Singlet reactions

This key paper was followed by a flurry of activity in this area, spanning several years." " "" A variety of workers reported attempts to deconvolute the temperature dependence of carbene singlet/triplet equilibria and relative reactivities from the influence of solid matrices. Invariably, in low-temperature solids, H-abstraction reactions were found to predominate over other processes. Somewhat similar results were obtained in studies of the temperature and phase dependency of the selectivity of C-H insertion reactions in alkanes. While, for example, primary versus tertiary C-H abstraction became increasingly selective as the temperature was lowered in solution, the reactions became dramatically less selective in the solid phase as temperatures were lowered further. Similar work of Tomioka and co-workers explored variations of OH (singlet reaction) versus C-H (triplet reaction) carbene insertions with alcohols as a function of temperature and medium. Numerous attempts were made in these reports to explain the results based on increases in triplet carbene population... 

Hammond and co-workers(1) used the following reasoning in ruling out the possibility of singlet reaction and in implicating the triplet. 

Thus the triplet states of the two diastereomers react to yield different product distributions although this effect is far less marked for the triplet than for the singlet reaction, which is essentially stereospecific. The singlet reaction could be either concerted or due to an extremely shortlived biradical. Since the product distributions of the triple reaction of these two diastereomers are different, it is clear that cleavage must occur before complete equilibration. Thus the lifetime of the aliphatic ketone derived biradical must be considerably shorter than the corresponding biradical derived from an aryl ketone. 

The quantum yield for the formation of the cycloaddition product has been found to be temperature dependent, increasing by a factor of approximately three as the temperature is lowered from 65 ( = 0.24) to 5°C ( = 0.69). Photolysis of mixtures of the olefin and f/my-stilbene in the presence of sensitizers yielded no cycloaddition product (42) but rather only m-stilbene. This suggests that the cycloadduct is produced via a singlet reaction. This conclusion is supported by the fact that tetramethylethylene quenches fluorescence from the /rans-stilbene singlet. A plot of l/ (42) vs. 1/[TME] (TME = tetramethylethylene) is linear. The slope of this plot yields rate constants for cycloadduct formation which show a negative temperature dependence. To account for this fact, a reversibly formed exciplex leading to (42) was proposed in the following mechanism<82) ... 

Table 3.13. Percent Singlet Reaction as a Function of they C—H Bond Strength 7 ...

Compound yDc-n, kcal/mole Percent singlet reaction... 

The preceding discussion applied to aromatic ketone triplet reactions. With aliphatic ketones the situation is quite dilferent. As stated previously, aliphatic ketones undergo type II cleavage from both the excited singlet and the triplet state. By studying the reaction with and without added quencher, one can determine the characteristics of the reaction for each state, that is, the singlet reaction can be studied in the presence of a strong triplet quencher while the triplet reaction characteristics can be obtained by the difference between the reaction without quencher and that when quencher is added. For example, for the reaction... 

The higher degree of stereospecificity in the singlet reaction relative to that of the triplet is not unexpected in light of the greater lifetime of the triplet bira cal allowing bond rotation or bond breaking to yield acetone and an isomerized olefin. 

Comparison of these experimental results with the calculated charge densities (S0 and Si) at the 2 and 3 positions (Table 11.5) shows that this is the expected result. Except for those compounds discussed below, the failure to observe quenching with triplet quenchers and reaction in the presence of a photosensitizer indicated singlet reactions. Compound (89) was found to also undergo benzophenone-photosensitized substitution, indicating that the triplet state of this compound is also reactive. The reaction, however, was less clean than that observed in the direct photolysis. Similarly, 1,6-dinitro-naphthalene was found to undergo both direct and benzophenone-photosensitized substitution ... 

As will be discussed later, it is possible a4> that the thermolysis involves a metal-nitrene complex whereas the photolysis involves the free nitrene. The product distribution is not affected by the presence of a photosensitizer, but since ferrocene itself is both an efficient triplet quencher as well as a sensitizer 26,27) jt is very difficult to probe the spin state of ferrocenyl nitrene at the moment of reaction. The cycli-zation appears to be a singlet reaction since the yield of 27 in benzene solution is essentially unaffected by oxygen or the presence of hydro-quinone a5>. 

Only a few of the many interesting possibilities will be discussed in this section. Examples of some of these have already been given in Eqs. 23—27, 29—31, 38, and 39. The unsensitized, probably singlet reactions in Eqs. 38 and 39 will be covered first. 

The chemical properties of BFL are very similar to those of FL. The greatest difference is that under similar conditions there is more singlet reaction from BFL than from FL. This observation is reflected in the estimate of AGsr obtained, exactly as it was for FL, from the observed rate of reaction with methyl alcohol in acetonitrile. For BFL, use of (28) gives A(7st = 1.0 kcal mol 1. This value implies that there is a significant amount of BFL in the equilibrium mixture, and that the effect of the conjugating benzo-substituent is to stabilize the singlet carbene more than the triplet. 

