2-Methyl-hexa-1,5-diene

Figure 4. 13C-NMR spectrum of a CDCls solution of poly(5-methyl-1,4-hexa-diene) prepared with a EtgAlCl/S-TiCl, catalyst at 0°C in pentane solvent, ambient temperature. Reproduced, with permission from Ref. 13, Copyright 1979, American Institute of Physics.

BD = butadiene 1,4-HD = 1,4-hexadiene T/C = ratio of trans-1,4- to cis-1,4-hexa-diene 3-MeP = 3-methyl-1,4-pentadiene 2,4-HD = 2,4-hexadiene. r The yield of 2,4-hexadiene increases with increasing conversion. 

The next higher homologs of 28-30 have all been prepared without difficulty as shown in Scheme 5.37. Single and double addition of dibromocarbene to 1,5-hexa-diene (biallyl) (240) leads to the adducts 241 and 242, respectively, which on methyl-lithium treatment are debrominated/rearranged to 32 and 34 in the usual way [43, 96, 97]. 

While linear dimerization of dienoic esters can also be accomplished with nickel-AMP systems, other functionalized dienes undergo little or no conversion. The reaction of methyl hexa-2,4-dienoate, 81, furnishes diastereomeric trienoic diesters (82) in high yields (equation 43). 

Isobutyl-5-methylhex-1-yn-3-ol 72(45%) 20 12 2.0 40/20 1 -Bromo-3-isobutyl-5-methyl-hexa-1,2-diene (52 %) 66(1.7)... 

Exercise 21-21 The Cope rearrangement is a type of sigmatropic rearrangement that occurs with 1,5-dienes. An example is the rearrangement of 3-methyl-1,5-hexa-diene to 1,5-heptatriene ... 

Zair, T. Santelli-Rouvier, C. Santelli, M. Palladium-mediated cydization of 1,5-hexa-dien-3-ols to l-methyl-l,3-cydopentadienes. Tetrahedron Lett. 1991, 32, 4501—4502. 

Thermal decomposition of 1-butene provides a more complex product spectrum than is obtained from either cis- or trans-2-butenes. Between 550° and 760°C in a flow system with nitrogen dilution (3), methane, propylene, butadiene, and ethylene were major products as well as hydrogen, ethane, 1-pentene, 2-pentene, 3-methyl-1-butene, and 1,5-hexa-diene. In studies in a static system (4), cyclohexadienes, benzene, cyclopentene, cyclopentadiene, toluene, orthoxylene, and cyclohexene were observed among the liquid products of the reaction over the temperature range 490°-560°C. 

Diene condensation of 3,4-methylenedioxy-j8-nitrostyrene and methyl hexa-3,5-dienoate afforded the adduct (XXI). Catalytic reduction of the olefinic and nitro groups occurred with spontaneous formation of the lactam (XXII). Lithium aluminum hydride reduction of XXII followed by Pictet-Spengler cyclization afforded ( + )-a-lycorane, identical in its IR-spectrum with (— )-a-lycorane. 

Chromium. The complexes Cr(arene)(CO)s are highly selective catalysts for the hydrogenation of dienes to monoenes. A recent study concentrated on the reduction of methyl hexa-2,4-dienoate (sorbate), and the mechanism was found to be similar to that for reaction with iron-carbonyl catalysts. It does not involve rate-determining loss of the arene to give Cr(CO>3 as the active species, as suggested earlier for a different catalytic system, but rather the arene becoming bi- or uni- rather than ter-dentate, so that the diene can be co-ordinated ... 

The irradiation of 2,5-dimethylfuran in the presence of mercury vapor gave a complex mixture of products. Carbon monoxide and propene were removed as gaseous products. Then, cis- and rran.s-l,3-pentadiene, isoprene, 1,3-dimethylcyclopropene, 2-pentyne, 2-ethyl-5-methylfuran, hexa-3,4-dien-2-one, 1-methyl-3-acetylcyclopropene, and 4-methylcyclopent-2-enone were obtained (Scheme 8) (68JA2720 70JA1824). The most abundantproduct was the cyclopentenone 19, the second was the 1,3-pentadiene 12, while the third product was the cyclopropenyl derivative 18. 

