Bridgehead Carbocations: Solvolysis of a Series of 5-Substituted Bicycle [3.1.1]heptyl Bromides. Nucleophilic Solvent Assistance to Ionization of 1-Bromobicyclo[3.1.1]heptane

1995 ◽  
Vol 48 (5) ◽  
pp. 967 ◽  
Author(s):  
EW Della ◽  
GM Elsey
Keyword(s):  
Transition State ◽  
Linear Relationship ◽  
Ring Opening ◽  
Deviant Behaviour ◽  
Donor Property ◽  
Methoxy Derivative ◽  
Methoxy Substituent ◽  
Anchimeric Assistance

The synthesis of a range of 5-substituted bicyclo [3.1.1] heptyl bromides for solvolytic studies is described. It is found that the substituent has a profound effect on the rate of solvolysis of the system and acts principally in accordance with the magnitude of its inductive/field constant σI. The most spectacular example of the effect of the substituent is provided by the COOMe group which leads to a retardation in the rate of methanolysis by a factor of 6.5°105. While a linear relationship in the plot of log k and σI is generally obeyed, as expected for a mechanism mediated by the bicyclo [3.1.1] heptyl bridgehead cation, two of the bromides, 1-bromobicyclo[3.1.1] heptane and its 5-methoxy derivative, show deviant behaviour and react more rapidly than predicted on the basis of the Hammett plot. Evidence is presented to show that the enhanced rate of the parent is the result of nucleophilic assistance by the solvent. Anchimeric assistance in the solvolysis of 5-methoxybicyclo[3.1.1] heptyl bromide is attributed to the powerful p-donor property of the methoxy substituent which stabilizes the transition state in a unique concerted ring-opening and ionization step.

On the Conrotatory Ring Opening of 3-Carbomethoxycyclobutene Vis-À-Vis 3-Carbomethoxy-1,2-Benzocyclobutene and the Predominant Inward Opening of 3-Dimethylaminocarbonyl-1,2-Benzocyclobutene

2018 ◽  
Author(s):  
Veejendra Yadav ◽  
Dasari L V K Prasad ◽  
Arpita Yadav ◽  
Maddali L N Rao
Keyword(s):  
Transition State ◽  
Ring Opening ◽  
Electron Process ◽  
Ring Opening Reaction ◽  
Stable Species ◽  
Electron Event

<p>The torquoselectivity of conrotatory ring opening of 3-carbomethoxycyclobutene is controlled by p<sub>C1C2</sub>→s*<sub>C3C4</sub> and s<sub>C3C4</sub>→p*<sub>CO</sub> interactions in the transition state in a 4-electron process as opposed to only s<sub>C3C4</sub>→p*<sub>CO</sub> interaction in an apparently 8-electron event in 3-carbomethoxy-1,2-benzocyclobutene. The ring opening of 3-carbomethoxy-1,2-benzocyclobutene is sufficiently endothermic. We therefore argue that the reverse ring closing reaction is faster than the forward ring opening reaction and, thus, it establishes an equilibrium between the two and subsequently allows formation of the more stable species <i>via</i> outward ring opening reaction. Application of this argument to 3-dimethylaminocarbonyl-1,2-benzocyclobutene explains the predominantly observed inward opening.</p>

An NBO-TS predictive approach for torquoselectivity of ring opening in 3-substituted cyclobutenes

2017 ◽  
Author(s):  
Arpita Yadav ◽  
Dasari L V K Prasad ◽  
Veejendra Yadav
Keyword(s):  
Transition State ◽  
Ring Opening ◽  
Natural Bond Orbital ◽  
Theoretical Calculations ◽  
Lone Pair ◽  
Product Distribution ◽  
Acceptor Substituent ◽  
Important Contributor ◽  
Predictive Approach ◽  
Donor Substituent

<p>The torquoselectivity, the inward or outward ring opening of 3-substituted cyclobutenes, is conventionally guided by the donor and/or acceptor ability of the substituent (S). It is typically predicted by estimating the respective ring opening transition state (TS) barriers. While there is no known dissent in regard to the outward rotation of electron-rich substituents from the approaches of TS calculations, the inward rotation was predicted for some electron-accepting substituents and outward for others. To address this divergence in predicting the torquoselectivity, we have used reliable orbital descriptors through natural bond orbital theoretical calculations and demonstrated that (a) interactions <i>n</i><i><sub>S</sub></i>→s*<sub>C3C4</sub> for a lone pair containing substituent, s<sub>S</sub>→s*<sub>C3C4</sub> for a s-donor substituent, s<sub>C3C4</sub>→p*<sub>S</sub> for a resonance-accepting substituent and s<sub>C3C4</sub>→s*<sub>S</sub> for a s-acceptor substituent constitute the true electronic controls of torquoselectivity, and (b) reversibility of the ring opening event is an additional important contributor to the observed product distribution.</p>

