Allene structure

Unstable chlorides were converted to stable SnCU complexes. In their IR spectra there is an intense absorption band in the 1900 cm region, which is consistent with the band of allenic system (structure C). Unlike unstable chlorides A and B, the SnCl4 complexes are stable and, when kept in an inert atmosphere, remain intact for several days. The allenic structure of the immonium salt was confirmed by studying the mercuration of the same aminobutenynes (74DIS). 

The allenic stannanes can be transmetallated by treatment with SnCl4, a reaction that results in the formation of the a propargyl stannane. If the transmetallation reaction is allowed to equilibrate at 0°C, an allenic structure is formed. These reagents add stereospecifically to the aldehyde through cyclic TSs.194... 

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. 

Although the observed products are consistent with initial formation of 4a, Untch proposed that they might arise via the intermediacy of cyclic allene 3a.23 Subsequent experiments by W.M. Jones and co-workers supported the initial formation of 3a in the dehydrohalogenation reactions, but left open the possibility of a rapid equilibrium between cycloheptatetraene (3a) and cycloheptatrienylidene (4a, Scheme 9).22,24 It was also found that fusion of benzene or naphthalene rings to different positions on the seven-membered ring strongly influenced whether a carbenic or allenic structure predominates.25... 

To prepare the parent bisallene 118, 116 is first converted into its Grignard reagent (known from spectroscopic studies to possess the allenic structure), from which, presumably, by the addition of cuprous chloride the organocopper intermediate 117 is generated. Addition of further 116 subsequently provides a mixture of 118 and propargylallene (l,2-hexadien-5-yne) (29) (see below) in a 2 3 isomer ratio [44],... 

The treatment of 23 with methyllithium in the presence of furan gave rise to the tetracyclic product 26, which is obviously a [4 + 2]-cycloadduct of furan to the 1,2-cyclopentadiene derivative 25 [27]. The feature that the oxanorbornene system of 26 carries its saturated substituent in the endo-position is analogous to the [4 + 2]-cycloadducts of furan to all six-membered cyclic allenes (see Section 6.3). Balci et al. [36] also provided evidence for the generation of l-phenyl-l,2-cyclopentadiene. They postulated this species to be an intermediate in the reaction of l-phenyl-2-iodocydo-pentene with potassium tert-butoxide in benzene at 240 °C, which resulted in the formation of 1-phenyl- and 1,2-diphenylcyclopentene. Both products were considered as evidence in favor of the diradical nature rather than the allene structure of 1-phe-nyl-1,2 -cyclopentadiene. 

DPIBF, the resulting 119 was optically active. This suggests that the progenitor of 119 has the allene structure 117 rather than other conceivable constitutions [80, 81]. Even early quantum-chemical calculations on 117 showed a strongly bent, chiral structure, although the enantiomerization barrier was not correctly estimated [19]. 

The isopyridine 179 (3<52-lH-pyridine) is the result if the oxygen atom of 180 is replaced by an NH group. Owing to the better electron-donor quality relative to that of an oxygen atom, the NH group could have the effect that the zwitterion 179-Zj is more stable than the allene structure 179, even in the gas phase. Experiments and quantum-chemical calculations support this expectation. 

These reactions point to the polar character of 179-Za and 293-Za. Already the first calculations resulted in a virtually planar geometry and thus in the conclusion that 179-Zj and likewise 293-Zj are the ground states [144]. In consequence, the allene structures 179 and 293 are not energy minima. Utilizing a higher level of theory,... 

In addition to the energetics of 162, 179-Za and 180, Scheme 6.42 contains those of the boraisobenzene 178, which represents the last possible system of that kind with one heteroatom of the second row of the periodic table. The allene structure 178 was indeed calculated to be the ground state, but AG298 of the zwitterion 178-Z2 is only 2 kcal mol-1 greater. The polarization of that state is opposite to that of 179-Za. To allow an aromatic jt-electron sextet to form, the cr-orbital at the central carbon atom of the allene system must remain empty [120], Experiments directed towards the observation of 178 or a derivative thereof seem to be unknown. 

