4- cyclohexa-2.5-dien

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. 

When a bidentate phosphine is used as a ligand for the reaction of J-keto esters or /i-diketones, no dimerization takes place. Only a 2-butenyl group is introduced to give 68[49,62], Substituted dienes such as isoprene, 1,3-cyclohexa-diene, and ocimene react with carbon nucleophiles to give a mixture of possible regio- and stereoisomers of 1 1 adducts when dppp is used as a ligand[63,64]. 

The best way to understand how orbital symmetry affects pericyclic reactions is to look at some examples. Let s look first at a group of polyene rearrangements called electrocyclic reactions. An electrocyclic reaction is a pericyclic process that involves the cycli/ation of a conjugated polyene. One 7r bond is broken, the other 7t bonds change position, a new cr bond is formed, and a cyclic compound results. For example, a conjugated triene can be converted into a cyclohexa-diene, and a conjugated diene can be converted into a cyclobutene. 

Because of its convenience and simplicity this procedure is the method of choice for laboratory preparation of 1,3-cyclohexa-diene This olefin is an intermediate of some importance because it offers a route via the Diels-Alder reaction to a variety of bicyclic compounds4i 7 10... 

The synthesis of chiral a-substituted allylborane 2 via the hydroboration of 1,3-cyclohexa-diene with diisopinocampheylborane has been reported21. 

Jod- und Brom-Atome werden auch hier reduktiv abgcspalten1. Natrium-bis-[2-tnethoxy-athoxy]-dihydri-do-aluminat reduziert sterisch stark gehinderte Nitro-benzole untcr partieller Ringhydrierungzu Cyclohexa-dien-on-oximen ... 

In fl-trimethylsilylcarboxylic acids the non-Kolbe electrolysis is favored as the carbocation is stabilized by the p-effect of the silyl group. Attack of methanol at the silyl group subsequently leads in a regioselective elimination to the double bond (Eq. 29) [307, 308]. This reaction has been used for the construction of 1,4-cyclohexa-dienes. At first Diels-Alder adducts are prepared from dienes and P-trimethylsilyl-acrylic acid as acetylene-equivalent, this is then followed by decarboxylation-desilyl-ation (Eq. 30) [308]. Some examples are summarized in Table 11, Nos. 12-13. 

The partial arene derivative hydrogenation into cyclohexene or cyclohexa-diene as intermediates is also investigated. The process developed by Asahi Chemical Industry in Japan is an example of the selective formation of cyclohexene [6]. In the future, this reaction could be an active area of research due to the potential of the intermediate in organic synthesis. 

Zanarotti, A. Preparation and reactivity of 2,6-dimethoxy-4-allylidene-2,5-cyclohexa-dien-l-one (vinyl quinone methide). A novel synthesis of sinapyl alcohol. Tetrahedron Lett, 1982, 23, 3815-3818. 

The reactions of 4-nitrobenzodifuroxan 242 with a series of common dienes, such as cyclopentadiene, cyclohexa-diene, isoprene, 2,3-dimethylbutadiene, and 1-acetoxybutadiene, with ethoxymethyleneacetylacetone were found to proceed very readily to afford stable cycloadducts, which are the result of highly stereoselective normal electron-demand (NED) Diels-Alder reactions. Due to the additional activation provided by the two adjacent furoxan rings, the nitroalkene double bond of compound 242 is also prone to undergo NED reactions with less reactive dienic structures, such as the enol form of ethoxymethyleneacetylacetone and the in situ generated 2-ethoxy-4-(2-furfur-yl)buta-l,3-diene <2004TL1037, 2005T8167>. 

By organoboration of bis(alkynyl)plumbanes 85, the compound 86, stabilized by a jt-bonding interaction, is prepared180. (Scheme 22a). The cation 86 is unstable and, at room temperature, decomposes rapidly to rearrange into a l,4-plumbabora-2,5-cyclohexa-diene, 87. 

FIGURE 2. The mechanism of the nickel-catalyzed addition of hydrogen cyanide to 1,3-cyclohexa-diene. Reproduced by permission of the Royal Society of Chemistry from Reference 46... 

Synonyms AI3-09068 AIDS-153 B 1266 1,4-Benzoquine Benzoquinone Benzo-l,4-quinone 1,4-Benzoquinone p-Benzoquinone BRN 0773967 Caswell No. 719C CCRIS 933 Chinone Cyclohexadienedione 1,4-Cyclohexadienedione 2,5-Cyclohexadiene-l,4-dione 1,4-Cyclohexa-diene dioxide 1,4-Dioxybenzene EINECS 203-405-2 EPA pesticide chemical code 059805 NCI-C55845 NSC 36324 Quinone 4-Quinone RCRA waste number U197 UN 2587 USAF P-220. 

Application of electron impact ionization mass spectrometry (EI-MS) techniques for the analysis of 1,2-thiazines has waned since the publication of CHEC-II(1996). In one recent example of this technique, bicycle 44 was ionized at 70 eV and 180°C to afford radical cation 53, 54 via loss of CO2, and W-sulfinyl compound 55 and 1,3-cyclohexa-diene radical cation 56 via a retro-[44-2] reaction in the gas phase (Scheme 5) <2002TA2407>. 

Effects of the magnitude of AGst on the hydrogen atom abstraction reactivity are seen more clearly in the reaction of cyclophane DPCs (20) with 1,4-cyclohexa-diene. Thus, feobs increases by a factor of 7 as the number of methylene units is decreased and AGst decreases from 3 to 0.5 kcal/mol. This observation is interpreted in terms of some participation of the singlet for those carbenes with sufficient small energy gap." ... 

According to the frontier orbital theory,525 electron-withdrawing substituents lower the energies of the lowest unoccupied molecular orbital (LUMO) of the di-enophile thereby decreasing the highets occupied molecular orbital (HOMO)-LUMO energy difference and the activation energy of the reaction. 1,3-Butadiene itself is sufficiently electron-rich to participate in cycloaddition. Other frequently used dienes are methyl-substituted butadienes, cyclopentadiene, 1,3-cyclohexa-diene, and 1,2-dimethylenecyclohexane. 

Hydrocarbons with easily abstractable hydride ion such as 1,4-cyclohexa-diene,227 cycloheptatriene,223 or compounds possessing tertiary or benzylic hydro-gen217,218,223 230 231 are suitable substitutes for silanes. In fact, this was the case in the first ionic hydrogenation observed by Ipatieff and coworkers. In an attempt to carry out alkylation of p-cymene with 3-methylcyclohexene, hydrogenation of the latter occurred, where p-cymene was the hydride ion source ... 

However, benzene and its derivatives can be reduced to cyclohexa-dienes by solutions of metals such as Li, Na, K, Zn, and Hg in a weakly acidic solvent, such as liquid ammonia, amines, or ether-alcohol mixtures. This general type of reaction is known as the Birch reduction after the Australian chemist, A. J. Birch. With benzene, reduction with metals leads to 1,4-cyclo-hexadiene ... 

The regioselectivity during hydride abstraction afforded by an ester substituent attached to cyclohexa-diene has also been exploited for total synthesis. The synthesis of (+)- and (-)-gabaculine in homochiral... 

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