Allenes from cyclopropenes

The kinetics and mechanisms of the gas phase pyrolysis of cyclopropene in the temperature range 193-243°C has been examined experimentally and theoretically The major and minor products of reaction are propyne and allene respectively propyne results from a unimolecular isomerization with an activation energy of 147.3 kJ mol Moreover, cyclopropene is most likely the important intermediate in the thermal isomerization of allene to propyne These observations are accommodated by the reactions of equation 65. The activation energy for the conversion of allene into cyclopropene is 269 kJmol" and that for the reverse process is 182 kJmol ... 

The direct irradiation of fiiran in gas phase gave carbon monoxide, methylacetylene, and allene, while cyclopropene was detected only in traces [68 JCP(48)2185]. The direct irradiation leads to the formation of the excited singlet state, in which case cyclopropene is not the main product then 3 is formed, starting from the excited triplet state. The direct flash photolysis of furan in gas phase gave mass... 

Allylic cations (180) were also generated by LFP of allenes (174) in TFE.86 Deuterium labels revealed that the cations 180 originate predominantly from vinylcarbenes (177), which are formed from 174 by way of a 1,2-H shift. Protonation at the central carbon of the photoexcited allenes87 is a minor reaction path with 174a,b,d. Vinylcarbenes are also known to arise in photolyses of cyclopropenes, 175 — 177.85bi88 However, LFP of 175 in protic media proved to be rather inefficient in generating allylic cations, presumably due to low quantum yields. 

Treatment of the oxime 63 with methyl lithium at -20° gave 2,3-dimethyl-1-hydroxy pyrrole (65) in 50% yield, along with 20% of the allene oxime (64), which is the product isolated from this type of reaction when the cyclopropene contains additional alkyl groups (80TL2893). 

The reaction of the dimethyl-derivative (27) with butoxide ion might be expected to produce the chlorocyclopropene (28) however, in practice two eliminations occur to produce (31) and the carbene (30), which can be trapped by an added alkene. Both products may be derived from (28), by a 1,4- or a formal 1,2-elimination respectively a study using a 14C-label at C-l of (27) showed that the carbene (30) was formed with the label exclusively at C-l, suggesting elimination via (29)32). However, in a related study, the isolated cyclopropene (28) labelled with 12C at C-l has been shown to react with methyl lithium to produce the carbene (30) labelled only at C-2 this suggests either that the reaction of (28) with butoxide follows a completely different course to that with methyl lithium, or that (28) is not involved in the reaction of (27) with base33). In a similar reaction the dichloride (32) has been shown to react with t-butoxide in DMSO to produce the allene (33) the product may be explained in terms of initial elimination to produce (34), followed either by rearrangement to the alkyne (35) and then elimination or by direct 1,4-elimination as in (36), followed in either case by a prototropic shift. Whatever the mechanism, a 12C-label at Ca in (32) is found at Ca in (33) 33). 

A number of groups have studied one or more of the C3H4 isomers [213, 677—680, 806]. It has been proposed that the loss of H- from the allene ion proceeds via two pathways to give two different (C3H3)+ structures [213] (cf. loss of Cl- from the propargyl chloride ion). A kinetic shift has been determined for formation of (C3H3)+ from allene [806]. Some evidence was found that the cyclopropene and propyne ions isomerised to the allene structure before they decomposed [678]. 

The formation of propyne and allene by pyrolysis of cyclopropene arises from opposite [1,2]H shifts in diradicals 191 or 192 The substantially larger activation energy (by some 24.5 kJ mol" ) for formation of allene reflects differences in the transition state structures for the two processes. Thus, the propyne-forming reaction requires the migrating hydrogen atom to span a single bond (see 194), whilst in the allene-forming process a double bond is involved and a more strained situation ensues (see 195). The formation of but-2-yne from 3-methylcyclopropene is similarly rationalized but the... 

The higher members of the [l,n,l]-eliminations are also of preparative importance. Thus, the Doering allene synthesis 14) leads to bicyclobutanes (e.g. 66, 28 %) 45), if bulky substitution as in 65 favours the [l,3,l]-elimination. The related cyclopropene syntheses from 6746) and 6946, respectively (40 and 6% 68 in derivatized form lithiation and carboxylation), are to be classified as [l,3,(2)l]-eliminations of bromine (reductive) as well as of hydrogen chloride. The thermolysis or the photolysis of diazo... 

