L,4-diacetoxy-2-butene

The most important reaction is the oxidative addition of two moles of acetic acid to butadiene to form 1,4-diacetoxy-2-butene (21) with the reduction of Pd2+ to Pd°. In this reaction, 3,4-diacetoxy-l-butene (127) is also formed. In order to carry out the reaction catalytic with regard to Pd2+, a redox system is used. This reaction attracts attention from the standpoint of industrial production of 1,4-butanediol. For this purpose, the formation of 127 should be minimized. Numerous patent applications have been made (examples 113-115), but no paper treating the systematic studies on the reaction has been published. 

The selectivity in the formation of 1,4-diacetoxy-2-butene (1,4-DAB) is considerably enhanced when tellurium compounds are used as cocatalysts. Thus a heterogeneous catalyst, prepared by impregnation of Pd(N03)2 and Te02 dissolved in HN03 over active charcoal (Pd/Te = 10), can be used for the oxidation of butadiene (by 02 in AcOH at 90 °C) to 63% trans-l,4-DAB, 25% cis-1,4-DAB and 12% 3,4-diacetoxy-l-butene. Conventional soluble catalysts such as Pd(OAc)2/Li(OAc) are much less selective in the formation of 1,4-DAB 429 The gas-phase 1,4-diacetoxylation of butadiene in the presence of Pd-Te catalysts is currently being industrially developed by Mitsubishi and BASF 430... 

To increase total yield of 1,4-butanediol, isomerization of 3,4-diacetoxy-1-butene, which is a major by-product in oxidative acetoxylation, has been examined. Isomerization of 3,4-diacetoxy-l-butene to l,4-diacetoxy-2-butene requires only Iwtppm Pd-phosphite catalyst [21]. Therefore, catalyst recovery is not necessary in view of the production cost. 

The oxidative diacetoxylation of butadiene withPd(OAc)2 affords 1,4-diacetoxy-2-butene (214) and 3,4-diacetoxy-l-butene (215). The latter can be isomerized to the former. The commercial process for l,4-diacetoxy-2-butene (214) was developed in AcOH using a Pd catalyst, which contains Te supported on carbon. Importantly, the Pd stays on carbon without dissolving into AcOH. 1,4-Butanediol (216) and THF are produced commercially from 214 [96]. 

Palladium(II)-promoted oxidative 1,4-difunctionalization of conjugated dienes with various nucleophiles is a useful reaction [98], The reaction is stoichiometric with respect to Pd(II) salts, but it can be made catalytic by use of Pd(0) reoxidants. 1,4-Difunctionalization with the same or different nucleophiles has wide synthetic application. The oxidative diacetoxylation of butadiene with Pd(OAc)2 proceeds by acetoxypalladation to generate the 7i-allylpalladium 136, which is attacked by acetoxy anion as the nucleophile, and (E)-, 4-diacctoxy-2-butcnc (137) is formed with 3,4-diacetoxy-1-butene (138) as the minor product. The commercial process for 1,4-diacetoxy-2-butene (137) by the reaction of butadiene, AcOH and O2 has been developed using a supported Pd catalyst containing Te. 1,4-Butanediol (139) and THF are produced commercially from l,4-diacetoxy-2-butene (137) [99]. 

Diacetates of 1,4-butenediol derivatives are useful for double allylation to give cyclic compounds. l,4-Diacetoxy-2-butene (126) reacts with the cyclohexanone enamine 125 to give bicyclo[4.3.1]decenone (127) and vinylbicy-clo[3.2.1]octanone (128)[85,86]. The reaction of the 3-ketoglutarate 130 with cij-cyclopentene-3,5-diacetate (129) affords the furan derivative 131 [87]. The C- and 0-allylations of ambident lithium [(phenylsulfonyl)methylene]nitronate (132) with 129 give isoxazoline-2-oxide 133, which is converted into c -3-hydroxy-4-cyanocyclopentene (134)[S8]. Similarly, chiral m-3-amino-4-hyd-roxycyclopentene was prepared by the cyclization of yV-tosylcarbamate[89]. 

