Oxidative coupling reaction olefin

The palladium(II)-mediated oxidative coupling of olefins with oxygen-nucleophiles (ROH water, alcohols, carboxylic acids) is a stoichiometric reaction with respect to Pd(II), resulting in an oxygenated product and Pd(0). To convert Pd(0) back to Pd(II) and start a new reaction cycle, a reoxidation reaction (which can itself be stoichiometric or catalytic) using a terminal oxidant is required. In this way, the overall process becomes catalytic with respect to the expensive Pd salt. 

Scheme 1. Palladium-mediated oxidative coupling reactions of olefins and water.

The general description of this oxidative coupling reaction is given by Reaction 19 in Table II. These reactions are involved in the metathesis of olefins or alkynes (J9). We have not found any rules allowing or forbidding the reaction. Matrix 5 shows only the structures of complexes found in the literature which give such reactions d10-M (M = Ni) (40), d8-MLs (41), d6-ML6 (42), and d2-MLs (43). For the d° and d4 systems we were unable to find any examples. 

Reactive acceptors are anodically available as radical cations in a wide variety by the oxidative Umpolung of donors. This way two donors can be coupled in one step, if one of them is converted to an acceptor by electron transfer. Chemically, two or more steps are required for this operation. Thus electrochemistry can save steps in synthesis. Of general synthetic value is the generation of latent immonium ions for electrophilic addition [48] and the oxidative coupling of olefins [49] and Chapter 22, In common chemical synthesis there is, on the other hand, a highly developed experience and a large arsenal of reagents to execute selective Se and Ae reactions with a wide choice of substrates. 

This chapter discusses the basic mechanistic aspects of these carboxylation processes and brings into focus the mechanistic details of the oxidative coupling reaction between CO2 and the unsamrated substrate (olefin, conjugated diene, cumulene, alkyne) with the purpose of highlighting, whenever possible, the relevance and role of CO2 coordination to the metal center in these transformatiOTis. 

The oxidative coupling reaction, 20.57-20.58, offers an interesting contrast to the (olefin)2Fe(CO)3 rearrangement in Section 18.3. Both reactions involve... 

Pd/Cu-coupled catalysis has been used in many Wacker-type olefin oxidations other than those that involve Markovnikov methyl ketone formation from terminal olefins [la,b, 21]. Pd/Cu-coupled aerobic oxidation systems have also been widely appfied to other sp and sp carbon oxidations. Selected examples of these oxidations, including those involving carbon nucleophiles, oxidative carbo-nylations and oxidative coupling reaction, are pictured in Scheme 5.7 [22, 26]. 

The aldehyde function at C-85 in 25 is unmasked by oxidative hydrolysis of the thioacetal group (I2, NaHCOs) (98 % yield), and the resulting aldehyde 26 is coupled to Z-iodoolefin 10 by a NiCh/CrCH-mediated process to afford a ca. 3 2 mixture of diaste-reoisomeric allylic alcohols 27, epimeric at C-85 (90 % yield). The low stereoselectivity of this coupling reaction is, of course, inconsequential, since the next operation involves oxidation [pyridinium dichromate (PDC)] to the corresponding enone and. olefination with methylene triphenylphosphorane to furnish the desired diene system (70-75% overall yield from dithioacetal 9). Deprotection of the C-77 primary hydroxyl group by mild acid hydrolysis (PPTS, MeOH-ClHhCh), followed by Swem oxidation, then leads to the C77-C115 aldehyde 28 in excellent overall yield. 

One other point to note in regard to this study (141) is that any evidence of oxidative addition, particularly with the chloro-olefins, was absent. The similarity of the spectra, coupled with the nonobservation of any bands in the visible region, as well as the observation of vc-c in the region commonly associated with 7r-complexation of an olefin (141, 142), all argue in favor of normal ir-coordination, rather than oxidative insertion of the metal atom into, for example, a C-Cl bond. Oxidative, addition reactions of metal atoms will be discussed subsequently. 

Recently, Larock and coworkers used a domino Heck/Suzuki process for the synthesis of a multitude of tamoxifen analogues [48] (Scheme 6/1.20). In their approach, these authors used a three-component coupling reaction of readily available aryl iodides, internal alkynes and aryl boronic acids to give the expected tetrasubsti-tuted olefins in good yields. As an example, treatment of a mixture of phenyliodide, the alkyne 6/1-78 and phenylboronic acid with catalytic amounts of PdCl2(PhCN)2 gave 6/1-79 in 90% yield. In this process, substituted aryl iodides and heteroaromatic boronic acids may also be employed. It can be assumed that, after Pd°-cata-lyzed oxidative addition of the aryl iodide, a ds-carbopalladation of the internal alkyne takes place to form a vinylic palladium intermediate. This then reacts with the ate complex of the aryl boronic acid in a transmetalation, followed by a reductive elimination. 

A list of examples in this section is not exhaustive rather, they have been chosen to illustrate the different approaches used for immobilization of the catalysts for important classes of organic reactions, namely hydrogenation, oxidation, and coupling reactions. Due to the major industrial importance of olefin polymerization (see Chapter 9.1), and although the objectives of immobilization of polymerization catalysts are rather different from the other examples, some references to this will also be given here. 

The olefin metathesis of 3-hydroxy-4-vinyl-l,2,5-thiadiazole 112 and a McMurry coupling reaction (Ti3+ under reductive conditions) of the aldehyde 114 were both unsuccessful <2004TL5441>. An alternative approach via a Wittig reaction was successful. With the use of the mild heterogenous oxidant 4-acetylamino-2,2,6,6-tetramethyl-piperidine-l-oxoammonium perfluoroborate (Bobbitt s reagent), the alcohol 113 was converted into the aldehyde 114. The phosphonium salt 115 also obtained from the alcohol 113 was treated with the aldehyde 114 to give the symmetrical alkene 116 (Scheme 16) <2004TL5441>. 

A total synthesis of the sesquiterpene ( )-illudin C 420 has been described. The tricyclic ring system of the natural product is readily quickly assembled from cyclopropane and cyclopentene precursors via a novel oxime dianion coupling reaction and a subsequent intramolecular nitrile oxide—olefin cycloaddition (463). 

Secondary phosphine oxides are known to be excellent ligands in palladium-catalyzed coupling reactions and platinum-catalyzed nitrile hydrolysis. A series of chiral enantiopure secondary phosphine oxides 49 and 50 has been prepared and studied in the iridium-catalyzed enantioselective hydrogenation of imines [48] and in the rhodium- and iridium-catalyzed hydrogenation functionalized olefins [86]. Especially in benzyl substituted imine-hydrogenation, 49a ranks among the best ligands available in terms of ex. 

When furan or substituted furans were subjected to the classic oxidative coupling conditions [Pd(OAc)2 in refluxing HOAc], 2,2 -bifuran was the major product, whereas 2,3 -bifuran was a minor product [12,13]. Similar results were observed for the arylation of furans using Pd(OAc)2 [14]. The oxidative couplings of furan or benzo[i]furan with olefins also suffered from inefficiency [15]. These reactions consume at least one equivalent of palladium acetate, and therefore have limited synthetic utility. 

Olefins with electron-donating substituents as the aUcoxy, acylamino, phenyl, or vinyl group can be coupled in methanol to give 1,4-dimethoxy dimers and/or dienes (Scheme 2). The first intermediate in this coupling reaction is a radical cation, which either by electrophilic addition to the olefin and subsequent le-oxidation (path A) [49] or by radical dimerization (path B) [50, 51] leads to a dimer dication that undergoes methanolysis or deprotonation. Representative examples of this coupling reaction are summarized in Table 7. 

---