Fragments, following

Schematic diagram of an orthogonal Q/TOF instrument. In this example, an ion beam is produced by electrospray ionization. The solution can be an effluent from a liquid chromatography column or simply a solution of an analyte. The sampling cone and the skimmer help to separate analyte ions from solvent, The RF hexapoles cannot separate ions according to m/z values and are instead used to help confine the ions into a narrow beam. The quadrupole can be made to operate in two modes. In one (wide band-pass mode), all of the ion beam passes through. In the other (narrow band-pass mode), only ions selected according to m/z value are allowed through. In narrow band-pass mode, the gas pressure in the middle hexapole is increased so that ions selected in the quadrupole are caused to fragment following collisions with gas molecules. In both modes, the TOF analyzer is used to produce the final mass spectrum.

Substituted pyrazoles fragment following pathways that are strongly dependent on the nature of the substituent. Thus iV- and C-phenylpyrazoles lead to iV-phenylaziridinium, -cyclopropenium and benzodiazepinium ions (68ZOR689, 78CHE1123), C-diphenylpyrazoles to the fluorenium ion (m/e 165) (690MS(2)739), and 1-phenylpyrazol-4-yl oximes to the 1-phenylpyrazolium ion (m/e 144) (69JCS(C)2497). 

The mass spectra of alcohols often completely lack a peak corresponding to the parent ion. This is due to extremely rapid loss of neutral fragments following initial ionization. For example, the mass spectrum of 2-methyl-2-butanol lacks a parent peak and contains strong peaks at M-15 (loss of CH3O and M-18 (loss of H2O). 

Other reactions that occur during hydrocracking are the fragmentation followed by hydrogenation (hydrogenolysis) of the complex asphaltenes and heterocyclic compounds normally present in the feeds. 

Seven-membered carbocycles are also available from the reaction of alkenylcarbene complexes of chromium and lithium enolates derived from methyl vinyl ketones [79b] (Scheme 65). In this case, the reaction is initiated by the 1,2-addition of the enolate to the carbene complex. Cyclisation induced by a [1,2]-migration of the pentacarbonylchromium group and subsequent elimination of the metal fragment followed by hydrolysis leads to the final cyclo-heptenone derivatives (Scheme 65). 

Schematic representation of one type of mass spectrometer. An electron beam fragments gas atoms or molecules into positively charged ions. The ions are accelerated and then deflected by a magnet. Each fragment follows a trajectory that depends on its mass.

Basically, the Fj concern bonds on one hand and polar effects on the other hand, as clearly treated by various authors [190,194,200-202]. Here, the fragmentation follows simple rules, but the number of Fj factors renders it impossible to use manually. A program called CLOGP has been developed [190,202-204], providing an easy way of calculating log with good agreement with experimental data. 

Electrospray mass spectrometry is a technique that allows pre-existing ions to be transferred from solution to the gas phase with minimal fragmentation, followed by conventional mass analysis. Recently electrospray mass spectra have been studied for a number of cationic phos-phinegold(I) complexes in dichloromethane/methanol solution.2525 For derivatives of the types [Au(PR3)2]+ and [Au(PR3)3]+, the intact cations were observed, but ions of the type [Au(PR3)4]+ where not detected in the gas phase. Even the three-coordinated [Au(PR3)3]+ are relatively unstable in the gas phase, and the ions [Au(PR3)2]+ are readily produced. 

The formation of this ketone is believed to proceed via internal abstraction of H in the initial peroxy radical (128 cf. p. 328), followed by migration of Me. It may be that the vigorous conditions employed now make a 1,2-alkyl shift feasible, or that the shift of Me may involve fragmentation followed by re-addition, rather than direct migration. 

Pederson and co-workers investigated the thermal behavior of 1,6,6aA4-trithiapentalene and some methyl-substituted derivatives using flash vacuum pyrolysis (FVP). The main products of the fragmentation, following loss of CS and/or CH2=C=S, were shown to be thiophene-3-thiones, or the thiol tautomers <1997J(P2)1261>. 

It is proposed that the B-state of Cu2 (bound in the gas phase) (57) is sufficiently strongly destabilized in the matrix to the extent that it is unstable with respect to dissociation to Cu(2D3/2) + Cu(2S1/2) fragments following photoexcitation of CU2 from the ground state, process (1) in above scheme. The extent to which the dissociation actually occurs depends on the local dynamics following photoexcitation and the details of the Cu2 rare gas potentials for the specific trapping site involved. 

Three possible mechanistic schemes can be suggested for this process. One involves elimination of the proton attached to the p-C atom of nitronate A or A followed by elimination of the OSi group from the intermediate anion (cf. Scheme 3.93). Another mechanism is associated with a 1,4-C,O-transfer of the proton from the p-C atom of nitronate A to the oxygen atom of the N—>0 fragment followed by elimination of silanol from hemiacetal B. The third mechanism is based on the concerted elimination of silanol from the minor cis isomer of SENA. 

Finally, it has been observed through pump-probe experiments [9] that the I2+ fragment following asymmetric dissociation of can itself be in an excited state. This also represents a very high level of electronic excitation. 

Linalool is an important fragrance and fragrance precursor (esters) with an annual production of over 15 000 t. Some 50% is made by semi-synthesis from a- and / -pinene, the other part is made synthetically starting from isobutene via 6-methylhept-5-en-2-one (9). Addition of an acetylene fragment followed by partial hydrogenation (Pd) leads to linalool. 

Few cases of incorporation of solvent fragments following hydrogen abstraction have become known. 

Scheme 2,2. Radical-cation fragmentation following the cyclization of substrate 33...

Vamos M, Ozboya K, Kobayashi Y (2007) Synthesis of bicyclic pyroglutamic acid featuring the Ugi reaction and a unique stereoisomerization at the angular position by Grob fragmentation followed by a transannular ketene [2-1-2] cycloaddition reaction. Synlett 1595-1599. Kreye O, Westermann B, Wessjohann LA (2007) A stable, convertible isonitrile as a formic acid carbanion [-COOH] equivalent and its application in multicomponent reactions. S3mlett 20 3188-3192... 

The strain of cycloproparenes is the principal obstacle that must be overcome in their synthesis. Cycloproparenes decompose usually at moderate temperatures, and they undergo ring-opening in the presence of electrophilic or metallic reagents. In contrast, they support even strongly basic conditions quite well. Accordingly, most successful cycloproparene syntheses use some base-induced elimination in the last step. Alternatively, flash vacuum pyrolytic or photochemical extrusion of a neutral fragment, followed by biradical closure or flash vacuum pyrolysis may be used. In these latter approaches the reaction conditions are neutral, and reactive products may be trapped at low temperatures. 

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