Addition triplet carbenes

There is a limitation to the use of stereospecificity of addition as a diagnostic test for singlet or triplet carbenes.When carbenes are generated by photolytic methods, they are often in a highly excited singlet state. When they add to the double bond, the addition is stereospecific but the cyclopropane formed carries excess... 

Studies have shown that carbene reactivity toward a wide variety of substrates is dramatically affected by the nature and multiplicity of the electronic state. - Similarly, the structure, electronic state, thermochemical stability, and reaction kinetics of both singlet and triplet carbenes can be significantly affected by the R-substituents. If R provides steric hindrance, the carbene center can be shielded to slow down inter-molecular reactions (kinetic stabilization). Additionally, bulky and/or geometrically... 

Addition reactions with alkenes to form cyclopropanes are the most studied reactions of carbenes, both from the point of view of understanding mechanisms and for synthetic applications. A concerted mechanism is possible for singlet carbenes. As a result, the stereochemistry present in the alkene is retained in the cyclopropane. With triplet carbenes, an intermediate 1,3-diradical is involved. Closure to cyclopropane requires spin inversion. The rate of spin inversion is slow relative to rotation about single bonds, so mixtures of the two possible stereoisomers are obtained from either alkene stereoisomer. 

A singlet carbene was proposed to account for this stereoselectivity. Attempts to produce triplet carbene by collisional deactivation with octafluorocyclo-butane were unsuccessful and stereospecific addition to olefin still occurred. However, nonstereospecific addition to olefins and larger amounts of olefinic (insertion) products result from irradiation of the phenyldiazomethane in a frozen m-butene matrix at — 196°C ... 

From an historical point of view, the earliest indication of spin-selective reactivity of carbenes was exhibited by the stereochemistry of the cyclo-propanation reaction. The Skell Hypothesis (Skell and Woodworth, 1956) suggests that a spin-prohibition requires the addition of a triplet carbene to an olefin to occur in at least two steps. In turn, the obligatory formation of an... 

Examination of DABA photolysis in cyclohexane instead of benzene solution leads to predictably different results. The laser spectroscopy shows that 3BA is formed but, in this solvent, the triplet carbene undergoes an additional rapid reaction to generate the mesitylbora-anthryl radical (BAH ). This radical is identified by comparison of its spectrum with that of an authentic sample prepared from dihydrobora-anthracene. The half-life of... 

The addition of a triplet carbene is similar to the addition of atomic oxygen. 

Photolysis of diazoaJkanes in liquid phase yields carbenes in a vibrationally relaxed state, since deactivation in solution immediately removes all excess vibrational energy. The addition of carbenes to the olefins, which results in nonstereospecific formation of cyclopropanes, must therefore result from the different multiplicity of carbenes — singlet or triplet. Since most of these multiplicity... 

Table 7 is a synopsis of cycloaddition reactions of various triplet carbenes to cis- or trans-olefins which result in nonstereospecific additions. 

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. 

In contrast to 2-alkylarylcarbenes, triplet carbonyl carbenes do not abstract H from 5- or e-CH bonds. Photolysis of diazo compounds (7) in methanol gave products due to Wolff rearrangement (8) and 0-H insertion (9). Sensitized photolysis led, in addition, to the H-abstraction product (10). Analysis of the results indicated that a large proportion of the insertion product (9) arises from the excited diazo compound and that spin inversion of the triplet carbene is faster than H-abstraction from the solvent. Intersystem crossing to the singlet state is a major reaction of all triplet carbonyl carbenes that are not rapidly scavenged intramolecularly. 

Biscarbene 34 was characterized by IR and UV/vis spectroscopy [49], The analysis of the experimental data showed that these are compatible with the presence of two phenylchlorocarbene (6) subunits in 34. This interpretation was further supported by the reactivity behavior of 34, which, like 6, is unreactive toward oxygen under conditions where triplet carbenes react fast. In contrast to its para isomer (22), 34 appears to undergo photochemical ring expansion analogous to that of 6[105]. In addition, the computed [RHF/6-31G(d)] IR spectrum of 34, which is in good agreement with the observed one, is based on the wave function for the singlet (cr /cr ) biscarbene (54 of Fig. 9). 

Fig. 10.1. Mechanisms for addition of singlet and triplet carbenes to alkenes.

Surprisingly, the critical experiment has been done infrequently over the last one-half of a century The requirements for an experiment that truly speaks to the issue at hand are that one be able to see the results of addition of both spin states of a single carbene, and these requirements rarely have been met. For example, the direct irradiation of methyl diazomalonate leads to the stereospecific addition expected of a singlet carbene, whereas the photosensitized decomposition of the diazo compound leads to formation of the triplet carbene and loss of the stereochemical relationship originally present in the reacting alkene. Rotational equilibration in the intermediate seems to be complete, as it makes no significant difference whether cis or trans alkene is used as starting material (Scheme 7.9). ... 

Assignment of the transient absorption spectrum typically is done by reference to the low-temperature spectra described above. It is also important to analyze the chemical reactions of the intermediates. For example, triplet carbenes are known to react with O2 very efficiently to give ketones. So, if the detected intermediate can be observed to react with O2, there is additional evidence for assignment as a triplet carbene (see Section 6.5). 

Product Studios. Alkenes are known as diagnostic reagents for spin state of reacting carbenes. Thus, the reaction of a singlet carbene with an olefin usually results in the formation of a cyclopropane through stereospecific addition to the C—C double bond, while a triplet carbene gives rise to a nonstereospecific addition product (see Section 7). 

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... 

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