Acyclic Conjugated Dienes

Sterically strained conjugated acyclic dienes and polyenes. 

Conjugated acyclic dienes (in contrast to cyclic dienes. Section 9.1.3.2), like alkenes conjugated to electron-withdrawing groups, give only linear oligomerization, resulting in acyclic polyene products. 

Although only the conjugated acyclic diene structures are shown, they may be readily modified to generate the corresponding 1,4- and 1,5-dienes. In this article, no systematic discussion of cyclic structures is intended. 

Table 13. Hydroformylation of conjugated acyclic dienes and trienes (Cobalt catalyst)...

It is possible to prepare 1-acetoxy-4-chloro-2-alkenes from conjugated dienes with high selectivity. In the presence of stoichiometric amounts of LiOAc and LiCl, l-acetoxy-4-chloro-2-hutene (358) is obtained from butadiene[307], and cw-l-acetoxy-4-chloro-2-cyclohexene (360) is obtained from 1.3-cyclohexa-diene with 99% selectivity[308]. Neither the 1.4-dichloride nor 1.4-diacetate is formed. Good stereocontrol is also observed with acyclic diene.s[309]. The chloride and acetoxy groups have different reactivities. The Pd-catalyzed selective displacement of the chloride in 358 with diethylamine gives 359 without attacking allylic acetate, and the chloride in 360 is displaced with malonate with retention of the stereochemistry to give 361, while the uncatalyzed reaction affords the inversion product 362. 

Binaphthol-derived titanium complexes [64], prepared from chiral ligands 65 (Figure 3.13), also performed very well in the cycloadditions of conjugated aldehydes with cyclic and acyclic dienes. Judging from the absolute configurations of endo and exo adducts, this catalyst should cover the re-face of carbonyl on its u tz-coordination to s-trans a,/l-unsaturated aldehydes, and hence dienes should approach selectively from the si-face. 

Various non-conjugated diene aldehydes react with Et3SiH/Ni(cod)2/PPh3 to give O-triethylsilylated cycloalkanols in low to high yields. Acyclic dienes can lead to the silylated cycloalkanols in moderate yields with the proper catalyst (Eq. 188).348 Bicyclic systems are also generated by this methodology (Eq. 189).349... 

The chloroacetoxylation is a quite general reaction and works well with a number of conjugated dienes. Some additional examples are given in Scheme 6 and in equations 15 and 16. The reaction is highly syn stereoselective for a number of cyclic dienes tried. Also, for acyclic dienes the reaction leads to a 1,4 syn addition and the reaction takes place with good stereospecificity (94 -96% syn). Thus (Zi,.E)-dienes give the R R isomer whereas (E,Z)-dienes produce the R S isomer (equations 15 and 16). The reaction has also been extended to include other carboxylic acids than acetic acid (chloroacyloxylation)33d. 

An exclusive 1,4-addition has also been observed88 in iodosulfonylation of conjugated dienes with in situ generated tosyl iodide. With symmetrical acyclic dienes the corresponding (5-iodobut-2-enyl sulfones were obtained. In the case of asymmetrical acyclic dienes, with the exception of isoprene, mixtures of regioisomeric products were isolated (equation 62). 

The addition of O2 to acyclic dienes proved to be strongly dependent on terminal substitution and the substituents at other positions of the conjugated system, and, furthermore, it must be accompanied by photoisomerization of ( ,Z)-dienes to ( , )-dienes because singlet oxygen adds exclusively to ( , )-dienes to yield endoperoxides (cf Tables 6(a), 6(b), and 7, and references therein). 

The few data available on conjugated dienes indicate that 1,3-cycloalkadienes undergo 1,2 addition,32 and acyclic dienes yield isomeric allylic alcohols resulting from 1,2 and 1,4 addition.33... 

The third important reaction of singlet oxygen is 1,4-cycloaddition to conjugated dienes, resulting in the formation of endoperoxides.360-364,371 -373 Acyclic dienes give monocyclic endoperoxides that exist in rapidly equilibrating half-chair conformations 410,411... 

Photochemical electrocycltc ring-closure in a 4-electron system works well for many acyclic dienes (2.17) and related cyclic systems 12.18). The situation with conjugated trienes is more complex, and they can act as 6-electron systems (2.19) leading to cydohexa-1,3-dienes, or as 4-electron systems (2.20) giving cyclobutenes. In addition they can undergo other photochemical reactions such as geometrical isomerization about the central double bond Iwhich must be c/s if a 6-electron electrocydic ring-closure is to take place). 

