Cyclopropenylidene

In contrast to the 4-hydroxy isomers, the thermally stable 5-hydroxy-THISs add to the C=C bond of cyclopropenylidenes (4. 18, 27. 28). The adducts eliminate carbonyl sulfide, and the strained bond breaks resulting in ring-expansion with formation of pyridin-4-ones. -thiones, or -imines. or 4-alkylidenedihydropvridines (20, X = 0. S.NR. or CRR ) (Scheme 19). 

Tlie smallest compounds among the azafulvalenes described to date are the azacalicenes, which are formed by combining a cyclopropenylidene moiety with a nitrogen-containing cyclopentadienylidene. Tlius, heating di-or triphenylpyrroles with diphenyl-methylsulfanyl-cyclopropenylium perchlorate in acetic acid gives the aza-triafulvalenium salts 33 and 34... 

In contrast to other terminal alkynes, the lithiated dimethylaminoethyne 40 does not give the corresponding alkynylcarbene but the cyclopropenylidene complex 41 (Scheme 7) [51]. Further addition of dimethylamine to 41 affords the substitution product 42 in excellent yield. This 2,3-bis(dimethylamino-cyclopropenylidene)pentacarbonylchromium (42) is extremely stable, and it cannot be transformed to the corresponding carbonyl compound, 2,3-bis... 

Cyclopropenylidene [2] has been obtained from two different precursors, [3] and [4], by their thermal decomposition in the gas phase (Reisenauer et al., 1984 Maier et al., 1987, 1989a). The subsequent UV photolysis (A = 313 nm and 254 nm) of this carbene in an argon matrix led to its isomerization to propargylene [5] and further to vinylidencarbene [6]. It should be noted that the carbene [5] was independently obtained by... 

The radio detection of a small molecule formed of three carbons and two hydrogens by Thaddeus et al. [1] in 1985 came as a surprize to all astrochemists cyclopropenylidene C3H2, last bom to the small world of detected interstellar species was soon to become famous, though competition is high in this world where exotism is common. 

Cyclopropenylidene is a singlet state of A] symmetry. In terms of symmetry adapted Lewis orbitals, its electronic configuration can be written as follows ... 

Fig. 19. The unimolecular reaction channels for the propargyl radical together with the heats of reaction. Assuming that H2 loss is associated with a substantial reverse barrier, formation of cyclopropenylidene, c-C3H2, is the most likely channel. (From Deyerl et a/.143)...

Two other carbenes, cyclopropenylidene(37) and cycloheptatrienyli-dene,<28> which have aromatic resonance hydrids (15) and (16), have been studied ... 

Scheme 2 shows the preparation of carbene complexes starting from [L M-ER3]" = [MeCp(CO)2Mn-SiMePlt2] and [(7r-arene)(CO)2Cr-SnPh3] [12,13]. By a related approach, a cationic cyclopropenylidene complex was prepared by reaction of Cp(CO)2FeSiMe3 with l-chloro-2,3-diphenylcyclopropenylium tetrafluoroborate [14],... 

Accordingly, the cyclopropenylidene anthrones 190/198 were converted by ferric chloride in hydroxylic solvents to the allene ketal 466, whose hydrolysis gives the allenic ketone 46 7288. The dioxolane 468 was obtained from the alkyl-substituted quinocyclopropene 190 in glycol and the ketone 467 in methanol. Apparently FeCl3 served not only as an oxidant, but also as a Lewis acid assisting solvent addition to C1 2 of the triafulvene. 

Because of its carbene functionality, our decision to consider only the parent hydrocarbons would mean that cyclopropenylidene would be ignored here even if monoolefins were of relevance. This philosophy accounts for our ignoring the energetics of the isomeric carbenes, propargylene (HCCCH) and propenylidene (CH2CC), in the earlier section on cumulenes in the current chapter. 

Alternatively, the translational energy threshold for endothermic proton transfer from MH+ to R can be measured using a flowing afterglow triple quadrupole instrument.127 These data define the proton affinity of M, relative to that of R. Thus, the PA of cyclopropenylidene was found to exceed that of ammonia by 23.3 1.8 kcal/mol (Table 6).128 In order to obtain absolute proton affinities, the enthalpies of formation of both the base and the conjugate acid must be known from other measurements (Eq. 9). Numerous reference compounds with known absolute PA are available.124... 

Cyclobutadiene (l)15 is the first member of the series of cyclic conjugated hydrocarbons (Kekule compounds) cyclopropenylidene (2), of cyclic conjugated carbenes trimethylenemethane (3), of the non-Kekule hydrocarbons. 

During all these studies on cyclobutadienes and tetrahedranes formed via carbenes as transient species we wondered whether matrix isolation IR spectroscopy might be a good tool for the direct observation of cyclopropenylidene (2) and trimethylenemethane (3). This is indeed the case. 

As has already been discussed (Section m.B.3) we were able to demonstrate that the three C3H2 isomers cyclopropenylidene (2), propargylene (36), and vinylidenecarbene (37), interconvert photochemically in low-temperature matrices. Unlike 36 vinylidenecarbene (37) was predicted to be a singlet.108-110 To aid the spectroscopic identification of S-37 we calculated (MP2/6-31G ) IR frequencies and intensities of this species.26 Comparison with the experimental IR spectrum (most intense band at 1952 cm-1) confirmed the allenic structure S-37. For T-37 a completely different IR spectrum was expected. An additional structural proof for S-37 was its reversible transformation into the other two C3H2 isomers S-2 and T-36. 

C. Cyclopropenylidene Complexes of Divalent Germanium, Tin, and Lead Amides... 

Scheme 7. Cyclopropenylidene complexes of divalent germanium, tin, and lead.

Cyclopropenylidene (R=H) is a cyclic carbene that was first detected in interstellar space [43], This small three membered cyclic and aromatic carbene 14, bearing two bulky Ti-donating diisopropylamino substituents, has been very recently isolated in the solid state (Scheme 4) [44],... 

Almost all stable carbenes behave as 2-electron-CT-donating ligands with a few exceptions. In particular, in almost all cases corresponding Rh(I) complexes were targeted due to the easy synthetic method. An exception is the cyclopropenylidene carbene, with an extremely acute carbene angle. In this case, a second equivalent of carbene squeezes into the rhodium center, eliminating a chloride anion, giving the cationic dicarbenic rhodium(l) complex [51] (Scheme 5). 

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