Norbornenes synthesis

Kinetics of norbornene synthesis and continuous reactor modeling study... 

Norbornene derivatives are common substrates for ring-rearrangement domino processes. Because of their highly strained structure, equilibrium is strongly shifted to the product and, owing to the highly stereocontroUed norbornene synthesis, it is granted easy access to new architectures which otherwise would be diflBcult to synthesize (Scheme 11.18). 

With the growing interest for the polynorbomene, photoresist polymer, and cyclic olefin copolymer, the synthesis norbornene or bicyclo[2,2,l]-2-heptene (NBN) has drawn significant attention because it is one of the most important precursor for these materials. Norbornene is produced by the reaction between ethylene and cyclopentadiene (CPD) via the Diels-Alder condensation process at elevated temperature and pressure [1,2]. 

An alternative synthesis of a thermally stable cyclopentadienyl functionalized polymer involved ring bromination of poly(oxy-2,6-diphenyl-l,4-phenylene), followed by lithiation with butyl lithium to produce an aryllithium polymer. Arylation of 2-norbornen-7-one with the metalated polymer yielded the corresponding 2-norbornen-7-ol derivative. Conversion of the 7-ol to 7-chloro followed by treatment with butyl lithium generated the benzyl anion which undergoes a retro Diels-Alder reaction with the evolution of ethylene to produce the desired aryl cyclopentadiene polymer, 6. 

Titanocene catalysts do not catalyze the hydrosilation of most internal olefins, although they can attach active olefins such as styrene, or norbornene to the growing polymer chain ends. The zirconocene-based catalysts, on the other hand, can be powerful hydrosilation catalysts and the remarkable copolymer synthesis shown in Equation 3 can be easily achieved under mild conditions (V7). 

A special class of synthesis is the utilization of retro-Diels-Alder (RDA) reactions. A double RDA sequence was used to prepare the pyrimido[l,2-A]pyridazin-3-one 118. In this versatile method both reactants of the parent compound were constructed from cyclopentadiene. The parent compound 117 contains two norbornene units and decomposes on heating in toluene in a double RDA reaction leaving two double bonds in the target heterocycle. Similarily, the parent compound 119 decomposes in a single RDA reaction to yield the benzologue, pyridazino[6,l-3]-quinazolin-10-one 120 (Scheme 13) <2000SL67>. 

Synthesis of block copolymers of norbornene derivatives, with different side groups, has been reported via ROMP [101]. Initially, exo-N-bulyl-7-oxabicyclo[2.2.1]hept-5-ene-2,3-dicarboximide was polymerized in acetone at room temperature with a ruthenium initiator (Scheme 40). The conversion of the reaction was quantitative. Subsequent addition of norbornene derivative carrying a ruthenium complex led to the formation of block copolymers in 85% yield. Due to the presence of ruthenium SEC experiments could not be performed. Therefore, it was not possible to determine the molecular weight... 

The living character of the ROMP promoted by the initiator Ru(CHPh)(Cl)2 (PCy3)2 (Cy = cyclohexane) was tested with the synthesis of diblock, triblock, and tetrablock copolymers of norbornene derivatives carrying acetyl-protected glucose, [2,3,4,6-tetra-O-acetyl-glucos-l-O-yl 5-norbornene-2-carboxylate], A or maltose groups, [2,3,6,2/,3/,4/,6/-hepta-0-acetyl-maltos-1-O-yl 5-norbornene-2-carboxylate], B, shown in Scheme 41 [102]. The AB, ABA, and ABAB structures were prepared by sequential addition of monomers with narrow molecular weight distributions to quantitative conversions. 

The first documented example of the living ROMP of a cycloolefin was the polymerization of norbornene using titanacyclobutane complexes such as (207) 510-512 Subsequent studies described the synthesis of di- and tri-block copolymers of norbornenes and dicyclopentadiene.513 However, functionalized monomers are generally incompatible with the highly electrophilic d° metal center. 

