Alkenes isomeric products with carbenes

Addition of carbenes to Jt-electron excessive aromatic compounds, or those which possess a high degree of bond fixation, is well established. Dihalocarbenes react with naphthalenes with ring expansion to produce benztropylium systems (Scheme 7.8). Loss of hydrogen halide from the initially formed product leads to an alkene which reacts with a second equivalent of the carbene to yield the spirocyclopropyl derivatives in high yield (>95%) [14, 50]. Insertion into the alkyl side chain (see Section 7.2) also occurs, but to a lesser extent [14]. Not unexpectedly, dichlorocarbene adds to phenanthrenes across the 9,10-bond [9, 10, 14], but it is remarkable that the three possible isomeric spiro compounds could be isolated (in an overall yield of 0.05% ) from the corresponding reaction with toluene [14]. 

The reaction of 3,3-disubstituted cyclopropenes with mono- and 1,2-disubstituted alkenes proceeds only with difficulty and leads to low yields of cyclopropanes. In the case of but-l-ene, an 8% yield, with hex-1-ene and hept-l-ene between 5 and 10% yield, and with cyclooctene about 10% of the cyclopropane product is formed. In these cases, the major product is the formal dimer of the intermediate ethenylcarbene complex, i.e. the corresponding (fj-hexatriene. When copper(I) chloride is used as catalyst rather than the copper halide/phosphane or phosphite system, about half the yield of the [2-f-1] cycloadduct is obtained along with an increased amount of the hexatriene. Mechanistically, these acyclic trienes could also be formed from an (alk-l-enyl)bicyclo[1.1.0]butane intermediate without any carbene being involved. Bicyclo[1.1.0]butanes are low yield (< 20%) byproducts of the thermal dimerization reaction of methyl 3,3-dimethylcyclopropenecarboxylate (1). On the other hand, bicyclo[l. 1. Ojbutanes, such as 3, are known to undergo isomerization to form 1,3-dienes. ... 

A Cu(I)-7V-heterocyclic carbene complex, IPrCu(DBM), catalyzes alkene aziridination with TcesNH2, Phl=0, and 4 A molecular sieves (eq 4). Reactions are performed with limiting alkene as- and frans-disubstituted olefins give isomeric product mixtures. In one example with styrene, TcesNH2, and PhI(OAc)2, 3 mol % of an Au(I) catalyst, [Au( Bu3tpy)](OTf), is found to catalyze aziridine formation. ... 

The reaction of metal-carbene complexes with electron-rich vinyl ethers occurs under milder conditions than the reaction with electron-poor unsaturated esters. The conditions are also milder than those required for ligand substitution of carbene complexes. The reaction products depend strongly on the external CO pressure with no added CO, alkene scission products predominate under 100 atm CO pressure, cyclopropanes are formed in 60% yield (Dotz and Fischer, 1972b). The ratio of isomeric cyclopropanes formed... 

As is the case with acyclic alkenes, geometrical isomerization becomes the major photoreaction observed for medium-sized cycloalkenes upon direct excitation, although the carbene-derived products are still obtained as minor products. Thus, direct irradiation of cyclohexene 31Z in aprotic... 

Thiocarbonyl derivatives of 1,3-dioxolanes and 1,3-oxathiolanes are readily isomerized to the 2-carbonyl compounds as shown in Scheme 20. Alkylation of the sulfur atom with alkyl halides usually leads to ring-opened products (Scheme 21) (69JOC3011). Most of the other chemistry of the sulfur derivatives has focused on desulfurization and subsequent generation of alkenes. The reaction is shown in equation (20) and proceeds with cis elimination via carbene intermediate (see Section 4.30.2.2.5) and is usually carried out with a phosphine (73JA7161) or a zero-valent nickel complex (73TL2667). 

In a large number of carbene and carbenoid addition reactions to alkenes the thermodynamically less favored syn-isomers are formed 63). The finding that in the above cyclopropanation reaction the anti-isomer is the only product strongly indicates that the intermediates are organonickel species rather than carbenes or carbenoids. Introduction of alkyl groups in the 3-position of the electron-deficient alkene hampers the codimerization and favors isomerization and/or cyclodimerization of the cyclopropenes. Thus, with methyl crotylate and 3,3-diphenylcyclopropene only 16% of the corresponding vinylcyclopropane derivative has been obtained. 2,2-Dimethyl acrylate does not react at all with 3,3-dimethylcyclopropene to afford frons-chrysanthemic add methyl ester. This is in accordance with chemical expectations 69) since in most cases the tendency of alkenes to coordinate to Ni(0) decreases in the order un-, mono-< di- < tri- < tetrasubstituted olefines. 

