1- Methylcyclopropene

Apparatus 21 three-necked, round-bottomed flask, fitted with a combination of a dropping funnel and a gas inlet, a gas-tight mechanical stirrer and a combination of a thermometer (this should dip into the liquid) and a very efficient reflux condenser. The top of the condenser is connected with a cold trap ( — 80 °C) via plastic tubes and a tube filled with anhydrous calcium chloride. A slow stream of nitrogen is passed through the apparatus. 

Apparatus 500ml three-necked, round-bottomed flask, fitted with a dropping funnel, a gas inlet (placed on the middle neck and reaching 1 cm above the level of 

In spite of the relatively low yields, reaction of an allylic chloride with an alkali amide in an organic solvent is the most simple way to prepare cyclopropene and some methyl derivatives. The transient carbenoid, generated by abstraction of an allylic proton, rearranges to the cyclopropene, either via a cycloelimination of chloride or via a free carbene  

The modest yields may be ascribed to a number of side reactions. Allylic halides are very prone to nucleophilic attack, which in this case will give rise to the formation of an allylic amine. One could also imagine a nucleophilic attack of the intermediary carbenoid on the allylic halide, or metallation of the cyclopropene by alkali amide and subsequent coupling of the cyclopropenylide with the allylic halide. The relatively high temperatures in the preparations of the cyclopropenes will certainly favour oligomerization, which may be a merely thermal one or an ionic process (similar to that observed by Dutch investigators [16] in the case of the parent hydrocarbon). Such reactions afford products which are considerably less volatile than the cyclopropenes and therefore will remain in the reaction mixture. One obvious subsequent reaction is the irreversible base-induced isomerization to methylenecyclopropane, which cannot be separated from the desired product [79]  

We have carried out the procedure for 1-methylcyclopropene several times with freshly prepared sodamide (see Chap. I, Exp. 7). In most cases the ratio of 1-methylcyclopropene to methylenecyclopropane was greater than 3, in one experiment even 10. The original paper reports the formation of almost pure 1-methylcyclopropene. The authors used commercial sodamide, which might be less reactive than the freshly prepared base, and therefore unable to cause isomerization. Since the protons in methylenecyclopropane are less acidic than the vinylic proton in 1-methylcyclopropene, the exocyclic isomer is not likely to interfere in metallation reactions with alkali amides in liquid ammonia or with LD A 

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Mercury-sensitized irradiation of 3-methylfuran gave 2-butyne, 1,2-pentadiene, 1-methylcyclopropene, 1-butyne, and 1,3-butadiene (Scheme 6) (68JA2720 70JPC574). In the direct irradiation, 1-methylcyclopropene was obtained in lower yields while both 3-methylcyclopropene and 2-methylfuran appeared (Scheme 6) (70JPC574). 

In some instances, with 1-methylcyclopropene as the 27t-component, isomeric mixtures of 4H-azepines, e. g. 39 and 40, are formed.84... 

A recent variation of these reactions uses 6/f-l, 3-oxazin-6-ones as the electron-deficient heterodiene in place of the triazine.113114 With cyclopropene at — 35 C oxazinone 45 furnishes the 4//-azepine 46 in excellent yield. Likewise, with 3-methylcyclopropene the 4-methyl derivative 46 (R = Me) is formed. Cycloaddition with 1-methylcyclopropene, however, generates a mixture of 7-tert-butyl, 2-methyl 3-methyl- and 5-methyl-4//-azepine-2,7-dicarboxylate in a 2 1 ratio and a 97 % overall yield. 

In contrast, the addition of 1-methylcyclopropene to 4-methyl-2-(trifluoromethyl)-6//-l,3-oxazin-6-one (47) is regioselective and yields only the 7-methyl-4//-azepine 48. 

Cycloaddition of cyclopropenes is catalyzed by transition metal complexes. 1-Methylcyclopropene 118 undergoes a facile PdCl2(PhCN)2-catalyzed cyclodimerization to dimethyltricyclo[3.1.0.02,4]hexanes 119. In contrast, cyclo-trimerization of 3,3-dimethylcyclopropene 120 occurs in the presence of a catalytic amount of Pd(PPh3)4 to give hexamethyl-frans-ff-trishomobenzene 121... 

The use of 1-methylcyclopropene (1-MCP) to extend the shelf life of cherry tomato (.Lycopersicon esculentum var. Cerasiforme) delayed the ethylene-induced climacteric peaks in mature green and breaker fruits, chlorophyll degradation, and accumulation of lycopene and carotenoids (Opiyo and Tie-Jin 2005). Higher 1-MCP concentrations inhibited the accumulation of lycopene and carotene such that the color of the fruit did not reach that of control fruit. 

Opiyo AM and Tie-Jin Y. 2005. The effects of 1-methylcyclopropene treatment on the shelf life and quality of cherry tomato (Lycopersicon esciilentum var. cerasiforme) fruit. Int J Food Sci Technol 40 665-673. 

