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Triplet Carbenes and Nitrenes

Triplet carbene and nitrene C—H insertion reactions occur with loss of stereochemical purity around the C from which the H is abstracted, in contrast to [Pg.247]

The singlet form of carbenes substituted with a carbonyl group is much lower in energy than the triplet, so the photolysis of a-diazocarbonyl compounds gives singlet carbenes. The photo-Wolff rearrangement (Chapter 2) of a-diazoketones occurs upon photolysis of these compounds. [Pg.248]

Upon photolysis of a diazo compound or an azide, a triplet carbene or nitrene is generated. The two unshared electrons in triplet carbenes have aligned spins and [Pg.262]

The symbol R2C should not be used to describe triplet carbenes. The in represents a pair of electrons, but the two electrons in triplet carbenes are unpaired. The symbol R2C can be used to describe either a singlet or triplet carbene. [Pg.263]

Methyl tert-butyl ether, MTBE, and ethyl tert-butyl ether, ETBE, are added to gasoline in order to increase the efficiency of gasoline combustion, which reduces the quantity of volatile organic compounds (VOCs) that escape into the atmosphere and cause smog. The chemical industry is very interested in using MTBE as a substitute for THF and diethyl ether, because autoxidation of THF and diethyl ether is a major safety problem for chemical companies. The industry is less interested in ETBE as a solvent substitute. [Pg.264]

The production of chlorofluorocarbons, or CFCs, has been banned by international treaty because of their deleterious effect on the ozone layer. The ozone layer absorbs much of the Sun s dangerous UV radiation before it reaches the Earth s surface. CFCs are extremely stable in the lower atmosphere (one reason why they are so useful), but when they reach the stratosphere they decompose, producing potent catalysts of ozone destruction. Ozone destruction is most evident above Antarctica during the spring, when this region is exposed to the Sun for the first time in months. Dichlorodifluoromethane (CF2CI2) is a typical CFC. [Pg.265]


Simple MO pictures also explain the differences in the bond dissociation energies (BDE) of methane and ammonia, and explain different triplet carbene and nitrene reactivities toward hydrogen atom donors. [Pg.505]

ESR parameters for triplet carbenes and nitrenes have been summarized, and it has been shown that phenylnitrene is produced predominantly (87-88%) in the singlet state by direct photolysis of phenyl azide in low-temperature matrices. The first spectroscopic observation of a singlet nitrene has been reported nanosecond-laser photolysis of 1-azidopyrene gives the So nitrene (A ax 450 nm) which has a lifetime of 22 nsec at room temperature (in benzene) and 34 nsec at 77 K in rigid solution. At room temperature it decays to the triplet ground state (Tj, A ax 415 nm) with a rate constant of about 4.4 x 10 sec T, is formed directly by biacetyl sensitized photolysis of the azide. The lifetime of the excited triplet (Tj) was about 7 nsec. Tj dimerizes to azopyrene. ... [Pg.233]

Figure 11 shows some of the geometrical parameters (computed at the MCSCF level of theory) for the five electronic states of 33. The local geometries of the carbene and nitrene subunits (bond angle at the carbene center, C—bond length of the carbene and the bond lengths between the diradical centers and the benzene ring) of the quintet state ( A ) are very similar to that of triplet phenylcar-bene and phenynitrene computed at the same level of theory (Fig. 12). The situation is less clear for the other two A states ( A and A ), but the geometry of... Figure 11 shows some of the geometrical parameters (computed at the MCSCF level of theory) for the five electronic states of 33. The local geometries of the carbene and nitrene subunits (bond angle at the carbene center, C—bond length of the carbene and the bond lengths between the diradical centers and the benzene ring) of the quintet state ( A ) are very similar to that of triplet phenylcar-bene and phenynitrene computed at the same level of theory (Fig. 12). The situation is less clear for the other two A states ( A and A ), but the geometry of...
When R = H in 25 the species is a protonated nitrene. Like carbenes and nitrenes, nitrenium ions can exist in singlet or triplet states.249... [Pg.204]

At least two types of nitrenium ions the nitrogen analogs of carbocations, can exist as intermediates, although much less work has been done in this area than on carbocations. In one type (65), the nitrogen is bonded to two atoms (R or R can be and in the other (66) to only one atom. " When R = H in 65 the species is a protonated nitrene. Like carbenes and nitrenes, nitrenium ions can exist in singlet or triplet states." ... [Pg.295]

When one estimates the heats of formation of aromatic nitrenes and carbenes from those of NH and CH2 with the aid of group additivity, the nitrenes will automatically become 10—15 kcal/mol more stable than the isomeric carbenes. These estimates are of necessity for the triplet species we do not know the resonance energies in the singlets. Nor do we know the singlet-triplet splittings, which may be different in carbenes and nitrenes. We have, therefore, also performed semi-... [Pg.242]