Synthesis of nitropolychlorinated dibenzo-p-dioxins (NPCDDs) and their photochemical reaction with nucleophiles

1995 ◽  
Vol 73 (6) ◽  
pp. 826-834 ◽  
Author(s):  
Simona G. Merica ◽  
Nigel J. Bunce
Keyword(s):  
Excited State ◽  
Quantum Yield ◽  
Linear Relationship ◽  
Nitro Group ◽  
Photochemical Reaction ◽  
Ring Opening ◽  
Sodium Ethoxide ◽  
Chemical Yield ◽  
Triplet Excited State ◽  
High Chemical

A series of nitropolychlorodibenzo-p-dioxins (NPCDDs) was synthesized by condensation between catechols and 2,6-dinitrohalobenzene derivatives. In the presence of sodium ethoxide in anhydrous ethanol, these underwent photochemical SN2Ar* substitutions meta to the nitro group in high chemical yield and moderate quantum yield. Both ring-opening and chloride replacement reactions were observed. The reactions involved the triplet excited state of the NPCDD, and showed a linear relationship between Φ−1 and [nucleophile]−1. Analogous reactions with KCN in methanol showed similar behaviour, but the products could not be isolated. Keywords: photosubstitution, SN2Ar*, dibenzo-p-dioxins.

On the Conrotatory Ring Opening of 3-Carbomethoxycyclobutene Vis-À-Vis 3-Carbomethoxy-1,2-Benzocyclobutene and the Predominant Inward Opening of 3-Dimethylaminocarbonyl-1,2-Benzocyclobutene

2018 ◽  
Author(s):  
Veejendra Yadav ◽  
Dasari L V K Prasad ◽  
Arpita Yadav ◽  
Maddali L N Rao
Keyword(s):  
Transition State ◽  
Ring Opening ◽  
Electron Process ◽  
Ring Opening Reaction ◽  
Stable Species ◽  
Electron Event

<p>The torquoselectivity of conrotatory ring opening of 3-carbomethoxycyclobutene is controlled by p<sub>C1C2</sub>→s*<sub>C3C4</sub> and s<sub>C3C4</sub>→p*<sub>CO</sub> interactions in the transition state in a 4-electron process as opposed to only s<sub>C3C4</sub>→p*<sub>CO</sub> interaction in an apparently 8-electron event in 3-carbomethoxy-1,2-benzocyclobutene. The ring opening of 3-carbomethoxy-1,2-benzocyclobutene is sufficiently endothermic. We therefore argue that the reverse ring closing reaction is faster than the forward ring opening reaction and, thus, it establishes an equilibrium between the two and subsequently allows formation of the more stable species <i>via</i> outward ring opening reaction. Application of this argument to 3-dimethylaminocarbonyl-1,2-benzocyclobutene explains the predominantly observed inward opening.</p>

An NBO-TS predictive approach for torquoselectivity of ring opening in 3-substituted cyclobutenes

2017 ◽  
Author(s):  
Arpita Yadav ◽  
Dasari L V K Prasad ◽  
Veejendra Yadav
Keyword(s):  
Transition State ◽  
Ring Opening ◽  
Natural Bond Orbital ◽  
Theoretical Calculations ◽  
Lone Pair ◽  
Product Distribution ◽  
Acceptor Substituent ◽  
Important Contributor ◽  
Predictive Approach ◽  
Donor Substituent

<p>The torquoselectivity, the inward or outward ring opening of 3-substituted cyclobutenes, is conventionally guided by the donor and/or acceptor ability of the substituent (S). It is typically predicted by estimating the respective ring opening transition state (TS) barriers. While there is no known dissent in regard to the outward rotation of electron-rich substituents from the approaches of TS calculations, the inward rotation was predicted for some electron-accepting substituents and outward for others. To address this divergence in predicting the torquoselectivity, we have used reliable orbital descriptors through natural bond orbital theoretical calculations and demonstrated that (a) interactions <i>n</i><i><sub>S</sub></i>→s*<sub>C3C4</sub> for a lone pair containing substituent, s<sub>S</sub>→s*<sub>C3C4</sub> for a s-donor substituent, s<sub>C3C4</sub>→p*<sub>S</sub> for a resonance-accepting substituent and s<sub>C3C4</sub>→s*<sub>S</sub> for a s-acceptor substituent constitute the true electronic controls of torquoselectivity, and (b) reversibility of the ring opening event is an additional important contributor to the observed product distribution.</p>

