Norbornene coupling

Three-component coupling with vinylstannane. norbornene (80). and bro-mobenzene affords the product 91 via oxidative addition, insertion, transme-tallation, and reductive elimination[85]. Asymmetric multipoint control in the formation of 94 and 95 in a ratio of 10 1 was achieved by diastereo-differ-entiative assembly of norbornene (80), the (5 )-(Z)-3-siloxyvinyl iodide 92 and the alkyne 93, showing that the control of four chiralities in 94 is possible by use of the single chirality of the iodide 92. The double bond in 92 should be Z no selectivity was observed with E form[86]. 

In an efficient diastereo-differentiative assembly of three components of norbornene, tv. v-alkenyl iodide, and KCN, the isomerization of the cis to the trans double bond takes place to give the coupled product 224. The isomerization is explained by the formation of the cyclopropane 222. its rearrangement to give a irans double bond in 223, and trapping with CN anion to give 224[168],... 

A similar dichotomy was observed in the titanium catalyzed polymerization of primary silanes coupled to the hydrogenation of norbornene (20). At low catalyst concentration (ca. 0.004H), essentially complete conversion of norbornene to an equimolar mixture of norbornane and bis-phenylsilyl- (and/or 1,2-diphenyl-disilyl)norbornane was observed. Under these conditions no evidence for reduction of titanium was obtained. At higher catalyst concentrations (> 0.02M) rapid reduction of the dimethyltitanocene to J, and 2 occurs and the catalytic reaction produces mainly polysilane (DPn ca. 10) and norbornane in ca. 80 per cent yields, and silylated norbornanes in about 20 per cent yield. 

Most studies on nickel-catalyzed domino reactions have been performed by Ikeda and colleagues [287], who observed that alkenyl nickel species, obtained from alkynes 6/4-41 and a (jr-allyl) nickel complex, can react with organometallics as 6/4-42. If this reaction is carried out in the presence of enones 6/4-43 and TM SCI, then coupling products such as 6/4-44 are obtained. After hydrolysis, substituted ketones 6/4-45 are obtained (Scheme 6/4.12). With cyclic and (5-substituted enones the use of pyridine is essential. Usually, the regioselectivity and stereoselectivity of the reactions is very high. On occasion, alkenes can be used instead of alkynes, though this is rather restricted as only norbornene gave reasonable results [288]. 

Fused pyrazole compounds have been prepared from A-alkyl substituted pyrazoles. For example, a palladium-catalyzed/norbornene-mediated sequential coupling reaction involving an aromatic sp2 C-H functionalization as the key step has been described, in which an alkyl-aryl bond and an aryl-heteroaryl bond were formed in one pot <060L2043>. A variety of highly substituted six-membered annulated pyrazoles 59 were synthesized in a one-step process in moderate yields from IV-bromoalkyl pyrazoles 57 and aryl iodides 58. 

A tandem palladium-catalysed ort/io-alkylation/intramolecular Heck reaction coupling sequence was used effectively to access in fair yields the tetrahydro 1-benzoxepines 67 from the iodoaryl precursor 66 and the appropriate alkyl bromide. The norbornene plays a relay role in the proposed reaction cycle <06JOC4937>... 

Arylation of C-H bonds is achieved by coupling reactions of C-H bonds with aromatic compounds such as halides, triflates, and organometallic reagents. Early works in this field involve the reaction of aryl halides with norbornene. As shown in Scheme 5, the coupling reaction of bromobenzene with norbornene in the presence of Pd(PPh3)4 as a... 

Sodium tetraphenylborate can be used for this transformation.407 In addition, three-component coupling products are obtained in the presence of norbornene (Equation (59)). 

Bazan and Schrock were the first to use ROMP of norbornene to prepare star polymers [108], As the coupling agent of the living polymer they used a norbor-nadiene dimer that plays the role of difunctional core-forming monomer as shown below ... 

A series of bridged dinuclear [(bpy)2Ru(p-L)Ru(bpy)2] " and [(bpy)2Ru(/i-L)Os(bpy)2] complexes have been prepared in which the bridge coordination domains are phen ligands and the spacer is constructed by the stereoselective coupling of units through norbornene and epoxide functionalities. " " ... 

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)). 

When /J-hydride elimination in the carbopalladation relay is completely blocked as in norbornene 116 and norbornadiene 117, the coupling with 6 -bromostyrene 113 furnished the respective cyclopentannelation products 118 and 119 exclusively and in good yields. 