It is known that tr-allylpalladium acetate is converted into allyl acetate by reductive elimination when it is treated with CO[242,243]. For this reason, the carbonylation of allylic acetates themselves is difficult. The allylic acetate 386 is carbonylated in the presence of NaBr (20-50 mol%) under severe conditions, probably via allylic bromides[244]. However, the carbonylation of 5-phenyl-2,4-pentadienyl acetate (387) was carried out in the presence of EtiN without using NaBr at 100 °C to yield methyl 6-phenyl-3,5-hexadienoate (388)[245J. The dicarbonylation of l,4-diacetoxy-2-butene to form the 3-hexenedioate also proceeds by using tetrabutylphosphonium chloride as a ligand in 49% yield[246]. 

Diacetoxylation of 1,3-butadiene is a process that drew much attention since the product l,4-diacetoxy-2-butene may be converted to 1,4-butanediol and tetrahydro-furan by further transformations (see Section 9.5.2). The liquid-phase acetoxylation of 1,3-butadiene in a Wacker-type system yields isomeric 1,2- and cis- and trans-1,4-diacetoxybutenes ... 

Diacetoxy-2-butene. Mitsubishi commercialized a new proces, the acetoxy-lation of 1,3-butadiene, as an alternative to the Reppe (acetylene-formaldehyde) process for the production of l,4-diacetoxy-2-butene. l,4-Diacetoxy-2-butene is tranformed to 1,4-butanediol used in polymer manufacture (polyesters, polyurethanes). Additionally, 1,4-butanediol is converted to tetrahydrofuran, which is an important solvent and also used in polymer synthesis. 

A somewhat similar hydrogenation problem arose in a different approach to 1,4-butanediol and tetrahydrofuran.345 In the process developed by Mitsubishi, 1,3-butadiene first undergoes Pd-catalyzed diacetoxylation to yield 1,4-diacetoxy-2-butene. To avoid the further transformation of the diol as in the abovementioned process, l,4-diacetoxy-2-butene is directly hydrogenated in the liquid phase (60°C, 50 atm) on traditional hydrogenation catalysts to produce 1,4-diacetoxybutane in 98% yield, which is then hydrolyzed to 1,4-butanediol. 

The C5 aldehyde intermediate is produced from butadiene via catalytic oxidative acetoxylation followed by rhodium-catalyzed hydroformylation (see Fig. 2.30). Two variations on this theme have been described. In the Hoffmann-La-Roche process a mixture of butadiene, acetic acid and air is passed over a palladium/tellurium catalyst. The product is a mixture of cis- and frans-l,4-diacetoxy-2-butene. The latter is then subjected to hydroformylation with a conventional catalyst, RhH(CO)(Ph3P)3, that has been pretreated with sodium borohydride. When the aldehyde product is heated with a catalytic amount of p-toluenesulphonic acid, acetic acid is eliminated to form an unsaturated aldehyde. Treatment with a palladium-on-charcoal catalyst causes the double bond to isomerize, forming the desired Cs-aldehyde intermediate. 

Reaction of an enamine with this palladium catalyst with l,4-diacetoxy-2-butene leads to a bicyclic ketone. Thus reaction of l-piperidinocyclopentene (4) gives bicyclo-[4.2.l]non-3-ene-9-one (5) in 28% yield. This reaction involves formation of an intermediate allyl-substituted enamine, which can also form a tt-allylpalladium complex. 

The cycloaddition reactions of various nitrile oxides, e.g., benzonitrile oxide with 3,4-dihydroxy-, 3,4-diacetoxy- or 3,4-di(trialkylsilyloxy)-l-butene, also lead to 4,5-dihydroisooxazole derivatives138,139,142 - 145,1 57. 

