Norbomene dimerization

Recently, some doubt has been raised as to whether these reactions involving metal complexes really are concerted electrocyclic reactions, since products and intermediates have been observed "which have no place in the previous mechanistic scheme 258> [see also the discussion of the norbomene dimerization in section H1],... 

Diels-Alder Reactions. The important dimerization between 1,3-dienes and a wide variety of dienoplules to produce cyclohexene derivatives was discovered in 1928 by Otto Diels and Kurt Alder. In 1950 they won the Nobel prize for their pioneering work. Butadiene has to be in the j -cis form in order to participate in these concerted reactions. Typical examples of reaction products from the reaction between butadiene and maleic anhydride (1), or cyclopentadiene (2), or itself (3), are <7 -1,2,3,6-tetrahydrophthaHc anhydride [27813-21 -4] 5-vinyl-2-norbomene [3048-64-4], and 4-vinyl-1-cyclohexene [100-40-3], respectively. 

A photosensitized dimerization of an isolated olefin, norbomene, has been reported by Scharf and Korte.<3) Irradiation in acetone or in the presence of acetophenone (Et = 74 kcal/mole) produced dimers (5) and (6) as major products. However, benzophenone (Et = 69 kcal/mole) failed to sensitize the reaction to (5) and (6), but in ether solution led to the quantitative formation of benzpinacol and in benzene to the oxetane (7) in 80% yield. Sensitizers of intermediate energy, such as xanthone (Et — 72 kcal/mole), demonstrated a competition between energy transfer to form triplet norbomene and cycloaddition to form the oxetane ... 

Aus Norbomen konnten nach dieser Methode zwei Dimere (46) und... 

Nitrosyl formate may be generated in situ by treating isopentyl nitrite with anhydrous formic acid. This reagent evidently adds to olefins such as cyclohexene, styrene, norbomene, trans-3-hexene, and 2,3-dimethylbutene to give nitrosoformates. While the first three olefins were converted into dimeric products, 2,3-dimethylbutene produced a 50-50 mixture of the monomer and the dimer (as a blue oil). The product of the reaction with trans-3-hexene contained some of the corresponding oximino formate. 

Copper-catalyzed [2 + 2] photocycloadditions are related to the latter reactions. These transformations have been extensively studied, frequently in the context of application to organic synthesis [21], When irradiated in the presence of copper(I) triflate, norbomene 12 was effidently transformed into its dimer 13 (Scheme 5.3, reaction 4) [22]. Although complexes such as III are involved in the reaction mechanism [22, 23], it is unclear whether MLCT (metal to ligand charge transfer) or LMCT (ligand to metal charge transfer) excitation induces the transformation. 

Due to its versatile applicability, the CuOTf-catalyzed [2 + 2] photocycloaddition was used successfully to study the topology of the intermolecular and intramolecular dimerization of norbomene derivates. When a racemic mixture of compound 27 is transformed in the presence of CuOTf, a 1 1 mixture of two stereoisomers (28a,b) is... 

The 2+ aqua ion catalyzes4 several reactions in solution such as the dimerization of ethylene via the intermediates [Ru(C2H4)(H20)5]2+ and [Ru(C2H4)2(H20)4]2+ that were isolated as tosylates.5 It also catalyzes polymerization of norbomenes.6... 

Hydroamination. The chiral BINAP-IrCl dimer induces the enantioselective addition of aniline to norbomene (95% ee) at 75°. The reaction is facilitated by fluoride ions I activity increase by 6.5-fold). 

In cases where unsubstituted porphyrins are used, the reactions are complicated by dimerization and disproportionation processes of the porphyrins (P), as illustrated for a Cr complex in the epoxidation of norbomene (Nb) [513]. 

A chelating diphosphite prepared from biphenol and PCI3 provides a very stable Ni(0) complex that catalyzes the hydrocyanantion of butadiene without excess ligand. Although the stability of this catalyst is enhanced, the amount of butadiene dimerization byproducts is significant. A related nickel catalyst prepared using a chiral chelating diphosphite based on I -2,2 -binaphthol provides enantioselectivity in the hydrocyanation of norbomene. The major product, J -exo-2-cyanonorbomane, was obtained in an enantiomeric excess of up to 38%. 

The first example of an o/t/20-alkylation/Mizoroki-IIcck coupling was reported by Catellani [4] in 1997. Using the PNP dimer as a catalyst in the presence of an aryl halide, norbomene, an alkyl iodide, a terminal olefin and a base at room temperature, 1,2,3-trisubstituted benzenes (Scheme 16), were synthesized through alkylation of a palladacycle of type 35, followed by Mizoroki-Heck coupling with an arylpalladium(II) species of type 36. Although the synthetic scope of the reaction was limited, the importance of the report reveals an unprecedented catalytic transformation where two aryl C-H bonds are converted to sp2-sp3 C-C bonds followed by a standard Mizoroki-Heck coupling. The 1,2,3-trisubstitution pattern generated in the products would be very difficult to obtain via conventional methods. 

