3.3- shift product

When the C—C bond is part of an a,p-unsaturated ketone moiety as in (38) (Scheme 8), direct (i.e. TT.ir excitation) triggers the ODPM reaction a stereoisomeric mixture of (39) and the 1,3-shift product (40) is formed in 93% and 6% yield, respectively, at 91% conversion of starting material. Compounds (40) and (39) are suggested to be generated from the singlet and triplet state, respectively. This finding... 

Direct irradiation of bicyclo[2.2.2]oct-5-en-2-one (11) in diethyl ether gave the 1,3-shift product, bicyclo[4.2.0]oct-4-en-7-one (12), which on further irradiation lost ketene without any detectable reverse 1,3-shift, to give eyclohexa-1,3-diene. On the other hand, acetone sensitized photolysis of bicyclo[2.2.2]oct-5-en-2-one (11) gave the 1,2-shift photoproduct, tricyclo[3.3.0.0 ]oc-tan-3-one (13), implicating a triplet excited state for this reaction and a singlet excited state for the 1,3-shift. ... 

Photolysis of bicyclo[3.2.2]octa-3,6-dien-2-one derivatives, for example the adduct of tropone with benzyne 33 a in water gave the three-membered ring derivative 35 a, while photolysis in acetonitrile gave the cyclopentanone derivative 37 a which must arise from the 1,3-shift product 36a.This different behavior in these two solvents was rationalized in the following way the efficient [3,3] shift of 33 a to cyclopropyl ketene 34 a is thermally reversible, so that in the absence of a nucleophile 35a is not observed. In that case, the less efficient 1,3-shift to form 36a and then 37a is the only detectable reaction. The ketene 34a was detected as the major product of photolysis of adduct 33 a at low temperature methanolysis of 34 a at — 80 "C gave only the enfifo-configurated ester 35 a. 

The lack of [1,3] shift products and the relative ease of the thermal rearrangement have often been cited as evidence for the concerted nature of open-chain substrates. On the other hand, certain bicyclic systems undergo [1,3] oxy-Cope as well as [3,3] oxy-Cope rearrangements. Thus, thermolysis of srn-7-vinylnorbornenol (6) gives predominantly the [1,3] shift product 7722. 

There is, however, one experiment which indicates that the planar TMM species is not involved directly in the formation of the 1,3-shift product. Baldwin found that optically ring deuterium labeled 2-methoxymethylmethylenecyclopropane gives geometric isomers and the 1,3-shift product the latter being formed without any direct racemization (Scheme 5.15)." ... 

Why this system undergoes both one- and two-center epimerization at C2 and C3 without giving some 1,3-shift product with epimerization at C2 is unclear, especially in light of the strong computational support for accessibility of the planar TMM singlet, vide infra. Further, pyrolysis of methyl-labeled 6-methylenebicyclo[3.1.0]-hexanes revealed the likelihood of at least partial intervention of the planar TMM species. 

However, the formation of 3-methylcyclopentene requires cleavage of the less substituted bond which was acknowledged to be less likely by the authors. Subsequently, the 1,3-shift product was identified as 4-methylcyclopentene, and it was formed with log = 13.67 - 48 640/2.3/ r. ... 

Recognizing that cyano rotation is preferred over isopropenyl rotation and suggesting that the direction of rotation about Cl might be so as to rotate the cyano group outward, the same orthogonal biradical was proposed to account for the major 1,3-shift product in each case. Closure of this intermediate by a least motion... 

In a related example, Swenton found evidence for only the sr 1,3-shift product with optically active fxJ6>-5,6-diphenylbicyclo[3.1.0]hexene at 130°C. Here log k = 12.6 — 32400/23RT (Scheme 7.33). The lower activation energy is consistent with increased stabilization of a non-planar biradical by a phenyl group and the lack of other products can be reasonably attributed to a higher activation energy for formation of a planar species due to the presence of vicinal phenyl groups which must interact sterically in the transition state for conformational interconversion. 

The bisallyl biradical, however, has been demonstrated to undergo rapid rotation around the central bond. Thus, pyrolysis of optically active T gives optically inactive 1,3-shift product at only 9% conversion further, starting Tracemized to the extent of... 

Subsequent work by Baldwin allowed the quadrisection of stereopathways by using non-racemic ra 5 -propenyl compounds. The results are almost, but not quite consistent with a common biradical with some stereochemical bias suggesting that complete randomization about all bonds has not been achieved prior to ring closure. Given in Scheme 7.101 are just the 1,3-shift products with their stereopathway from each isomer. 