Synthesis of l,4-diacetoxy-2-butene by stoichiometric reaction of a halide complex was considered in an early period [8], and then some catalysts were developed. Although there are a series of Phillips patents, which include the InBrg-LiBr catalyst system, the l,4-diacetoxy-2-butene production rate was low and the 1,4-selectivity did not exceed 80%. The reaction of this system was summarized by Stapp [9] for example, the reaction using Cu(OAc)2-LiX-based catalysts proceeds by a copper-based redox cycle (Scheme 10.1). In addition, 20s-CuBr2-KBr, CuBr2-NaBr, and Ag(OAc)2-LiOAc were known for diacetoxylation, but either 1,4-selectivity or reaction rate was low. Furthermore, l,4-dichloro-2-butene is obtained in the production of chloroprene from 1,3-butadiene. 

Exploration of Basic Catalyst Components The study of direct oxidative acetoxyla-tion of 1,3-butadiene began with the use of Wacker-type homogeneous catalyst Pd(OAc)2-CuCl2 [10]. This catalyst system gave low l,4-diacetoxy-2-butene selectivity, and there was a problem in separating the catalyst. After that, liquid-and vapor-phase methods using a Pd-based catalyst were studied in parallel. Catalyst activity was greatly improved by the addition of Bi or Sb to the Pd catalyst in the gas-phase reaction [11]. However, catalyst activity was reduced by the adhesion of resin by-product derived from unsaturated aldehydes on the catalyst surface. Various improvements have been tried in the gas phase, but catalyst robustness has never met industrial requirements. 

Enyne metathesis/metallotropic [l,3]-shift domino processes are also valuable for natural product synthesis [33c,d]. Reaction of substrate 168 with cis-l,4-diacetoxy-2-butene in the presence of Grubbs catalyst 2 generated the intermediate ruthenium alkinyl carbene through a relay RCM with the hberation of 2,5-dihydrofuran followed by metallotropic [l,3]-shift and terminating (Z)-selective CM with the co-olefin to yield the conjugated enediyne 169 (Scheme 2.58) [33c]. The antitumor active Panax ginseng constituent (3R,9R,10R)-panaxytriol was readily synthesized from 169 in six steps. 

In a subsequent report, the CAAC-based metathesis catalysts were examined for selectivity in the formation of Z E olefins, as well as their activity for ethenolysis [12]. Both of these processes require kinetic selectivity to produce the thermodynamically less-favored Z and terminal olefins, respectively. It was discovered that the CAAC catalysts displayed improved conversion to the Z olefin EIZ= 1.5—2.5 after 70% conversion) for the cross metathesis of cis-l,4-diacetoxy-2-butene with allylbenzene, relative to that observed using the classical NHC- and phosphine-based systems ElZ = 3-4) at comparable conversion (Scheme 4.3). 

Lee et al. have used a relay metathesis trigger to further enable their studies of metallatropic [l,3]-shifts. For example, both of the relay-activated diene-diynes 98 underwent CM with (Z)-l,4-diacetoxy-2-butene (99) to give ene-diyne 100 (Scheme 9.24) [30]. This process was designed to involve insertion of ruthenium onto the relay subunit in 98 to give 101, followed by rapid migration of [Ru] to the isomeric species 102 and 103 and drainage of the most reactive of these, 103, from the equilibrium manifold by final CM with 99. 

Structure 4 is an intermediate for manufaeturing vitamin A (Scheme 2). The annual demand for vitamin A is about 3000 tons. Major producers are BASF, Hoffmann-La Roche and Rhone-Poulenc Animal Nutrition [55]. At an early stage in the synthesis BASF and Hoffmann-La Roche are using a hydroformylation step to synthesize 4 starting from l,2-diacetoxy-3-butene (5) and 1,4-di-aeetoxy-2-butene (6), respectively [56, 57]. The selectivity toward the branched product in the BASF process is achieved by using an unmodified rhodium carbonyl catalyst at a high reaction temperature. The symmetry of 6 in La Roche s process does not lead to regioselectivity problems. Elimination of acetic acid and isomerization of the exo double bond (La Roche) yields the final product 4 in both processes. 

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