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Cyclopropenes, ions

The relative stability of the anions derived from cyclopropene and cyclopentadiene by deprotonation is just the reverse of the situation for the cations. Cyclopentadiene is one of the most acidic hydrocarbons known, with a of 16.0. The plCs of triphenylcyclo-propene and trimethylcyclopropene have been estimated as 50 and 62, respectively, from electrochemical cycles. The unsubstituted compound would be expected to fall somewhere in between and thus must be about 40 powers of 10 less acidic than cyclopentadiene. MP2/6-31(d,p) and B3LYP calculations indicate a small destabilization, relative to the cyclopropyl anion. Thus, the six-7c-electron cyclopentadienide ion is enormously stabilized relative to the four-7c-electron cyclopropenide ion, in agreement with the Hixckel rule. [Pg.526]

As was shown for the mechanism of quinocyclopropene formation in acetic anhydride75 (see p. 20), acylation of the cyclopropenone is reasonable for the primary reaction step, then the O-acyl-cyclopropenium ion 74 forms methylene cyclopropene 73 through addition of the anion of the C-H acidic component and elimination of acetic acid. [Pg.22]

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. [Pg.6]

The strain of 1 has been determined experimentally from silver-ion catalyzed methanolysis to be ca. 68 kcal/mol, and that of cyclopropa[/j]naphthalene to be 65-67 kcal/mol. Combustion calorimetry gave a value of 67.8 kcal/mol. For dicyclopropa[/>,g]naphthalene (115) a lower limit of 166 kcal/mol was found. These energies are well reproduced by ab initio, and even by semiempirical calculations. Thus 3-2IG calculates a strain energy of 70 kcal/mol while 3-2IG gives 71.6. At the MP2/6-31G level the strain of 1 is 71.3 kcal/mol, while that of cyclopropene amounts to 57.4 kcal/mol. This latter value compares well with the experimental one, which is 52.6 kcal/mol. While semiempirical calculations have been found unreliable for cycloproparene structures, the calculated strain energies are usually close to reality for MINDO/3, MNDO, and force-field-SCF calculations. The strain energies of the dicyclopropabenzenes (100, 102) have been predicted to be 133 and 140 kcal/mol, respectively, and that of tricyclo-propabenzene (260) to be 217 kcal/mol (3-21G). ... [Pg.73]

Theoretical calculations predict that the highest occupied Walsh orbital of the cyclopropene ring in 1 should stabilize a positive charge at C2, ° and this prediction was experimentally confirmed. Appearance energy measurements for the loss of Br from ionized 2-bromobenzocyclopropene (202 X = Br) suggest that the benzo-cyclopropen-2-ylium ion 291 is stabilized by more than 27.6 kcal/mol over the phenyl cation 292. Calculations (3-2IG ) give a stabilization energy of 23 and 22 kcal/mol of 291 over 292 and over the m-isomer 293, respectively. ... [Pg.83]

Isomerization of cyclopropenes. Irradiation of the cyclopropcne 1 results in the alkcncs 2 and 3, formed by cleavage of the h bond. Exposure of 1 to silver ion results in two different alkenes, 4 and 5, formed by cleavage of the a bond. [Pg.354]

The contrasting reluctance of the three-membered ring -it system to take on four electrons is illustrated by the very low acidity of triphenyl cyclopropene (54), estimated to be roughly 18 powers of ten less than that of triphenylmethane.33 A number of ions and hetero-atom large ring systems are also known.34... [Pg.40]

The interesting zwitterionic compound (39) with the cationic component a butadien-2-yl cation was obtained by reaction of l,4-di(t-butyl)butadiyne with 2 mol of di(r-butyl)aluminium hydride, with the structure being established by X-ray analysis.80 The reactions of the l,2-diferrocenyl-3-(methylthio)cyclopropenylium ion with carbanions derived from active methylene compounds were investigated.81 Products were derived by ring opening of the cyclopropene ring after the initial carbanion addition. The bis(ferrocenylethynyl)phenylmethylium cation (Fc-C=C-)2C+Ph (Fc = ferrocenyl) was prepared 82 This cation proved to be much less stable than its bis-ethenyl analogue (Fc-CH=CH-)2C+Ph. [Pg.212]

As one would expect because of the increasing strain, cyclopropenes readily reacted with silver ions, leading to ring-opening products. For example, dialkylcy-clopropenecarboxylates gave mainly E, Zs-dienoates together with some isomers when... [Pg.90]

The mechanistic aspects of the silver(I)-promoted rearrangement of cyclopropene derivatives have been investigated, confirming preferential attack of Ag + on the a bond to give an argentocarbenium ion. This intermediate is trapped by methanol, leading to a vinylsilver intermediate that is deuterated (Scheme 3.20).33,34... [Pg.91]

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). [Pg.144]

Reaction of the sulphonium salt (52) with cyclopentadienide ion leads to methylene-cyclopropene, though this can only be trapped in low yield by addition to cyclo-pentadiene 47). [Pg.147]