The involvement of trimethylenemethane diradicals in deazetization of diazoalkane-allene adducts or trimethylene diradicals in the deazetization of the adducts of acyclic alkenes often leads to mixture of regioisomers and stereoisomers and from the standpoint of cyclopropane syntheses, this is undesirable. Far fewer problems of this type attend deazetization of the adducts of cyclic or polycyclic alkenes and, furthermore, even a modest amount of strain in the system activates the alkene to diazoalkane addition so that there is no need for activating substituents on the double bond. Cyclopropene is highly reactive towards diazoalkanes (see also Section 1.1.5.1.5.3.1.) and cycloaddition reactions of this type provide a ready entry into the bi-cyclo[1.1.0]butane series. The addition of diphenyldiazomethane to cyclopropene gave 4,4-diphenyl-2,3-diazabicyclo[3.1.0]hex-2-ene (1), which on photolysis gave a mixture of 2,2-diphenylbicyclo[1.1.0]butane (2) and 1,1-diphenylbuta-l,3-diene (3). ... 

Although the endothermicity of the 1- 2 interconversion may be overcome both photochemi-cally and thermally (see below), it is only by photoisomerization that cyclopropenes have been prepared from allenes in isolable amounts. As shown in the table below, the allene-cyclopropene rearrangement has been employed predominantly for the preparation of highly substituted and bicyclic cyclopropenes, respectively. It appears that the ring strain of the starting cycloal-lenes is a prerequisite for the success of the interconversion. Furthermore, in order to suppress [2 + 2] cyclodimerization of the substrates a bulky substituent in the cyclic or acyclic precursor is necessary. Low reaction temperatures serve the same purpose. 

The photochemical isomerization of the parent system 1 to cyclopropene (2) was first detected in an argon matrix at 8 K. The allene photoisomerizations most likely involve singlet intermediates employing benzene as a triplet sensitizer leads to complex mixtures containing structurally completely different isomers. A two-step mechanism whereby hydrogen migrates first to the central allene carbon atom, followed by either cyclization or a second 1,2-hydrogen shift from a vinylcarbene intermediate, accounts for the overall features of the allene cyclization. ... 

A related reaction leads directly from cyclopropenylium salts to allenes without the isolation of a cyclopropene. ... 

Diazopropane is a potential source of ge w-dimethyl groups. It undergoes 1,3-dipolar addition to acetylenes and allenes/ and the adducts can be photolyzed to give cyclopropenes or methylenecyclopropanes/ respectively. In certain cases the adducts from a-substituted acetylenes give good yields of allenes and conjugated dienes on photolysis. ... 

Ruden et al calculated the indirect nuclear spin-spin coupling constants of allene, cyclopropane, and cyclopropene at different levels of electronic-structure theory and compared them with experimental equilibrium constants, which were obtained from experiment by subtracting evaluated vibrational contributions. It was found that, even in a relatively small basis set, the coupled-cluster singlesand-doubles (CCSD) method provides very good results. SOPPA consistently performed better than RASSCF. Hybrid DFT performed as well as SOPPA for the one-bond coupling constants, while, for the other coupling constants, it provided results of similar quality as CCSD. 

The Jcc and Jcc couplings of allene and two sterically strained hydrocarbons, cyclopropane and cyclopropene, have been calculated by Ruden et at different levels of electronic-structure theory and compared with each other and with the experimental equilibrium constants obtained from experiment by subtracting the calculated vibrational contributions. 

Computational studies have compared substituent effects on the stability of ketenes, allenes, diazomethanes, diazirines, and cyclopropenes. Ketenes belong to the first generation of reactive intermediates along with carbocations, carbanions, radicals, and carbenes, and are intensively studied members of the cumulene family, with many useful synthetic applications. Ketenes were first recognized in 1905, when diphenylketene, a stable and isolable example, was obtained from the dehalogenation of the a-bromodiphenylacetyl bromide (Scheme 7.37). The most characteristic reaction of ketene is cycloaddition, as in the formation of p-laclams. 

Allenes, 1,3-dienes, cyclopropanes, cyclopropenes, and even acetylenes can also serve as the starting materials for 7r-allylpaUadium complexes. Some representative examples are given in Scheme 19. It is interesting to note that while sterically hindered t-BuC=CBu-t displaces the two ethylene ligands from [Pd(CH2=CH2)Cl2]2 to give the corresponding alkyne-Pd complex (Sect. B.ii, Table 5), sterically less hindered PhC=CBu-t reacts with the same Pd complex to generate a 7r-allylpalladium species (Scheme 19). ... 

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