Methylenecyclobutane rearrangement

An alternative mechanistic interpretation of the methylenecyclobutane rearrangement is the postulation of a 2,2 -bisallyl (or 2,3-dimethylenebutane-l,4-diyl) diradical or zwitterionic intermediate.83... 

Preparative applications of methylenecyclobutane rearrangements are described in Section 3. 

With the hypothesis that allene-olefin cycloadditions and methylenecyclobutane rearrangements have a common intermediate comes the prediction that common stereochemical modes for the various rotations involved will prevail. The cycloaddition model outlined above involves disrotatory ring closure with the same sense of rotation about C(4)C(5) and C(5)C(2). For the methylenecyclobutane rearrangement, the same stereochemistry is anticipated for the reverse reaction. 

In analogy with the spiropentane to methylenecyclobutane rearrangement, one may expect that kinetic studies on 1-methylenespiropentane will be on interest. Thermal reactions of substituted methylenespiropen-tane have just been reported 33>. 

The electron transfer spiropentane-methylenecyclobutane rearrangement of 50 to 51 and 52 occurs under a variety of photochemical conditions, but unlike in the direct photolytic reaction, neither 53 nor 54 is formed. Spiropentanes (50a-c) are good electron donors and form colored EDA complexes with TCNS. The EDA complex of the more potent electron donor, 50a (E° = +1.17 V vs. SCE), and TCNE exhibits CT absorption maxima at 398 and 598 nm in dichlo-romethane. The long-wavelength band of the less electron-rich do-... 

The methylenecyclobutane rearrangement also appears to proceed via biradicals since the dimethyl products described above equilibrate prior to rearrangement to the other methylenecyclobutane. Interestingly, the extent of the formation of the ultimate product, 3-methylene-l,5-hexadiene, from the methylenecyclobutane rearrangement of bicyclo[3.2.0]hept-l-ene varied inversely with pressure suggesting that it was produced from vibrationally hot methylenebicyclo[2.1.1]hexane. 

Oxidative rearrangement takes place in the oxidation of the 1-vinyl-l-cyclo-butanol 31, yielding the cyclopentenone derivative 32[84], Ring contraction to cyclopropyl methyl ketone (34) is observed by the oxidation of 1-methylcyclo-butene (33)[85], and ring expansion to cyclopentanone takes place by the reaction of the methylenecyclobutane 35. [86,87]... 

It is reasonable to assume that 5 and styrene generate the diradical 527, which collapses to give 528 at 30 °C under kinetic control. At temperatures of 75 °C and above, the last step is reversible and an equilibrium is established in that 528 and the methylenecyclobutane derivative 529 maintain a ratio of 1 6 [215]. The [2 + 2]-cydoadduct 530 of 5 to phenylacetylene should be formed analogously via a diradical of the type 527 and is closely related in its structure to the cephalosporin derivatives 437 and 438 (R= Ph, Scheme 6.89). In addition to 530, 2-phenylindene was obtained, which has to be considered as the product of the thermal rearrangement of 530 [216]. Akin to such a process, 526 [194] and 529 [215] were converted into indane derivatives on heating. 

This section includes preparations of cyclobutanes, methylenecyclobutanes and cyclobu-tanones, which involve a rearrangement of the carbon framework or a valence isomerization. There are three types of rearrangements ... 

The transformation of spiropentane to methylenecyclobutane can be regarded as a ring enlargement of a cyclopropane derivative (Section 7. A.3.3.), while the transformations of methylene-cyclobutanes can be classified as a rearrangement from one cyclobutane to another (Section 1.A.5.2.4.2.). The mechanistic aspects of these rearrangements were first studied in 1961 by Frey7y and Chesick.80... 

The degenerate rearrangement of methylenecyclobutane occurs with inversion at C2 and suprafacial migration across one side of the allylie unit. However, the rearrangement is 65% antarafacial with respect to the allylie component.82... 

Noncationic Rearrangements Including Bicyclobutane to Cyclobutene and Spiropentane to Methylenecyclobutane... 

The thermal rearrangement of spiropentanes to methylenecyclobutanes proceeds via two successive bond cleavages. First, a peripheral bond breaks to give a 1,3-diradical 28, and then a radical bond breaks to give a 1,4-diradical 29, until ring closure yields the product(s). 

Ring contraction also resulted on irradiation (185 nm) of cyclopentenes, to form methylenecy-clobutanes in low yield. Thus, photolysis of cyclopentene gave methylenecyclobutane (36) in 27% and bicyclo[2.1.0]pentane in 28% yield,110 via an intermediate carbene shown by rearrangement of [3,4-2H2]cyclopentene. Labeling in the product was found almost exclusively in endocyclic positions,111 Analogously, 1 -methyl-1 -vinylcyclobutane (37) was obtained from 1,2-dimethylcyclopentene in 20% yield.112... 

