Isomerization of carbene

Diazirines were detected when there was broad activity in the carbene field. From their structure, cleavage to nitrogen and carbene was foreseeable, and this was shown to occur on photolysis as well as on thermolysis. As early as 1962, Frey and Stevens in a series of papers reported on photolysis of simple diazirines. According to these authors, diazirines are especially fit for the study of excited intermediates and their stabilization products. Products of isomerization of carbenes, i.e., olefins and cyclopropanes, are formed containing more energy than is necessary for their further decomposition. Their stabilization by loss of energy to partners competes with stabilization by subsequent reactions. 

Kinetic investigations of alkylchlorodiazirine thermolysis were carried out in the gas phase 70JCS(A)1916). Chloromethyldiazirine (232) decomposition follows first order kinetics giving nitrogen and vinyl chloride, easily interpretable as isomerization of a carbene. 

Photolysis of chromium alkoxycarbenes with azoarenes produced 1,2- and 1,3-diazetidinones, along with imidates from formal azo metathesis (Eq. 21) [85, 86]. Elegant mechanistic studies [87-89] indicated the primary photoprocess was trans-to-cis isomerization of the azoarene followed by subsequent thermal reaction with the carbene complex. Because of the low yields and mixtures obtained the process is of little synthetic use. 

Nevertheless, a more traditional approach to the stabilization of carbenes and the investigation of their spectral properties deals with the direct generation of carbenes in low-temperature matrices, e.g. by the photolysis of diazo-compounds or ketenes. The method allows stabilization of carbenes in their ground electronic state, prevents intramolecular isomerization and also facilitates direct spectroscopic monitoring of their chemical transformations in low-temperature matrices. 

UV photolysis (Chapman et al., 1976 Chedekel et al., 1976) and vacuum pyrolysis (Mal tsev et al., 1980) of trimethylsilyldiazomethane [122]. The silene formation occurred as a result of fast isomerization of the primary reaction product, excited singlet trimethylsilylcarbene [123] (the ground state of this carbene is triplet). When the gas-phase reaction mixture was diluted with inert gas (helium) singlet-triplet conversion took place due to intermolecular collisions and loss of excitation. As a result the final products [124] of formal dimerization of the triplet carbene [123] were obtained. 

Gault and coworkers [ 149] have observed that the distribution of products obtained by hydrogenolysis and isomerization of methylcyclopentane was the same as those obtained with hexane. They proposed two competing mechanisms a selective mechanism implying an a, a, p, j0-tetra-adsorbed species and a non-selective mechanism implying coordinated olefin and bis-carbene intermediates (Scheme 38). 

The cis-trans isomerization of alkenyl ligands in transition metal alkenyl compounds is proposed to occur via zwitterionic carbene intermediates.46 According to this, the low contribution of the form b to the metal-dienyl bond in Os (Z)-CH=CHC(Me)=CH2 K1-0C(0)Me (C0)2(P,Pr3)2 could explain why this compound does not evolve into its (if )-isomer. 

Photolysis of acyldisilanes at A > 360 nm (103,104) was shown, based on trapping experiments, to yield both silenes 22 and the isomeric siloxy-carbenes 23, but with polysilylacylsilanes only silenes 24 are formed, as shown by trapping experiments and NMR spectroscopy (104,122-124) (see Scheme 4). These silenes react conventionally with alcohols, 2,3-dimethylbutadiene (with one or two giving some evidence of minor amounts of ene-like products), and in a [2 + 2] manner with phenyl-propyne. Ketones, however, do not react cleanly. Perhaps the most unusual behavior of this family of silenes is their exclusive head-to-head dimerization as described in Section V. More recently it has been found that these silenes undergo thermal [2 + 2] reactions with butadiene itself (with minor amounts of the [2 + 4] adduct) and with styrene and vinyl-naphthalene. Also, it has been found that a dimethylsilylene precursor will... 

We theoretically studied the reactions of stable West silylenes 32 and 73 with phosphorus ylide H2C=PMe3.74 Similarly to the simplest analogs of carbenes, these compounds can form betaines in which the negative charge is localized on the silicon atom and the positive charge is localized on the phosphorus atom. These betaines can thermally decompose to form silenes (direction A, Scheme 39) or be isomerized to ylides via direction B. 

