Pyrolysis alkynes

Alkylidenes (alkylidene carbenes) are valence isomers of alkynes. They have been prepared by alkyne pyrolysis, by homologation of ketones, and by generation of alkenyl anions bearing oc-leaving groups. Generated by any of these means, an alkylidene will insert intramolecularly into a remote C- H bond to form a new C—C bond and thus a cyclopentene. A concerted two-electron process, this reaction proceeds with retention of absolute configuration at the C - H site. 

The chemistry of 7 is relatively unexplored as compared to the Group 6 and 9 molecules. It is dominated by reactions with phosphines and alkynes. Pyrolysis of Cp2V2(CO)5 in THF in the absence of phosphines gives... 

Certain a-substituted alkyltetrazoles on pyrolysis yield nitrogen and an alkyne by the mechanism shown in Scheme 53 <77AHC(2i)323). 

The addition of phthalimidylnitrene (374) to simple alkynes affords 1-azirines in yields of 1-15% (Scheme 10). In this reaction, which is of no real preparative value, the symmetrical 2-azirines (375) were suggested as the most plausible intermediates and unequivocal proof of the existence of such species was demonstrated from a series of 1,2,3-triazole pyrolysis reactions <71CC1518). Extrusion of nitrogen from the regioisomeric 4,5-disubstituted 1,2,3-triazoles (376) during flash vacuum pyrolysis furnished identical product mixtures which included both regioisomeric 1-azirines (377). 

Fluorinated alkynes are readily prepared by treatment of the mtermediate formed by acylation with a second equivalent of ylide followed by pyrolysis [63. 64, 65] (equation 57)... 

C/o50-carboranes are the most numerous and the most stable of the carboranes. They are colourless volatile liquids or solids (depending on mol wt.) and can be prepared from an alkyne and a borane by pyrolysis, or by reaction in a silent electric discharge. This route, which generally gives mixtures, is particularly useful for small c/o50-carboranes (n = 5-7) and for some intermediate c/ow-carboranes (n = 8-11), e.g. 

Phosphorus ylids are quite common (see 16-47) and keto-phosphorus ylids (RCOCH=PPh3) are also known. When these compounds are heating (flash vacuum pyrolysis, FVP) to great than >500°C, alkynes are formed. Simple alkynes can... 

The flash vacuum pyrolysis of alkynes, arynes, and aryl radicals has been reviewed. A discussion of secondary reactions and rearrangements is included. The pyrolysis of cyclopentadienes has also been examined. The rates for the initial C—H bond fission and the decomposition of C-C5H5 have been calculated. A single-pulse shock study on the thermal decomposition of 1-pentyl radicals found alkene products that are formed by radical isomerization through 1,4- and 1,3-hydrogen migration to form 2- and 3-pentyl radicals. The pyrrolysis of f-butylbenzene in supercritical water was the subject of a report. ... 

The first preparative use of intramolecular C—H insertion in organic synthesis was based on the observation that on flash vacuum pyrolysis, a conjugated alkynyl ketone such as 1-(1-methyl-cyclopentyl)-2-propynonc is smoothly converted to a mixture of the cyclizcd enones 1 and 223. This elegant reaction apparently proceeds via isomerization of the alkyne to the corresponding alkylidene carbcne, followed by subsequent intramolecular C-H insertion. It should be noted that despite a 3 2 statistical predominance of primary C-H bonds over secondary C—H bonds, a marked preference for insertion into the latter (methylene) is observed. 

Cyclopenta[c]quinolizines (284) reacted with alkynic esters in boiling nitrobenzene to give cyclopenta[cflf][3.3.3]cyclazines (285), the alkoxycarbonyl groups of which could be readily removed by alkaline hydrolysis followed by vacuum pyrolysis of the resulting acids (69JCS(C)239>. 

Extrusion of a silylene from a silirane or silirene is of course the inverse of silylene addition to alkenes or alkynes, respectively. The reversibility of most silylene reactions allows the inverse of addition to 1,3-dienes to also be employed as a silylene source. The first such reaction was reported by Chernyshev and coworkers, who found that transfer of SiCl2 units from l,l-dichlorosilacyclopent-3-enes was a unimolecular process and hence was likely to consist of silylene extrusion and readdition (equation 38)82. Dimethylsilylene extrusion has been found in the pyrolysis of silacyclopentenes and other products of... 

TABLE 1. Pyrolysis of acyl phosphorus ylides 253 to afford alkynes 255... 

In cases where R1 is a stabilizing group (entries 1-11) conventional pyrolysis is satisfactory, but the use of FVP has allowed the extension of the reaction to cases with R1 = H or alkyl (entries 13, 18-20) where conventional pyrolysis does not work. For R1 = C02Et, simply increasing the furnace temperature leads to loss of the ester group to give terminal alkynes and 1,3-diynes (entries 15, 17) as already noted in Section IV.B. The E-Z isomerization which occurs for styrylalkynes at higher temperatures (entry 20) was also noted previously in Section II.C. 

---