Vinylcarbene is known to close to cyclopropene.59 The reverse reaction is also possible Triplet-propene-l,3-diyl (frans-T-33 ) can be generated from cyclopropene 32 by irradiation in a bromine-doped xenon matrix at 10 K 1-methylcyclopropene (34) yields triplet-2-butene-l,3-diyl (Iruns-T-SS ).60-62 The concentration of 35 under these conditions is high enough to be able to detect this diradical IR spectroscopically. The experiments suggest that even the parent vinyl carbene 33 is detectable.61,62 Calculations ((U)B3LYP/6-31G )61,62 not only allow the comparison of theoretical and experimental IR spectra but also... 

The concentrations of acetates and butanoates seemed to increase during ripening of Valery bananas [40]. This was confirmed by an investigation in which bananas were treated with the ethylene antagonist 1-methylcyclopropene (1-MCP) [22]. The volatiles were recovered by a Tenax TA trap. The 1-MCP treatments caused quantitative changes in the amounts and the composition of... 

The cyclohepta[e][l,2,4]triazinone (382) reacted with cyclopropene to give compound (383), which showed no tendency to tautomerize to the azaheptalene derivatives (383a) or (383b) (81UP21900). Reaction of (382) with 1-methylcyclopropene affords the condensed norcaradiene (384), which tautomerizes very slowly, particularly in the presence of silica gel, to the methylene derivative (385). The azanorcaradiene (384) reacts with a second molecule of 1-methylcyclopropene to form the polycyclic compound (386). 

In this section we compare the enthalpy of formation of isomeric pairs of cyclic olefins, one with an exocyclic double bond and the other with the double bond endocyclic. We start with 1-methylcycloalkenes and related methylenecycloalkanes, species genetically 18a and 18b, respectively. From prior experience with cyclopropanes and the thermochemical consequences of replacement of sp3 carbons by sp2 carbons in three-membered rings52, we expect 1-methylcyclopropene to be considerably less stable than methylenecyclopropane because the former compound has two trigonal carbons within the ring while the latter has but one. And so it is found56 the former has a gas-phase enthalpy of formation 43.1 2.2 kJ mol-1 more positive than the latter. 

Incorporating an. s/ 2-hybridizcd carbon into a three-membered ring leads to more angle strain than incorporation of an. s/r -hybridizcd carbon. 1-Methylcyclopropene has two sp2-hybridized carbons in a three-membered ring and so has substantially more angle strain than methylenecyclopropane. 

The higher degree of substitution at the double bond in 1-methylcyclopropene is not sufficient to offset the increased angle strain, and so 1-methylcyclopropene is less stable than methylenecyclopropane. 

Gycloaddition reactions of several pyran-2-one derivatives with interesting outcomes include coumalic acid 262 and 1-methylcyclopropene-l-carbonitrile 261 to give 263 (Equation 27) <2006RJ0969>. 

The position of the alkyl substituent in the product indicates that cyclisation occurs with rearrangement of the double bond, ie., by 1,1-elimination and formal formation and cyclisation of a vinylcarbene. Although the overall yields are not always good, the reagents are readily available and large quantities of the simple alkylcyclopropenes can be produced. 1,2-Dimethylcyclopropene has been prepared in a similar process by treatment of methallyl chloride with two equivalents of phenyl lithium, followed by quenching with methyl iodide presumably, the initial reaction leads to 1-methylcyclopropene which is converted in situ to the 2-lithio-species 10). The elimination of HBr from brominated alkylidenemalonates also leads to cyclopropenes, though in low yield U) ... 

Reaction of t-butyl hypochlorite with 1-methylcyclopropenes can lead to efficient formation of 2-chloro-l-methylenecyclopropanes through trapping of an intermediate allylic radical 287 288) ... 

Acetylenes and cyclopropenes (37) are related to each other in the same formal way as olefins and cyclopropanes are. Recall that cyclopropanation of ethylene is almost slightly endothermic and the endothermicity was asserted to increase by some (3 2) kJ moT per alkyl group. Cyclopropanation of acetylene to form cyclopropene (37, X = X = H) is endothermic by (48.9 2.6) kJ mol. Cyclopropanation of propyne (monomethy-lacetylene) to form 1-methylcyclopropene (37, X = Me, X = H) has an increased endothermicity of (58.7 1.4) kJ mol. By contrast, the cyclopropanation of 2-butyne (dimethy-lacetylene) using a derived value for the enthalpy of formation of 1,2-dimethylcyclopropene (37, X = X = Me) has an accompanying endothermicity of only 41 kJ moT. We suspect that the last value is in error and so suggest remeasurement of the enthalpy of formation of dimethylcyclopropene as well as measuring the enthalpy of formation of other cyclo-propenes. ... 

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