For comparison, let us also consider the experimentally well-documented51 singlet reactions of H2 with group 9 metal carbene cations to give metal dihydride insertion products, e.g., for M = Ir,... 

Skell s hypothesis proved to be extremely useful in carbene chemistry even though it was frequently opposed. The principal significance of these rules is represented in the scheme below. The singlet reaction occurs in a concerted step, the cis-addition product being formed in a stereospeciiic manner. In the triplet addition, which is a two-step reaction, rotation is thought to be faster than intersystem crossing (spin inversion) and ring closure, i.e., which would... 

The first systems with meta-activation deliberately investigated by sensitization (and quenching) experiments were m-nitroanisole in liquid ammonia (van Vliet et al., 1970) cmd l-methoxy-6-nitro-naphthalene in alkaline medium (Beyersbergen van Henegouwen and Havinga, 1970), In these two cases indications of a singlet reaction were found. 

Runs in pure piperylene as solvent, where we assume all triplet reaction is quenched, indicate quantum yields for singlet reaction of 0.025 and 0.10 for 2-pentanone and 2-hexanone, respectively.326 With the reasonable assumption that the rate of intersystem crossing is similar... 

Total quantum yields in the absence of quencher for 2-pentanone and 2-hexanone are 0.20 and 0.25, respectively.326 Subtraction of the quantum yields for singlet reaction from the total quantum yields furnishes values of triplet quantum yields. Stern-Volmer plots of reciprocals of the triplet quantum yields versus piperylene concentration indicate that triplet 2-hexanone is five times as reactive as triplet 2-pentanone.273 Rough measurements have been made with methoxyacetone which suggest that at least 60% of its total photoelimination occurs from an unquenchable singlet state.273... 

Before proceeding further we should note that in equation 7 there are two diradicals. One is Diradical I , labeled I, and the other is Diradical II , labeled II. In the case of triplets both are reaction intermediates, while in the case of the singlet reactions these appear more likely to be mere points on the excited state hypersurface32-33 42. 

Aliphatic ketones generally undergo Type II reaction from their singlet and triplet states simultaneously, as can be seen from the fact that only part of the reaction can be efficiently quenched by 1,3-dienes. The rates of intersystem crossing for most aliphatic ketones are similar to one another, and therefore the percentage of the singlet reaction depends on how fast it can occur 5-methyl-2-hexanone (55), which has a relatively weak y-C—H bond, reacts mostly from the singlet state, but 2-pentanone (56) reacts mostly from the triplet.90... 

It is not surprising to us that the current CIDEP study was not able to pinpoint the role of the excited singlet reaction. Nevertheless, the results of the solid state experiments at 77 K discussed earlier support the recent CIDNP product analysis of similar lignin model reactions which strongly indicate a primary direct 0-cleavage reaction from the excited singlet state (9,11-12). 

Before the concerted vs. two-step question was further elucidated, another basic mechanistic puzzle was raised. One research group found that type II cleavage of 2-pentanone was quenched by biacetyl [6], which was known to quench excited triplets rapidly. Another group found that the reaction of 2-hexanone was not quenched under the same conditions [7]. The two groups obviously differed as to which excited state undergoes the reaction. The apparent conflict was neatly solved by the revelation that each of the two ketones reacts from both states, with 2-hexanone undergoing more unquenchable singlet reaction than 2-pentanone [8,9]. 

There is a well established way to enforce only excited singlet reaction, namely the inclusion of a triplet quencher at a concentration sufficient to... 

The selectivity C/D is controlled by the geometry of the conical intersection for the singlet reaction and by the optimal ISC-geometry of the 2-oxatetramethylene biradical (3X) for the triplet reaction. The situation is described for the propionaldehyde/2,3-dihydrofuran photocycloaddition reaction. At low concentrations (0.005 M, triplet conditions), the diastereo-selectivity (endo-159 / exo-159) approaches a maximum endo/exo value of 85 15 (Sch. 56). At high concentrations (0.5 M, singlet conditions), the diastereoselectivity decreased to 52 48. 

The photochemistry of tethered alkenes is more predictable than the nontethered situation. There is evidence of Ti-stacking for closely held moieties which presumably improves the orbital interactions between the alkenes. Exciplex formation is likely involved when the reacting groups are within 5 angstroms. Cornil et al. have shown that k systems can couple within this distance [11]. This exciplex could lead to a concerted cycloaddition from the excited state which would be consistent with the observed products. Although stepwise addition (see Sch. 3) cannot be ruled out even in these tethered singlet reactions. Ring closure must be very rapid if diradicals are involved, since no radical-trapped species have been found. 

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