Imines, either acyclic or macrocyclic but invariably multidentate, have a rich coordination chemistry that has been investigated at length. The 7r-accepting ability of imine donors results in the stabilization of lower oxidation states relative to their saturated amine analogs, and there exist many air-stable divalent imine complexes of Co, in contrast to amine relatives. The hexa-methyl-diene (52) has been the most intensively studied ligand of this class, particularly when complexed with Co. In addition, Co complexes of the dimethyl (53),295,296 tetramethyl (54),297 pentamethyl (55)298 and octamethyl (56)299 macrocyclic dienes are also known. In the presence of... 

Benzene-l,4-diols are oxidized to quinones by benzyltrimethylammonium tribromide under mild conditions in almost quantitative yields [6]. With an excess of the tribromide further reaction produces the 2-bromo-l, 4-quinones. This oxidation is in contrast to the analogous reaction of phenols, which produces bromophenols (see Section 2.3). Hindered 4-methyl-phenols are oxidized to the corresponding benzyl alcohols, benzaldehydes, bromomethyl derivatives and 4-bromo-4-methylcyclo-hexa-2,5-dien-l-ones [7]. Benzylic alcohols are oxidized under neutral or basic conditions to yield the corresponding aldehydes (>70%) oxidation with an excess of the reagent produces the benzoic acids (>90%) [8],... 

Although it is generally agreed that the thermal isomerization of bicyclo[2.2.0]hexanes to hexa-l,5-dienes takes place via diradical intermediates,113 118 121,123 125 experimental evidence has been obtained which implies otherwise.115,116 While a radical stabilization energy of approximately 4 kcal mol"1 was obtained for the pyrolysis of methyl 4-chlorobicy-clo[2.2.0]hexane-l-carboxylate (28 b) to methyl 5-chlorohexa-l,5-diene-2-carboxylate (29b),115116 as related to the parent 2-chlorohexa-1,5-diene (29a),115-l16-118 kinetic studies have indicated that there is a small but significant increase in activation energy of about 1 kcal mol-1 for the gas-phase and solution pyrolysis of l-chloro-4-methylbicy-clo[2.2.0]hexane (28c), as compared to l-chlorobicyclo[2.2.0]hexane (28a).115-116 In the light of this result, the commonly accepted diradical mechanism must be questioned and it is likely that the isomerization of these compounds occurs via a concerted process. 

The Cope rearrangement of fluorine-containing hexa-1,5-dienes has been studied extensively by Dolbier and co-workers.3-5 In contrast to the acceleration obtained in the oxy-Cope rearrangement (i.e.. the irreversible transformation of 3-hydroxy-l,5-dienes to the corresponding 6-oxoalkenes). substitution of fluorine at positions 1 or 3 has very little influence on the kinetic system shown below, and is comparable to 3-methyl substitution.4 Temperatures between 236 and 278 C were required for the kinetic studies of 1 and 2. 

Two simple applications may be mentioned. With cyclohexene, 3-bromo-cyclohexene is obtained in a satisfactory yield (Expt 5.68), the latter upon dehydrobromination with quinoline affords an 80-90 per cent yield of cyclo-hexa-1,3-diene (Expt 5.13). Methyl crotonate yields the valuable synthetic reagent methyl y-bromocrotonate (Expt 5.69) this latter compound permits the introduction (in moderate yield) of a four-carbon atom chain at the site of the carbonyl group by the use of the Reformatsky reaction (compare Expt 5.170) ... 

This can be illustrated by the acylation of linear56,58) and cyclic57 vinylsilanes. It is quite simple to prepare methylvinylketone (59)56 or l-methyl-6-TMS-hexa-2,5-diene-4-one (70)58. In the latter case the starting material is bis(trimethylsilyl)-ethene (62) which can either be achieved via hydrosilylation (vide supra)30 or by the more classical approaches59-61. Furthermore, because of the high stereospecifi-ty, compounds like E- cinnamaldehyde (71) are obtainable in good yields and with superb optical purity62. ... 

The Diels-Alder reaction between (2 ,4Z)-hexa-2,4-diene with the dieno-phile 2-methoxycyclohexa-2,5-diene- 1,4-dione yields four stereoisomers. The Diels-Alder reaction is a stereospecific reaction, therefore the configuration of both the starting materials is relocated in the product. Since the reaction is a concerted [4 + 2] cycloaddition, all the products here have always cis-fused rings and because the configuration of (2 ,4Z)-hexa-2,4-diene remains unaltered, both methyl groups in the product will be trans to each other. The various stereoisomers result because the diene and the dienophile may approach each other from both sides and in two distinct orientations. Note that the reaction occurs almost exclusively at the electron poor double bond of the dienophile. In order to deduce the relationships the... 

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