Allylic perpendicular

The 7t-electron delocalization requires proper orbital alignment. As a result, there is a significant barrier to rotation about the carbon-carbon bonds in the allyl cation. The results of 6-31G/MP2 calculations show the perpendicular allyl cation to be 37.8 kcal/mol higher than the planar ion. Related calculations indicate that rotation does not occur but that... 

The rapid fabrication of covalently bonded ID functional molecular lines with predefined location, direction, and length provides a means to make a predesigned interconnection of molecular lines running along and across the dimer rows. Indeed, the perpendicularly connected allyl mercaptan and styrene lines or allyl mercaptan and acetone lines have been fabricated on the H-Si(l 00)-2 X 1 surface. °° 2 ... 

Although at first glance addition to the central carbon and formation of what seems like an allylic carbonium ion would clearly be preferred over terminal addition and a vinyl cation, a closer examination shows this not to be the case. Since the two double bonds in allenes are perpendicular to each other, addition of an electrophile to the central carbon results in an empty p orbital, which is perpendicular to the remaining rr system and hence not resonance stabilized (and probably inductively destabilized) until a 90° rotation occurs around the newly formed single bond. Hence, allylic stabilization may not be significant in the transition state. In fact, electrophilic additions to allene itself occur without exception at the terminal carbon (54). 

The vinyl cation analog of an allylic carbonium ion is an allenyl cation 242, where the empty p orbital on the unsaturated carbon overlaps with the perpendicular n bond of the allenyl system. Allenyl cation 242 is of course a resonance form of the well known alkynylcarbonium ion,... 

Two main structural types have been identified for allyl alkali metal species solvated ions in the form of CIPs where a delocalized anion with metal coordination is perpendicular to the ligand plane,130-134 or unsolvated allylic lithium compounds displaying localized ligand systems with NMR spectra closely resembling those of alkenes.135-138... 

Tetramethylene-ethane (TME), or 2,2/-bis-allyl diradical 81, was suggested as an intermediate in the thermal dimerization of allene, as well as in the interconversions of 1,2-dimethylenecyclobutane 82, methylenespiropentane 83, bis-cyclopropylidene 84 and other bicyclic systems (equation 30)45. The isolation of two different isomeric dimethylene cyclobutanes 87 and 88 (in a ca 2 I ratio) after the thermal rearrangement of the deuteriated 1,2-dimethylene cyclobutane 85 suggests that the rearrangement proceeds via a perpendicular tetramethyleneethane diradical (2,2/-bisallyl) 86 (equation 31)45. 

Stabilized allyl radical will be stabilized further if substituents are introduced. This stabilization occurs to different degrees in the ground state and the transition structure for rotation. In the ground state the substituent acts on a delocalized radical. Its influence on this state should be smaller than in the transition structure, where it acts on a localized radical. In the transition state the double bond and the atom with the unpaired electron are decoupled, i.e. in the simple Hiickel molecular orbital picture, the electron is localized in an orbital perpendicular to the jt(- c bond. 

Structure C is usually written with a pentavalent phosphorus this description, however, should merely denote that the phosphorus lone pair is delocalized and no allenic bond with two perpendicular jr-systems has been formed. H2C=P(H)=CH2, a typical representative of structure C phosphorus, can be described by an allyl-like r-system, ° with the p -type lone pair of a planar tricoordinate phosphorus participating in the 4-electron-3-center jT-bonding, without invoking d-orbital participation. " Thus, the C -type bonding of phosphorus involves tricoordination, with a planar bonding environment, whereby the lone pair is available for jT-bonding. 

Addition of silyl radicals to cumulenes and their isoelectronic derivatives has mainly been studied by EPR spectroscopy. The adducts of MesSi radical with the two substituted allenes 54 and 55 have been recorded [73,74]. The attack occurs at the central atom affording unconjugated allyl-type radicals. In particular the adduct radical with 55 has been described as a very persistent perpendicular allyl radical [74]. 

All 1,3-dipoles contain an allyl anion type n system, i.e., four electrons delocalized over three parallel atomic n orbitals, but, in addition, 1,3-dipoles of the propargyl-allenyl type contain an additional n bond in the plane perpendicular to the allyl anion MO (Fig. 1). 

The trimethylsilyl substituent in 2-(trimethylsilyl)allyl cations is enforced into a perpendicular conformation and thus fi-C—Si hyperconjugative stabilization is essentially not possible. The carbocationic centre Cl (<513C = 295.0 ppm) and the terminal methylene carbon C3 of the allyl system (<513C = 166.6 ppm) in 348a are deshielded by 11.5 and 1.2 ppm, respectively, compared with the silicon-free analogue 348 (613C = Cl 283.5 C3... 

C—H bond and the reacting C=C double bond (see Scheme 25). For example, in substrates 93 and 95 the major ene adducts (74% and 90%, respectively) are formed by vinylic hydrogen atom abstraction, whereas the minor ene adducts (26% and 10%, respectively) are produced by allylic hydrogen abstraction. The percentage of allene hydroperoxide is lower (43%) in the photo-oxidation of 94. In this substrate, the steric hindrance at the stereogenic center is lower than in 93 or 95, and the diene is therefore less twisted and the a --rr interaction less profound, or by analogy, the vinylic hydrogen atom is less perpendicular to the double bond. 

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