Pyrolysis of Aryl Azides. XI. Enhanced Neighboring Group Effects of Carbonyl in a Locked Conformation

1992 ◽  
Vol 45 (12) ◽  
pp. 1991 ◽  
Author(s):  
LK Dyall ◽  
JA Ferguson
Keyword(s):  
Transition State ◽  
Nitrogen Loss ◽  
Bond Formation ◽  
Carbonyl Oxygen ◽  
Oxidative Cyclization ◽  
Nitrobenzene Solution ◽  
Aryl Azides ◽  
Neighboring Group ◽  
Neighbouring Group Participation ◽  
Cyclic Product

Rates of pyrolysis in nitrobenzene solution have been measured for 1-azido-9H-fluoren-9- one, 1-azido-9H-xanthen-9-one, 1-azidoacridin-9(10H)-one and 1-azidoanthracene-9,1O-dione; relative to azidobenzene at 120� these were respectively 5.68, 1750, 5090 and 18400. The lack of neighbouring group participation for the first azide is related to the large distance between carbonyl oxygen and the inner azido nitrogen atom, and the data argue against a published proposal that the transition state is stabilized by electrostatic attraction. In the remaining azides, the 'locked conformation' leads to much larger neighbouring group assistance than is observed for freely rotating ortho groups such as benzoyl (krel79). Only the last azide yields an isoxazole on pyrolysis , the second and third ones providing the first reported examples of neighbouring -group-assisted pyrolysis in which no cyclic product is obtained. These results are interpreted in terms of an electrocyclic mechanism in which the transition state is early and N---0 bond formation is less advanced than other changes in bonds. Much of the rate enhancement is attributed to an electron distribution which favours nitrogen loss. 1-Aminoanthracen-9(10H)-one, 1-amino-9H-fluoren-9-one and 1-amino-9H-xanthen-9-one do not yield the corresponding isoxazoles when oxidative cyclization is attempted.

Pyrolysis of aryl azides. II. Naphthyl azides

1972 ◽  
Vol 25 (3) ◽  
pp. 599 ◽  
Author(s):  
G Boshev ◽  
LK Dyall ◽  
PR Sadler
Keyword(s):  
Nitro Group ◽  
Steric Effect ◽  
Kinetic Studies ◽  
Oxidative Cyclization ◽  
Nitrobenzene Solution ◽  
Aryl Azides ◽  
Phenyl Azide ◽  
Group Participation ◽  
Neighbouring Group Participation ◽  
Group Effects

Kinetic studies of the pyrolyses of 1- and 2-naphthyl azides reveal neighbouring group participation by nitro and phenylazo substituents. The acceleration observed in 1-nitro-2-naphthyl azide, 1-phenylazo-2-naphthyl azide, and 2-nitro-1-naphthyl azide is 1730-, 211-, and 23.6-fold, respectively, in nitrobenzene solution at 120�. These effects are all smaller than that of the 2-nitro group in phenyl azide (3370-fold), which is ascribed to the steric effect of the peri hydrogen at C8 in naphthyl azides. The sizes of these neighbouring group effects correlate with the success of oxidative cyclization of ortho-substituted naphthylamines with (diacetoxyiodo)benzene.

Pyrolysis of Aryl Azides. XII. Mechanistic Implications of the Very Small Neighboring Group Effects Across the 2,3-Bond of 2-Azidonaphthalene

1994 ◽  
Vol 47 (6) ◽  
pp. 1031 ◽  
Author(s):  
LK Dyall ◽  
JA Ferguson
Keyword(s):  
Transition State ◽  
Dilute Solution ◽  
Nitrobenzene Solution ◽  
Aryl Azides ◽  
Neighboring Group ◽  
Spectroscopic Evidence ◽  
Infrared Spectroscopic ◽  
Group Effects

