Protonated cyclopropene

Stable than cation 35,2 kcalmoT less stable than corner protonated cyclopropene (39) and 1 kcal mol less stable than the perpendicular allyl cation (36). It has been speculated that 34 is the transition state for the stereomutation of the planar allyl cation This is confirmed by MP2/6-31G(d,p) calculations ", which show that the cyclopropyl cation is located at a first-order transition state possessing one imaginary frequency of symmetry that describes a disrotatory movement of the CH2 groups in line with a stereomutation process of the allyl cation. Alternatively, stereomutation can also follow a stepwise route via perpendicular allyl cations 36 that are somewhat more stable than 34 (Table 26) and therefore lead to lower stereomutation barriers. ... 

Loss of nitrogen occurs to give the cyclopropene (16), which loses a proton to yield the charged species Loss of HCN leads to the unsaturated... 

Products of a so-called vinylogous Wolff rearrangement (see Sect. 9) rather than products of intramolecular cyclopropanation are generally obtained from P,y-unsaturated diazoketones I93), the formation of tricyclo[2,1.0.02 5]pentan-3-ones from 2-diazo-l-(cyclopropene-3-yl)-l-ethanones being a notable exception (see Table 10 and reference 12)). The use of Cu(OTf), does not change this situation for diazoketone 185 in the presence of an alcoholl93). With Cu(OTf)2 in nitromethane, on the other hand, A3-hydrinden-2-one 186 is formed 160). As 186 also results from the BF3 Et20-catalyzed reaction in similar yield, proton catalysis in the Cu(OTf)2-catalyzed reaction cannot be excluded, but electrophilic attack of the metal carbene on the double bond (Scheme 26) is also possible. That Rh2(OAc)4 is less efficient for the production of 186, would support the latter explanation, as the rhodium carbenes rank as less electrophilic than copper carbenes. 

The inverse functionalization of the two components for a Wittig reaction has been described by Russian authors70 who combined the cyclopropenylide 70 with aldehydes to give the unstable methylene cyclopropenes 71 characterized by protonation as cyclopropenium salts 72 ... 

As Table 10 shows, vinylic three-ring protons in cyclopropenone itself and in monosubstituted cyclopropenones appear at remarkably low field in the region of 0.9-1.6 r. Comparison with protons at the double bond of covalent cyclopropenes (e.g. 200 3.34 t174 ) and cyclopropenium cations (210 —0.35 r146 202 ... 

The 13C—H coupling constants of methyl (213 Hz23 ) and phenyl (216 Hz55 ) cyclopropenone are in the order of those obtained for cyclopropene vinylic protons (200/201 218 Hz/221 Hz174 ) and reflect an s-contribution of more than 40% in the carbon hybrid orbital of the vinyl C—H bond. 

Selective reduction of the cyclopropenone carbonyl group to a CH2 group has been described for diphenyl cyclopropenone utilizing its protonation product 294 or the diphenyl chloro cyclopropenium cation 292, which yielded 1,2-diphenyl-A1,2-cyclopropene (293) on treatment with trimethylamine borane210 ... 

In some cases the C /C2 double bond in methylene cyclopropenes and calicenes was found to show dienophilic functionality towards diene components. Thus, di-ethylamino butadiene combines with 497 to give the Diels-Alder adduct 507, whose proton-catalyzed elimination of amine interestingly did not lead to the dibenzo heptafulvalene 508, but to the methylene norcaradiene derivative 509293 ... 

The ratio of isomeric ethers is strongly affected by polar substituents which induce an asymmetric distribution of charge in allylic cations. Photolysis of methyl 2-diazo-4-phenyl-3-butenoate (20) in methanol produced 24 in large excess over 25 as the positive charge of 22 resides mainly a to phenyl (Scheme 8).19 As would be expected, proton transfer to the electron-poor carbene 21 proceeds reluctantly intramolecular addition with formation of the cyclopropene... 

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. 

As cyclopropanes have olefin-like properties, the presence of a double bond in the three-membered ring adds some acetylenic character to the methine protons. X-ray crystallographic and gas-phase structural studies on cyclopropenes have been reviewed by Allen11 and recently by Boese5. 

Isolation of 3-cyclopropenyl metal compounds by this method has been achieved so far for iron and rhenium metals only. Thus, the reaction of Na[CpFe(CO)J (NaFp) with cyclopropenylium salts at -70 °C, in THF, gave 3-Fp-cyclopropene complexes (equation 194)2 267. The X-ray crystal structure of the most stable iron complex 3-Fp-C3Ph3 exhibits a regular cyclopropene C—C single and double bond distances (151 and 129 pm), and a characteristic distance of 208 pm for the Fe—C (T-bond267. The H NMR (CS2) spectrum of the 3-Fp-C3Ph,H complex displays a singlet at S = 2.63 ppm, of the cyclopropen yl proton at the 3-position. ... 

Characteristic of f-olefin and cyclopropene complex formation are the upfield shifts observed in the NMR resonances of the olefinic protons (<5 =3.30-5.29 ppm) and carbons (<5 = 59.9-72.4 ppm). For the phosphite complexes, these upfield shift resonances decrease as the steric bulk of the imido ligand increases, corresponding to weaker binding of the cyclopropene in the more sterically crowded molecule. Difference NOE spectroscopy... 

Suppose one added hydrogen as H2 to the cyclopropene double bond of 1-methoxy-2-phenyl-3,3-dimethylcyclopropene. Explain how the proton nmr of the product can be used to infer whether the hydrogen added to the double bond in the suprafacial or antarafacial manner. (Review Section 9-10G and 9-1 OH.)... 

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