The first report of ROMP activity by a well-characterized Mo or W species was polymerization of norbornene initiated by W(CH-t-Bu)(NAr)(0-f-Bu)2 [122]. In the studies that followed, functionality tolerance, the synthesis of block copolymers, and ring-opening of other monomers were explored [30, 123]. Two important issues in ROMP concern the cis or trans nature of the double bond formed in the polymer and the polymer s tacticity. Tacticity is a consequence of the presence of two asymmetric carbons with opposite configuration in each monomer unit. The four ROMP polymers (using polynorbornene as an example) that have a regular structure are shown in Scheme 3. 

As had been observed in the synthesis of carbohydrate-substituted polymers of different lengths, the reactivity of the monomers was an important parameter in generating the triblock polymers. If the mannose-substituted 7-oxanor-bornene derivative was first polymerized, followed by the galactose-derivatized norbornene and the mannose-substituted norbornene monomers, two distinct sets of products were observed. These were identified by modification of the resulting polymers by acetylation, and analysis of the products by GPC. With this protocol, it was found that the product was composed of short polymers (DP=... 

An important procedure for the synthesis of cyclopentenones is the so-called Pauson-Khand reaction, which constitutes a formal [2 + 2 + 1] cycloaddition of an alkene, an alkyne, and carbon monoxide. Due to the increase in structural diversity of the available starting materials, the reaction has become an attractive target for scientific investigations [1-8]. The first successful example was reported by Pauson, Khand et al [9] in 1973 for the conversion of norbornene with the phenylacetylene-hexacarbonyldicobalt complex to give the corresponding cyclopentenone in 45% yield (Eq. 1). 

A versatile new and general method was developed for the synthesis of six-membered 1,3-heterocycles. In comparison with the general reaction conditions of retrodiene reactions, a very mild retro Diels-Alder (RDA) decomposition was found to occur when the norbornene-d/eAro- or -diendo-fused dihydrooxazinone 474 was heated at melting temperature or refluxed in different solvents (e.g., chlorobenzene). Cyclopentadiene splitt off and the 2-aryl-6//-l,3-oxazin-6-ones 475, earlier unknown representatives of the simple 1,3-oxazines, were obtained (84S345, 84T2385). 

A variety of substituted seven-membered annulated pyrroles can be synthesized in a one-step process in good yields from readily accessible N-bromoalkyl pyrroles 75 and aryl iodides. The synthesis is based on a palladium-catalyzed/ norbornene-mediated sequential coupling reaction involving an aromatic sp C-H functionalization as the key step. The proposed mechanism suggests that orffzo-alkylation with the formation of intermediate 76 most likely precedes aryl-heteroaryl coupling (Scheme 15 (20060L2043)). 

Studies by the submitters have indicated that the procedure reported here is the preferred method for the preparation of bicyclo[3.2.1]octan-3-one. It employs readily available, inexpensive reagents, and the overall yield is good. In addition, the method can be used for the synthesis of the difficultly accessible next higher homologues of bicyclo[2.2.2]oct-2-ene as well as for derivatives of norbornene. Bicyclo[3.2.2]nonan-3-one and l-methylbicyclo[3.2.1]octan-3-one have been prepared by a similar route6 in 60% and 47% yields, respectively (based on adduct). However, the preferred procedure for the formation of the dichlorocarbene adduct of bicyclo[2.2.2]oct-2-ene is that of Seyferth using phenyltrichloromethylmercury. Even in this case the overall yield is moderate (37%). 

Poly (norbornene)-supported Co(III)-Salen complexes (44) and poly (styrene)-supported Co(III)-Salen complexes (45,46) were synthesized via newly developed procedures. In particular, a new, high-yielding, one-pot synthesis of non-symmetrical salens was developed (46). Hydrolytic kinetic resolutions were carried out at room temperature and the products were characterized by chiral GC. 

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