Cycloadditions chloride dimer

This reaction can also be used for the synthesis of substituted 1-benzoxepins with one modification instead of the 4/T-benzopyran the 2/7-isomer must be used. 2-[Diazo(phosphoryl)meth-yl]-2//-benzopyrans decompose in the presence of ))3-allylpalladium chloride dimer with elimination of nitrogen to give 1-benzoxepins 2.192 In some cases, the reaction takes a different course and gives 2-methylene-2//-benzopyrans 3.192 In this respect, the bicyclic system behaves differently to the monocyclic diazo(pyranyl)methane. The 2-isomers of the latter structure could not be isolated and gave l//-l,2-diazepines.190 The 4//-benzopyrans do not form benzoxepins but undergo an intramolecular [2+1] cycloaddition to 3,4-dihydro-2,3,4-metheno-2//-ben-... 

Brummond et al. disclosed that acyclic allenynes could be efficiently converted into tetracyclic compounds via consecutive rhodium-catalyzed Alder-ene and double Diels-Alder cycloaddition reactions [47]. The former reaction transforms alkynyl allenes such as 45 into triene-ynes (46) using rhodium biscarbonyl chloride dimer. 

Two transition metal catalyzed [27t + 2strained hydrocarbons, reacts with exo-tricyclo[3.2.1.0 " ]oct-6-ene (5) to give tetracyclo[3.3.0.0 .0 ]octane (6) in quantitative yield. The endo-isomer is unreactive. ... 

The trans isomer is more reactive than the cis isomer ia 1,2-addition reactions (5). The cis and trans isomers also undergo ben2yne, C H, cycloaddition (6). The isomers dimerize to tetrachlorobutene ia the presence of organic peroxides. Photolysis of each isomer produces a different excited state (7,8). Oxidation of 1,2-dichloroethylene ia the presence of a free-radical iaitiator or concentrated sulfuric acid produces the corresponding epoxide [60336-63-2] which then rearranges to form chloroacetyl chloride [79-04-9] (9). 

Scheme 6.8 gives some examples of ketene-alkene cycloadditions. In Entry 1, dimethylketene was generated by pyrolysis of the dimer, 2,2,4,4-tetramethylcyclobutane-l,3-dione and passed into a solution of the alkene maintained at 70° C. Entries 2 and 3 involve generation of chloromethylketene by dehydrohalo-genation of a-chloropropanoyl chloride. Entry 4 involves formation of dichloroketene. Entry 5 is an intramolecular addition, with the ketene being generated from a 2-pyridyl ester. Entries 6, 7, and 8 are other examples of intramolecular ketene additions. 

A new Y solvolysis scale has been developed for benzylic species with extensive charge delocalization, based upon the solvolyses of some benzhydryl bromides and /-butyl(2-naphthyl)methyl bromides.39 Chlorides have negative salt effects on the ionization of benzhydryl bromide in 7-butyrolactone.40 The X-ray structure of the dimerization product of l,8-bis(dhnethylammonio)-4-naphthyl(phenyl)methyl carbocation has been determined it appears to be formed via a 4n + 2n-cycloaddition mechanism 41... 

We have reported the first electroactivity of a thioketene dimer compound [116]. The CV measurement of 2,4-dibenzylidene-l,3-dithietane (31), which was prepared by a basic dimerization of phenylthioketene derived from ben-zyltriphenylphosphonium chloride, showed irreversible two-step oxidation peaks at 0.25 and 0.61 V vs Ag/Ag+, indicating that 31 acts as a stronger electron donor than 2,6-bisphenyl-l,4-dithiafulvene (30) and TTF (2). The dimer (31) can form a 1 1 CT complex with TCNQ in DMSO. Cycloaddition polymerization of bisthioketene derived from p-xylenebis(triphenylphosphoni-um chloride) gave a -conjugated polymer (32) with thioketene dimer unit in the main chain (Scheme 12). This polymer was the first polymer contain-... 

