Dienes cross-conjugated—

Figure 11.15. Typical chemical groupings in a sulphur-vulcanised natural rubber network, (a) Monosulphide cross-link (b) disulphide cross-link (c) polysulphide cross-link (j = 3-6) (d) parallel vicinal cross-link (n = 1-6) attached to adjacent main-chain atoms and which have the same influence as a single cross-link (e) cross-links attached to common or adjacent carbon atom (f) intra-chain cyclic monosulphide (g) intra-chain cyclic disulphide (h) pendent sulphide group terminated by moiety X derived from accelerator (i) conjugated diene (j) conjugated triene (k) extra-network material (1) carbon-carbon cross-links (probably absent)...

A variety of conjugated dienones are reduced by lithium-ammonia, presumably via dienyl carbanions analogous to the allyl carbanions encountered in enone reductions. Cross-conjugated l,4-dien-3-ones afford 4-en-3-ones as the major reduction products, indicating that the cyclohexadienyl carbanion (55) protonates largely at C-1. Some protonation at C-5 does occur as shown by examination of the NMR spectrum of the crude reduction product derived from the 17-ethylene ketal of androsta-l,4-diene-3,17-dione. The 17-ethylene ketal of androst-4-ene-3,17-dione is formed in 75%... 

A"" -3-Ketones are more reactive than cross-conjugated A ""-3-ketones. A"" -3,3-CycIoethylenedioxy compounds can be easily prepared by acid-catalyzed reaction with ethylene glycol or by exchange dioxolanation. 3,3-Cycloethylenedioxy-A -dienes can be prepared from 3,3-cycloethy-lenedioxy-A -enes by allylic bromination and dehydrobromination. Acid hydrolysis yields A"" -3-ketosteroids. ... 

Base-induced elimination of sulphinate from homoallylic sulphones , from y-ketosulphones , and from 1,2-bissulphones has been used in synthetic sequences ranging from the preparation of retinoic acid and of its methyl ester , to a novel pentannulation sequence that leads to a range of cross-conjugated dienes , as exemplified by equation (69). The overall yield for the two steps was 63%. 

The literature of diene and polyene photochemistry provides many cases of synthetically useful reactions. As a result, certain arbitrary decisions have been made regarding what is covered in this chapter. For example, intramolecular [2 + 2]-photocycloaddition reactions of a, >-dienes can be formally included under the general rubric of diene photochemistry. However, we have chosen to restrict our discussion to dienes and polyenes which constitute a self-contained chromophore, viz. conjugated, cross-conjugated and 1,4-diene systems. Likewise, arene-olefin photocycloadditions will not be considered. These two broad classes of photoreactions have been applied extensively in synthesis, and have been the subject of recent reviews3,4. 

Methods have been presented, with examples, for obtaining quantitative structure-property relationships for alternating conjugated and cross-conjugated dienes and polyenes, and for adjacent dienes and polyenes. The examples include chemical reactivities, chemical properties and physical properties. A method of estimating electrical effect substituent constants for dienyl and polyenyl substituents has been described. The nature of these substituents has been discussed, but unfortunately the discussion is very largely based on estimated values. A full understanding of structural effects on dienyl and polyenyl systems awaits much further experimental study. It would be particularly useful to have more chemical reactivity studies on their substituent effects, and it would be especially helpful if chemical reactivity studies on the transmission of electrical effects in adjacent multiply doubly bonded systems were available. Only further experimental work will show how valid our estimates and predictions are. 

If the alkenes and acetylenes that are subjected to the reaction mediated by 1 have a leaving group at an appropriate position, as already described in Eq. 9.16, the resulting titanacycles undergo an elimination (path A) as shown in Eq. 9.58 [36], As the resulting vinyltitaniums can be trapped by electrophiles such as aldehydes, this reaction can be viewed as an alternative to stoichiometric metallo-ene reactions via allylic lithium, magnesium, or zinc complexes (path B). Preparations of optically active N-heterocycles [103], which enabled the synthesis of (—)-a-kainic acid (Eq. 9.59) [104,105], of cross-conjugated trienes useful for the diene-transmissive Diels—Alder reaction [106], and of exocyclic bis(allene)s and cyclobutene derivatives [107] have all been reported based on this method. 

Furthermore, PtCl2 prove to be active for the conversion of allenynes. New types of products are then observed. Depending on the substrate, here either the cross-conjugated 236 or the bicylic 1,3-dienes 238 are the products (Scheme 15.74) [144]. 

The second example is an intermolecular crystal-state reaction. Cross-conjugated 1,5-disubstituted 1,4-dien-3-ones in solution undergo both cis-trans photoisomerization and photodimerization, yielding complex mixtures of products, including die all-trans-substituted cyclobutane 2 in the case of 1,5-diphenyl-1,4-pentadien-3-one. In contrast, dienones such as 3a in whose crystals adjacent molecules lie parallel and strongly overlapped react in the solid to give 3b as the sole photoproduct. This isomerically pure tricyclic diketone results, formally, from an eight-center dimerization. It is not formed in the reaction in solution, and could be prepared by other methods only with considerable difficulty (4). 

