Cyclopentenes, from vinylcyclopropanes

An illustrative example for the generation of cyclopentenes from vinylcyclopropanes is the formation of bicyclo[3.3.0]oct-l-ene 10 from 1,1-dicyclopropylethene 9 by two consecutive vinylcyclopropane cyclopentene rearrangements. ... 

Franzen34 photolyzed CH2N2-butadiene mixtures in the pressure range 31-335 mm., with butadiene in excess by a factor of 2-15. Franzen also observed cyclopentene as a product, the ratio of cyclopentene to vinyl cyclopropane decreasing from 0.25 at 35 mm. to 0.095 at 335 mm. Franzen proposed that some of the cyclopentene resulted from 1,4 addition of methylene to butadiene, on the grounds that all excited vinyl-cyclopropane should be collisionally deactivated at pressures as high as 335 mm. However, the ratio of cyclopentene to vinylcyclopropane obtained by Franzen at 335 mm. is close to that predicted by the ratio of rate constants for reactions (63) and (64) calculated by Frey.44... 

So far the di-ir-methane photoisomerization has found few applications in synthesis, despite the fact that vinylcyclopropanes are versatile intermediates. Several reviews discuss Aese compounds from a general point of view. For example, vinylcyclopropanes are prone to rearrange to cyclopentenes. Hence, in sequence, these two rearrangements may serve as a way to produce cyclopentenes from 1,4-dienes. ... 

Fig. 3. Energy diagram for the formation of cyclopentene and 1,3-pentadiene from vinylcyclopropane.

Other functionalized cyclopentenes are produced at moderate temperatures from vinylcyclopropanes, e.g. 15 and... 

Cyclopentene ring from vinylcyclopropanes Stereospecific cyclopentene annelation... 

Adducts derived from cyclopropyl-TMM reactions are versatile synthetic intermediates. Alkylidenecyclopropanes have been proven useful in further Pd-cata-lyzed transformations [4], On the other hand, vinylcyclopropanes can undergo smooth thermal ring-expansion to cyclopentenes. Thus, a total synthesis of 11-hy-droxyjasionone (27) was achieved with the cyclopropyl-TMM cycloaddition as the crucial step, and the thermal rearrangement of the initial adduct (28) as an entry to the bicyclo[6.3.0]undecyl compound (29), a key intermediate in the synthetic sequence (Scheme 2.9) [19]. 

In addition to cyclopentenes, other types of compounds may be formed upon heating of vinylcyclopropanes. For example pentadienes 6a/b may be formed by a competitive route from a diradical intermediate. 

It will be noted that the isomerization to cyclopentene proceeds with a considerably lower energy of activation than the other cyclopropane isomerizations so far discussed. As a result these reactions have been investigated kinetically at temperatures about 100° lower than those not having a vinyl substituent. A number of substituted vinylcyclopropanes have been studied and the Arrhenius parameters for their isomerizations to substituted cyclopentenes determined. The results are shown in Table 4. From the results in Table 4 it can be seen that the isomerizations... 

Vilsmeier reaction, 49, 1 56, 2 Vinylcyclopropanes, rearrangement to cyclopentenes, 33, 2 Vinyllithiums, from sulfonylhydrazones, 39, 1... 

One of the classical papers in this area is Neureiter s discovery of the vinyl-cyclopropane to cyclopentene rearrangement [185]. This important process was first observed on a l,l-dichloro-2-vinylcyclopropane derivative, which on heating yielded a chlorocyclopentadiene, presumably by the loss of hydrochloric acid from the initially formed 4,4-dichlorocyclopentene. 

The thermal rearrangement of vinylcyclopropane to cyclopentene was uncovered in I96090 91. That vinylcyclopropanes, like other cyclopropanes, may undergo cis, trans iso-merizations was inferred in 1964 when trans-l-vinyl-2-methylcyclopropane was thermally converted to mostly (4Z)-1,4-hexadiene, a product formed at much lower temperatures from cw-1-vinyl-2-methylcyclopropane92. The reversible interconversion of the cis and trans isomers of l-vinyl-2-d-cyclopropane (equation 2) was reported soon thereafter, in 196793"96. Additional examples, including cases showing both geometrical isomerization and enantiomerization processes, soon followed. 

FIGURE 4. Empirical correlation of AG (k,2) for vinylcyclopropane to cyclopentene rearranfements for tram-1 -R l-2-R, -cyclopropanes with the sum of radical stabilization energies for R CH2 and R2CH2 . The 15 examples are taken from References 59, 62, 64, 144,150-154,180,181 and 207-211... 

However, carbenes react with dienes to give vinylcyclopropanes 6.302, avoiding the [2 + 4] cycloaddition with a linear approach giving cyclopentenes 6.299. We have seen that the cyclopropane-forming reaction is allowed when it uses a nonlinear approach 6.130, but we need to consider why the nonlinear approach is preferred when the linear approach giving a cyclopentene could profit from overlap to the atomic orbitals with the two large coefficients at the ends of the diene. 

A similar acceleration has most recently been observed in the rearrangement of vinylcyclopropanes of type (39 Scheme 8). This fluoride-mediated vinylcyclopropane-cyclopentene isomerization proceeds at -78 C to give (40) in 85% yield this is to date the mildest condition available. Two possible intermediates, the enolate anion (39a) or the diradical anion (39b), may be responsible for such acceleration in analogy to the enolate anion accelerated divinylcyclobutane rearrangement recently reported." The mechanism of this transformation is unclear but may involve anion acceleration similar to that observed in the rearrangement of sulfonyl anions derived from (42 Scheme 8). By comparison the thermolysis of (39) produced exclusively the endo isomer of (41) at 580... 

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