Singlet state of carbenes

Alkyl groups are attractive kinetic protectors for triplet carbenes. However, they are potentially reactive toward triplet carbenes and, hence, will not be able to shield the reactive center completely. In this respect, we need to explore protecting groups that are almost completely unreactive toward triplet carbenes. Halogens are generally reactive toward the singlet state of carbene but are not reactive with the triplet. 

Moderate amounts of nondynamic correlation are also handled surprisingly well by CC calculations. Thus, even if CASSCF indicates significant contributions from excited configurations, P, to the correlated ground state wavefunction, Pg-, this does not always adversely affect the CC results, particularly those obtained at the CCSD(T) level. Good examples are provided by CC calculations on closed-shell singlet states of carbenes, where a doubly excited... 

Thermal decomposition of alkyldiazirines proceeds through the lowest singlet state of the carbene 79JA5082). 

If the reaction rates of a specific carbene with various quenchers are studied in the same solvent, and with small concentrations of Q, K will be constant. Relative reactivities for the singlet state of a spin-equilibrated carbene can thus be derived. However, few researchers have varied the acidity of ROH, estimated kinetic isotope effects, and compared alcohols with ethers (Table 4). The data indicate proton transfer to diarylcarbenes (139d, 139k, 205, 206)112-117 and diadamantylcarbene (207).118... 

Phenylcarbene (la). Just as in triplet methylene (CH2), in triplet phenylcarbene (3A"-la) one electron occupies the p-jr atomic orbital on the carbene carbon and one electron occupies the in-plane a hybrid orbital. However, in the lowest singlet state of CH2 and of phenylcarbene ( A -la), both electrons occupy the hybrid a orbital, because this orbital is substantially lower in energy than the p-jt AO. 

An important conclusion, independent of the actual magnitude of AGSX, is that the electron donating methoxy-substituents on the fluorenylidene nucleus lower the energy of the singlet state of the carbene more than that... 

Time-Resolved IR Spectroscopy. More recently, time-resolved IR (TRIR) experiments have been used to characterize species with lifetimes of micro-and even nanoseconds. Since IR spectra provide structural information in more detail than UV, this technique will be more powerful than TRUV-vis if one can find a carbene that can be detected and studied by this technique. To date, however, only one carbene has been studied by using TRIR. The matrix IR study shows that the planar triplet and twisted singlet states of 2-naphthyl(methoxycartbonyl) carbenes (NMC, 17) show distinctly different IR bands (see Section 3.1.4). Both NMC and NMC are detected by TRIR in solution and their kinetics have been studied. Such experiments provide clear cut data for the reaction kinetics as well as energetics of both states (see Sections 4.2 and 4.3... 

Huron and Platz recently smdied the photochemistry of 13 in solution by LFP. The triplet state of 19 absorbs at 400 nm in 1,1,2-trifluorotrichloroethane with a lifetime of 1-2 ps. The triplet is formed within 10 ns of the laser pulse. Relaxation of the singlet to the triplet state of 19 is fast relative to the related process in aryl-nitrenes and is comparable to a carbenic process. As we will see later when we discuss intersystem crossing rates of singlet arylnitrenes, this difference is most likely due to the closed-shell electronic configuration of the singlet state of 19. 

In all cases, the calculations identify the singlet species as the more stable one at the PMP-4 level, the singlet states of FpP—C—SiFp and (FpNpP—C—SiFp are 5.6 and 13.9 kcalmol-1, respectively, below the triplet states. Thus, all computational evidence points to a description of (R2N)2P—C—SiR3 as a species with a P,C multiple bond, with an emphasis on the ylide structure II in which both charges are stabilized by the respective neighboring substituents. An X-ray structure determination of one of the isolable [bis(dialkylamino)phosphino](silyl)carbenes is not yet available but, as will be shown in... 

Structural Parameters and Multiplicities of Lowest Singlet (a2) and Triplet (ap) States of Carbenes RR C Predicted by Extended Hiickel Theory ... 

These results were interpreted with reference to an intramolecular heavy atom effect the authors concluded the carbene is formed from the excited singlet state of the molecule, in competition to intersystem crossing. A direct proof of this statement could, however not be obtained [24], It should be noted here that alternative explanations for the effect of halogen substitution on photolysis quantum yields have been put forward for the 4-haloanilines (see below). 

In contrast to the carbene-chemistry there is ample evidence that in some cases the singlet state of nitrene will be stabilized by inert solvents, e.g. CeFe and CH2CI2. The stabilization of nitrene singlet states by symmetrical interaction of the nitrene with two lone pairs has been proposed by Gleiter and Hoffman 95>. 

Tlie decomposition of diazo compounds can be run under very difierem conditions. Their photochemical decomposition has been largely investigated in the gas phase and in solution in the presence of olefinic and saturated hydrocarbons as substrates the fundamental concepts concerning the relative reactivity and selectivity of singlet-triplet states of carbenes are essentially the result of such studies and they have formed the basis for the determination of the correla tion between the spin state of the carbene and its selectivity and reactivity in imerliun and cycloaddition reactions. 

Table 6.2 Calculated energy difference between singlet and triplet states of carbenes, A s t [89, 90]...

The TTyir singlet states of the cyclopropenes (101) are reactive and lead to re2irrangement products by way of a carbene (102) mechanism as outlined above. Typical examples of the reaction are shown in the Scheme 8, where it can be seen that the rearrangement affords the isomeric substituted furans (103) and (104) from (101, X = 0) and pyrroles (105) and (106) from (101, X =NMe). This latter reaction also yields the diene-substituted pyrrole (107), which is good evidence for the proposed carbene mechanisms. The thiophene derivative (101, X = S) yields a single product (108) on photoexcitation. 

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