Of cyclopropene

Oxirene (2) is one of a number of heterocycles in which the CH2 group of cyclopropene has been replaced by a group or element associated with Groups V or VI of the periodic table. Replacement of the CHj group of cyclopropene by an NH group gives l//-azirine... 

Liquid-phase photolysis of furan atroom temperature occurred in very low yields (1 % conversion), giving a mixture of Diels-Alder adducts deriving from the reaction of cyclopropene-3-carbaldehyde and formylallene with furan (85JOC3034). 

Cyclopropanes are now readily available and have become useful, through hydrogenolysis, for synthesis of compounds containing quaternary carbons, em-dialkyl, r-butyl, and angular-methyl substituents (779), compounds often available only with difficulty otherwise (.77,5i,55,750,756), Cyclopropanes can be formed in good yields by hydrogenation of cyclopropenes (26). 

A gaseous stream of cyclopropene was passed through a stirred suspension of methyl 5,6-diphenyI-1,2,4-triazine-3-carboxylate (1.46 g, 5 mmol) in anhyd El20 (20 mL) for 15 min after which time an homogeneous solution was obtained. The solvent was removed to give an oil [yield 1.4 g (92%)] which solidified, and the solid was recrystallized (MeOH) mp 75- 79 C. 

The Diels-Alder reaction of cyclopropenes with 1,2,4,5-tetrazines (see Vol.E9c, p 904), a reaction with inverse electron demand, gives isolable 3,4-diazanorcaradienes 1, which are converted into 4H-1,2-diazepines 2 on heating. The transformation involves a symmetry allowed [1,5] sigmatropic shift of one of the bonds of the three-membered ring, a so-called walk rearrangement , followed by valence isomerization.106,107... 

Correlation diagrams include the product orbitals while perturbation approaches require knowledge of the empty orbitals of the reactant. However, the occupied molecular orbitals of diazirine (II), compared with those of cyclopropene (I), do seem to give some indication of a preferred thermal decomposition of (II) compared with the rearrangement of (I). Moreover these molecular orbitals are a typical illustration of the localization obtained in the presence of an electronegativity perturbation. 

The relevant orbitals to consider are the molecular orbitals 5Aj and 6A, of cyclopropene (III.46) which correlate with the lowest acc orbital of cyclopropane (3A, III.56) and with one of the higher degenerate <7CC orbitals (3E, III.56). The cyclopropene orbitals are very similar to their cyclopropane counterparts. In particular the 6Aj orbital is a linear combination of all three crcc bond orbitals and extends over the entire cyclopropene molecule. Its amplitude, shown in Fig. 46, is fairly accurately represented by the... 

Padwa A., Fryxell G. E. Cyclization and Cycloaddition Reactions of Cyclopropenes Strain Org. Chem. 1991 1 117-166... 

Whereas the Rh2(OAc)4-catalyzed addition of diazoalkanes to propargyl alcohols readily gives the insertion of the carbene into the 0-H bond, with only a small amoimt of cyclopropenation of the resulting propargylic ether [54] the 2-diazopropane 59 reacts at 0 °C with l,l-diphenyl-2-propyn-l-ol 62a in dichloromethane and exclusively gives, after 10 h of reaction, only the adduct 63a isolated in 75% yield and corresponding to the regioselective 1,3-dipolar cycloaddition of the 2-diazopropane to the alkyne C - C bond (Scheme 15). 

The photochemical study of 3H-pyrazoles was carried out in the search for a route to cyclopropenyl tertiary alcohols. Irradiation of 63a in dry dichloromethane at 300 nm and at room temperature for 0.5 h led to the exclusive formation of the gem-dimethylcyclopropene 65 (Scheme 17). The formation of cyclopropene 65 arises from the loss of N2 and cycUzation of the vinylcarbene intermediate (III). 

Under the catalytic action of Rh2(OAc)4, formation of a propargylic ether from a terminal alkyne (229, R1=H) is preferred as long as no steric hindrance by the adjacent group is felt162,218>. Otherwise, cyclopropenation may become the dominant reaction path [e.g. 229 (R1 = H, R2 = R3 = Me) and methyl diazoacetate 56% of cyclopropene, 36% of propargylic ether162)], in contrast to the situation with allylic alcohols, where O/H insertion is rather insensitive to steric influences. 

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 theoretically interesting phenyl hydroxy cyclopropenone (57) was prepared by Famums1, s2 according to the general principle of cyclopropene ring closure developed by Closss3) from 53 via the vinyl carbene 54 and phenyl trichloro cyclopropene (55). 

Intriguingly 13C—H coupling of cyclopropenes 203/204 substituted at C3 by strongly electron-withdrawing groups comes even closer to the value (265 Hz173 )... 

Very recent work111 has shown that the predominant formation of the endo adduct in the reaction between cyclopropene and isotopically substituted butadiene could be attributed to an attractive interaction between a C—H bond of cyclopropene and the jt bond being formed in the diene moiety. 