Torque control by metal-orbital interactions

2006 ◽  
Vol 78 (2) ◽  
pp. 415-423 ◽  
Author(s):  
Masahiro Murakami ◽  
Yasufumi Miyamoto ◽  
Munehiro Hasegawa ◽  
Ippei Usui ◽  
Takanori Matsuda
Keyword(s):  
Transition State ◽  
Ring Opening ◽  
Stereoselective Synthesis ◽  
Torque Control ◽  
Orbital Interactions ◽  
Silicon Carbon ◽  
Highest Occupied Molecular Orbital ◽  
Ring Opening Reaction ◽  
Novel Method ◽  
Metal Orbital

The silyl substituent of 3-silylcyclobutene prefers inward rotation rather than outward rotation during a thermal ring-opening reaction, giving the Z-isomer predominantly. This intriguing behavior was explained by assuming electron-accepting interactions between the low-lying σ*-orbital of the silicon-carbon linkage and the highest occupied molecular orbital (HOMO) of the opening cyclobutene system, which are possible only in the inward transition state. On the basis of this finding, a novel method for the stereoselective synthesis of functionalized 1,3-butadiene derivatives from cyclobutenones was developed. Boryl substituents exhibit even stronger preference for inward rotation than silyl substituents as a result of electron delocalization from the cyclobutene HOMO into the vacant p-orbital of boron at the inward transition state.

Alkaline hydrolysis of glycol esters - A newly suggested criterion in the assessment of anchimeric assistance

1974 ◽  
Vol 27 (11) ◽  
pp. 2325 ◽  
Author(s):  
M Balakrishnan ◽  
Rao G Venkoba ◽  
N Venkatasubramanian
Keyword(s):  
Transition State ◽  
Solvent Effects ◽  
Alkaline Hydrolysis ◽  
Triethylene Glycol ◽  
Binary Solvent ◽  
Solvent Mixtures ◽  
Acetone Water ◽  
Anchimeric Assistance ◽  
Hydrolysis Of ◽  
Suggested Criterion

Solvent effects vis a vis structural effects on the alkaline hydrolysis of various di-and mono-benzoates of glycols have been studied in binary solvent mixtures of dimethyl sulphoxide-water, ethanol-water and acetone-water. It is observed that the higher the stabilization of the transition state by the neighbouring group, the greater is the susceptibility of the reaction to dipolar aprotic solvent acceleration[i.e. k(Me2SO)/k(EtOH) value]. The possibility of employing such solvent effects to evaluate the extent of anchimeric assistance in ester hydrolysis where the neighbouring group can stabilize the transition state is examined. The studies have been extended to triethylene glycol derivatives and it is suggested that dipolar aprotic-protic solvent effects could be used as a kinetic probe for the conformation of the molecule.

COMPUTATIONAL INSIGHTS ON THE LONE PAIR INDUCED BARRIER MODULATION IN THE THERMAL REARRANGEMENT OF 6-HALO-2-PYRONES

2007 ◽  
Vol 06 (02) ◽  
pp. 233-243 ◽  
Author(s):  
LAKSHMINARAYANAN AKILANDESWARI ◽  
PONNAMBALAM VENUVANALINGAM
Keyword(s):  
Transition State ◽  
Ring Opening ◽  
Natural Bond Orbital ◽  
Lone Pair ◽  
Thermal Rearrangement ◽  
Lone Pairs ◽  
Sigmatropic Shift ◽  
Tandem Process

2-pyrones undergo intramolecular thermal rearrangement resulting in the migration of groups at 3-position to 5-position and vice-versa. Rearrangement of 6-halopyrone is a tandem process involving electrocyclic ring opening and closure (ERO & ERC), rotation, sigmatropic shift. It has been modeled at MP2/6-31g (d) level to understand the migratory aptitude of the halogens. Computations show that electrocyclic transition state and the corresponding intermediate which could not be located in 2-pyrone rearrangement have been located for 6-fluoropyrone and 6-chloropyrone. Halogens effectively modulate the barriers of all the steps involved and NBO (Natural Bond Orbital) analyses clearly reveal the involvement of lone pairs in activating rotation and sigmatropic shift, and deactivating ERO and ERC.

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