Aromatic compounds from alkynes

In general, unsaturated heterocyclic compounds are generated from the reactions of allyl-type 1,3-dipole precursors with either alkynes or alkenes as well as the reactions of propargyl/allenyl-type 1,3-dipoles with alkenes, while aromatic compounds are often accessible by the cycloaddition reactions of propargyl/allenyl type 1,3-dipoles and alkynes (Scheme 16.2). To obtain aromatic compounds from the former type of cycloaddition reactions, the resulting unsaturated cycloadducts are required to undergo further aromatization, such as oxidation and elimination of small molecules (i.e., H2O, CH3COOH, CO2, etc.). 

CoF is used for the replacement of hydrogen with fluorine in halocarbons (5) for fluorination of xylylalkanes, used in vapor-phase soldering fluxes (6) formation of dibutyl decalins (7) fluorination of alkynes (8) synthesis of unsaturated or partially fluorinated compounds (9—11) and conversion of aromatic compounds to perfluorocycHc compounds (see Fluorine compounds, organic). CoF rarely causes polymerization of hydrocarbons. CoF is also used for the conversion of metal oxides to higher valency metal fluorides, eg, in the assay of uranium ore (12). It is also used in the manufacture of nitrogen fluoride, NF, from ammonia (13). 

Saturated hydrocarbons are stable. Only cycloalkanes with a tight ring are unstable. Alkenes and alkynes have a strong endothermic character, especially the first homologues and polyunsaturated conjugated hydrocarbons. This is also true for aromatic compounds, but this thermodynamic approach does not show up their real stability very well. Apart from a few special cases, the decomposition of unsaturated hydrocarbons requires extreme conditions, which are only encountered in the chemical industry. 

Intermolecular [4+2]-cycloaddition of vinylallenes with alkynes is efficiently mediated by means of an electronically tuned rhodium catalyst (Scheme 16.81) [91]. A five-membered rhodacycle is formed from the vinylallene. Coordination followed by insertion of an alkyne to the rhodacycle generates a seven-membered rhodacycle, from which rhodium(I) is eliminated reductively to produce a cyclohexatriene, leading to the aromatic compound. 

The use of several rhodacyclopentadiene complexes in syntheses of polycyclic aromatic compounds has been described by Miiller. In general, alkynes displace the rhodium center to give substituted arenes. In this way, complex 279 [from l,2- PhC=CC(0) 2C6H4] reacts with PhC=CC=CR (R = Me, Ph) to give 280 (R = Me, Ph) (Scheme 61). ... 

Quite recently, some mononuclear ruthenium complexes such as [(p-cymene)RuX-(CO)(PR3)]OTf (X = Cl, OTf, R = Ph, Cy) have been found to work as catalysts for the propargylation of aromatic compounds such as furans, where some ruthenium complexes were isolated as catalytically active species from the stoichiometric reactions of propargylic alcohols (Scheme 7.27) [31]. The produced active species promoted the propargylation of furans vdth propargylic alcohols bearing not only a terminal alkyne moiety but also an internal alkyne moiety, indicating that this propargylation does not proceed via allenylidene complexes as key intermediates. 

Why did he think this was so One answer is that alkanes are available as raw material for the chemical industry, and new reactions by which they can be converted into functionally substituted organic compounds are likely to be of considerable interest to the industrial chemist. A second answer is that the nature of any interaction between an alkane and a transition metal must be quite different from that of other hydrocarbons (i.e., alkenes, alkynes, and aromatic compounds) having 7r-electrons that can play a dominant role. 

Cyanogen Iodide (ICN) has been used extensively for the cyanation of alkenes and aromatic compounds [12], iodination of aromatic compounds [13], formation of disulfide bonds in peptides [14], conversion of dithioacetals to cyanothioacetals [15], formation of trans-olefins from dialkylvinylboranes [16], lactonization of alkene esters [17], formation of guanidines [18], lactamization [19], formation of a-thioethter nitriles [20], iodocyanation of alkenes [21], conversion of alkynes to alkyl-iodo alkenes [22], cyanation/iodination of P-diketones [23], and formation of alkynyl iodides [24]. The products obtained from the reaction of ICN with MFA in refluxing chloroform were rrans-16-iodo-17-cyanomarcfortine A (14)... 

Chiral Metal Atoms in Optically Active Organo-Transition-Metal Compounds, 18, 151 13C NMR Chemical Shifts and Coupling Constants of Organometallic Compounds, 12, 135 Compounds Derived from Alkynes and Carbonyl Complexes of Cobalt, 12, 323 Conjugate Addition of Grignard Reagents to Aromatic Systems, I, 221 Coordination of Unsaturated Molecules to Transition Metals, 14, 33 Cyclobutadiene Metal Complexes, 4, 95 Cyclopentadienyl Metal Compounds, 2, 365... 

