Phenols, telomerization

Scheme 10 Bimetallic telomerization mechanism proposed by Keim [65] for acetic acid or phenol telomerization, adapted from [25]...

These telomerization reactions of butadiene with nucleophiles are also catalyzed by nickel complexes. For example, amines (18-23), active methylene compounds (23, 24), alcohols (25, 26), and phenol (27) react with butadiene. However, the selectivity and catalytic activity of nickel catalysts are lower than those of palladium catalysts. In addition, a mixture of monomeric and dimeric telomers is usually formed with nickel catalysts ... 

The most characteristic reaction of butadiene catalyzed by palladium catalysts is the dimerization with incorporation of various nucleophiles [Eq. (11)]. The main product of this telomerization reaction is the 8-substituted 1,6-octadiene, 17. Also, 3-substituted 1,7-octadiene, 18, is formed as a minor product. So far, the following nucleophiles are known to react with butadiene to form corresponding telomers water, carboxylic acids, primary and secondary alcohols, phenols, ammonia, primary and secondary amines, enamines, active methylene compounds activated by two electron-attracting groups, and nitroalkanes. Some of these nucleophiles are known to react oxidatively with simple olefins in the presence of Pd2+ salts. Carbon monoxide and hydrosilanes also take part in the telomerization. The telomerization reactions are surveyed based on the classification by the nucleophiles. 

The telomerization of butadiene by means of water in ILs was described by Dullius et Rottger et al. report a process for the telomerization of acyclic olefins having at least two conjugated double bonds, or their mixtures, using a palladium-carbene complex as catalyst in an IL solvent. The nucleophiles included water, alcohols, phenols, polyols, carboxylic acids, ammonia and primary and secondary amines. The acycylic olefins could be either 1,3-butadiene or isoprene. 

The telomerization of dienes in a two-phase system was first described in a patent (100). Water was used as the solvent for the catalyst, with sulfonated phosphane ligands providing the water solubility. Water, alcohols, phenols, acids, amines, and acetylacetic add were used as nucleophiles. 

Krotz, A. Vollmiiller, F. Stark, G. Beller, M. Salt-free C-C coupling reactions of arenes Pd-catalyzed telomerization of phenols. Chem. Commun. 2001, 195-196. 

Generally, octatriene formation is favored by higher temperatures, higher phosphine and/or butadiene concentrations and, importantly, by an increase in steric bulk of either the ligand or the nucleophile. Indeed, Harkal et al. showed a selectivity switch from telomerization products to 1,3,7-octatriene formation by altering the steric demand of the /V-heterocyclic carbene ligand in the reaction of butadiene with isopropanol under further identical reaction conditions [48]. For the more basic nucleophiles, such as the alcohols, the telomer products are stable under experimental conditions, i.e. product formation is irreversible, but for more acidic substrates such as phenol, product formation is reversible and more 1,3,7-octatriene will be formed after the substrate has been depleted. 

Fig. 18 The major products of the telomerization of 1,3-butadiene with phenol...

Behr et al. [114] investigated the selective formation of octadienyl phenol ethers in a liquid-liquid biphasic loop reactor. Although the Pd/TPPTS system showed good activity and selectivity (86% conversion with 74% selectivity towards the telomer after 5 h) in a lab-scale batch reactor, telomer yields in the loop reactor were insufficient for efficient phase separation. Telomerization with phenol was therefore deemed unsuitable for this type of reactor. The authors also noted that the C-allylated octadienyl phenol product rather than the telomerization product became the main product after 6 h of reaction, attributing this to a metal-catalyzed Claisen-type rearrangement. 

Estrine B, Soler R, Damez C, Bouquillon S, Henin F, Muzart J (2003) Recycling in telomerization of butadiene with methanol and phenol Pd-KF/Al203 as an active heterogeneous catalyst system. Green Chem 5 686-689... 

Krotz A, Vollmuller F, Stark G, Beller M (2001) Salt-free c-c coupling reactions of arenes palladium-catalyzed telomerization of phenols. Chem Commun 195-196... 

The preparation of different (NHC) - Pd°(dvds) complexes allowed the authors to make a systematic comparison of structure/activity for the telomerization reaction [228]. This study showed that electron-withdrawing substituents on the carbene backbone destabilizes the catalyst and therefore enhance its reactivity. These catalysts are applicable to primary and secondary alcohol as well as phenols and represent the first industrially viable catalyst system for palladium-catalyzed telomerization of butadiene with alcohol. 

Palladium-phosphine complexes such as Pd [PPh3 ]4 or, most conveniently, Pd(OAc)2 and PPh3 are used. Usually, these telomers are obtained in high yields. Nucleophiles such as water, carboxylic acids, alcohols, phenols, ammonia, amines, enamines, nitroalkanes, and active methylene and methyne compounds participate in telomerization. Also, carbon monoxide and hydrosilanes are involved in the reaction to give telomers. These easily available telomers are trifunctional and extremely useful starting materials for simple synthesis of certain types of natural products. 

Although the telomerization of dienes in a two-phase system has been intensively investigated with compounds containing active hydrogen such as alcohols, amines, phenols, acids, etc., the selective and productive telomerization of butadiene continues to be a challenge. It is only recently that primary octadi-enylamines have been obtained with selectivity up to 88% in the telomerization of butadiene with ammonia using a two-phase toluene/water system and Pd(OAc)2/tppts as the catalyst [Eq. (23)] [125]. 

In the presence of suitable cocatalysts such as alcohols, phenols, or secondary amines, 1,3-diolefins are oligomerized to linear dimers or trimers by the same nickel-ligand systems, which are effective for cyclooligomerization (see 14.5.2.5.1). Reactions may be accompanied by telomerization. Typical examples are given in Table 1. 

The linear telomerization reaction [1] of dienes (the taxogen) with nucleophiles (the telogen) such as alcohols, amines, carboxylic acids, active methylene compounds, phenols, or water provides an elegant method for the synthesis of various useful compounds (Eq. 1). 

In what follows, the telomerization of butadiene with acetic acid, alcohols, phenol, C—H-acidic compounds and nitroalkanes will be considered. Also some examples of carboxytelomerization and the telomerization of substituted dienes will be given. In all reactions trifunctional compounds are formed which contain two double bonds and one functional group. 

---