Cyclopropane, excited, from

The molecule decomposes by elimination of CF, which should occur with equal probabilities from each ring when energy is randomized. However, at pressures in excess of 100 Torr there is a measurable increase in the fraction of decomposition in the ring that was initially excited. From an analysis of the relative product yield versus pressure, it was deduced that energy flows between the two cyclopropyl rings with a rate of only 3x10 s In a related set of experiments Rabinovitch et al [116] studied the series of chemically activated fliioroalkyl cyclopropanes ... 

Butadiene. The reaction of methylene with butadiene was studied by Frey44 under experimental conditions similar to those in the case of allene, except that lower pressures were required to avoid butadiene polymerization. Products formed by attack of methylene on the C—H bonds were cis and vinyl-cyclopropane resulting from addition of CH2 to the carbon-carbon double bond underwent collisional deactivation or isomerization to cyclopentene and C dienes, with the exception of isoprene. 

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... 

The photoelimination of nitrogen from 1-pyrazolines is one of the most thoroughly investigated photoreactions and it has been used extensively in the synthesis of cyclopropane derivatives.334 Both stereospecific and non-stereospedfic processes have been observed and these are believed, at least in simple 1-pyrazolines, to correspond to singlet and triplet excited states, respectively. Two reaction pathways have been proposed in the azoalkane 405335 direct excitation via a thermally activated S, state affords the C6H6 isomers 406 to 409, whereas triplet-sensitized excitation results in a tem-... 

The Y-intercept ( 3 /k Eq. 13) of the reciprocal correlation for the photolysis of diazirine 21-C1 in TME was 2.18, which translated into a 68% incursion of diazirine excited state in the genesis of the rearrangement products, 22-C1 and 23-C1 carbene 17-C1 only accounted for 32% of these products.28 A similar conclusion followed from the ratio of rearr/addn (68 32) at a high concentration (6.7 M) of TME in pentane, where carbene 17-C1 was almost completely diverted to the cyclopropane, and 22 and 23 were exclusively derived from the excited diazirine.28... 

In addition, 18-19% of isobutene and chloroacetylene formed via fragmentation. Photolysis of the diazirine in up to 9 M trimethylethylene in pentane led to a sharp decrease in 27 and 28 (to 32% and 8.5%), along with 40% of cyclopropanes formed via the capture of 19. However, the yield of isobutene and chloroacetylene was unchanged, indicating that these products did not stem from the carbene, but arose directly by fragmentation of its excited diazirine precursor.45... 

If Scheme 2 accurately represented the PhCH2CCl chemistry, curvature in the addn/rearr vs. [alkene] correlation would persist when the carbene was generated from 37. The absence of curvature in this case counts against Scheme 2 (and the CAC mechanism), but accords with the RIES mechanism, Scheme 3. Elimination of the diazirine precursor eliminates the diazirine excited state. From 37, both cyclopropane formation and 1,2-H rearrangement proceed from a single (carbene) intermediate, and addn/rearr vs. [alkene] is linear.25... 

Among simple hydrocarbon ions, the distonic trhnethylene radical cation. CH2CH2CH2+ (7 +) is accessible upon internal excitation of the cyclopropane molecular ion or by loss of formaldehyde from the tetrahydrofuran... 

It was demonstrated in Section 3 that spiroannulation of cyclopropane to four- and higher-membered rings does not cause an increase in the strain energy of the molecule. It is not surprising, then, that the chemistry of these compounds does not differ from that of simple alkyl substituted cyclopropanes. Since exciting reviews on the chemistry of cyclopropanes are available42, the authors of this chapter will limit the description of properties of SPC of these types to some of their interesting applications. 

On the basis of triplet sensitization experiments, this rearrangement was shown to occur from the excited singlet state and to involve the biradical intermediate 111. Interestingly, vinyl cyclopropane esters 113-114 underwent geometric isomerization at a much faster rate than rearrangement on direct excitation (Scheme 8.34) [57, 58]. However, the triplet-sensitized reaction of these esters give both cyclopentene and isomerization products. 

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