3- Butenyl cation

As pointed out by the pioneer in those studies C4H has some of the characteristics expected for a very rapidly equilibrating mixture of classical cyclopropylmethyl, cyclobutyl, and 3-butenyl cations and yet other characteristics which wholly belie any description that implies conventional charge distributions or geometries derived from structural representations using solid lines representing two-electron bonds . The rapidly equilibrating ions referred to are shown in 59a, 59b and 59c respectively, and the structures with unconventional charge distribution are the bicyclobutonium ion (60) or the bisected structure (61). 

Another interesting case is the preferred conformation of 1-butenyl cation for which the two conformations, Cs and Ts, are shown below ... 

Cycloprop-2-en-l-ylmethanol derivatives (341) ring expand under acidic conditions to cyclobutenols in a reaction thought to proceed through a homoaromatic cyclo-butenyl cation 271) ... 

Experimental evidence concerning the relative stabilities of cation conformations is limited. The few examples known, however, strongly support our analysis. For example, the hydrolysis of a-methyl allyl chloride under Sn 1 conditions affords exclusively the tram cotyl alcohol, presumably via a transoid butenyl cation ... 

Olah has prepared a series of cyclobutenyl cations in superacid solution, either by dimerization of alkynes or by ionization of appropriate cyclobutene derivatives, and has studied them by H and C n.m.r. spectroscopy. The 1,3-diphenylcyclo-butenyl cations (354) exhibit properties characteristic of allyl cations with negligible l,3-7t-overlap. The charge remains principally at the ends of the allyl system and C-1 and C-3 resonate at lower field than C-2 in the C n.m.r. spectra. With substituents at the 1- and 3-positions less effective than phenyl at stabilizing positive charge, e.g. 

Figure 11 -29 Mass spectra of (A) 1-butene, showing a peak at m/z = 41 from cleavage to give the resonance-stabilized 2-propenyl (allyl) cation (B) 2-hexene, showing similar cleavage between C4 and C5 to give the 2-butenyl cation, with m/z = 55.

Branched and internal alkenes fragment similarly at allylic bonds. Figure 11-29B shows the mass spectrum of 2-hexene, in which the base peak at m/z = 55 corresponds to formation of the resonance-stabilized 2-butenyl cation. 

K 1), corresponding to a free energy of activation of 13.7,17.2 and 22.2 keal/mol, respectively. Theoretical investigations rationalize the dependence of the equilibrium on the nature of the cation for the 2-butenyl anion 129. 

Chelation control does not operate in the addition of 2-butenyl Grignard reagents to a-oxyalkanals, since with (racemic) 2-benzyloxy-, 2-(benzyloxymethoxy)- and 2-(t< rt-butyldi-methylsilyoxy)propanal similar ratios of isomers are formed28. Several cations were investigated, but the best choices, e.g., allylboronates or allylsilanes, were not included in this study. 

The radical cations 15 + and 16 + add methanol exclusively from the exo face forming exo-methoxynorbornyl and nortricyclyl free radicals, which undergo rapid cyclopropylcarbinyl-butenyl interconversions (119 120 121 see... 

Next, we considered the activation of 13 towards hydrolysis by K-complexation of a cationic metal unit to the electron-rich diene system. On the basis of the well-known palladium-mediated addition of nucleophiles to alkenamines, it was anticipated that the enol ether function in 13 would add H2O in the presence of Pd(II).21 Interestingly, exposure of 13 to a slight excess of Pd(OAc)2 led to the isolation of 14 (Scheme 8). This material suggested the exploitation of the existing Pd-C linkage for carbon-carbon bond formation with an appropriate A-side chain. In particular, the intramolecular syn insertion of the allylic double bond in the rrans-butenyl substituent in 15b and subsequent syn (3-hydride elimination would give the desired E-alkene 17. This proposal was examined using alkene 15a as a model system, synthesized in a manner similar to 13. Upon exposure to Pd(OAc)2 under the conditions... 

Ring-closing metathesis (RCM) reactions of iV-alkenylpyridinium salts are possible using Grubbs catalyst and this strategy has been used to access dihydroquinolizinium cations from iV-(2 -butenyl)-a-vinylpyridinium triflates in good yield <20040L4125>. 

The dominant contributor to the reactivity of vinylcyclopropanes in any radical reaction is the form (4a), the cyclopropylcarbinyl radical system. The opening of a cyclopropylcarbinyl radical to a butenyl radical is among the fastest radical processes known, with a rate constant of 1.3 x 10 sec". - The various stereoelectronic effects of this rearrangement have been reviewed. The structure of (4a), deduced from its ESR spectrum - and in agreement with calculations (STO-36 basis set), is in the bisected conformation shown, predicted to be 1.4 kcal mol more stable than its perpendicularly oriented counterpart. Above -KX) T only the butenyl radical (4b) can be detected. Substituent efiects do not seem to operate here when the substituents are on the cyclopropane (i.e. product stabilization). The cy-clopropylcaibinyl cation and anion have structures similar to (4a), bisect conformations (5) and (6), respectively. A concise summary of solvolytic and mechanistic data for system (5) has recently appeai Reviews of cyclopropylcarbinyl anions and carbenes are also available. - ... 

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