The other necessary reaction for a BN to VN isomerization is a well precedented 1,5 H shift to convert the linearly conjugated substituted cyclopentadiene (LCC) into the cross conjugated cyclopenta-diene (CCC). The relative lability of BN relative to VN is thus a reflection of the stabilizing conjugation of the substituent in the vinyl isomers and the fact that the formation of LCC from BN is more favorable than the formation of CCC from the retro Diels Alder of VN. The relative energetics for all of these processes is represented in a combined reaction profile diagram shown in Figure 1. 

Reduction of the aromatic nucleus in AjjV-dimethylbenza-mide occurs by an initial single electron transfer to give a radical anion. Protonation of the radical anion generates a radical and a second electron transfer gives the amide enolate 1. Protonation of the cross-conjugated trienolate moiety in 1 occurs carbonyl group to give the cyclohexa-1,4-diene 2. ... 

Bis-allylic oxidation of 23 and related cyclohexa-1,4-dienes provides a convenient and general preparation of cyclohexa-2,5-dien-l-ones (Scheme 7). These cross-conjugated die-nones are substrates for a variety of photochemical rearrangement and intramolecular cycloaddition reactions. Amide-directed hydrogenations of dienones 24a and 24b with the homogeneous iridium catalyst afford cyclohexanones 25a and 25b, containing three stereogenic centers on the six-... 

The Diels-Alder reaction between a 2-fluoroacrylic acid derivative of 8-phenyl-menthol (83) and cyclopentadiene shows high exo- and jr-diastereofacial selectivity (Scheme 30). The C(2) of endocyclic cross-conjugated 2-(acylamino)-l,3-dienes exerts excellent diastereofacial control on the Diels-Alder addition with electron-deficient dienophiles to produce octahydroquinolines. ... 

Vinyl iodide coupled to 1 gave the rather sensitive and reactive cross-conjugated diene 243 in 37-43% yield. When a dienophile 244-R was added to the reaction mixture prior to work-up, the corresponding monoadducts of type 247 were isolated in 15-65% yield. Heating a mixture of 1, vinyl iodide, and a dienophile 244-R in the presence of palladium catalyst yielded the corresponding bisadducts 250 resulting from a domino Diels-Alder addition onto the cross-conjugated triene 243 (Scheme 57) [147,148]. 

Unfortunately, this process gave good results only with bulky substituents at the acetylenic terminus in 104 and, surprisingly, Jeffery s conditions—but without a quaternary ammonium salt—were found to work best. With an additional alkenyl substituent at the acetylenic terminus as in the cyclohexenyl-substituted 2-bromonona-l,8-dien-6-yne 109, the co-cyclization with bicyclopropylidene 12, under these conditions, leads to 110. This transformation involves two consecutive 67r-electrocyclizations of the initially formed cross-conjugated pentaene 111, which was not isolated (Scheme 30). ... 

Conjugated dienes, 11 Conjugation, 11, 368 Conrotation, 345 Coordination polymerisation, 322 Copolymerisation, 322 Cope reaction, 268 Cope rearrangement, 354 Cracking, 112, 305 Cram s rule, 235 Cross-linking, 323... 

Conjugated dienes take part readily in triplet-sensitized photodimerization. and the products obtained from buta-1.3-diene (2.691 include a (4 + 2) adduct as well as slereoisomeric (2 + 2) adducts. The reaction is non-concerted. and a rationalization for the products is provided on the basisof the formation of a biradical intermediate as shown (which is the most stable of the three possible biradicals that might be formed in the first step), by the attack of triplet diene on ground-state diene. Cross-addition takes place in some systems, such as myrcene (2.70) where a triplet diene group attacks the alkene within the same molecule direct irradiation of myrcene gives mainly... 

As just described, Zimmerman has reported one instance of a dienone rearrangement which definitely does not fit Chapman s general picture. Schuster has provided two reports410,411 of cross-conjugated cyclohexadienones which eliminate radical species. With the trichloro-methyl-substituted ketone 34, both cleavage to the cresol and rearrangement to lumiproduct are quenched by dienes.411 Stern-Volmer quenching plots indicate that the rate at which the excited triplet reacts exceeds 10° sec"1 for both reactions.412... 

An interesting instance of the ring-opening metathesis polymerisation of cyclic trienes is the polymerisation of the novel conjugated diene 3,4-diisopropylidene-cyclobutane in the presence of a bis(cyclopentadienyl)titanacyclobutane derivative as a catalyst, which affords a linear cross-conjugated polymer [156] ... 

It was demonstrated some time ago that vinylic and cross-conjugated double bonds of protoporphyrin (IX) dimethyl ester (80a) constitute a diene system capable of undergoing Diels-Alder reactions with activated dieno-philes (68CC697). Isobacteriochlorins 81 and 82a were obtained in a reaction with dimethyl acetylenedicarboxylate (ACDE) and tetracyanoethene (TCNE), respectively (73JCS(PI)1424). 

---