In this section, compounds are described that differ from 6 by the replacement of one or more methylene groups by heteroatoms or heteroatom groups. Quantum-chemical calculations on such species have not been carried out. However, on the basis of the results discussed in the above sections and depicted in Schemes 6.42 and 6.64, there is no doubt that all reactive intermediates under consideration are genuine allenes. After all, the tether across the allene subunit is larger in the corresponding compounds in this section than in 3<52-lH-pyridine (179), 3<52-pyran (180) and 3<52-thiopyran (299) owing to the absence of a double bond. Thus, compared with these models, the allene structures in this section suffer from less strain and are hence stabilized relative to their zwitterionic states. 

Two types of derivatives of 1,2-cyclohexadiene with two heteroatoms were proposed as reactive intermediates more than 20 years ago. Lloyd and McNab [168] observed the reaction of the 5-bromo-l,2-dihydropyrimidinium ions 411 with thiourea in refluxing ethanol to give the bromine-free cations 413. Suspected as intermediates, the 5d2-dihydropyrimidines 412 were initially considered as zwitterions of the type 414-Zj. However, quantum-chemical calculations on the parent systems suggested an unambiguous preference of the allene structure 414 over the zwitterion 414-Za [169]. 

In addition, the C3-H coupling constant (from a gated decoupling NMR experiment) of 161.8 Hz in 106 compared with 162 Hz in allenyllithium vs 167.5 Hz in methoxyallene and 168 Hz in allene is also in agreement with an allenic structure. However, neither the C-H coupling constant nor the NMR chemical shifts distinguish between the alternatives that 106 has a nonclassical 1,3-bridged structure 108 (M = Li) or an O-coordinated allenic structure (109). Hence the 6Li, -HOESY NMR technique which can be used to detect close proximities (ca < 3.5 A) between XH and 6Li nuclei was applied. The HOESY spectrum of a-lithiomethoxyallene in THF solution (in which 106 is dimeric) is shown... 

Attractive Compounds. The male produced sex pheromone of the dried bean beetle, Acanthoscelides obtectus, is an unusual methyl ester, methyl (R,2 )-2,4,5-tetradecatrienoate 204 [372] (Scheme 23). The compound was among the first pheromones identified from male beetles, and only very recently other insect volatiles showing allenic structures have been described [184]. Careful head... 

The intramolecular 2 - - 2 - - 1-cycloadditions of allene, alkyne (106), and carbon monoxide yield a -methylene-(107) or 4-alkylidene-cyclopentenones (108) depending on the allene structure or the reaction conditions (Scheme 4i).i59.i6o The cobalt-catalysed 4 - - 2 - - 2-cycloaddition of norbornadienes (109) with buta-1,3-dienes readily produces cycloadducts (110) when a bimetal system is used (Scheme A2) ... 

While the mono- and dianions of internal acetylenes stiU have allenic structures, polylithiation of terminal acetylenes results in anions having a different structure, as previously reported (Scheme 9). 

Introduction of the allene structure into cycloalkanes such as in 1,2-cyclononadiene (727) provides another approach to chiral cycloalkenes of sufficient enantiomeric stability. Although 127 has to be classified as an axial chiral compound like other C2-allenes it is included in this survey because of its obvious relation to ( )-cyclooctene as also can be seen from chemical correlations vide infra). Racemic 127 was resolved either through diastereomeric platinum complexes 143) or by ring enlargement via the dibromocarbene adduct 128 of optically active (J3)-cyclooctene (see 4.2) with methyllithium 143) — a method already used for the preparation of racemic 127. The first method afforded a product of 44 % enantiomeric purity whereas 127 obtained from ( )-cyclooctene had a rotation [a]D of 170-175°. The chirality of 127 was established by correlation with (+)(S)-( )-cyclooctene which in a stereoselective reaction with dibromocarbene afforded (—)-dibromo-trans-bicyclo[6.1 0]nonane 128) 144). Its absolute stereochemistry was determined by the Thyvoet-method as (1R, 87 ) and served as a key intermediate for the correlation with 727 ring expansion induced... 

In allene structures, strong absorption is observed near 850 cm-1, arising from =CH2 wagging. The first overtone of this band may also be seen. Some spectra showing alkene features are shown in Appendix B trichloroethylene (No. 12) and tetrachloroethylene (No. 13). 

The driving force for conjugation may be a significant factor to stabilize the allene structure in the isomerization of aryl-substituted alkynes145 [Eq. (4.30)146] ... 

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