Cyclopropenes bromination

The first stable derivative of methylene cyclopropene was the quinocyclopropene 108 reported in 1964 by Kende98 it was prepared from the cyclopropenium cation 106 which underwent pyrolysis and bromination with NBS to the p-hydroxy-phenyl substituted cation 107, which gave quinocyclopropene 108 by deprotonation ... 

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... 

Bromine complexes of ethylene and cyclopropene have been isolated in argon matrices and studied by IR spectroscopy and ab initio methods. Two methods, namely MP2 and BLYP, were compared and found suitable for the calculation of IR spectra as they exhibited only minor differences.25... 

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) ... 

Tetrakis(trifluoromethyl)furan was obtained in nearly quantitative yield from 3-trifluoroacetyl-l,2,3-tris(trifluoromethyl)cyclopropene on heating in a Pyrex ampoule to 250°C in the presence of bromine (78TL1015). 

Additions to the double bond of cyclopropenes have recently found considerable attention from both a synthetic and a mechanistic point of view. For example, Lehmkuhl et al.57) have developed a stereoselective synthesis of chrysanthemic acid via a 1,2-Grignard addition to 3,3-dimethylcyclopropene. Nesmeyanova et al.58) have found that the bromination of 3,3-dimethylcyclopropene possibly proceeds via a concerted, electrocyclic trans-addition. 

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 addition of bromine to methyl 2,3-diphenylcycloprop-2-ene-l-carboxylate, in the dark or diffuse light, in chloroform or acetic acid solution, gave the tran.v-addition product like cyclopropene itself, whereas in carbon tetrachloride or chloroform and under irradiation, cis addition from the face opposite to the ester substituent predominated. ... 

In the case of 2,3-dichloro-l,l-dimethylcyclopropene, addition of bromine/carbon tetrachloride to a solution of the cyclopropene in diethyl ether at — 90 to — 95 °C gave both cis-and /rfl .v-addition products in a 3 2 ratio (78%). Addition of bromine to 2-bromo-3-chloro-1,1-dimethylcyclopropene also occurred at low temperatures to give the corresponding cyclopropane (60%). °... 

Addition of bromine to a number of other cyclopropenes is reported to lead only to ring-opened or ring-expanded products. This reaction is highly dependent on the substitution thus... 

In the dehydrobromination of 9-bromobicyclo[6.1.0]nonane (37), which has no alkyl group geminal to the bromine, the initial product is a cyclopropene intermediate in which the double bond undergoes rapid base-catalyzed migration to a position exo to the cyclopropane ring. An 80% yield of product was thereby obtained in which the major component (95%) was bicyclo[6.1.0]non-l -ene (38). ... 

Removal of the elements of BrOEt from l-bromo-2-ethoxycyclopropane by reaction with magnesium in tetrahydrofuran gave cyclopropene, which was trapped by addition of bromine or by cycloaddition to cyclopentadiene, in each case in 20-25% yield. 

The products of the dibromo addition to 3,8-methanocyclopropa[10]annulene depend on the reaction conditions.At 0 C bromine reacted initially in a, A-cis addition to C4-C7 which was followed by cis addition across the cyclopropene bond to give a tetrabromide 4 (18%), the structure of which was established by X-ray crystallography. In this particular case, ring opening of the norcaradiene adduct did not occur. 

Attention was drawn [2] to the contrast between triphenylcyclo-propenium bromide which exists as an ionic salt and triphenylmethyl bromide in which the bromine atom is covalently bound. Yet strain factors, and the increased electronegativity of the ring carbon atoms in the cyclopropene derivative, due to the different hybridisation in the three-membered ring, should favour ionic dissociation of the triphenylmethyl compound rather than the triphenylcyclo-propenium derivative. The stable existence of this cyclopropenium cation was thus evidence for the extra stability conferred on this ion by its delocalised system of two 7r-electrons. 

Maier, G. and Senger, S., Bromine complexes of ethylene and cyclopropene matrix-lR-spectro-scopic identification, photochemical reactions, ab initio studies, Liebigs Ann. Chem., 317,1997. 

In much the same way, triplet but-2-ene-l,3-diyl (67 R = Me) was generated on UV-irradiation of 1-methylcyclopropene (66 R = Me) in bromine-doped Xe matrices and was characterized by IR spectroscopy. " Under similar conditions, cyclopropene seemed to yield propene-l,3-diyl (67 R = H), although the interpretation of the spectral data was less certain in this case. Theoretical calculations accorded with the experiments in predicting that formation of the diyls is the lowest energy ground-state pathway for ring opening of each of the cyclopropenes. 

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