While the PNP dimer was an efficient catalyst for the ort/toalkylation/ Mizoroki-Heck reaction, the practicality of the transformation is lessened by the fact that the PNP dimer is not commercially available, and can be quite difficult to prepare. Thus, Catellani adapted the reaction conditions to include commercially available and air-stable Pd(OAc)2 as the catalyst source [46], Under these conditions, the ortho-u kylation/Mizoroki-I Ieck coupling of aryl iodides containing a pre-existing ortho substituent could be carried out. The reaction required higher temperatures, and the addition of KOAc to promote the carbopalladation of norbomene [47] and encourage the o/t/zo-alkylation pathway vs a direct Mizoroki-Heck coupling. 

Asymmetric hydrosilylation of norbomene (2). This reaction can be effected in high regio- and enantioselectivity by reaction of 2 with CliSiH catalyzed by (allyl)chloropalladium dimer complexed with (R)-l. The product can be converted to (lS,2S,4R)-norbornanol (4) in 96% ee. 

The generation of an aryl phosphenite (150) from a 7-phospha-norbornene derivative (151) by thermolysis or photolysis has been investigated. Thermolysis gave a condensate containing some (150) besides dimers and trimers photolysis, however, gave no fragmentation products from (151) or from similar phospha-norbornenes unless alcohols were present. Photolysis of 7-phospha-norbomenes is therefore not a route to two-co-ordinate phosphorus compounds, as assumed earlier. 

Another example of chemical reaction which gives different products in ball mill than solution chemistry is phthalazine addition to fullerene [11], In the ball-milling conditions, intermolecular [4+2] cycloaddition of phthalazine 16 takes place, followed by spontaneous nitrogen elimination from 1 1 adduct 17. This adduct further in sohd state undergoes intermolecular cycloaddition and formation of the corresponding dimer 20 (Scheme 7.5). On the other hand, when reaction is carried out in solution, an intramolecular [4+4] addition takes place with formation of product 20, followed by retro- [2+2+2] addition and formation of product 21. This reaction sequence is characteristic for fullerene chemistry, which is not observed in analogous cycloaddition reactions of phthalazines with norbomenes [12]. 

Analogously, it was observed that norbomene could be inserted into an arylpalladium bond and the resulting product was stabilized through the formation of a dimer or by appropriate ligands (Scheme... 

Aromatic iodides, unsubstituted or bearing substituents in meta and/or para positions, readily react with aliphatic iodides and a terminal alkene in the presence of norbomene, a palladium species such as phenylnorbomylpaUadium chloride (PNP) dimer and K2CO3 in DMF at room temperature yielding alkenylarenes symmetrically alkylated at both ortho positions (Scheme lO). As shown in Table 1, selectivity always is very high except when secondary alkyl iodides are used. Yields can be improved by prolonging the reaction time. 

The core-first approach was also adopted for the synthesis of star polymers. According to this procedure, the catalyst was reacted with the difunctional monomer to give the multifunctional initiator, followed by the addition of norbomene." Unfortunately, a multimodal product was obtained revealing the presence of linear chains, dimers, and star stmctures. 

Photocycloadditions. CuOTf is an exceptionally effective catalyst for 2n + 2Tf photocycloadditions of alkenes. Thus while CuBr promotes photodimerization of norbomene in only 38% yield, the same reaction affords dimer in 88% yield with CuOTf as catalyst (eq 11). A mechanistic study of this reaction revealed that although both 1 1 and 2 1 alkene Cu complexes are in equilibrium with free alkene and both the 1 1 and 2 1 complexes absorb UV light, only light absorbed by the 2 1 complex results in photodimerization. In other words, photodimerization requires precoordination of both C=C bonds with the Cu catalyst. Thus the exceptional ability of CuOTf, with its weakly coordinating trillate counter anion, to form r-complexes with as many as four C=C bonds is of paramount importance for its effectiveness as a photodimerization catalyst. 

As a nonconjugated diene, dicyclopentadiene forms first a monomeric rT.-ir-complex, which is attacked by nucleophiles at the norbomene double bond giving the 6-bonded dimeric complex, jjjg diene has an acyclic double bond this is also preferentially... 

Norbomene undergoes polymerization and dimerization to four isomeric dimers (36) in the presence of WCle or ReClg, and when the reaction is carried out in benzene norbomylbenzene is also obtained. Kinetic studies show that while the alkylation... 

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