Cyclopropylcarbinyl, ring

In the alkylative cyclization of the 1,6-enyne 372 with vinyl bromide, formation of both the five-membered ring 373 by exn mode carbopalladation and isomerization of the double bonds and the six-membered ring 374 by endo mode carbopalladation are observed[269]. Their ratio depends on the catalytic species. Also, the cyclization of the 1,6-enyne 375 with /i-bromostyrene (376) affords the endo product 377. The exo mode cyclization is commonly observed in many cases, and there are two possible mechanistic explanations for that observed in these examples. One is direct endo mode carbopalladation. The other is the exo mode carbopalladation to give 378 followed by cyclopropana-tion to form 379, and the subsequent cyclopropylcarbinyl-homoallyl rearrangement affords the six-membered ring 380. Careful determination of the E or Z structure of the double bond in the cyclized product 380 is crucial for the mechanistic discussion. 

Opening of the cyclopropyl ring by the cyclopropylcarbinyl-liomoallyl arrangement, a scheme developed earlier for prosta-... 

The cyclopropylcarbinyl cation is characterized, like the previous molecules, by the existence of a high labile 2p orbital which can interact with orbitals of appropriate symmetry in the ring. We can single out the important interactions in the two extreme configura-... 

This is less common than rearrangement of carbocations, but it does occur (though not when R = alkyl or hydrogen see Chapter 18). Perhaps the best-known rearrangement is that of cyclopropylcarbinyl radicals to a butenyl radical. The rate constant for this rapid ring opening has been measured in... 

Cyclopropylcarbinyl radicals (5) are alkyl radicals but they undergo rapid ring opening to give butenyl radicals." The rate constant for this process has been measured by picosecond radical kinetic techniques to be in the range of 10 M s for the parent to lO Af s for substituted derivatives. This process has been observed in bicyclo[4.1,0]heptan-4-ones. ... 

Fragmentation of cyclopropylcarbinyl radicals has been incorporated into several synthetic schemes.357 For example, 2-dienyl-l,l-(dimethoxycarbonyl)-cyclopropanes undergo ring expansion to cyclopentenes. 

Cyclopropylcarbinols. Treatment of cyclopropylcarbinols 15 (R = Ph, C-C3H5) with trifluoroacetic acid in dichloromethane leads to the rapid formation of ring-opened 4-substituted 3-butenyl-l-trifluoroacetate esters 16 (Eq. 20).130 Cyclopropylcarbinyl trifluoroacetates are not formed. Ring opening is facilitated by phenyl substituents. Addition of organosilicon hydrides to the reaction mixture favors the formation of cyclopropylmethanes 17 and suppresses the formation of the ring-opened esters.130... 

Vinylcyclopropanes represent particularly useful functionality. They do permit a ring expansion to cyclobutanes via the cyclopropylcarbinyl cation manifold (Eq. 9). Equally important, such systems suffer smooth thermal rearrangement to cyclopen-... 

Cyclopropylcarbinyl-cyclobutyl ring expansions (Eq. 5) are facilitated by the presence of the heteroatom substituent in the order O > S > Se. In this case, the heteroatom stabilized cyclobutyl cation (see Eq. 39) can suffer hydrolysis to give the... 

The mechanism of this reaction is still not clear, but the key steps are probably a cyclopropylcarbinyl to cyclobutyl ring enlargement [45] with subsequent ring enlargement of the cyclobutane derivative 46. In fact, such cyclobutane derivatives 46c,d could easily be prepared in 86 and 94% yield, respectively, by stirring dichloromethane solutions of 42c,d in the presence of AI2O3 at 20 °C, and 46 c, d quantitatively isomerized into 47 c, d upon heating in DMSO at 100 °C for 2 h. 

Artemisyl, Santolinyl, Lavandulyl, and Chrysanthemyl Derivatives.— The presence of (41) in lavender oil has been reported earlier. Poulter has published the full details of his work (Vol. 5, p. 14) on synthetic and stereochemical aspects of chrysanthemyl ester and alkoxypyridinium salt solvolyses (Vol. 3, pp. 20—22) and discussed its biosynthetic implications. Over 98% of the solvolysis products are now reported to be artemisyl derivatives which are formed from the primary cyclopropylcarbinyl ion (93) which results from predominant (86%) ionization of the antiperiplanar conformation of (21)-)V-methyl-4-pyridinium iodide the tail-to-tail product (96 0.01%) may then result from the suprafacial migration of the cyclopropane ring bond as shown stereochemically in Scheme 3. This is consistent with earlier work (Vol. 7, p. 20, ref, 214) reporting the efficient rearrangement of the cyclobutyl cation (94) to (96) and its allylic isomer, via the tertiary cyclopropylcarbinyl cation (95). ... 

Rate constants for ring openings of substituted cyclopropylcarbinyl radicals show the same types of kinetic effects as seen in radical additions to substituted alkenes with an obvious relationship to the thermodynamics of the reaction (Fig. 4.16). Minor steric effects influence the kinetics, however, and... 

Figure 4.16. First-order rate constants for ring openings of substituted cyclopropylcarbinyl radicals at ambient temperature. [Data from (71, 113, 115-121).]...

When the reaction is followed by NMR, the initial 1 1 ratio between the isomers changes as the cyclopropyl isomer rearranges to the cyclopropylcarbinyl counterpart. Further rearrangement of the latter finally afforded ring-opened [)f-5-methylenebicyclo[2.2.1]-hexane]PtCl2 complex. 

The cyclopropylcarbinyl anion is stabilized by phenyl groups attached to the carbinyl carbon toward ring opening, provided that potassium or sodium is the counterion. However, replacement of the potassium by lithium or magnesium leads to immediate... 

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