The addition of alkoxycarbonylcarbene derived by catalysed decomposition of methyl diazoacetate to several simple, and in particular terminal, alkynes leads to low yields S7), but the reaction with 1 -trimethylsilylalkynes proceeds reasonably efficiently subsequent removal of the silyl-group either by base or fluoride ion provides a route to l-alkyl-3-cyclopropenecarboxylic acids. In the same way 1,2-bis-trimethylsilyl-ethyne can be converted to cyclopropene-3-carboxylic acid itself58 . The use of rhodium carboxylates instead of copper catalysts also generally leads to reasonable yields of cyclopropenes, even from terminal alkynes 59). [Pg.149]

Treatment of (205) with a large excess of potassium t-butoxide in THF leads to ethers and related compounds their formation may be rationalised in terms of the formation of a cyclopropene (206) which can undergo a complex set of rearrangements to naphthylcarbenes which are then trapped by alkoxide ion l41, l43). [Pg.169]

Reaction of cyclopropenes with bases such as alkoxide or amide ions often leads to a methylenecyclopropene by removal of an allylic hydrogen and reprotonation 6 9-71) though other reactions such as nucleophilic addition (see Section 5) or metallation at a vinylic position (see Section 2) may compete. Thus the ester (209) is isomerised by KOH to (210), and under more vigorous conditions to (211)144) ... [Pg.170]

The addition of an electrophile to the cyclopropene double bond formally leads to a cyclopropyl cation this may be expected to undergo ring opening to an allyl ion unless it is rapidly trapped by a nucleophile. In some cases, however, electrophilic attack may occur at one of the a-bonds, leading directly to an allylic cation. [Pg.173]

Silver-ion induced isomerisation of cyclopropenes gives products which contrast sharply with those derived by photolysis. Thus reaction of (244) with catalytic quantities of silver perchlorate in benzene leads to a quantitative yield of (245), whereas the photochemical process leads to the l-methyl-2-phenyl regioisomer. The alcohol (246)... [Pg.175]

Lithium aluminium hydride reduction of 1,2-disubstituted cyclopropene-3-carboxy-lates occurs initially at the ester group, but with additional reagent good yields of cis-1,2-disubstituted-rranj-3-methanols are obtained 176). The reduction of the double bond is regioselective, leading to the more stable carbanion, and the attack of the hydride ion exclusively cis to the 3-substituent may be explained in terms of initial formation of an alkoxyaluminium complex followed by intramolecular hydride transfer 176). In the case of cyclopropene-1-carboxylates, direct reduction to the saturated alcohol occurs thus (253) is converted to the rranj-alcohol177) ... [Pg.177]

Early reports of the reactions of cyclopropenes with amide ions indicated complex products derived by addition of an intermediate 2-aminocyclopropylanion to unreacted cyclopropene 1). However, amines themselves can add to cyclopropenes ... [Pg.180]

Reaction of cyclopropenes with cyclopropenium ions leads to a three carbon ring expansion to aromatic derivatives, presumably through ring opening of an initially formed cyclopropenyl-cyclopropylcation 276) ... [Pg.198]

Product analysis shows that the thermal and metal induced reactions follow similar courses. The former reaction may proceed by opening of one ring to produce a formal vinyl carbene, followed by ring expansion the latter may be rationalised in terms of electrophilic attack by silver ion at one of the cyclopropenes to produce a carbenium ion (355) — that is a silver-coordinated equivalent of the carbene — followed by ring expansion of the second cyclopropene. [Pg.199]

The reaction of either cis- or trans-1 with potassium /-butoxide in tetrahydrofuran at 25 °C leads to a /-butyl ether (2), apparently arising by attack of /-butoxide ion on an intermediate l,4-di-/-butylmethylenecyclopropene. If the reaction is carried out at low temperature and the volatile materials are distilled directly into a cold trap, the cyclopropene can be trapped, albeit in low yield (10 %), by added cyclopentadiene or detected directly by low-temperature NMR23. In a related example, a l,l-dihalo-2-bromo-3-methylcyclopropane (2a) leads to products which are also apparently derived through an intermediate 1 -chloro-3-methylenecyclopropene which undergoes nucleophilic addition (See Ref. 80). [Pg.1354]

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]. [Pg.98]

Although the reactions have been generally described in terms of a carbenium ion mechanism, this does not altogether explain the catalytic behavior of the alkali metal ion-exchanged zeolites or the selectivity behavior. An ionic mechanism of the type previously described for cyclopropene dimerization would seem to be more appropriate for the alkali metal ion-exchanged zeolites, where the activity does seem to correlate qualitatively with the electrostatic field (e/r) exerted by the cation. [Pg.38]


See other pages where Cyclopropenes, ions is mentioned: [Pg.187]    [Pg.525]    [Pg.43]    [Pg.12]    [Pg.32]    [Pg.588]    [Pg.41]    [Pg.254]    [Pg.616]    [Pg.512]    [Pg.65]    [Pg.512]    [Pg.104]    [Pg.707]    [Pg.146]    [Pg.153]    [Pg.178]    [Pg.180]    [Pg.194]    [Pg.1260]    [Pg.1368]    [Pg.1373]    [Pg.62]   
See also in sourсe #XX -- [ Pg.40 ]




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Cyclopropenations

Cyclopropene

Cyclopropenes

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