According to recent quantum mechanical calculations, die importance of secondary orbital interactions, which have also been frequently used to explain die endo diastereoselectivity of Diels-Alder reactions, seems to be questionable and to be reserved for special cases like the addition of cyclopropene to various dienes. T. Karcher, W. Sicking, J. Sauer and R. Sustmann, Tetrahedron Lett., 33, 8027 (1992) R. Sustmann and W. Sicking, Tetrahedron, 48, 10293 (1992) Y. Apeloig and E. Matzner,./. Am. Chem. Soc., 117, 5375 (1995). 

The elusive diazoalkenes 6 and 14 are unlikely to react with methanol as their basicity should be comparable to that of diphenyldiazomethane. However, since the formation of diazonium ions cannot be rigorously excluded, the protonation of vinylcarbenes was to be confirmed with non-nitrogenous precursors. Vinyl-carbenes are presumedly involved in photorearrangements of cyclopropenes.21 In an attempt to trap the intermediate(s), 30 was irradiated in methanol. The ethers 32 and 35 (60 40) were obtained,22 pointing to the intervention of the al-lylic cation 34 (Scheme 10). Protonation of the vinylcarbene 31 is a likely route to 34. However, 34 could also arise from protonation of photoexcited 30, by way of the cyclopropyl cation 33. The photosolvolysis of alkenes is a well-known reaction which proceeds according to Markovnikov s rule and is, occasionally, associated with skeletal reorganizations.23 Therefore, cyclopropenes are not the substrates of choice for demonstrating the protonation of vinylcarbenes. 

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. 

UV irradiation (A. > 305 nm) of cyclopropene 3a results in the ring-opening and formation of ketene 4a. Two reaction pathways for the 3a - 4a rearrange-... 

Rearrangement of the 3,5-dimethylated carbene Id would yield the destabilized cyclopropene 3d with a methyl group in the bridgehead position 1, and consequently no detectable amount of cyclopropene 3d is formed during irradiation of Id. Indeed, whereas the 3,5-dimethyl substituted carbene Id is 3.4 kcal mol-1 more stable than the 2,6-dimethyl isomer lb, the stability is reversed for the cyclopropenes, as 3d is found to be 6.5 kcal mol-1 higher in energy than 3b at the B3LYP/6-31G(d) level of theory (Table 3). 

The palladium-isocyanide catalyst is also effective for the silastannation of cyclopropenes (Equation (115)).158 The reaction proceeds smoothly under mild conditions (RT, 10-30 min) with high face selectivity to give tetrasubstituted... 

To direct a solvolytic ring opening, 2-methoxycyclopropyllithium (6) was developed as a chain extension conjunctive reagent. The failure of P-elimination to occur in 6 presumably derives from the high strain of cyclopropene and poor orbital overlap for elimination. The aldehyde adducts smoothly solvolyze to give p,Y unsaturated aldehydes (Eq. 22) 23) which are best initially isolated as their hemithioacetals. 

In addition to the ring opening of cyclopropenes noted above, vinylketene complexes 103 have been prepared by (1) ligand initiated carbonyl insertion of vinyl carbene complexes 104 and (2) benzoylation of ,/3-unsaturalcd acyl ferrates 105 (Scheme 20)114. X-ray diffraction analysis of these vinylketene complexes indicates that the structure may be best represented as a hybrid between an /j4-dicnc type complex (103) and an jj3-allyl r/1 acyl complex (106). The Fe-Cl distance (ca 1.92 A) is shorter than the Fe-C2, Fe-C3, or Fe-C4 distances (ca 2.1-2.2 A)113a-C. In addition, the C—C—O ketene array is not linear (bend angle ca 135°). 

The reasons for the ewrfo-selectivity of Diels-Alder reactions are only useful for the reactions of dienophiles bearing substituents with lone pairs without a Lewis basic site no secondary orbital interactions are possible. But even in reactions of pure hydrocarbons the ewrfo-selectivity is observed, requiring alternative explanations. For example, the ewrfo-preference of the reactions of cyclopropene with substituted butadienes have been rationalized on the basis of a special type of secondary orbital interactions70. Apart from secondary orbital interactions which are probably the most important reason for the selec-tivities of Diels-Alder reactions, recent literature also advocates other interpretations. 

The most common method for the generation of the metal alkylidene species seems to be a-elimination from an intermediate dialkyl-metal species. This procedure gives the most active catalysts. Above we mentioned the addition of other carbene precursors, which leads to active catalysts. Other methods to generate the metal alkylidene species involve alkylidene transfer from phosphoranes [16] or ring-opening of cyclopropenes [17], In Chapter 16.4 we will describe a few compounds that are active by themselves as metathesis catalysts. 

Fig. 3.29. Isomerization of cyclopropene complexes into vinylcarbene complexes [5831.

The intermolecular reaction of alkynes with acylcarbene complexes normally yields cyclopropenes [587,1022,1060-1062]. Because of the high reactivity of cyclopropenes, however, in some of these reactions unexpected products can result. In particular intramolecular cyclopropanations of alkynes, which would lead to highly strained bicyclic cyclopropenes, often yield rearrangement products of the latter. In many instances these products result from a transient vinylcarbene complex, which can be formed by two different mechanisms (Figure 4.3). 

Compare the chemical shift of cyclopropene vinyl hydrogens (7-0, Wiberg and Nist, 1961) with normal alkenes ( 6 0). 

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