Cross-experiments

Similar results were obtained by Ogino and co-workers. After the initial cleavage of the CO from the metal, a silyl group migration occurs. It was shown by cross experiments that in this case the silyl substituent migrates and not the silylene. In a subsequent step, the silylene is then displaced by CO. Unfortunately, no trapping experiments of the silylenes in solution have been reported. 

In 1971, a short communication was published [54] by Kumada and co-workers reporting the formation of di- and polysilanes from dihydrosilanes by the action of a platinum complex. Also the Wilkinson catalyst (Ph3P)3RhCl promotes hydrosilation. If no alkenes are present, formation of chain silanes occurs. A thorough analysis of the product distribution shows a high preference for polymers (without a catalyst, disproportionation reactions of the silanes prevail). Cross experiments indicate the formation of a silylene complex as intermediate and in solution, free silylenes could also be trapped by Et3SiH [55, 56],... 

The intramolecular mechanism of this reaction is proved by cross experiments for the metallation and 1,3-rearrangement reaction. In the same manner it is possible to yield the benzoxasiloles 11 (Eq.(4)). The first step in the reaction of chloromethylsilylethers of o-bromophenol 10 with sodium in boiling toluene is the metallation, followed by 1,3-rearrangement and intramolecular cyclization [8]. 

This method is equally applicable to atoms 26) and to molecules 22). In molecules the Zeeman splitting depends on the quantum number / of the total angular momentum and therefore the fluorescence from a single rotational level (v, f) need be observed. Because of this necessarily selective excitation, these molecular level-crossing experiments can be performed much more easily with lasers than with conventional light sources and have been sucessfully performed with Naj 2 > and NaK 29). 

On heating to 140°C in the absence of a solvent, the Al-acyl compounds 18 rearrange quantitatively to the isomeric structures 17. " The rate of reaction is increased by bulky ring substituents, being immeasurably slow when R = R = R = Me it also depends on the nature of R Crossing experiments have shown the migration to be intermolecular. ... 

Fig. 24. Schematic representation of the fluorescence yields in a level-crossing experiment, (a) A Lorentzian yield curve, (b) A dispersion yield curve.

It was pointed out by Bixon, Jortner, and Dothan9 that the treatment presented above of the level-crossing experiment is applicable only to the... 

The possible reversibility of the C-alkylation of nitronate anions under S l conditions was studied by examining the degree of interconversion of the C-alkylation product 3 and 4 in crossed experiments with the nitronate ions 5 and 669. 

Similar crossed experiments demonstrated the irreversibility of the association step between / -nitrobenzyl radicals and anions of / -keto esters69. 

At least ten chemical taxa of Tanacetum vulgare have been revealed by analysis of the leaf monoterpenoids.449,450 These breed true from individual plants over up to 7 years, and, of the two most common monoterpenoid types, that producing camphor was shown to be dominant over that producing thujone by crossing experiments. Four taxa were similarly identified for Lindera species,451 and the occurrence of such races in many species and genera would probably be revealed by analysis of large numbers of individual plants rather than of oil obtained from pooled populations. 

ESMS has been used to characterize the intermediate Nin-complexes formed in the coupling reaction of 2-bromo-6-methylpyridine in the presence of Raney nickel (Scheme 1) [45]. The composition of the intermediate had already been determined previously by elemental analysis, but the ES mass spectra, showing a strong peak for the ion [Ni2(dmbp)2Br3]+, pointed to a dimeric structure. It was concluded that this ion was formed by the loss of Br from the dimeric structure 1. An alternative explanation is that the intermediate has the more common four-coordinate structure 2, and that the observed peak was due to the ion-paired species [Ni2(dmbp)2Br2]2++Br. The dimeric nature of the intermediate was confirmed by a cross experiment when mixtures of differently substituted pyridines were reacted, mixed ligand dinickel species were observed in the ES mass spectra. 

Summary Trichlorosilyl germanes MejGeSiCb (1) and Me2Ge(SiCh)2 (2) can be considered as new sources for SiCl2/SiCl3 transfer reactions under mild, aprotic conditions. Transfer reactions with chloro- and diphosphanes (RR PCl R, R = Pr 4a and R = Bu, R = Pr 4b (R2P)2 R = 30 5a, R = Pr 5b) and amine-catalyzed cross experiments with alkyl(chloro)germanes R. .flGeCln (R = Me, Et and n = 1,2) lead by... 

To test this hypothesis, several silylene/silyl transfer experiments were carried out with the compounds 1 and 2. Here we report the cross experiments with alkyl(chloro)germanes, transfer experiments with chloro- and diphosphanes, and trapping reactions with three phosphaalkenes. 

Scheme 2. Cross experiment between MesGeSiCb (1) and EtsGeCl.

Aldehydes react very fast and at low temperature (between -70 and 0 C), whereas ketones require higher temperatures and longer reaction times (Tables 2 and 3). In cross experiments performed using benzaldehyde and acetophenone with MeTi(0 )3 in 1 1 1 molar ratios, only the aldehyde was consumed, leading almost quantitatively to its corresponding methylphenylcarbinol." Such features are fully exploited in the synthesis of macrocyclic lactones like (36 equation 18). ... 

Chemoselectivity is affected by electronic factors as well. Cross experiments show that small changes in the electrophilic nature of the carbonyl group are responsible for marked differences in the nucleophilic reactivity of Ti and Zr reagents (equations 22 and 23). The electronic nature of the nucleophilic moiety of the metal reagent also affects the chemoselectivity of the nucleophilic addition. This is the case for some special titanium(IV) triisopropoxide derivatives [e.g. (53)-<56)] which are selective towards aldehydes and give high addition yields, due to their resonance-stabilized residues acting as nucleophiles. ... 

Ti reagents are chemoselective and usually react with a,3-unsaturated carbonyls faster than with their saturated analogs, as is shown by several cross experiments (equation 37)." An exception to this generality was observed in the case of cyclohex-2-enone versus cyclohexanone, the latter being much more reactive (equation 22, Section 1.5.3.1.1). 

Chemoselectivity of these reagents, tested in cross experiments (equations 41 and 42), parallels the one already discussed for the allyl derivatives. ... 

Population of the CO d A, v =5 level by absorption of Fourth Positive radiation from a CO discharge has been attributed (30) to rotational perturbations between the d A and A" n states. The existence of these perturbations has been demonstrated by Slanger and Black (33) who investigated the emissions from the three sublevels of the d A state to the a II state following absorption by CO of d A -> X E" " radiation from a CO discharge. This work has been extended by these authors (34) to perturbations between A II and e E . Elastic cross-relcixation was demonstrated between A II and d3A and e j ". Perturbations coupling the a 3r+ state to v = 1 of A II have been observed by Burnham et al. (35) in a level-crossing experiment. 

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