In this account httle has been said about the spin states of the carbenes and nitrenes which react, and it hcis been tacitly assumed that rearrangement occurs from singlet states, whereas amine and azobenzene formation most probably occurs from triplets. Since we are dealing with thermal reactions, the law of spin conservation 88) would predict that the first-formed species are singlets. However, this is claimed not to be absolutely necessary, for the direct formation of triplets. [Pg.246]

The reactivity and lifetime of triplet PN and triplet PC differed remarkably in the inner phase of 6. Whereas PC reacts with 6 already at 15K and persists probably only a few minutes at this temperature, the lifetime of incarcerated PN is 13.6min at -3°C. Both PC and PN preferentially insert into inward-pointing acetal C-H bonds of 6 to produce 115 and 134, respectively (Figures 41 and 43). The difference in reactivity toward C-H insertion between triplet carbenes and an isoelectronic triplet nitrenes is well known and is believed to proceed through a nitrogen rehybridization in the rate-limiting H-abstraction step of the nitrene. Rehybridization is not needed in the carbene reaction. [Pg.921]

Abstract In the examples of the irreversible reactions, the original molecule is broken down on exposure to UV light and releases a part of the molecule to become a new prodnct. The reverse reaction cannot occur. The triplet carbenes are formed from the bis(2,4,6-triphenyl)diazomethane derivatives and the arylnitrenes are produced from the arylazide derivatives on exposure to UV hght The unstable carbenes and nitrenes were observed after the crystal was irradiated with UV light at 80 K. Moreover, the further reaction processes were observed from the intermediate nitrenes to the final products, azobenzene derivatives or five-membered ring derivatives. The acid-base complex formation between arylazides and amine derivatives is very effective to observe the reaction processes of nitrenes. [Pg.187]

The generation and the evaluation of the singlet-triplet gap have been investigated by DFT (density functional theory) and CCSD(T) calculations for an array of carbenes and nitrenes. ... [Pg.179]

The forward reaction (ring formation) is limited to the reactions of high energy species such as carbenes and nitrenes with the ethylenic molecule. Concerted cyclo-addition reactions are limited to a few singlet carbenes ( CRa) and nitrenes ( NR) and these reactions are completely stereospecific. Equation (6.125) represents the general situation, and lack of stereospecificity is usually taken as evidence for the incursion of triplet reactions. [Pg.198]

In contrast, the much lower enthalpy computed for 3 lb, compared to 3lc, means that the N-H BDE of the anilinyl radical 8b is much lower than the C-H BDE of the 3-pyridylmethyl radical 8c. The results in Table 5 show that this is indeed the case, not only for R=Ph and R =3-pyridyl, but also for R=R =Ph and R=R —H.77 The data in Table 5 indicate that, not just for lb and lc but in general, triplet nitrenes are ca. 20 kcal/mol more thermodynamically stable than comparably substituted triplet carbenes. [Pg.237]

In the case of carbenonitrenes, the nitrene subunit has an intrinsic preference for a triplet configuration, and, thus, for a A" ground state, a carbene with a very strong preference for a closed-shell singlet ground state is required. Such a case has not been realized yet. On the other hand, if both subunits are carbenes, then. [Pg.158]

The states can be approximated as the combination of a triplet carbene with a nitrene in which its ct- and -rr-electrons have opposite spins (Fig. 14). If the latter configuration is taken to correspond to an open-shell singlet nitrene (as... [Pg.169]

Nitrenes,239 R—N, are the nitrogen analogs of carbenes, and most of what we have said about carbenes also applies to them. Nitrenes are too reactive for isolation under ordinary conditions. Alkyl nitrenes have been isolated by trapping in matrices at 4 K,24n while aryl nitrenes, which are less reactive, can be trapped at 77 K.241 The ground state of NH, and probably of most nitrenes,242 is a triplet, though nitrenes can be generated in both triplet and singlet states. In additions of EtOOC—N to C=C double bonds two species are involved,... [Pg.202]


See other pages where Triplet Carbenes and Nitrenes is mentioned: [Pg.194]    [Pg.194]    [Pg.262]    [Pg.263]    [Pg.263]    [Pg.247]    [Pg.247]    [Pg.194]    [Pg.194]    [Pg.262]    [Pg.263]    [Pg.263]    [Pg.247]    [Pg.247]    [Pg.37]    [Pg.254]    [Pg.453]    [Pg.237]    [Pg.274]    [Pg.829]    [Pg.37]    [Pg.37]    [Pg.129]    [Pg.13]    [Pg.37]    [Pg.95]    [Pg.247]    [Pg.248]    [Pg.253]    [Pg.168]    [Pg.176]    [Pg.453]    [Pg.488]    [Pg.20]    [Pg.219]    [Pg.505]    [Pg.176]    [Pg.35]    [Pg.176]   


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And carbenes

Carbenes and nitrenes

Nitrene

Nitrene, 117 carbene

Nitrenes

Nitrenes carbenes

Triplet carbene

Triplet carbenes

Triplet nitrenes

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