Two molecules of norbornene 116 are incorporated in its coupling with / -bromostyrene to yield the bisnorbornane-annelated methylenecyclopentane derivative 120. But under different conditions, the reaction proceeds with the reverse 2 1 stoichiometry to give the cyclohexadiene-annelated norbornane derivative 121 (Scheme 32). ... 

Scheme 32 Two different reaction nnodes in the cross-coupling of / -bromostyrene with norbornene. ...

Exo-endo additions of organic azides to the norbornene skeleton and other bridged bicyclic systems can be easily differentiated from the NMR spectra of the adducts. The endo protons (exo adducts) of the norbornane do not couple with the bridgehead hydrogens.386 The endo protons in position 3a and 7a of adduct 65 exhibit an AB spectrum. The lack of coupling with the bridgehead protons is a result of a dihedral angle of 82°.25,96 Based on this, the exo form of cycloadduct 66 exhibits symmetrical doublets centered at 8 4.60 and... 

The protons in the 1,2-thiazetidine T-oxide ring 22 appear as a pair of doublets as expected for fit-coupling in a four-membered ring, but significantly they are not coupled to the vicinal bridgehead protons of the norbornene component thereby confirming the fuco-fusion <1997SL167>. 

The ruthenium-catalyzed addition of C-H bonds in aromatic ketones to olefins can be applied to a variety of ketones, for example acetophenones, naphthyl ketones, and heteroaromatic ketones. Representative examples are shown in the Table 1. Terminal olefins such as vinylsilanes, allylsilanes, styrenes, tert-butylethy-lene, and 1-hexene are applicable to this C-H/olefin coupling reaction. Some internal olefins, for example cyclopentene and norbornene are effective in this alkylation. The reaction of 2-acetonaphthone 1 provides the 1-alkylation product 2 selectively. Alkylations of heteroaromatic ketones such as acyl thiophenes 3, acyl furans, and acyl pyrroles proceed with high yields. In the reaction of di- and tri-substitued aromatic ketones such as 4, which have two different ortho positions, C-C bond formation occurs at the less congested ortho position. Interestingly, in the reaction of m-methoxy- and m-fluoroacetophenones C-C bond formation occurs at the congested ortho position (2 -position). 

One of the first examples of ruthenium-catalyzed C-C bond formation afforded the synthesis of cyclobutenes, from norbornene derivatives with dimethyl acetylenedicarboxylate, and was reported by Mitsudo and coworkers [45, 46] by using various catalysts such as RuH2(CO)[P(p-C6H4F)3]3 or RuH2(PPh3)4. More recently, the complex Cp RuCl(COD) has shown to be an excellent catalyst for the [2+2] cycloaddition of norbornenes with various internal alkynes [45] (Eq. 33) and with a variety of substituted norbornenes and norbornadienes [47]. The ruthenacycle intermediate, formed by oxidative coupling, cannot undergo /1-hydride elimination and leads to cyclobutene via a reductive elimination. 

Chromans and isochromans can be obtained by the reductive Pd-catalysed Heck cyclisation of 4-(2-bromophenoxy)-2-methylbut-l-ene and 3-(2-bromobenzyloxy)-2-methylpropene respectively <07TL2307>. Bromopropyl derivatives of 3,5-dihydroxy-iodobenzene and of 3,5-dihydroxymethyliodobenzene undergo sequential intramolecular alkylations and an intermolecular Heck coupling on treatment with an acrylate under microwave irradiation and Pd-catalysis mediated by norbornene. The products are 5-alkenyl derivatives of pyrano[3,2-g]benzo-2/7-1 -pyrans and pyrano[4,3-/]bcnzo-l77-2-pyrans (Scheme 13) <07JOC775>. 

Earlier, a one-pot procedure based on consecutive Pd-catalyzed aryl-aryl coupling followed by N-C bond formation had been described for the synthesis of phenanthridones and their thiophene analogs <2004OL4759>. The starting materials were 2-iodotoluene and 3-bromothiophene-2-carboxamides or 3-bromobenzo[3]thiophene-2-carboxamides (Equations 40 and 41). The conditions were rather critical in order to prevent the nucleophilic amide group from coordinating with the palladium, tri-2-furylphosphine had to be added to the reaction mixture. Norbornene serves to form a palladacycle which then reacts with the bromoamide. 

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