Conversion of cyclopropane

Thus, reactions affording either cyclopropanes or propylenes would most likely represent forms of termination of metathesis activity. As a corollary, any catalytic conversion of cyclopropanes to metathesis olefins via Eq. (26) would seem to require decomposition of the metal-carbene species in order to regenerate a naked metal species (M ) capable of further reactions with cyclopropanes. Of course, bimolecular carbene decomposition to yield an olefin as in Eq. (11) (e.g., ethylene from 2M=CH,) is one accepted process which could account for regeneration of M ... 

Previous work had demonstrated the remarkable conversion of cyclopropane to ethylene (15) and of substituted cyclopropanes to a-olefins (16) over the metathesis catalyst PhWCl3-AlCl3, and results from that work inferred that a mixture of W=CHC2H5 and W=CH2 species would be generated from ethylcyclopropane. When a mixture of ethylcyclopro-... 

As mentioned above, the conversion of cyclopropane to propene radical cation has been investigated by ab initio calculations. The general course of this reaction was confirmed, or anticipated, by product studies in the electron transfer-sensitized conversion of 1,1,2,2-tetraphenylcyclopropane (37) to 1,1,3,3-tetra-phenylpropene (38). The sequence of the key steps, migration versus ring opening cannot be derived from the results. In the case of 37, the four phenyl substituents may actually favor a ring-opened bifunctional radical cation. 

The pyrolysis of substituted cyclopropane leads to three types of unimolecular isomerizations (see Fig. 3). The first kinetic study of the conversion of cyclopropane into propylene (reaction a) was undertaken by Trautz and Winkler in 1922 ... 

In this context, we mention two zeolite-induced conversions of cyclopropane derivatives. Incorporation of tra -l,2-diphenylcyclopropane trans-Vi) and its 3,3-D2-isotopomer into the channels of a redox-active pentasil zeolite (Na-ZSM-5) generated exo,exo-l,3-diphenylallyl radical (24 ) and its 2-Di-isotopomer. This conversion is a zeolite-specific reaction it requires a series of reactions, including oxidation, ring opening, and deprotonation [70]. 

During the past few years a very considerable amount of work has been done on the decompositions and isomerizations of cyclic compounds. Perhaps the most important reason for the recent interest in these reactions of cyclic hydrocarbons is that a number of them occur in a reasonably simple manner, so that they are particularly suitable for the evaluation of theories of unimolecular reactions. The simplest of these processes is the conversion of cyclopropane into propene... 

The first in-situ application of FT PFG NMR during chemical reaction was reported by Hong et al. [17] for the conversion of cyclopropane to propene in zeolite X. The self-diffusivities of the two species in the adsorbed mixture were found to be essentially the same, and they remained constant during the course of the reaction. One of the important results was that for such a system the diffusivities under reaction conditions could be extrapolated from the diffusivities determined at lower temperatures where no chemical reaction took place. 

Besides rearrangements, ligand exchange, formation of alkanes, alkenes and other products, release of cyclopropanes is one of the most important reactions of metallacyclobutanes. Of course, this latter reaction is only useful in cyclopropane synthesis if the product is not identical with the starting material used to form the metallacyclobutane. Nevertheless, the discovery of a complex formed from hexachloroplatinic acid and cyclopropane and later structural elucidations have initiated intensive investigations on the conversion of cyclopropanes to metallacyclobutanes and release of cyclopropanes from the latter. These results have been thoroughly discussed in several reviews. " Therefore in this section only some general aspects of cyclopropane formation from metallacyclobutanes and selected synthetically useful methods are discussed. 

The cyclopropane ring, due to its ring strain, can be considered as a functional group comparable to the double bond with the synthetic potential to generate functionalized three carbon chains via ring opening. Besides thermal-, photochemical-, oxidative-, reductive-, radical-, nucleophile- and Lewis acid or electrophile-mediated activation, the conversion of cyclopropanes mediated by transition metals plays an important role in synthetic uses of small-ring compounds. In most of these cases, prior to conversion, a complexation of the cyclopropane system by the transition metal is necessary. 

The conversion of cyclopropane to propene in the gas phase is a first-order reaction with a rate constant of 6.7 X 10... 

The molecularity of a reaction is the number of molecules reacting in an elementary step. These molecules may be of the same or different types. Each of the elementary steps discussed above is called a bimolecular reaction, an elementary step that involves two molecules. An example of a unimolecular reaction, an elementary step in which only one reacting molecule participates, is the conversion of cyclopropane to propene discussed in Example 13.3. Very few termolecular reactions, reactions that involve the participation of three molecules in one elementary step, are known, because the simultaneous encounter of three molecules is a far less likely event than a bimolecular collision. 

Figure 33 Time dependence of the relative amount of cyclopropane and propene during the conversion of cyclopropane to propene in NaX, together with values for their self-diffusion coefficients (1 cyclopropane cyclopropene) at 473 K. (From Ref. 208.)...

The interaction of many hydrocarbons (both aliphatic and aromatic) with zeolites has been investigated. HY zeolite catalyses the conversion of cyclopropane at room temperature to isobutane. The proposed mechanism involves a non-classical protonated cyclopropane ion intermediate. At 200 cyclopropane isomerizes to propene and also forms aromatic species. Adsorption and transformation of but-ene has been widely studied. It is useful to draw a distinction between hydroxylated and dehydroxylated samples. On hydroxylated samples but-ene isomerizes and also oligo-The -OH groups vibrating at 3640 cm" were found to be... 

The ability of PPG NMR, to monitor simultaneously the mobihty of different components [159,164] makes it a very effective tool for studying the mobility of the reactant and product molecules during chemical reaction. Figure 19 shows the results of in-situ PPG NMR measurement during the conversion of cyclopropane to propene in zeohte Na-X [165]. hi addition to the diffusivity of the reactant molecule (cyclopropane) and the product molecule (propene), also the time dependence of the relative amounts of the involved molecular species is presented. Since the conversion times are much larger than the intercrystalline exchange times as following from the diffusivities, the considered reaction may clearly be assumed to be reaction controlled. 

An example of such a first-order reaction is the conversion of cyclopropane into propylene... 

Rearrangements.—Cyclepropyl-Allyl Rearrangement. Further kinetic studies of this isomerization in the gas phase have been reported, and analytical least-motion forms have been applied to the reaction. An orbital description of a concerted pathway for the conversion of cyclopropane into propene has been supplied by Baldwin and Grayston, who have noted that in dehydroadamantanes the reaction proceeds smoothly even though orthogonal trimethylene radical geometry cannot be achieved. 

The conversion of cyclopropane to propylene is first order with a half-life of 18 minutes at 500 C. (a) What fraction of a given quantity of cyclopropane remains after 54 minutes (b) Starting with 2.00 g, how many grams remain after (i) 72 minutes, (ii) 45 minutes (c) How many minutes will be... 

PRACTICE EXAMPLE B The following mechanism can be used to account for the change in apparent order of unimolecular reactions, such as the conversion of cyclopropane (A) into propene (P), where A is an energetic form of cyclopropane that can either react or return to unreacted cyclopropane. 

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