For thermally induced [2 + 2] cycloadditions, the concerted mechanism is operative only in particular cases, such as in the reactions between an alkene or alkyne and a ketene. The ketene can be generated directly in the reaction mixture from the appropriate acid chloride with triethylamine. The cycloaddition reaction is stereospecific and occurs exclusively in a cis fashion. Although the intermolecular cycloaddition with ketene itself proceeds in poor yields due to the propensity of the unsubstituted ketene to undergo dimerization, it is quite an efficient reaction with ketenes containing electron-withdrawing substituents. Usually, a-chloro ketenes are employed as reagents formed in situ from the corresponding a-chloro acid chlorides. Typical examples are represented in the preparation of cycloadducts such as 378 and 379 (Scheme 2.127). The latter cycloadduct, prepared in modest yield (ca. 20%),... 

P-Disubstituted a-chloroenamines are readily converted into the corresponding keteniminium salts (33) in the presence of a Lewis acid (equation 68). Silver tetrafluoroborate, zinc, tin and aluminum chlorides have been successfully used. 3-Disubstituted keteniminium salts have been isolated and characterized. " Unlike the corresponding ketenes, they do not dimerize or polymerize. However, they readily undergo [2 + 2] cycloaddition reactions with unsaturated substrates. ... 

An example of complexation of Ji bonds is illustrated in the isomerization reaction shown in Sch. 39. Use of copper chloride enables the efficient conversion of the cis olefin 168 to the trans olefin 169 [77]. (CuOTf)2(C6Hg) can be readily prepared (or purchased) and has been used for [2 + 2] photocycloaddition. An example of norbor-nene dimerization is shown below in which the Cu(I) forms a n complex with two molecules of the olefin and enables a facile cycloaddition.[51]... 

Methylenedioxy-6-propenylphenol (42) underwent le oxidation followed by radical coupling resulting in the formation of dimeric o-quinone methide 43, which was further converted to carpanone (44) and seven-membered ether 45 in 11 and 44% yields, respectively, as shown in Scheme 8. The former is produced by an intramolecular [4 + 2] cycloaddition . Carpanone has also been synthesized using oxidants such as palladium chloride and molecular oxygen in the presence of Co(II)salen. ... 

Two syntheses involving cycloaddition reactions were the self-condensation of N-aryltrichloroacetimido chlorides, in the presence of antimony pentachloride, which gave 45-63% yields of 3-aryl-2,4-6wtrichloromethyl-3,4-dihydroquinazolines, and the dimerization of A-aryl dialkylketen-imines (71). The imines 71, in the latter reaction, required a temperature of 125°C for several days to give the 4-methylene-3,4-dihydroquinazolines (72) in ca. 50% yields. Quinazolines could not be formed in this reaction when both the ortho positions of the A-aryl group were substituted. 

Ketenes are usually prepared in situ by elimination from acid chlorides with a tertiary amine. Thus (6) gives dimethyl ketene. If no other reagent is present, dimer (5) is formed. If a nucleophile is present, product (3) is formed, whilst thermal 2 + 2 cycloadditions take place with alkenes to give cyclobutanones (7). ... 

Reports of studies on the reactions of diphosphenes have begun to appear. Like (137), (140) is stable towards methanol but is cleaved by hydrogen chloride. The less hindered (143) dimerizes at room temperature in a few hours, and gives cycloaddition products with Sg and cyclopentadiene (Scheme 11). Oxidation... 

The photochemical behavior of these oligomers was first examined with the dimers which were irradiated with a medium-pressure mercury lamp through a Pyrex filter to remove any wavelength below about 300 nm. In dilute methylene chloride solutions, all the dimers (la and Ila-IId) isomerized to the cis conformation until the cis/trans thermodynamic equilibrium mixture was reached. No other photoinduced reaction was detected in these conditions. Irradiation of concentrated solutions, as well as crystalline solid samples, gave rise to molecular dimerization through the alkenyl bridge with all the dimers, through the classical [n2+7t2] cycloaddition process, e.g. for dimer la ... 

Moving to a related type of [2 + 2] cycloaddition, not involving aldehydes or imines as partners, Calter and coworkers reported Cinchona alkaloid (TMSO-QN) catalysed asymmetric dimerisation of ketenes, generated in situ from the corresponding acid chlorides, yielding (3-lactones via a formal Claisen condensation (Scheme 14.29). The unstable ketene dimers were trapped with an alko>yamine to afford p-keto amides i.e. Weinreb amides) with variable yields and excellent enantioselectivities. 

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