Cyclic tetraenes

Under high concentration of alkynes, the cooligomerization of alkyne and butadiene in 2 2 ratio takes place using the Ni(0)catalyst, coordinated by (PhO)3P, to give the 12-membered cyclic tetraenes 48 [13], Acetylene and butadiene react in 2 1 ratio to give 5-vinyl-l,3-cyclohexadiene (49) using the Ni complex of Bu3P [14]. 

Thus, Sondheimer s famous synthesis of [18]annulene (145) employs the hydrogenation of an acetylenic precursor in the last step (Scheme 8-19). Recently, hydrogenation of cyclic diene-diynes has been used for the preparation of interesting cyclic tetraenes, such as tetrahomocy-clooctatetraene 146 [Eq. (23)] [Sa]. 

A5 = — 10.4e.u. Furthermore, protonation of the cyclononatetraenide anion, presumably giving the all cis-cyclic tetraene results in cw-dihydroindene at room temperature consistent with a disrotatory electrocyclization of the all c/ -tetraene. " However, it should be recognized that the cis-6A.O triene cannot give the all cis-tetraene in a concerted retro electrocyclization, so a biradical pathway must be accessible for this process (Scheme 10.12). 

Useful bi -phosphoranes can be obtained from a, >-dinitriles by an unusual sequence, involving reduction followed by coupling with a phosphorane (Scheme 23) a subsequent Wittig reaction with a dialdehyde leads to isomeric mixtures of cyclic tetraenes in good yield. ... 

Hydrocarbons. A variety of unsaturated hydrocarbons such as monoolefins, linear and cyclic dienes, cyclic tetraenes etc. have been found and structurally characterized as ligands bonded to metal clusters. 

The skeletal rearrangement of various strained cyclic compounds is carried out with a catalytic amount of soluble complexes of PdCl2. Namely, the rearrangements of bulvalene (67) to bicyclo[4.2.2]deca-2,4,7,9-tetraene (68)[54], cubane (69) to cuneane (70)[55], hexamethyl Dewar benzene (71) to hexa-methylbenzene (72)[56], and 3-oxaquadricyclanes[57] and quadricyclane (73) to norbornadiene[58-60] take place mostly at room temperature. Reaction of iodocubane (74) with a terminal alkyne catalyzed by Pd(0) and CuBr unexpectedly affords an alkynylcyclooctatetraene 75, without giving the desired cubylalkyne 76. Probably the rearrangement is a Pd-catalyzed reaction[61]. 

Section 11.18 Although cyclic conjugation is a necessary requirement for aromaticity, this alone is not sufficient. If it were, cyclobutadiene and cycloocta-tetraene would be aromatic. They are not. 

Cyclic ligands Square brackets designate that the ligand is cyclic and the number within the brackets is the number of atoms in the macrocyclic ring. Saturated macrocycles are designated by ane , while, for unsaturated ligands, ene is used preceded by a prefix to identify the number of double bonds (e.g. diene or tetraene). The donor atoms are then identified. Substituents attached to the macrocycle are indicated by either a prefix or suffix. 

Cyclic allenes undergo head-to-head cyclodimerization to give tricyclic 1,2-alkylidenccyclobu-tanes. Unsaturated cyclic allenes represent molecules with increased strain and often dimerize spontaneously. Whereas cyclonona-1,2-diene (28) requires heating of a neat sample to 138°C,38 the tetraene 31 has a half-life of 10 minutes at 0°C.39 The more strained cycloocta-l,2,4,6-te-traene (34) could not be isolated and only the dimeric cyclobutane 35 was formed on thermolysis of the tosylhydrazone precursor 33.40... 

Among cyclic polyenes, cyclic dienes, trienes and tetraenes have been ring-open polymerised via the metathesis reaction. Representative of the cyclodienes most commonly used for polymerisation are 1,5-cyclooctadiene, norbornadiene (bicyclo[2.2.1]hept-2,5-diene) and dicyclopentadiene as mono-, bi- and tricyclic diolefins respectively. Cycloocta-1,5-diene metathesis polymerisation is another approach to the preparation of 1,4-polybutadiene ... 

All the reactions described so far have mobilised six electrons, but other numbers are possible, notably a few [8 + 2] and [6 + 4] cycloadditions involving 10 electrons in the cyclic transition structure. A conjugated system of eight electrons would normally have the two ends of the conjugated system far apart, but there are a few molecules in which the two ends are held close enough to participate in cycloadditions to a double or triple bond. Thus, the tetraene 6.17 reacts with dimethyl azodicarboxylate 6.18 to give the [8 + 2] adduct 6.19, and tropone 6.20 adds as a 6-electron component to the 4-electron component cyclopentadiene to give the adduct 6.21. 

Schreiner and Prall examined a series of cyclic enyne-allenes (67) and find that ring strain lowers the barrier for both the Myers-Saito and Schmittel cycUzations. The lowest barrier and most exothermic reaction is found for the Myers-Saito cycUzation of the nine-membered ring. Cramer and Squires examined the Myers-Saito cycUzation of cyclonona-l,2,3,5-tetraen-7-yne (68) using the BD(T)/cc-pVDZ method. The activation enthalpy for 68 is about 10 kcal mol less than for 59, which they attribute to ring strain. They note that the cyclization of 68 leads to a [Pg.254]

Cyclic triene (nonaromatic) and tetraene reactions form a series of new complexes. While some cyclotrienes yield mostly unstable species, heating or hgand trapping can generally lead to stable, isolable compounds. Hydrogen transfer also plays an important role in the synthesis ofrf-rf sandwich complexes. With cot and Cr atoms a Ci Cr bonded Cr2(CgH8)3 complex is formed, while in the case of Ti-cot, a novel Triple Decker Sandwich is obtained. ... 

Cyclic oligomers of tetraenic propellanes with bistriazolediones were similarly prepared 25,2< . 

An example of a synthetic plicatitm in the natural products field ctmcems the ring opening of diene (375) to a ca. 1 1 mixture of starting material and triene (376), which upon hydrogenation yields dihy-drocostunolide (377). A synthesis of tetravinylethylene (379) was achieved by photoinduc (-78 Q equilibration of tetraene (378) (containing -4 1 of cyclic to acyclic form). ... 

The same procedure can be applied to f/ -bound tricarbonyliron complexes of cyclic trienes and tetraenes such as cycloheptatrienes, tropones and cyclooctatetraenes. 

The synthesis of perhydrophenanthrenes related to tri- and tetra-cyclic diterpenoids has continued to be an active area.167 Compound (94) has been synthesized as a possible intermediate for the synthesis of cafestanone.168 A synthetic approach to diterpenoids with an abnormal trans-syn ab ring junction [e.g. (95)] has been described.169 The syntheses of 13-methoxypodocarpatrien-19,20-dioic acid170 and the ll-hydroxyabieta-2,8,11,13-tetraen-l-one,171 isomeric with shonanol, have been described. 

HSO3F-SO2FCI al —120 °C proceeds to form the cycloheptatrienes 276 and 277 (equation 98) 5. However, under the same conditions, as well as in the presence of H3PO4, the analogous propargyl substituted diene 278 affords the much more active vinyl cation 279, thus changing the reaction pathway (equation 99). The cyclic ketone 280 obtained was also obtained upon treatment of the tetraene 281 with H3PO4 52. 

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