1.3- Butadiene substituted acyclic

On the other hand, the hydration of acyclic dienes such as 1 - or 2-substituted 1,3-butadienes affords an equilibrium mixture of allylic alcohols resulting from both 1,2- and 1,4-addition (equation 195).296... 

Diels-Alder reactions of the type shown in Table 12.1, that is, Diels-Alder reactions between electron-poor dienophiles and electron-rich dienes, are referred to as Diels-Alder reactions with normal electron demand. The overwhelming majority of known Diels-Alder reactions exhibit such a normal electron demand. Typical dienophiles include acrolein, methyl vinyl ketone, acrylic acid esters, acrylonitrile, fumaric acid esters (fnms-butenedioic acid esters), maleic anhydride, and tetra-cyanoethene—all of which are acceptor-substituted alkenes. Typical dienes are cy-clopentadiene and acyclic 1,3-butadienes with alkyl-, aryl-, alkoxy-, and/or trimethyl-silyloxy substituents—all of which are dienes with a donor substituent. 

A plethora of different acyclic and cyclic diaza dienes has been employed in aza Diels-Alder reactions. With regard to acyclic dienes, the main interest has focused on the cycloadditions of 1,3-diaza-1,3-butadienes. A current example of these transformations is the preparation of highly substituted pyrimidine derivatives such as 3-65 by cycloaddition of diaza-1,3-butadienes e.g. 3-64 with electron-deficient acetylenes (Fig. 3-20) [299]. 

Attempts to trap P-tert-butyl substituted phosphaalkene complexes 66c-e with 2,3-dimethylbutadiene have been unsuccessful, owing to the poor reactivity of the diene at low temperatures. This limitation is circumvented when the respective f-butyl-substituted complexes are exposed to electron-rich acyclic dienes such as l-methoxy-l,3-butadiene, l-(trimethylsilyloxy)- ,3-butadiene, and l-methoxy-3-(trimethylsilyloxy)-... 

Acyclic -butadiene Fe(CO)3 complexes have repeatedly demonstrated their enormous value for organic synthesis in the last few years [1], In this context, both the changed reactivity of the ligand and the steric effect(s) of the Fe(CO)3 fragment have been exploited for the stereocontrolled generation of new chirality centers in the neighborhood of the butadiene-Fe(CO)3 unit. It is important to note that unsym-metrically substituted complexes (e.g. of type A with R / R ) are chiral. 

The results obtained are reported in Table 8 and show that the methyl affinities (k4/k2) of MB, NB and TDE are comparable with those known for acyclic and cydic olefins, respectively, having approximately the same degree of substitution at the double bond. Conversely, the k4/k2 ratio for (III,a) is 5 times greater than that expected for two separated double bonds with comparable degree of substitution, but 1—2 orders of magnitude sillier than those observed for other dienic systems, e. g. butadiene, cydopoitadiene, etc. This result illustrates the opposite effects due to resonance stabilization and steric hindrance. 

The asymmetric hydrocyanation of dienes with substantial enantioselectivities has also been reported (Equation 16.10). Like the reactions of vinylarenes, these reactions have been reported with catalysts containing carbohydrate-derived phosphinites. Reactions of aryl-substituted dienes occur to form the products from 1,2-hydrocyanation. In addition to the reactions of purely acyclic dienes, such as 1-phenyl-l,3-butadiene, dienes containing an exocyclic vinyl group have been studied. These are substrates for products possessing... 

This method has not yet found widespread use for the preparation of allylboronates. In fact, uncatalyzed hydroborations of dienes tend to provide the undesired regioiso-mer with the boron atom on a terminal carbon, i.e., homoallylic boranes. By making use of certain transition metal catalysts, however, Suzuki and co-workers found that (Z)-allylic catecholboronates such as 22 can be obtained in high yield from various substituted butadienes (e.g., isoprene. Equation 11) [44]. Whereas a palladium catalyst is the preferred choice for acyclic dienes, a rhodium catalyst (Rh4(CO)i2) was best for the hydroboration of cyclohexadiene. A suitable mechanism was proposed to explain the high regioselectivity of this process. In all cases, a reaction quench with benzaldehyde afforded the expected homoallylic alcohol product from a tandem hy-droboration/allylation (Section 6.4.1.4). 

Aungst and Funk reported studies on the related a-siloxy- 3-substituted acroleins such as 84 almost contemporaneously (Scheme 18.17) [23]. Siloxyacrolein 84 was prepared by a retro-hetero-Diels-Alder reaction of dioxin derivative 83. Compared with 79, compound 84 bears an additional carbon substituent that would further stabilize siloxyallyl cation 85 and favored its formation, with the result that even acyclic dienes like butadiene underwent [4+3] cycloaddition effectively. 

Acyclic Series. The first complex in the acyclic series was prepared from butadiene by the thermal method. Heating isoprene and pentacarbonyliron at high temperature, however, is inefficient due to competitive Diels-Alder dimerization. Despite the formation of some bis(diene) iron carbonyl complexes on prolonged irradiation, the photochemical method is superior in this case. Complexation of acyclic dienes by Fe(CO)3 is limited to those that can adopt a cisoid conformation, with the syn substitution pattern normally preferred. 2,4-Hexadienolc acid, for example, ean be conveniently complexed by a photolytic procedure. Trialkylsilyl-substituted dienes have also been complexed. ... 

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