The trimerization of alkynes is a general and useful method for the preparation of aromatic compounds [152]. However, this method has serious limitations when three different alkynes are used, as numerous regioisomers may be formed. Taka-hashi and co-workers have reported the beginnings of a solution using zirconocy-clopentadienes prepared in situ from two different alkynes. Substituted arenes were obtained upon addition of a third alkyne to the organometallic complex in the presence of copper chloride [153] or a nickel complex [154], This approach is nevertheless limited by the fact that at least one of the alkynes must be symmetrical, and by... 

Because of the generally greater reactivity of >C=C<, -C=C- and aromatic rings toward OH radical addition, H-atom abstraction from alkyl or substituted alkyl groups in the alk-enes, alkynes, and aromatic compounds is generally of minor importance, and Kwok and Atkinson (1995) set the substituent group factors F(>C=C<), F(-C=C-) and F(-C6H5) equal to unity. 

Fig. 16.30. Pd(0)-catalyzed arytation of a copper acetytide at the beginning of a three-step synthesis of an ethynyt aromatic compound. Mechanistic details of the C,C coupling Step 1 formation of a complex between the catalytically active Pd(0) complex and the arylating agent. Step 2 oxidative addition of the arylating agent and formation of a Pd(II) complex with a cr-bonded aryl moiety. Step 3 formation of a Cu-acetylide. Step 4 trans-metalation the alkynyl-Pd compound is formed from the alkynyl-Cu compound via ligand exchange. Step 5 reductive elimination to form the -complex of the arylated alkyne. Step 6 decomposition of the complex into the coupling product and the unsaturated Pd(0) species, which reenters the catalytic cycle anew with step 1.

For the preparation of conjugated alkynes, one can alkenylate or arylate alkynes according to Section 13.3.4. Alternatively, metallated alkenes or metallated aromatic compounds also may be alkynylated, but this option will not be pursued further. We merely mention in passing that bromoalkynes and iodoalkynes are suitable alkynylat-ing agents and that these can be obtained in a one-step reaction from terminal alkynes ... 

The following reactions proceed with the participation of the allylic boron system (i) allylboration and protolytic cleavage of organic compounds with multiple bonds, (ii) allylboron-alkyne condensation,598 599 (iii) reductive mono-and trans-a,a -diallylation of nitrogen aromatic compounds, (iv) disproportionation processes between tribut-2-enylborane and BX3 (X = C1, Br, OR, SR). Allylboration of carbonyl compounds, thioketones, imines, or nitriles leads to the homoallylic alcohols, thiols, or amines (Equations (136) and (137). It is most important that 1,2-addition to aldehydes and imines proceeds with high diastereoselectivity so that ( )-allylic boranes and boronates give the anti-products, while -products are formed preferentially from (Z)-isomers. 

Q Propose single-step and multistep syntheses of ketones and aldehydes from alcohols, alkenes, alkynes, carboxylic acids, nitriles, acid chlorides, esters, and aromatic compounds. Problems 18-55, 56, 60, and 64... 

Alkynes undergo cycloaddition on irradiation with benzene or naphthalene derivatives or with other aromatic compounds. With a benzene derivative the product is usually a cyclo-octatetraene which results from thermal electrocyclic ring-opening of the bicyclo-octatriene formed initially by 1,2-addition of the alkyne to the benzene ring (equation 61) . The intermediate can be trapped using a dienophile such as tetracyanoethylene (equation 62) ". The first step of the photoaddition process involves excitation of the alkyne , and orbital symmetry considerations suggest that concerted 1,2-addition is allowed if the alkyne is excited but not if the benzene is excited ... 

Ghlorofluorination with T-chlorosaccharin-HF/pyridine <1995SL327>, iodofluorination with T-iodosaccharin-HF/pyridine, and iodomethoxylation with A -iodosaccharin <2000SL544> of alkenes, alkynes, and activated aromatic compounds have been described. Bromohydrin and iodohydrin derivatives were prepared from electron-deficient alkenes and A -halosaccharins <2005EJO2349>. 

Aromatic compounds can be prepared by cyclotrimerization of alkynes or triynes. Cyclotrimerization is possible by heating to 450-600°C with no catalyst. The spontaneous (no catalyst) trimeiization of t-BuC=CF gave 1,2,3-tri-tert-butyl, 5,6-trifluorobenzene (220), the first time three adjacent tert-butyl groups had been put onto a benzene ring. The fact that this is a head-to-head joining allows formation of 220 from two alkynes. The fact that 219 (a Dewar benzene) was also isolated lends support to this scheme. Three equivalents of 3-hexyne trimerized to hexaethylbenzene at 200°C in the presence of Si2Cl6. ... 

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