Triplet butadiene

Since a sensitizer (vide infra) is used in the reaction, it may also be called a photosensitized reaction. (Any photochemical reaction in which the exciting light is absorbed by some species other than the principal reactants.) Putting everything together generates the imposing descriptive name, photosensitized cycloaddition reaction. If the steps which constitute the mechanism of the reaction are discussed in detail, the prefix photo- becomes superfluous and the individual steps will be described in ordinary chemical terms. For example, if it is decided that the mechanism of the above reaction involves addition of butadiene triplets to butadiene, that step of the reaction would be simply called cycloaddition. 

Alkyl derivatives of 1,3-butadiene usually undergo photosensitized Z-E isomerism when photosensitizers that can supply at least 60 kcal/mol are used. Two conformers of the diene, the s-Z and s-E, exist in equilibrium, so there are two nonidentical ground states from which excitation can occur. Two triplet excited states that do not readily interconvert are derived from the s-E and s-Z conformers. Theoretical calculations suggest that at their energy minimum the excited states of conjugated dienes can be described as an alkyl radical and an orthogonal allyl system called an allylmethylene diradical ... 

As the triplet energy of the sensitizer becomes less than that necessary to excite either form of the butadiene (Et < 50 kcal/mole), it is proposed that energy is transferred via nonvertical excitation to lower energy twist forms of the diene triplets.(11> The product distribution again reflects the ground state population of s-cis and s-trans forms ... 

Triplet photoaddition of simple non-cyclic monoolefins is unknown. The sensitized dimerization of ethyl vinyl ether gives exclusively head-to-head adducts, Eq. 21, and probably should not be classed as an example of simple acyclic olefin. Usually the triplets have high energies and are severly twisted. 55> Some cyclic rigid molecules, Eq. 20, that do dimerize 63> do not incorporate substituents that allow regioselectivity to be determined. Butadiene gives principally head-to-head dimerization, Eq. 19, concordant with the PMO prediction, and so does indene, Eq. 22. The anti dimer that is formed would not be expected from a singlet excimer reaction. 

Ionizing radiation leads also to formation of excited molecules in the triplet state. Okazaki and coworkers74 calculated the yield of the triplets and found that the yields for conjugated dienes are significantly higher than those of monoalkenes. The yield for 1,3-butadiene is 2.66, whereas the values for 1-butene, 2-butene and 2-methylpropene are 1.51, 1.55 and 1.54, respectively. 

It is evident from Table 11 that the rate of addition of the triplet bis(methoxy-carbonyl) carbene is somewhat slower than that of the singlet. Another important general rule may also be deduced from Table 11 the triplet carbene adds to dienes about 3—4 times faster than to olefins the reactivity ratio of 2,3-dimethyl-butadiene-1,3/pentene-l is 9.6 for triplet and 2.8 for the singlet. This ratio may be compared with that for diphenylcarbene (1,3-butadiene/hexene-l), which is > 100. 

An interesting extension of the forementioned generalizations has recently been reported.43 When a dilute solution of 2,3-dimethyl-l,3-butadiene 11a is irradiated in the presence of a large excess of benzo-phenone, a product 16 arising from the photocycloaddition reaction is observed. This anomalous result has been explained as involving the 77,77- triplet of the diene, formed by triplet-triplet transfer from the n,n triplet of benzophenone, which then reacts with ground-state benzophenone. The critical steps are shown in structures 14-16. 

Rates and Activation Parameters. The first condensed-phase absolute rate measurement for a carbene-alkene addition was reported by Closs and Rabinow in 1976 flash lamp photolysis of diphenyldiazomethane generated (triplet) diphenylcarbene, which added to butadiene (in benzene) with k =... 

Triplet carbenes show enhanced reactivity toward alkenes that can stabilize the intermediate radical center. For example, the reactivity of 1,3-butadiene toward DPC is shown to be some a 100 times larger than that of 1-hexene. ... 

The bimolecular rate constant for the reaction of DPC with butadiene is determined to be 6.5 X 10 M s . Isoprene can be employed as a selective trap for triplet carbenes. Styrene is also shown to be an efficient trap for triplet carbene. (E)-p-Deutero-a-methylstyrene (89) is a very convenient reagent to diagnose the mult-plicity of the reacting carbene because it reacts with both singlet and triplet carbenes with different stereochemical outcomes. The stereochemistry of the adduct cyclopropane (90) can be easily judged by NMR (Scheme 9.28). For example, BA (22) reacts with styrene with total loss of stereochemistry, while in the reaction with dimethoxy FL (23a), the expected cyclopropane is obtained with complete retention of stereochemistry. The rate constants for the additions are (1.2 0.2) x 10 and... 

But for molecules whose triplet energy lies between 226 kJ and 251 kJ compound (3) is formed in large amounts. Et of cis-butadiene is 226 kJ. This suggests that since rotation around the central bond in the excited state is a slow process it will not effectively compete with dimerization. 

The products obtained upon direct excitation of butadiene,291 norbornadiene,292 cyclohexadiene,293 and other dienes294 are also completely different from those obtained by photosensitization, again demonstrating quite different chemical reactivity of excited singlet and triplet dienes. However, 1,l -bicyclohexenyl295 and 1,3-cycloocta-diene298 represent exceptions, undergoing both direct and sensitized photoisomerizations to cyclobutene derivatives. 

In the thermal reaction the [4 + 2] or Diels-Alder adduct is the major product, whereas in the photochemical reaction [2 + 2] cycloadditions dominate. Because the photochemical additions are sensitized by a ketone, C6H5-COCH3, these cycloadditions occur through the triplet state of 1,3-butadiene and, as a result, it is not surprising that these cycloadditions are stepwise, nonstereospecific, and involve diradical intermediates. 

Exercise 28-24 Why must the resonance forms 20a, b, c, etc. correspond to a singlet state Formulate the hybrid structure of a triplet state of butadiene in terms of appropriate contributing resonance structures. 

Photosensitized dienes dimerize to give 2 + 2 and 2 + 4 addition products. For example, triplet butadiene yields the three products in Equation 13.60,... 

Sensitizers whose triplet energies are less than 60 kcal mole 1 are of lower energy than the butadiene r-trans triplet and they must therefore seek out an r-cis diene (triplet energy of r-cis butadiene is 53 kcal mole-1) before they can transfer their energy. The biradical produced by attack of an. f-cis triplet on an r-trans ground-state molecule can form a cyclohexene without bond rotation (see Scheme 2), and thus the relative amount of cyclohexene increases. When the... 

Photosensitized dienes also add to simple olefins the relative amount of 2 + 2 and 2+4 addition product again depends on the energy of the triplet sensitizer. For example, in the photosensitized addition of butadiene to tri-fluorethylene (Equation 13.61), the percentage of the 2 + 4 adduct (52) increases from 0.5 percent when acetophenone is used as the photosensitizer to 22.5 percent when fluorenone is the photosensitizer.79... 

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