Stereochemical analysis of deuterated alkyl chains by charge-remote fragmentations of protonated parent ions

Zhang, Kangling; Bouchonnet, Stéphane; Serafin, Scott V.; Morton, Thomas Hellman

doi:10.1016/s1387-3806(03)00047-2

Cited by 8 publications

(12 citation statements)

References 11 publications

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“…a assessing specificity. [21][22][23] For all three systems 3-5 in Table 1, this figure of merit has a value r ¼ 1.9 at 8 V, slightly less than the value for ESI-CAD of a different pair of 4-d 1 -3-hexyl diastereomers 23 The results suggest a simple mechanism. Heterolysis of the C-N bond either expels neutral amine to yield a free alkyl cation or else forms an ion-neutral complex.…”

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confidence: 79%

“…This quotient provides a figure of merit, r , for assessing specificity. 21–23 For all three systems 3–5 in Table 1, this figure of merit has a value r = 1.9 at 8 V, slightly less than the value for ESI-CAD of a different pair of 4- d 1 -3-hexyl diastereomers 23 ( r = 2.2), where ion-neutral complexes do not intervene. Isotopic transposition might explain the lower value of r in the present case.…”

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confidence: 80%

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Stereospecificity of the Hofmann elimination in the gas phase

Figadère

Cao

Morton

2017

Eur J Mass Spectrom (Chichester)

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The Hofmann elimination of ammonium ions having a single positive charge is demonstrated to exhibit stereospecificity with regard to expulsion of neutral alkene. For the 3-hexyl series of threo and erythro 4-monodeuterated ions (3-hexylammonium ion; N,N,N-trimethyl- d-3-hexylammonium ion; N-ethyl-3-hexylammonium ion; N-methyl, N-ethyl-3-hexylammonium ion; N,N-dimethyl, N-ethyl-3-hexylammonium ion), the upper limit of the E:Z ratio of the expelled alkene (r) approaches 2 (the stereospecificity) with a deuterium isotope effect close to 2.0, although the effects of isotopic substitution diminish the E:Z ratio somewhat. Two fragmentations compete with that reaction: hydride shift (which gives the same products but with hydrogen scrambling) and loss of a neutral amine to give an alkyl cation. These competing reactions render our calculation approximate, but the results suggest a value not too far from the upper limit.

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confidence: 79%

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confidence: 80%

Stereospecificity of the Hofmann elimination in the gas phase

Figadère

Cao

Morton

2017

Eur J Mass Spectrom (Chichester)

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“…Alkenes are eliminated from secondary alkylphenyl ether ions via 1,2-syn transition states in parallel with complex-mediated 1,2-processes [23][24][25][26][27]. Parallel operation of two mechanisms was demonstrated by quantum chemical calculations and the stereospecificity/ isotopic exchange inferred by modeling experimental results.…”

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confidence: 94%

“…In the latter, the abstraction of a ␤ hydrogen/ deuterium in the elimination of C 4 (H,D) 8 from labeled 2-phenoxybutane ions was demonstrated by specific transfer of a ␤-H(D), and free rotation of partners was demonstrated by H-transfer from all positions of the side chain [26]. Morton and coworkers [27] established that dissociation of the 2-phenoxypropane ion, like the dissociation of CH 3 CHϭOH ϩ , has an onset below the threshold for complex-mediated dissociation [26]. Although this reaction has a 1,2-transition state, at the transition state the O-propyl C-O bond is essentially fully broken (2.688 Å long); the O has moved toward H t (ROH t ϭ 1.748 Å) and H t has moved only slightly away from its carbon (RCH t ϭ 1.163 Å [26].…”

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confidence: 99%

“…Although this reaction has a 1,2-transition state, at the transition state the O-propyl C-O bond is essentially fully broken (2.688 Å long); the O has moved toward H t (ROH t ϭ 1.748 Å) and H t has moved only slightly away from its carbon (RCH t ϭ 1.163 Å [26]. Computations indicate that breaking the OC bonds in the eliminations of 2-butene from neutral and from protonated mdimethylaminophenyl butyl ethers also precedes Htransfer from a ␤ carbon to O [27]. All three of the reactions just described begin with substantial CO bond stretching and are followed by transfer of a ␤-hydrogen to O.…”

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1,2-Eliminations from (CH₃)₂NH⁺CH₂CH₃ and (CH₃)₂NH₂⁺: Guided dissociations

McAdoo

2008

J. Am. Soc. Mass Spectrom.

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1,2-Eliminations are a varied and extensive set of dissociations of ions in the gas phase. To understand better such dissociations, elimination of CH 2=CH2 and CH 3CH3 from (CH 3hNH+CH2CH3 (1) and of CH 4 from (CH 3hNH2 + are characterized by quantum chemical calculations. Stretching of the CN bond to ethyl is followed by shift of an H from methyl to the bridging position in ethyl and then to N to reach (CH 3hNH2 + + CH 2=CH2 from 1. CH 3CH3 elimination by H-transfer to C 2Hs + to form CH 3NH+=CH2 + CH 3CH3 also takes place.(CH 3hNH2 + eliminates methane by CN bond extension followed by /3-H-transfer to give CH 2=NH+ + CH 4 • Low-energy reactions resembling complex-mediated 1,2-eliminations occur and constitute a hitherto largely unrecognized type of reaction. As in many complexmediated reactions, these reactions transfer H between incipient fragments. They are distinguished from complex-mediated processes by the fragments not being able to rotate freely relative to each other near the transition state for reaction, as they do in complexes. Most 1,2-eliminations are ion-neutral complex-mediated, occur by the just described lower energy reactions, have 1,l-like transition states, or utilize highly asynchronous 1,2 transition states. All of these avoid synchronized 1,2-transition states that would violate conservation of orbital symmetry. ( . Williams and Hvistendahl concluded that some 1,2-eliminations of H 2 occur by high-energy, symmetry-forbidden 1,2-trajectories and others by allowed 1,l-transition states [2,3]. Uggerud and coworkers further characterized 1,l-like transition states for 1,2-eliminations [4-6]. Uggerud concluded that orbital symmetry [7] cannot be used to predict the reaction trajectories for the 1,2-eliminations that do occur [4]. However, we conclude that formal 1,2-eliminations generally avoid violating conservation of orbital symmetry. They do this by occurring in separate steps (ion-neutral complex-mediated or parallel lower energy processes), utilizing non-forbidden Ll-like transition states, or passing through sufficiently asynchronous 1,2-transition states that they are not symmetry forbidden [8]. To categorize 1,2-eliminations, we here use theory to characterize dissociations of (CH 3hNH+CH2CH3 (1) to (CH 3hNH2 + + CH 2=CH2 , to CH 3NH+=CH2 + CH 3CH3 , to CH 3NH+=CH2 + 2 'CH 3 , and of (CH 3hNH2 + to CH=NH 2 + + CH 4 .The most common 1,2-elimination by cations in the gas phase is probably bond cleavage to form an ionAddress reprint requests to Dr. D. J.McAdoo, Department of Neuroscience and Cell Biology, University of Texas Medical Branch, 301 University Blvd., Galveston, TX 77555-1043, USA. E-mail: djmcadoo@utmb.edu neutral complex followed by H-transfer between incipient partners, often accompanied by isomerization [9][10][11][12][13][14][15][16]. Complexes are considered to be intermediaries when a bond is extended to the point that the partners can rotate relative to each other [10,11,17,18], and/or reactions occur between incipient fragments [10,12,15]. Such reactions are initia...

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