Methylenebicyclo[3.2.0]hept-2-ene rearranged to 5-methylenebicyclo[2.2.1]hept-2-ene at 250°C with logA = 13.7 — 39 600/2.The 6-dideuteriomethylene derivative was shown to rearrange to its 7,7-dideuterio isomer with logA = 13.4 - 40900/2.3/ r (a rate factor 8-10 slower than formation of the 2.2.1 isomers), presumably via a 1,3-shift of carbon. However, the two 2.2.1-dideuterio isomers were formed in a 60 40 and in a 56 44 ratio from the two 3.2.0-dideuterio isomers with the 1,3-shift product dominating over the 3,3-shift product in each case (Scheme 9.58). Finally, the two 2.2.1 isomers interconverted very slowly with log = 13.5 - 45 900/2.3/ r. 

To characterize the stereochemistry of the conversions, the x6>-7-methyl-6-methylenebicyclo[3.2.0]hept-2-ene derivative was pyrolyzed and found to give 88% inversion at Cl in the 1,3-shift product (mostly) /i(i(9-6-methyl-5-methylenebicy-clo[2.2.1]hept-2-ene. However, the endo-1-mtihyX 3.2.0 isomer also gave 60% retention in its 1,3-shift to give the endo-6-mQ hy 2.2.1 product. It would appear that the bisallyl biradical responsible for the major product is formed preferentially from... 

Hammond and DeBoer also found that partially optically active 1,3-shift product is formed from optically active starting material suggesting that the biradical(s) is(are) not thermally equilibrated under the reaction conditions. Further, Berson and... 

Dervan found that the 1,3-shift product is formed with 8% net inversion of configuration at the migrating carbon/ ... 

Finally, the l-m hoxy-l-syn (c/5-propenyl) derivative gave a 1 1 mixture of the endo- and exo-mo hy 3,3-product and a 1 2 mixture of the endo- and xo-methyl-1,3-shift products roughly 10 times slower than the /ran -propenyl compound suggesting that non-concerted reactions are involved. 

The reaction is a 3,3-shift necessarily via a boat-like transition state. A biradical path was ruled out since the 1,3-shift product, the standard cyclopentadiene dimer, was not observed. The transition state, however, probably closely resembles the biradical since the activation entropy is not strongly negative as is usually the case with chair-like 3,3-shifts (see Chapter 7, Section 4.1). 

Pyrolysis of cis- and rrafx -4-methylenebicyclo[5.2.0]nona-2-ene in a flow system at 500°C gave the retro 2 + 2 cycloaddition product, 5-methylene-l,3-cyclohepta-diene, as well as the 1,3-shift product, 2-methylenebicyclo[3.2.2]nona-6-ene (Scheme 11.87)/ ... 

A biradical was suggested as an intermediate for both reactions. At higher temperatures, the 1,3-shift product also underwent a retro 4 + 2 reaction. An allowed 2s + 2a pathway was considered for the retro 2 + 2 cycloaddition mechanism but was discarded on the basis of the supposition that the trans material would give substantially more of this product than the cis isomer. 

In an attempt to provide an example of an all-carbon aromatic Claisen rearrangement, cw-1-phenyl-2-vinylcyclopropane was pyrolyzed. However, at 150°C only geometric isomerization was observed with A// = 32kcal/mol and = -5.8 e.u. At still higher temperatures the 1,3-shift product, 4-phenylcy-clopentene, was formed with log k = 13.38 - 41 000/2.3RT (Scheme 12.16). ... 

More recent work established that the ratio of trans- to cw-l-phenyl-2-vinylcyclopropane at 216.4°C is roughly 4, and that the latter gives the 1,3-shift product with a rate constant roughly 2.3 times faster than the former. Furthermore,... 

In an attempt to prepare a precursor to interesting (CH)i2 hydrocarbons, the potential 3,3-shift of 7-carbethoxytricyclo[5.4.0.0 ]undeca-3,5,9-triene was examined. At 50°C it instead gave a 1,3-shift product with = 31 kcal/mol and AS = 13 e.u. At higher temperatures some evidence for the 3,3-shift product was obtained (Scheme 12.20). 

Other derivatives behave similarly making this one of the few cis-1,2-divinylcyclopropanes which does not undergo a 3,3-shift even though favorably arranged geometrically. Thermodynamic factors may be involved since the 3,3-shift product in one case has been demonstrated to revert to the tricyclo 5.4.0.0 system which then gives the 1,3-shift product (Scheme 12.21). ... 

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