The intramolecular 2 + 2-photo-cycloadditions of optically active allenesilanes (5) with enones and enoates produce silyl-substituted exo-methylenecyclobutanes (6) in high enantiometric excess. Photo-desilation leads to the parent unsaturated exo-methylenecyclobutanes (7) (Scheme 3).19 The cycloaddition of naphthoquinone to allyltrimethylsilane in the presence of Me2 A1C1 yields the expected 2 + 2-cycloadduct that slowly rearranges to the 2 + 3-adduct.20 hi the presence of bases, Cephalosporin triflates (8) undergo 2 + 2- and 4 + 2-cycloaddition with alkenes, alkynes, and dienes via an intermediate six-membered cyclic allene (9) (Scheme 4).21... 

Finally, as already mentioned, metallocene catalysts can polymerize a variety of olefins. In certain cases the structural features of the monomer lead to the formation of novel polymers. Two such examples are shown by reactions 6.6 and 6.7. It is clear that the polymerization processes involve considerable rearrangements of the bonds. Reactions 6.8 and 6.9 show the formal mechanisms of such rearrangements for 1,5-hexadiene and methylenecyclobutane, respectively. 

The two rearrangement products from the given substituted methylenecyclobutane are seen to be related as mirror images, which makes interpretation of the rearrangement of such an optically active system subject to much uncertainty a mixture of antipodes in the product might imply either a lack of stereochemical specificity or a competitive operation of both fully stereoselective rearrangement pathways. 

Disubstituted ethylenes undergo oxidation with a skeletd rearrangement. Methylenecyclobutane (17) was oxidized to cyclopentanone via ring expansion (Scheme 3). ... 

The ultimate in small-ring propellanes (C2 exists but I don t consider it to be [O.O.OJpropellane), [I.l.ljpropellane (96), was finally synthesized from 2,3-dibromo-bicyclo[l.l.l]pentane (95) with t-BuLi . It undergoes thermal rearrangement to 97 with a half-life of 5 min at 114°C. Thus, Wiberg s prediction that this compound ought to be relatively stable as compared to other small-ring propellanes is amply borne out. In acetic acid it affords l-acetoxy-3-methylenecyclobutane (98). 

Sodium on alumina. This high-surface sodium catalyst, prepared by adding sodium to dry alumina with stirring at 150° under nitrogen, is effective for isomerization of butenes and pentene-1. In contrast to acid-catalyzed isomerization, no skeletal rearrangement occurs. The results indicate some stereoselectivity. Thus butene-1 is isomerized initially to about equal amounts of cis- and trani-butene-2 eventually the thermodynamic equilibrium mixture rich in the trans-isomti is obtained. The catalyst was used to effect almost quantitative conversion of methylenecyclobutane into 1-methylcyclobutene. ... 

A phenylpalladium complex also causes ring-opening rearrangement of methylenecyclobutane to a ji-allylpalladium complex, which arises from p-carbon elimination of an intermediate (cyclobutylmethyl)palladium complex [82]. 

The thermal decompositions of several types of strained hydrocarbons produce diradicals containing the cyclopropylmethyl unit. 1,3-Diradicals of type 2 are formed on thermolysis of spiropentane (1) in the gas phase, in a sealed tube, or in a flow system. Diradical 2 not only ring closes back to spiropentane, but also rearranges by -scission to produce the new delocalized diradical 3, ring closure of which generates methylenecyclobutane 4) A competing reaction which yields allene and ethene is minor, provided the pyrolysis temperature is not allowed to rise higher than ca. 400 °C. 

Methylenecycloalkanes, 418 Methylenecyclobutane, 151 Meihylenecyclobutanes, 251 Methylenecyclobutanones, 223 Methylenecyclohexane, 421 Methylenecyclopiopanes, 42, 530 Methylene diesteis, 565 a-Methylenelactam rearrangement, 2-3 a-Methylene-y-lactones, 157,460, 466,... 

The three methylenecyclobutanes, 51,52, and 53, do not interconvert under direct irradiation or SET-photosensitized conditions. However, 3,3-diaryl-4,4-dideuterio-l-methylenecyclobutane (d -Sd) does undergo electron-transfer photoinduced degenerate methylene-cy-clobutane rearrangement when either DCN, DCA, or 2,6,9,10-tetracy-anoanthracene is used as sensitizer. These processes involve the allylically stabilized 1,4-cation radical intermediates dj-SS and d -58/-+28 tjjg 1 4-cation radical 58 is efficiently captured by molecular oxygen, giving rise to 73 as shown in Scheme 15. 

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