One of the many photoreactions of conjugated dienes is the formation of cyclobutenes,99 and this has been reported both in cyclic and acyclic dienes. The stereochemistry of such photocyclizations has been discussed in terms of orbital symmetry by Woodward and Hoffmann.100... 

While the process works for a great number of conjugated dienes, a few, such as 1,3-cyclopentadiene and those acyclic dienes that have an oxygen substituenl in an allylic position, did not give a chloroacetoxylation product.23 Control of the 1,4-relative stereochemistry and preparation of compounds analogous to the title compounds also work for acyclic dienes,23 5 This process was used to obtain remote stereocontrol in acyclic systems and applied to the synthesis of a pheromone.5... 

Upon UV irradiation in hydrocarbon solution, the hexacarbonyls of chromium, molybdenum, and tungsten react differently with conjugated dienes like 1,3-butadiene (la), ( )-l,3-pentadiene (lb), 2-methyl-1,3-butadiene (lc), ( , )-2,4-hexadiene (Id), ( )-2-methyl-l,3-pentadiene (le), 2-ethyl-1,3-butadiene (If), or 1,3-cyclohexadiene (Ig). Chromium hexacarbonyl (2) yields, with the acyclic dienes la-lf, tetracarbonyl-r/2-dienechromium(0) complexes (3a-3f) in a smooth reaction (8-10). With 1,3-cyclohexadiene, in addition to 3g, dicarbonylbis(>/4-l,3-cyclohexadiene)chromium(0) (4g) is obtained [Eqs. (7) and (8)j. During chromatography on silica gel, the 1,3-cyclohexadiene complex 3g dismutates readily to [Cr(CO)6] and 4g [Eq. (9)]. Under the same conditions with 2 1,3-cyclopentadiene (lh) yields, in a hydrogen-transfer reaction, the stable dicarbonyl- / 5-cyclopentadienyl-r/ 3-cyclopent-enylchromium (5) (11-13) [Eq. (10)]. 

Both ends of the conjugated diene must be close to the n bond of the dienophile for reaction to occur. Thus, an acyclic diene in the. v-trans conformation must rotate about the central C-C a bond to form the s-cis conformation before reaction can take place. 

The transformation of conjugated dienes to allylsilanes can also be carried out with asymmetric induction. The efficiency of the asymmetric synthesis for acyclic dienes is moderate, but for cyclic dienes it remains to be improved. Nevertheless, these reactions provide us with a method for the preparation... 

An intriguing competition arises in the context of cation radical cycloadditions (as in the context of Diels-Alder cycloadditions) which involve at least one conjugated diene component. Since both cyclobutanation and Diels-Alder addition are extremely facile reactions on the cation radical potential energy surface, it would not be surprising to find a mixture of cyclobutane (CB) and Diels-Alder (DA) addition to the diene component in such cases. Even in the cyclodimerization of 1,3-cyclohexadiene, syn and anti cyclobutadimers are observed as 1 % of the total dimeric product. Incidentally, the DA dimers have been shown not to arise indirectly via the CB dimers in this case [58]. The cross addition of tw 5-anethole to 1,3-cyclohexadiene also proceeds directly and essentially exclusively to the Diels-Alder adducts [endo > exo). Similarly, additions to 1,3-cyclopentadiene yield essentially only Diels-Alder adducts. However, additions to acyclic dienes, which typically exist predominantly in the s-trans conformation which is inherently unsuitable for Diels Alder cycloaddition, can yield either exclusively CB adducts, a mixture of CB and DA adducts or essentially exclusively DA adducts (Scheme 26) [59]. 

The adduct is a six-membered ring alkene. The diene can have the two conjugated bonds contained within a ring system as with cyclopentadiene or cyclohexadiene, or the molecule can be an acyclic diene that must be in the cis conformation about the single bond before reaction can occur. 

Although acyclic dienes in general gave a lower yield than the cyclic ones, the 1,4-stereocontrol obtained for internal conjugated dienes is of interest in synthesis. For example, E,E) and ( , Z)-hexa-2,4-dienes were stereoselectively transformed to the d,l and meso-1,4-diacetate, respectively vide infra). 

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