Intramolecularly Stabilized

Hydroxymethylmethyldiazirine (209 unprotonated) formed propionaldehyde as the sole product by thermal nitrogen extrusion 4-hydroxy-l,2-diazaspiro[2.5]oct-l-ene (218) formed a mixture of cyclohexanone (73%), cyclohexenol (21%) and cyclohexene oxide (5%). Thermal decomposition of difluorodiazirine (219) was investigated intensively. In this case there is no intramolecular stabilization possible. On heating for three hours to 165-180 °C hexafluorocyclopropane and tetrafluoroethylene were formed together with perfluorofor-maldazine 64JHC59). 

Difluoroaminofluorodiazirine (225) extrudes nitrogen at only 75 °C. Intramolecular stabilization gives trifluoromethylenimine (226) added tetrafluoroethylene is cyclopropa-nated. This type of dichotomy is not often found in carbene chemistry alkylcarbenes undergo intramolecular stabilization as a rule, whereas intermolecular stabilization is observed exclusively with alkoxycarbonylcarbenes and with difluorocarbene. In the latter case CF2 attacks its precursor when no other reaction partner is present. 

In chlorodiazirine photolysis there is also evidence of competition between intramolecular stabilization and reaction with added chemicals. Apparently carbenes are stabilized by... 

A special case of the internal stabilization of a cationic chain end is the intramolecular solvation of the cationic centre. This can proceed with the assistance of suitable substituents at the polymeric backbone which possess donor ability (for instance methoxy groups 109)). This stabilization can lead to an increase in molecular weight and to a decrease in non-uniformity of the products. The two effects named above were obtained during the transition from vinyl ethers U0) to the cis-l,2-dimethoxy ethylene (DME)1U). An intramolecular stabilization is discussed for the case of vinyl ether polymerization by assuming a six-membered cyclic oxonium ion 2) as well as for the case of cationic polymerization of oxygen heterocycles112). Contrary to normal vinyl ethers, DME can form 5- and 7-membe red cyclic intermediates beside 6-membered ringsIl2). 

Thus, the preferred intramolecular stabilization of the cationic chain end by the formation of 5- and 3-membered cycles, which is possible for DME but impossible for vinyl ethers, can explain the characteristics of the cationic polymerization of DME in solvents of low polarity. 

Besides such intermolecular stabilized compounds, intramolecular stabilized compounds have also been reported. Rettig et al. described the synthesis of phosphine-stabilized Al—N monomers by using a tripodal ligand [101]. More recently, Raston et al. reported on the synthesis of compounds of the type [H2A1E(H)R]2, containing only weak Al—H bridges [102]. 

An important contribution to silylium ion chemistry has been made by the group of Muller, who very recently published a series of papers describing the synthesis of intramolecularly stabilized silylium ions as well as silyl-substituted vinyl cations and arenium ions by the classical hydride transfer reactions with PhjC TPEPB in benzene. Thus, the transient 7-silanorbornadien-7-ylium ion 8 was stabilized and isolated in the form of its nitrile complex [8(N=C-CD3)]+ TPFPB (Scheme 2.15), whereas the free 8 was unstable and possibly rearranged at room temperature into the highly reactive [PhSi /tetraphenylnaphthalene] complex. ... 

The reductive coupling of of dienes containing amine groups in the backbones allows for the production of alkaloid skeletons in relatively few steps [36,46,47]. Epilupinine 80 was formed in 51% yield after oxidation by treatment of the tertiary amine 81 with PhMeSiEh in the presence of catalytic 70 [46]. Notably, none of the trans isomer was observed in the product mixture (Eq. 11). The Cp fuMcTIIF was found to catalyze cyclization of unsubstituted allyl amine 82 to provide 83. This reaction proceeded in shorter time and with increased yield relative to the same reaction with 70 (Eq. 12) [47]. Substitution of either alkene prevented cyclization, possibly due to competitive intramolecular stabilization of the metal by nitrogen preventing coordination of the substituted olefin, and resulted in hydrosilylation of the less substituted olefin. 

Unexpectedly, tho hydrolysis of jV-(2,4-dinitrobenzoyl)imidazole at 25 °C was found to be slower by a factor of 25 in comparison to iV-(4-nitrobenzoyl)imidazole. This lower reactivity of iV-(2,4-dinitrobenzoyl)imidazole was explained by a combination of steric crowding at the reaction center and intramolecular stabilization of the reactant state.[19]... 

Figure 3 Structures of analogously substituted, intramolecularly stabilized triorganylmetallanes 46 and 47, illustrating the different coordination modes of gallanes and indanes.

Figure 52 Synthesis and structure of the intramolecularly stabilized dihydroxygallane 847.

Intramolecular stabilization via side-chain donor ligands has also been employed in the nitrogen-stabilized... 

The third class of host defense peptides, the extended peptide class, is defined by the relative absence of a defined secondary structure. These peptides normally contain high proportions of amino acids such as histidine, tryptophan, or proline and tend to adopt an overall extended conformation upon interaction with hydrophobic environments. Examples of peptides belonging to the extended class include indolicidin, a bovine neutrophil peptide, and the porcine peptide fragment, tritpticin. These structures are stabilized by hydrogen bonding and van der Waals forces as a result of contact with lipids in contrast to the intramolecular stabilization forces found in the former peptide classes. 

The bonding situation in the stannanorbornyl cation 62 is reminiscent of the intriguing zwitterions of the types 63 and 64 in which triorganotin cations are intramolecularly stabilized by side-on coordination with an alkynyl group. The zwitterions 63 with a [3-l-1] coordination for the tin atom are characterized by a significantly deshielded Sn NMR resonance and sizeable j(SnC) couplings to the alkynyl carbon atoms (for = alkyl and R = Et 5" Sn = 160-215 J(SnC ) = 41 8Hz, J(SnC ) = 60-80 Coordination with the amino... 

The intramolecular stabilization of germyl cation 34 by a remote aryl substituent was demonstrated by the NMR chemical shifts of the coordinated aryl ring. The chemical shift pattern found for the coordinated arene ring of 34 is characteristic for arenium ions and it closely resembles that found bissilylated arenium ions 78. 

Solid State Structure of Intramolecularly Stabilized Cations... 

Owing to the high Lewis acidity the group 14 organometallic cations are polymerization catalysts par excellence. so Silanorbonyl cations and triethylsilyl arenium have been shown to be efficient catalysts for metal-free hydrosilylation reactions. Chiral silyl cation complexes with acetonitrile have been applied as cata -lysts in Diels Alder-type cyclization reactions °792 intramolecularly stabilized tetracoordinated silyl cations have been successfully used as efficient catalysts in Mukaiyama-type aldol reactions. 

Van Koten and coworkers also applied this strategy to obtain intramolecularly stabilized 1,4-dilithiated pincer ligands of the type 72. l,4-Dilithio-2,3,5,6-... 

Concerning the possible rearrangement of the lithiooxirane into the alkoxy carbene 155, calculations have also shown that the activation energies of the 1,2-H shifts (to cyclopentanone enolate or cyclopentenol) are extremely high (at least 23 kcalmol" ) from 155, whereas they are much lower (between —0.4 kcalmol" and 8.8 kcalmol" ) from carbene 154. This is explained by a strong intramolecular stabilization of the carbene by the alcoholate moiety, as depicted in Scheme 66. This stabilization could signify that the formation of a carbene from the carbenoid is a disfavored process, and that the carbenoid itself is involved in the rearrangement reaction. 

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