The rapid isomerization of c/s-4-methyl-2-pentene relative to productive metathesis suggests further information regarding the isomerization process. If regenerative metathesis proceded selectively via an isopropyl carbene. and assuming that the isopropyl groups maintained equatorial... 

Alternatively, if regenerative metathesis involved preferentially a methyl-substituted carbene, the prefered metallocyclobutane would favor isomerization of the starting olefin. 

Irradiation causes ring closure by valence isomerization of 1,3-diphosphacyclobutane-2,4-diyl 42 (R = 2,4,6-tri(tert-butyl)phenyl) to 2,4-diphosphabicyclo[1.1.0]butane 43 which on thermolysis yielded the gauche-1,4-diphosphabutadiene 44 <99AG(E)3028>. The same group of workers have isolated the carbene 45 (R as above) as the lithium salt of a trimethylalane complex 46 <99AG(E)3031>. 

The reaction of carbenes with alcohols can proceed by various pathways, which are most readily distinguished if the divalent carbon is conjugated to a tt system (Scheme 5). Both the ylide mechanism (a) and concerted O-H insertion (b) introduce the alkoxy group at the originally divalent site. On the other hand, carbene protonation (c) gives rise to allylic cations, which will accept nucleophiles at C-l and C-3 to give mixtures of isomeric ethers. In the case of R1 = R2, deuterated alcohols will afford mixtures of isotopomers. 

It was obvious, that the parent carbene 21 should be isolable in an argon matrix at 10 K. The barrier for the isomerization of 21 to imidazole 22 is calculated (B3LYP/6-311G(d,p)) to be rather high (41.5 kcal mol-1).45 Because of the interaction of the nitrogen lone pairs with the empty p-orbital of the car-benic center, carbene 21 should be a singlet molecule (S/T-gap = 81.6 kcal mol-1). 

It was also found that the ring expansion could be accomplished photo-chemically, from either phenyldiazomethane or triplet phenylcarbene.7 Both the thermal and photochemical ring expansions were found to be reversible,5c, thus providing rare examples of carbene-to-carbene interconversions. One remarkable example of this reversibility is the interconversion of the isomeric tolylcarbenes upon pyrolysis — the ultimate products of which include styrene and benzocyclobutene (Scheme 3).6,8,9... 

Cycloaddition of Carbenes to conjugated Nitro Olefins (28). From the above it is evident that there is another synthetic route to nitronates (24 g) with the use of carbenium intermediates based on [1 + 2]-cycloaddition of carbenes RR C to conjugated nitro olefins (28) followed by isomerization of intermediate nitrocyclopropanes (23 g). However, this strategy was used only in one study (see Scheme 3.30). 

A similar isomerization of an allenyl ketone, catalyzed by a Cr(CO)sL complex, is most probably the mechanistic key step of the palladium-catalyzed conversion of chromium carbene complexes and propargyl bromide to furans. In control experiments different aryl and alkyl allenyl ketones 96 isomerized to the furans 99 in the presence of 10 mol% of Cr(CO)5(NEt3) in good yields (Scheme 15.31) [70],... 

These isomerization reactions are of great interest to theoreticians because the role of many factors (metal, substituents on the organic fragment, ancillary ligands) on the outcome of the reaction can be studied through computations. The purpose of this chapter is to describe the theoretical studies carried out on the isomerization of alkyne to vinylidene and alkene to carbene in the presence of transition metal fragments. 

C(sp2)-H energies are close. Likewise, the transformation of an alkene to a substituted carbene corresponds mostly to the loss of the it component of a C=C double bond. The loss of these bonds is clearly costly in energy but a key point to the present story is that the energy cost is different for the two systems. Loss of one of the it bonds of an alkyne corresponds to 40-50 kcal.mol 1, whereas the loss of the it bond in an alkene amounts to 70-80 kcal.mol 1. Therefore the isomerization of primary alkyne to vinylidene and of alkene to substituted carbene are both endothermic with the latter having the larger endothermicity. 

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