This method can be extended to applications with halo-substituted allyl dichlorides. Chloro(2,2-dichlorovinyl)carbenes 6 have been formed by deprotonation with lithium tetramethylpiperid-ide, but yields were improved when sodium hexamethyldisilazanide was used (see Houben-Weyl, Vol. E19b, Table 97). 1,2,3,3-Tetrachloroprop-l-ene can also easily be deprotonated, but in this case subsequent decomposition formed two isomeric carbenes, as observed in the product ratio after cyclopropanation.2 However, treatment of 1,1,2,3,3-pentachloropropene (11) with a base gave only the [2 + 2] dimer 14 of tetrachloroallene 13, generated by jS-elimination of lithium chloride.2 Other pentahalopropenes with a fluorine in the 3-position (e.g. 15) reacted with x-elimination, forming the corresponding cyclopropanes 17 in the presence of alkenes (see Houben-Weyl, Vol. E 19b, Table 97). 

In the same work, however, the authors found what they were looking for in 10,10-diflurobicyclo[4.3.1]deca-l,3,5-triene (32), readily obtainable from in-dane. This precursor was used to generate CF2 both thermally, as had already been demonstrated, and by photolysis with a medium-pressure mercury arc, and difluorocyclopropane products were obtained in both cases from a series of alkenes, mostly in moderate to very good yields. There was an interesting twist to this discovery, because a previous attempt to utilize (32) as a source of CF2 in flash-photolysis experiments had failed to yield appreciable quantities of the carbene. It was thus surmised that photons are capable of eliminating the carbene only from the norcaradiene valence tautomer (33), and that isomerization of (32) to (33) is exclusively a thermal process. 

These generalizations about stereochemistry are not applicable to gas-phase reactions. The reason is that most simple carbene-alkene-addition processes are highly exothermic, since two bonds are formed and none is broken. The initial adduct may be formed with sufficient energy to undergo isomerization, resultiilg in loss of stereospecificity. In solution, the excess energy is dissipated very rapidly to the medium, and changes in stereochemistry do not usually occur after formation of the product. 

Isomerization most commonly proceeds according to the first two mechanisms. The carbene mechanism was proposed for the isomerization of 1-alkenes in the presence of [Pd2Cl6] in acetic acid. In the products of isomerization of 3- /2-1-octene with the application of this catalyst neither migration of deuterium to the terminal carbon atom nor deuterium transfer from the solvent to the hydrocarbon was observed which support the carbene mechanism. The absence of deuterium at the first carbon excludes 1,3-hydrogen migration and therefore the 71-allyl mechanism. The fact that there is no deuterium transfer from the solvent to the olefin indicates that the formation of hydrido complexes involving the solvent also does not take place, which eliminates the first... 

Irradiations of alkenes are generally conducted with a low-pressure mercury lamp (185 nm), an ArF excimer laser (193 nm), or a zinc resonance lamp (214 nm). Highly substituted alkenes can also be irradiated with a medium-pressure lamp and quartz optics (>200 nm). Because of the presence of two or more close-lying singlet excited states, irradiation of alkenes usually results in several competing photoprocesses. Thus, for example, irradiation of 2,3-dimethyl-2-butene (1) in hydrocarbon or ether solvent results in rearrangement to the carbene-derived products 4 and 5 and the double bond isomer 9. E,Z-Isomerization also occurs but is not apparent in this case because of the symmetry of alkene 1. Similar behavior is exhibited in the gas phase. In alcoholic or aqueous media, the nucleophilic trapping products 8 are formed in competition with the carbene-derived products 4 and 5. ... 

Double bond migration generally competes with ILZ-isomerization and the formation of carbene-derived products, unless it is structurally inhibited as in 2-norbomene (14) and homobrexene (38). It presumably occurs in cyclohexene (27) and cyclopentene (32) but is not observable. In an alkene such as 43, in which the double bond is exocycKc to a five-membered ring, migration is the dominant photoprocess and occurs... 

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