Alkene and Alkyne Oligomerization

In the case of reactions such as valence isomerization, metathesis reactions of alkenes and alkynes, oligomerization or cyclooligomerization of olefins, metallacycloalkanes are of special importance. Their catalytic efficiency depends on the ease of the M—C bond cleavage, which is the result of reductive elimination of the organic substrate or of /J-hydrogen transfer. Also a- or / -C—C bond rupture has been reported. Heterocycles with an aliphatic carbon skeleton and a donor atom adjacent to the metal are suitable model compounds for the study of individual catalytic steps and structural properties. In connection with the activation of C—H bonds, cyclometa-lation has become a very general reaction and was reviewed in 1977. ... 

Review W. Reim, A. Behr, M. Roper, Alkene and alkyne oligomerization, cyclooligomerization and telomerization reactions, in Comprehensive Organometallic Chemistry, (Eds. G. Wilkinson, F. G. A. Stone, E. W. Abel), I rgamon, Oxford, 1984. 

The oligomerization, cooligomerization and metathesis of small alkenes and alkynes and much of cobalt-catalyzed hydroformylation fall within the scope of the companion work Comprehensive Organometallic Chemistry and are not dealt with here. 

Lewis acid catalyzed versions of [4 4- 2] cycloadditions are restricted to functionalized dieno-philes. Nonfunetionalized alkenes and alkynes cannot be activated with Lewis acids and in thermal [4 + 2] cycloadditions these suhstrates usually show low reactivity. It has been reported that intcrmolecular cycloaddition of unactivated alkynes to dienes can be accelerated with low-va-lent titanium, iron or rhodium catalysts via metal-mediated - -complex formation and subsequent reductive elimination39 44. Usually, however, low product selectivities are observed due to side reactions, such as aromatization, isomerization or oligomerization. More effective are nickel-catalyzed intramolecular [4 4- 2]-dienyne cycloadditions which were developed for the synthesis of polycycles containing 1.4-cyclohexadienes45. Thus, treatment of dienyne 1, derived from sorbic acid, with 10mol% of Ni(cod)2 and 30 mol % of tris(o-biphenyl) phosphite in tetrahydrofuran at room temperature affords bicyclic 1,4-dienes 2, via intramolecular [4 + 2] cycloaddition, with excellent yield and moderate to complete diastereocontrol by substituents attached to the substrate. The reaction is sensitive towards variation in the catalyst and the ligand. 

Finally, it is possible to cyclo-dimerize, trimerize, cyelo-trimerize, oligomerize and co-oligomerize 1,3-butadiene with other alkenes and alkynes using catalysts based on Ni, Ti and Rh. A variety of large rings can be obtained in this way with high turnover numbers. ... 

Isopropoxy-4/f-l,3,2-benzodioxaborin (18) has been synthesized and found to react with a variety of hydroxy compounds to replace the isopropoxy group on boron. Aryl boric acids, RB(OH)2, reportedly condense with diols of the type HO(CH a)nOH to yield oligomeric rings (19) of varying size. The reaction of allenyl borates (20) with hydrogen bromide to yield alkenes and alkynes has been published. ... 

Many of the complexes discussed in the previous sections are catalysts for alkyne oligomerization. In fact, alkyne dimerization and trimerization (see Cyclodimerization -tri-merization Reactions) at a cobalt center is recognized as one of the most synthetically useful catalytic reactions mediated by a homogeneous transition metal complex. The cobalt complexes most useful and extensively studied are CpCoL2, where L is CO, alkene, diene, or phosphine. The complex types... 

We have shown in this review that neutral and cationic organoactinide complexes have been extensively studied, in the last decade, as catalysts for several organic transformations [9-12, 111]. Polymerization of alkenes[112,113], oligomerization of terminal alkynes [55, 114], hydrosilylation of terminal alkynes [41], and 1,1-insertion of isonitriles into terminal alkynes [28] comprise some other studied processes not presented here. However, due to the high oxophilicity of the actinide complexes (as mentioned above), substrates containing oxygen have been excluded because of the expected and predictable oxygen—actinide interaction. 

The cis-1,2-addition of M-X bonds to unsaturated A=B bonds and its reverse, the -elimination of X from M-B-A-X, are fundamental elementary steps of catalytic reactions such as hydrogenation, hydroformylation, oligomerization, polymerization, hydrosilation, hydrocyanation, or alkene isomerization processes, as well as the Heck reaction. Most of the reactions described in the literature involve M-H or M-C bonds, and alkenes or alkynes. Besides them there are processes where the unsaturated substrate is different from alkene or alkyne This includes CO2, CS2, aldehydes and ketones, imine, or nitrile. Also, there are processes involving M-Si, M-Sn, M-B, M-N, M-P, or M-M bonds. The insertion of alkenes into M-carbene bonds is not essentially different in their intimate mechanism, but it is not discussed in this chapter. 

Cyclization, Isomerization, and Coupling Reactions. Inter-(eq 42) and intramolecular (eq 43) cyclotrimerizations of alkynes are mediated by Wilkinson s catalyst. This is an extremely efQcient route to ring fused systems. Similarly, Diels-Alder-Uke [4 + 2] cyclization processes are promoted by RhCljPPhsjs dienophile components in these reactions need not be electron deficient, and they can be an alkene or alkyne (eqs 44 and 45) AUenes oligomerize in pathways determined by their substituents For instance, four molecules of aUene combine to give a spiro-cyclic system (eq 46), but tetraphenylallene isomerizes to give an indene (eq 47). ... 

Silylpalladium complexes have attracted attention due to their relevance in the mechanism of Pd complex catalyzed reactions such as hydrosilylation of alkenes and dienes, bis-silylation of dienes and alkynes, carbosilylation of alkynes, cross-coupling of organic halides with disilanes, and ring-opening oligomerization and polymerization of cyclic disilanes and polysilanes. 

---