2-naphthylcarbenes

Scheme 27 Cocyclization of the 1-naphthylcarbene complex 128 with ferf-butylphospha-alkyne 129 [91]...

This effect was first observed for the pairs of 1- and 2-naphthylcarbenes (12). ° Since then, reports of geometric isomerism in triplet carbenes have appeared with increasing frequency two sets of triplet signals having similar but nonidentical ZFS parameters are observed. The spectra are assigned to the two conformations of the carbene in which the ct orbital at the divalent carbon and the aromatic moiety are coplanar. When the distribution of the spin in the 7t orbital is unsymmetric, the dipole spin-spin interaction of the p electron with the electron localized in the CT orbital is different for the two conformations. Consequently, the ZFS parameters will be different and in cases in which the differences are sufficiently large, it is possible to observe the spectra of the two isomers. 

IR bands for the both states. For example, singlet and triplet states of 2-naphthyl-(methoxycarbonyl)carbenes (NMCs, 17) show very different IR spectra (Scheme 9.4) (1590, 1625, 1640 cm for NMC and 1660 cm for NMC). Both states are observable in this case. In the singlet state, the carbomethoxy group assumes a conformation perpendicular to the naphthylcarbene plane to avoid destabilization of the empty carbenic 2p atomic orbital by the electron-withdrawing carbonyl group, while in the triplet, the methoxycarbonyl group is in the same plane to delocalize an unpaired electron. For this reason, there is a barrier between the two states and hence both of them are observable under these conditions. 

Treatment of (205) with a large excess of potassium t-butoxide in THF leads to ethers and related compounds their formation may be rationalised in terms of the formation of a cyclopropene (206) which can undergo a complex set of rearrangements to naphthylcarbenes which are then trapped by alkoxide ion l41, l43). 

The benzannulation of naphthyl carbene complexes may be used as a direct route to functionalized phenanthrenes (Scheme 34) [37a]. While it is obvious that 1-naphthyl carbene complexes such as 87 lead to phenanthrenes 88 (see also Section 8.3.2, Scheme 19), chromium 2-naphthylcarbenes 89 offer two alternatives for annulation, giving either phenan-threne or anthracene derivatives. Generally, angular benzannulation affording phenanthrenes 90 is favored over linear benzannulation to give anthracene derivatives 91. A rationale for this regiopreference is the higher electron density at C-l compared with that at... 

With respect to the arylcarbenes, however, the carbene addition is not stereospecific the aryl-cyclopropanes are generally formed as mixtures of diastereomers. In almost all cases reported, the sj -isomer predominates, and this is the case even when the corresponding uw/z-isomer is far less sterically crowded. Thus, when 1-naphthylcarbene reacted with cyclopentene, the corresponding cyclopropane 6-(l-naphthyl)bicyclo[3.1.0]hexane (4) was obtained in 62% yield with endojexo ratio of 84 16. ... 

With respect to the carbene, arylcyclopropane formation occurs with rather low stereospecificity. In most cases the ewt/o-isomer predominates, which is in accordance with the general observation made when arylmethylenes from other sources are added to alkenes. Thus, photolysis of phenyldiazomethane dissolved in cyclohexene gave 7-phenylbicyclo[4.1.0]heptane (2) in 34% yield with a endojexo ratio of 1.1. Preference for the e (/o-isomer is also observed when monoarylcarbenes are reacted with cyclohepta-l,3,5-triene, but when 1-naphthylcar-bene, 2-naphthylcarbene and 9-anthrylcarbene are reacted with cycloocta-l,3,5,7-tetraene the exo-isomer predominates. ... 

The dihydropyrroles are conceivably formed via a nitrile ylide, an intermediate resulting from a reaction between 1-naphthylcarbene and acetonitrile. 

Cyclizations ofcarbenes and nitrenes to form four-membered rings are extremely rare, but when no other options are available for the reactive species, such processes will occur. Thus, cyclobuta[de] naphthalene is formed in better than 40% yield from 1-naphthylcarbene in the gas phase. The same... 

Although the ring-contraction paths followed by phenylcarbene and phenylnitrene are different, it is noteworthy that the naphthylcarbenes take a course not without similarity to phenylnitrene both ring expansion and... 

These reactions are very interesting from the point of view of synthesis of novel heterocyclic compounds. Analogs are not found in the isosteric car-benes, however. 2-Diazomethylpyridine N-oxides (330) give rise to 2-acyl-pyridines (332) by photolysis or thermolysis.373,374 In view of the surprisingly facile formation of cyclobuta[de]naphthalene from 1-naphthylcarbene (see Eq. 69) the intermediate 10 jr-electron pyridooxazete 331 does not seem unreasonable (Eq. 94). Other pathways are possible too, however.273... 

Irradiation of the benzodiazocycloheptatriene (15) (Scheme 4) in argon matrices at 10 K provided access to the naphthylcarbene rearrangement manifold and generated primarily (17), together with minor amounts of the carbenes (16) and (18). ° A reversible photoequilibrium was established in which (17) was converted into (18) at 290 nm and regenerated by light of X = 360 nm. Similar photolytic behaviour was observed for (19). 

It is known that most carbenes, including arylcarbenes, undergo intermolecular electrophihc addition to double bonds 87). Then we might expect that if arylcarbenes cyclize intramolecularly to bicycloheptatrienes [e.g. Eqs. (12), (13)], this should be an electrophilic addition, and the carbene should add preferentially to the double bond of highest bond order 68>. Gas-phase experiments with substituted naphthylcarbenes indicated that such was the case 88). 

Thus, the 2-naphthylcarbene 76 added only to the 1,2-double bond of naphthalene, leading to 2-vinylnaphthalene (77). Carbene 78 did not add to the wesiker... 

The LUMO energy in naphthalene is lower than in benzene, and the carbene can now act as a nucleophile with respect to both rings. The naphthylcarbenes always prefer to attack the 1-position in naphthalene which has the highest electron density and the lowest localization energy for electrophilic substitution. 

LFP of 1-naphthyldiazomethane (10) in acetonitrile, the first solvent we studied, produced a transient spectrum with X = 386 nm. The transient was formed in an exponential process with a time constant of 100 10 ns and decayed over several microseconds. Our initial impulse was to attribute the transient absorbing at 386 nm to triplet 1-naphthylcarbene formed from invisible singlet 1-naphthylcarbene 11 with a time constant of 100 ns. [21]... 

Ylides of singlet 1-naphthylcarbene ( 11) with acetone, triethylamine and pyridine were soon observed. [21] Fluorenylidene was also found to react with coordinating solvents to form ylides. [22]... 

The transient spectra of neither singlet nor triplet 1-naphthylcarbene have ever been observed upon LFP of 10. The kinetics of these invisible carbenes were obtained from the growth of their ylide reaction products and not by direct observation of either singlet or triplet 1 -naphthylcarbene. 

The study of the 1-naphthylcarbene system is an example of the use of probe kinetics, first employed in great detail by Scaiano and coworkers in the study of radicals and biradicals. [23]... 

The situation with 1-naphthylcarbene is by no means unique. The invisibility of singlet and triplet non-aromatic carbenes makes studying their kinetics even more challenging. In fact, in our opinion the most interesting carbenes lack useful chromophores for laser flash photolysis studies. 

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