Diene coupling carboxylic acids

Dicyclopentadiene has two cyclopentene rings with different reactivities. In the reaction with nickel complexes and carbon dioxide only the more reactive norbornene ring couples with CO2, whereas the unstrained ring proved to be unreactive [14,16]. The last example in Figure 6 is cyclooctatetraene, which stands in a temperature-dependent equilibrium with bicyclo[4.2.0]octatriene. Both isomers undergo an oxidative coupling with nickel and CO2. Decomposition by hydrochloric acid leads to cycloocta-2,4,6-triene-carboxylic acid and bicyclo-[4.2.0]octa-2,4-diene-7-carboxylic acid [14]. 

Carbon dioxide instead of aldehydes can be involved in Ni(0)-promoted reductive coupling reactions (Equations (76) and (77) Scheme 90).434,434a 434c A stoichiometric amount of Ni(COD)2/DBU reacts with C02 and dienes, alkynes, or allenes to afford a metallacycle intermediate. This metallacycle reacts with organozinc compounds or aldehydes in one-pot to give carboxylic acid derivatives. As shown in Scheme 90, double carboxylation occurs in the presence of dimethylzinc, where the stereochemical outcome is opposite to that of the reaction with diphenylzinc. 

The linear telomerization reaction of dienes was one of the very first processes catalyzed by water soluble phosphine complexes in aqueous media [7,8]. The reaction itself is the dimerization of a diene coupled with a simultaneous nucleophilic addition of HX (water, alcohols, amines, carboxylic acids, active methylene compounds, etc.) (Scheme 7.3). It is catalyzed by nickel- and palladium complexes of which palladium catalysts are substantially more active. In organic solutions [Pd(OAc)2] + PPhs gives the simplest catalyst combination and Ni/IPPTS and Pd/TPPTS were suggested for mnning the telomerizations in aqueous/organic biphasic systems [7]. An aqueous solvent would seem a straightforward choice for telomerization of dienes with water (the so-called hydrodimerization). In fact, the possibility of separation of the products and the catalyst without a need for distillation is a more important reason in this case, too. 

The stoichiometric head-to-head oxidative coupling of alkynes with CpRuBr(COD) affords a metallacyclic biscarbene complex [22], This process has been used to initiate catalytic formation of the RCH=CH-CH=C(Y)R backbone, to produce Junctional dienes from alkynes by addition of H-Y. The complex [Cp RuCl(COD)[ successfully catalyzes this new chemical transformation, involving the combination of two molecules of alkynes and one molecule of carboxylic acid to afford functional conjugated dienes (Scheme 4) [23]. 

Scheme 4. Functional dienes from alkyne/carboxylic acid coupling.

The precatalyst Cp RuCl(COD) allowed the head-to-head oxidative dimerization of terminal alkynes and the concomitant 1,4-addition of carboxylic acid to stereoselectively afford 1-acyloxy-l,3-dienes in one step under mild conditions [89] (Eqs. 67,68). The first step of the reaction consists in the oxidative head-to-head alkyne coupling via the formation of a ruthenacycle intermediate that behaves as a mixed Fischer-Schrock-type biscarbene ruthenium complex, allowing protonation and nucleophilic addition of the carboxylate. 

The precatalyst 17 was reported to promote a coupling reaction of two molecules of phenylacetylene or its derivatives 65 with carboxylic acids, leading to (l ,3i )-l,4-diaryl-l-acyloxybuta-1,3-dienes 66 in various yields (Scheme 4.24) [57]. Amino acids, as well as diacids, can also be employed as carboxylic acid components. A mechanism involving the addition of a carboxylic acid to the ruthenacydopentatriene intermediate 67 was proposed for this stereoselective coupling. 

Carboxylic acids can be converted by ancxlic decarboxylation into radicals and/or carbocations. The reaction conditions are simple an undivided beaker-type cell as reaction vessel, controlled current supplied from an inexpensive d.c. power supply and meAanol as solvent are in most cases sufficient. A scale-up is fairly easy and the yields are in general good. By the radical pathway 1,/i-diesters, -diketones, -dienes and -dihalides, chiral intermediates for synthesis, pheromones and unusual fatty acids are accessible in just a few steps. The addition of the intermediate radicals to double bonds affords additive dimers, whereby four building units, two alkyl radicals from the carboxylates and two alkenes, can be coupled in one step. Five-membered hetero- or carbo-cyclic compounds can be prepared by intramolecular addition starting from unsaturated carboxylic acids. 

Support-bound C-C-C-C fragments serve as components for cyclization in Diels-Alder and hetero-Diels-Alder reactions (HD). In normal Diels-Alder reactions these dienes should preferably be electron rich. The simplest way to introduce the diene is to couple a commercial diene or a diene synthesized by solution phase methodology to a support-bound group. This method, though efficient, restricts the user to a rather limited set of support-bound dienes. In several examples, 2,4-pentadiene-l-carboxylic acid and some of its derivatives were coupled to support-bound amines to give the support-bound diene, which is not particularly reactive in [2 -I- 4]-cycloadditions [301, 302]. 

Removing the aromatic ring from the diene makes good sense once we know about Suzuki coupling as it allows us to build the two halves of the molecule separately. There is a free choice as to whether we put the boron atom on the diene or on the aromatic ring, but Marko put it on the diene because he planned to make the vinyl boronate 248 by hydroboration of an alkyne 249. Note the protection of the phenol and the carboxylic acid as methyl ether and ester respectively. 

An unusual coenzyme A-coupled hydratase reaction occurs during the anaerobic degradation of benzoic acid by Thauera aromatica, where cyclohexa-l,5,-diene-1-carboxylate CoA hydratase (E.C. 4.2.1.100) adds a water molecule to the cyclohex-adiene functionality, resulting in the formation of 6-hydroxycyclohex-l-enecarbonyl CoAI41l. 

Also other routes were prosecuted to synthesize carboxylic acids by nickel induced coupling of dienes and CO2. If piperylene is reacted with carbon dioxide and the resulting nickel complex is worked up with maleic anhydride (MSA), sorbic acid is formed in yields up to 40 % [46,47] (Equation 5). 

Recently, the Chou group reported on the palladium-catalyzed decarboxylative coupling of substituted 2,5-cyclohexadiene-l-carboxylic acids. The products obtained are 5-arylcyclohexa-1,3-dienes. ... 

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