Olefins Ortho’-effect

Cycloaddition reactions of (E)-l-acetoxybutadiene (18a) and (E)-l-methoxy-butadiene (18b) with the acrylic and crotonic dienophiles 19 were studied under high pressure conditions [9] (Table 5.1). Whereas the reactions of 18a with acrylic dienophiles regioselectively and stereoselectively afforded only ortho-enJo-adducts 20 in fair to good yields, those with crotonic dienophiles did not work. Similar results were obtained in the reactions with diene 18b. The loss of reactivity of the crotonic dienophiles has been ascribed to the combination of steric and electronic effects due to the methyl group at the )S-carbon of the olefinic double bond. 

The postulated mechanism involves a directing effect of the carbonyl group to the metal center, ideally positioning this metal for insertion into the ortho-G-H bond. The resulting ruthenium hydride undergoes hydridometallation of the olefin followed by reductive elimination to give the new C-C bond. 

The effect of monofluorination on alkene or aromatic reactivity toward electrophiles is more difficult to predict Although a-fluonne stabilizes a carbocation relative to hydrogen, its opposing inductive effect makes olefins and aromatics more electron deficient. Fluorine therefore is activating only for electrophilic reactions with very late transition states where its resonance stabilization is maximized The faster rate of addition of trifluoroacetic acid and sulfuric acid to 2-fluoropropene vs propene is an example [775,116], but cases of such enhanced fluoroalkene reactivity in solution are quite rare [127] By contrast, there are many examples where the ortho-para-dueeting fluorine substituent is also activating in electrophilic aromatic substitutions [128]... 

Ohashi et al. [128] found that the yields of ortho photoaddition of acrylonitrile and methacrylonitrile to benzene and that of acrylonitrile to toluene are considerable increased when zinc(II) chloride is present in the solution. This was ascribed to increased electron affinity of (meth)acrylonitrile by complex formation with ZnCl2 and it confirmed the occurrence of charge transfer during ortho photocycloaddition. This was further explored by investigating solvent effects on ortho additions of acceptor olefins and donor arenes [136,139], Irradiation of anisole and acrylonitrile in acetonitrile at 254 nm yielded a mixture of stereoisomers of l-methoxy-8-cyanobicyclo[4.2.0]octa-2,4-diene as a major product. A similar reaction occurred in ethyl acetate. However, irradiation of a mixture of anisole and acrylonitrile in methanol under similar conditions gave the substitution products 4-methoxy-a-methylbenzeneacetonitrile (49%) and 2-methoxy-a-methylbenzeneacetonitrile (10%) solely (Scheme 43). 

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). 

Chatani and coworkers reported the effective carbonylation of the C-H bond in the aromatic ring via Ru3(CO)12-catalyzed reaction of olefins and CO with heteroaromatics (Eq. 101) [159] and substituted benzene (Eq. 102) [160]. For more examples of the acylation of five-membered heteroaromatic compound see Ref. [ 161 ]. The reaction is closely related to the process of the ortho alkylation of substituted aromatic compounds and involves an additional step of CO insertion. 

The complex is an extremely active hydrogenation catalyst for terminal olefins 1 it is much less effective for the isomerization of olefins.1 Treatment of the complex with D2 results in the deuteration of the ortho-phenyl positions of the phosphine ligands14 as well as the Ru—D bond.1,4 It also serves as a catalyst for the preparation of selectively orf/fo-phenyl-deuter-ated triphenylphosphine.4... 

Isocoumarins.2 Isocoumarins can be prepared from benzoic acids by ortho-thal-lation followed by olefination promoted by PdCl2. Reaction with simple alkenes requires 1 equiv. of PdCl2 and a base-catalyzed reaction to effect the final cyclization. The reaction with vinyl halides or acetates generates isocoumarins directly and requires only catalytic amounts of PdCl2. 

Another reaction of some synthetic utility is the insertion of olefins into aromatic C-H bonds [9d]. This reaction is catalyzed by ruthenium compounds and requires a coordinating group (typically, ketone) on the aromatic ring. The group binds to the metal and the ortho C-H bonds are activated due to the resulting chelate effect. 

Polymerization experiments carried out using Pd(dpm)2 also showed that norbornene was polymerized quite effectively in the absence of AlEt3 at a 4000 1 9 monomer Pd B(C6F5)3 ratio (run 6, Tab. 4.2). The polymer was soluble enough at 50°C in ortho-dichlorobenzene to obtain a H NMR spectrum. No olefinic resonances were observed, confirming the formation of addition poly(norbornene). However, the solubility difficulties encountered prevented further characterization. 

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