Cyclobutyl derivatives

Phenylmercury halides give the corresponding phenylmercury derivative, CgH5HgC(CF3)3 (61%) [156], and perfluorocyclobutene gives the corresponding cyclobutyl derivative [156], Mechanistically, the reaction could be interpreted as formation of the fluorocarbanion via nucleophilic addition of fluoride ion to the fluoroolefin followed by capture of the intermediate fluorocarbanion by the mercury salt [156]. The regiochemistry of the reaction is consistent with this mechanism [156] (equation 119). 

Chloro-4-cyanobutane undergoes a high-yielding intramolecular cyclization under basic solidrliquid two-phase conditions in the presence of tetra-n-butylammo-nium chloride to form cyclobutyl cyanide 1-chloro-4,8-dicyanooctane is formed as a by-product (ca. 10%). No cyclization occurs in the absence of the ammonium salt or when aqueous sodium hydroxide is used [29]. Attempts to produce the cyclobutyl derivative in a one-pot reaction of 1,4-dichlorobutane with sodium cyanide/sodium hydroxide gave only a 9% yield, with 1,4-dicyanobutane (63%) and l-chloro-4-cyanobutane (18%). A similar intramolecular cyclization of (3-chloropropyl-thio)acetonitrile yields 2-cyanotetrahydrothiophene (80%) [30]. 

Wie gezeigt wurde, reagieren bei Desaminierungs- und Solvolysereaktionen Cyclopropylmethyl-Derivate der Formel (14) sowie Cyclobutyl-Derivate der Struktur (9) ohne Umlagerung (75, 22—26, 59). Wird in (14) R = CHjF, so andert sich nichts am Bild der Umlagerung (75) erst bei R = CFg reicht der induktive Effekt aus, um bei Solvolyse-... 

It has been shown that the cyclization step of 3-halopropylmalonic esters to give cyclobutane-1,1-dicarboxylatcs, e.g. 3. is rate-determining.4 A convenient synthesis of cyclobutyl derivatives via hydroboration has been reported.5 In this case, 4-tosyloxybut-1-yne was bishydroborated with 9-borabicyclo[3.3.1]nonane (9-BBN), and this was followed by treatment with methyllithi-um to give 9-cyclobutyl-9-BBN 4 in 65% yield.5 24... 

Thus, the ring-opening reaction of cyclobutyl derivatives is usually not used for preparative purposes. It is, on the contrary, an undesirable back reaction in the synthesis of cyclobutanes by homoallyl rearrangement (see Section 7.A.2.I.).3... 

SCHEME 12. Isodesmic reactions based on cyclobutyl derivatives 49-52. Reaction energies from MINDO/3 (first entry) and EHT calculations (second entry) in kcal mol173... 

In 1951 Roberts observed that most cationic reactions of cyclopropylcarbinyl and of cyclobutyl derivatives give the same products in nearly the same ratio.73 For example, cyclopropylcarbinyl and cyclobutyl amines on deamination form the products shown in Equation 6.31.74 Moreover, when allylcarbinyl tosylate (63)... 

This would make it appear that cyclopropyl and cyclobutyl derivatives each solvolyze to give ions that are similar in structure to the starting material. Solvent capture may occur at this stage. If it does not, the first-formed ion rearranges. 

On the other hand, there is now a good deal of evidence that the solvolysis of most cyclobutyl derivatives does lead directly to the cyclopropylcarbinyl cation. For example, orbital symmetry considerations (Section 11.3) indicate that the conversion of cyclobutyl cations into cyclopropylcarbinyl cations should occur by disrotatory ring opening as shown in Figure 6.11 but any steric factors that would hinder such a process decelerate most cyclobutyl solvolyses. Thus 86 See note 84(b). 

If two different C4H7 + ions may exist, which is the more stable The fact that most cyclobutyl derivatives seem to solvolyze directly to the cyclopropylcarbinyl cation strongly suggests that that ion is the more stabilized. Nuclear magnetic resonance studies, however, give conflicting information. The spectrum of the unsubstituted cyclopropylcarbinyl cation in superacid solution is most... 

The [l,2]-alkyl migration A —> B of Figure 14.7 converts a cation with a well-stabilized tertiary carbenium ion center into a cation with a less stable secondary carbenium ion center. This is possible only because of the driving force that is associated with the reduction of ring strain a cyclobutyl derivative A is converted into a cyclopentyl derivative B. 

Changing the medium drastically from water to pentane, does not significantly affect the reaction course, and the cyclopropane derivatives still account for ca. 94 % of the products in this reaction . On the other l nd, cyclobutyl derivatives are the sole products in the acid-catalyzed hydration of 3-methylbicyclobutanecarbonitrile (equation 54) and in the methanolysis of methyl 3-methylbicyclobutanecarboxylate (equation 55) . ... 

The partial data presented above seem to indicate that substituents which are capable of stabilizing a carbocationic center at a bridgehead position will favor the formation of cyclobutyl derivatives, whereas such a substitution on the methylene carbons results in cyclopropylcarbinyl derivative formation. This is consistent with the observation that the acid-catalyzed methanolysis of 3-/ -bicyclobutanecarbonitrile (35) yields 58 % and 42 % of... 

The nature of the cation formed in the first step is highly controversial. On the basis of solvolytic studies, in which cyclopropyl as well as cyclobutyl derivatives are obtained, Roberts and coworkers suggestedthat the bicyclobutonium ion is in fact non-classical with a delocalized a structure (43). On the basis of NMR studies, classical as well as non-classical structures (44, 45 and 46) were suggested for this species and its derivatives. 

In cases where two bonds are cleaved, it is often suggested that the cleavage occurs in one step. This assumption is based on the fact that a stepwise reaction would lead to the formation of either cyclopropyl or cyclobutyl derivatives which are usually inert under the conditions employed. However, it is well established that the pervasive hydrogen species chemisorbed on transition metal surfaces is the hydrogen atom . Thus the mechanism could still be stepwise since the cyclic intermediates are obtained as highly reactive radicals which can further add hydrogen in a series of successive steps. 

In contrast to metallation of the cyclobutyl derivative in Eq. (af), the reaction of the cyclopropyl analogue with n-BuLi leads to addition ... 

In a study of Troger s base derivatives 68 [R = H, Me RR = (CH2)3] in acid media (see Section II,B,3), Greenberg et al. found the dication of the dimethyl derivative (186, R = Me) to have nonidentical benzylic methylene groups and nonequivalent methyl NMR resonances. These results are consistent with the open iminium ion structure 69 (R = Me), and contrast with the results with Troger s base itself (68, R = H) and its spiro (cyclobutyl) derivatives 68 [RR = (CH2)3], in which the protonated closed structure 186 [RR = (CH2)3M] is favored (84JOC1127). 

Both the selenoxide and the selenonium ylide routes have been applied to cyclobutyl derivatives, themselves readily available 57) from selenoacetals of cyclo-butanones on one hand and primary alkyl halides, epoxides, or carbonyl compounds on the other. 

The solvolysis of both cyclopropylcarbinyl and cyclobutyl derivatives proceeds with rearrangement to give mixtures of cyclopropylcarbinyl (515), cyclobutyl (516), and allylcarbinyl derivatives (517)401,402. Under certain conditions the product distributions are very similar, starting from either (57. ) or (514) (Table 16). These data... 

Table 16. Products from S jl reactions of cyclopropylcarbinyl and cyclobutyl derivatives...

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