endo that is, the more bulk) side of the alkene is under the ring, and this is probably true for open-chain dienes also. However, exceptions are known, and in many cases mixtures of exo and endo addition products are found. It has been argued that facial selectivity is not due to torsional angle decompression. ... 

Bis-l,2,4-triazole-3,5-diones such as 110 have also been used in Diels-Alder reactions, and give bispyridazines.171 The pyridazine derivative 111 is formed in quantitative yield from PTAD and 2,7-dimethyl-2,3,5,6-octatetraene,172 and the azadiene, 4-aza-l,3,5-triphenylpenta-2,4-diene, also reacts readily with PTAD to give 112.173 There are many other examples of Diels-Alder additions of ADC compounds to simple acyclic dienes which proceed entirely as expected the above selection has been limited to reactions of synthetic potential and with novel features. 

Cyclic dienes also react readily with ADC compounds although in many cases the initial adducts are not isolable. 1-Substituted pyrid-2-ones give Diels-Alder adducts with ADC compounds, although 2-pyridone itself gives only the substitution product (e.g., 113).174 2-Pyrone gives a 1 2 adduct with PTAD, since the initial adduct (114) rapidly loses C02 to generate a diene which then reacts with more PTAD.175 The initial adducts of ADC compounds with cyclopentadienones,176 and 3,4-dimethyl-1-phenylphos-phole 1-sulfide (115)177 also regenerate a diene by loss of CO and PhP=S, respectively. 

Acyclic dienes also undergo the palladium-catalyzed cyclization with the Mn02/BQ oxidation system22. Thus, simple 1,5-hexadiene afforded a 72% isolated yield of cyclized products 18, 19 and 20, with an isomer distribution of 65 25 10, respectively (equation 8). In general, the selectivity and/or yield was lower for the acyclic dienes. 

The reaction tolerated a variety of functionality, including ester and ether groups on the alkyl-substituted alkene at least two carbons away from the double bond, and raefa-nitro or para-methoxy substituents on the styrene. As expected, cross-metathesis occurred selectively at the less hindered monosubsti-tuted double bond of dienes also containing a disubstituted alkene (Eq. 8). 

The elimination from the zirconium alkoxide B (Scheme 8.23) to give the 1,4-diene also proceeds through cationic activation. An independently prepared sample of pure B (X = Cl) would not undergo elimination unless a catalytic amount of AgC104 (or TMSC104, which is the probable chain carrier in this elimination reaction) was added. If AgAsF6 is used as the promoter for the reaction sequence, only the first (addition) step takes place and no elimination to the diene is observed [51],... 

Danishefsky s diene also reacted with 130a at 130 °C to give phenol derivatives 132, which might arise from the initially formed adduct 131 through hydrolysis and subsequent aromatization. 

It is Hkely that the diene also remains coordinated during the hydrosilylation reactions of phenylacetylene with HSiEts, catalyzed by [Ir(diene)(NCMe)(PR3)]BF4 complexes [PR3 = P Pr3, PMe3 diene = 1,5-cyclo-octadiene, tetraflorobenzobarre-lene (TFB)]. However, detailed studies on the [Ir(COD)(NCMe)(PMe3)]BF4 complex show that cyclo-octadiene carmot be considered, in this case, as an ideal innocent Hgand because it transform into complexes containing cyclo-octadiene, cyclo-octadienyl or cyclo-octenyl Hgands in a variety of coordination modes, due to an... 

Besides products from homoallyl rearrangement (see Section 2.1.), solvolysis of 5-tosyloxypen-ta-1,2-dienes also gives products from 1234-1243 rearrangement. It is possible to discriminate between the different rearrangement products if the / -allenic p-toluenesulfonates 1 arc substituted at Cl or C2. Thus, an apparent exchange of the positions of R1 and R2 occurs in the solvolysis. 

The reaction with conjugated dienes also depends on the substituent, e.g. formation of 19 and 20.34... 

Conjugated dienes also undergo addition reactions by radical-chain mechanisms. Here, the addition product almost always is the 1,4 adduct. Thus radical addition of hydrogen bromide to 1,3-butadiene gives l-bromo-2-butene, presumably by the following mechanism ... 

Conjugated dienes also undergo photochemical cycloaddition reactions. Related thermal cycloadditions of alkadienes have been discussed in Sections 13-3A, 21-10A, and 21-10D, but the thermal and photochemical reactions frequently give different cyclic products. Butadiene provides an excellent example of the differences ... 

In selective poisoning or selective inhibition, a poison retards the rate of one catalysed reaction more than that of another or it may retard only one of the reactions. For example, there are poisons which retard the hydrogenation of olefins much more than the hydrogenation of acetylenes or dienes. Also, traces of sulphur compounds appear selectively to inhibit hydro-genolysis of hydrocarbons during catalytic reforming. 

Photosensitized dienes also add to simple olefins the relative amount of 2 + 2 and 2+4 addition product again depends on the energy of the triplet sensitizer. For example, in the photosensitized addition of butadiene to tri-fluorethylene (Equation 13.61), the percentage of the 2 + 4 adduct (52) increases from 0.5 percent when acetophenone is used as the photosensitizer to 22.5 percent when fluorenone is the photosensitizer.79... 

Nonconjugated dienes also undergo aminomercuration-demercuration to generate pyrrolidines, piperidines and their heterocyclic analogs (equations 156-158).211-2,8... 

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). 

Dienes also condense with phenolic compounds, but the product is usually a chroman. Isoprene 91 condenses with phenol to yield 2,2-dimethylchroman LXXVI, and phytadiene 92 Condenses with trimethyl-... 

In view of the observed inactivity of a, a-disubstituted olefins towards polymerisation with Ziegler-Natta catalysts, it is interesting to note that a, co-diolefins substituted at unsaturated carbon atoms, such as e.g. 2,5-dimethyl-l,5-hexa-diene, also undergo cyclopolymerisation, analogously to unsubstituted parent monomers [2,446], This can be interpreted in terms of a reaction pathway analogous to that shown in scheme (89). The insertions in the cyclopolymerisation appear to be facilitated by the nature of such a process. 

The photochemistry of 1,3-dienes can be highly dependent on the diene structure and reaction conditions. Important variables include the ground state conformation [22,23], the reaction concentration, the use (or not) and properties of a triplet sensitizer [14] or an electron acceptor [18], and solvent polarity. The simplest dienes also often yield the most complex chemistry. For example, 1,3-butadiene 3 undergoes unimolecular isomerization in dilute solution to give only cyclobutene 4 and bicyclobutane 5 (Sch. 2), and polymerization in concentrated solution [24]. At intermediate... 

C2-symmetric cyclic alkenyl sulfoxide (187), with cyclopentadiene, proceeded under mild conditions to give a single diastereomeric adduct (188) in excellent yield (Scheme 48). Other acyclic dienes also gave single diastereomeric adducts often without the necessity of using Lewis acids. The Ws-sulfoxide moiety can be readily deprotected using a two-step sequence of sulfoxide reduction followed by hydrolysis of the dithiolane to give the enantiomerically pure norbomenone (189). 

In petrochemical applications, dienes also hamper the normal running of the installations and catalysts. In these uses, however, olefins and sulfur compounds also present a number of drawbacks, with respect to extraction solvents, for example. Given the... 

The results summarized in Table 1 and discussed above were all obtained upon excitation with visible light into the Si band of diones. Excitation into the S2 band using ultraviolet light results in bisdecarbonylation but interpretation is complicated by the fact that possible intermediates and product dienes also absorb in this region of the spectrum. Mechanistic details have not been investigated yet. 

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