2-Azidonaphthalenes with nitro, acetyl, benzoyl and methoxycarbonyl substituents in the 3-position have been synthesized and then pyrolysed in nitrobenzene solution. At 120°, the rates (relative to 2-azidonaphthalene) are respectively 27.9, 24.8, 5.00 and 3.67. These very small neighbouring group effects are consistent with a transition state which has considerable quinonoid character. No evidence of cyclic products was obtained from the two azides with nitro and ester neighbouring groups. With the two azido ketones there was infrared spectroscopic evidence of clean cyclization in dilute solution, to form the corresponding naphth [2,3-c] isoxazole. Our attempts to isolate these two naphthisoxazoles led to decomposition, though 3-methylnaphth[2,3-c] isoxazole has been reported previously ( Friedrichsen and Kaschner 1977).

Participation of 19-ester groups in the cleavage of 2α,3α- and 5α,6α-steroid epoxides. A case of competition between participation and external nucleophile attack

1979 ◽  
Vol 44 (5) ◽  
pp. 1496-1509 ◽  
Author(s):  
Pavel Kočovský ◽  
Václav Černý
Keyword(s):  
Perchloric Acid ◽  
Hydrobromic Acid ◽  
Ester Group ◽  
Carbonyl Oxygen ◽  
Cyclic Carbonate ◽  
Cyclic Ether ◽  
Normal Reaction ◽  
Neighboring Group ◽  
Ether Oxygen ◽  
The Difference

Acid cleavage of the acetoxy epoxide IIIa with aqueous perchloric acid or hydrobromic acid gave two types of products, i.e. the diol Va or the bromohydrin VIa, and the cyclic ether VIII. The latter compound arises by participation of ether oxygen of the ester group. On reaction with perchloric acid the epoxide IVa gave the diol XIIIa as a product of a normal reaction and the isomeric diol Xa as a product arising by intramolecular participation of the carbonyl oxygen of the 19-acetoxy group. Participation of the 19-ester group is confirmed by the formation of the cyclic carbonate XI when the 19-carbonate IVb is treated analogously. On reaction with hydrobromic acid, the epoxide IVa gave solely the bromohydrin XIVa as a product of the normal reaction course. Discussed is the similarity of these reactions with electrophilic additions to the related 19-acetoxy olefins I and II, the mechanism, the difference in behavior of both epoxides III and IV, the dependence of the product ratio on the nucleophility of the attacking species, and the competition between participation of an ambident neighboring group and an external nucleophile attack.

Participation of 19-substituents in electrophilic additions to 6,7-unsaturated 5α-cholestane and B-homo 5α-cholestane derivatives; A case of competition between the participation of an ambident neighboring group and external nucleophile attack

1980 ◽  
Vol 45 (2) ◽  
pp. 559-583 ◽  
Author(s):  
Pavel Kočovský ◽  
Ladislav Kohout ◽  
Václav Černý
Keyword(s):  
Perchloric Acid ◽  
Hydrobromic Acid ◽  
Cyclic Ether ◽  
Cyclic Ethers ◽  
Neighboring Group ◽  
Acid Cleavage ◽  
Neighbouring Group Participation ◽  
Aqueous Perchloric Acid ◽  
The Difference ◽  
Electrophilic Additions

Hypobromous acid action upon the 6,7-unsaturated 19-substituted 5α-cholestans Va-Vc results in the formation of two types of products, the cyclic ethers IX as products of 5(O)n participation of the 19-substituent, and the bromohydrins X. All these compounds are formed from the 6α,7α-bromonium ions Va'-Vc'. Under the same conditions the B-homo-5α-cholestane derivatives VIIa-VIIc afforded solely the cyclic ethers XIV as products of 5(O)n participation of the 19-substituent in the cleavage of the bromonium ions VIIa'-VIIc'. Acid cleavage of the 6α,7α-epoxides VIb and VIc with aqueous perchloric acid or hydrobromic acid gave two types of products, i.e. the cyclic ethers XI and the diols XII or bromohydrines XIII. The cyclic ethers XI arise by 5(O)n participation of the 19-substituent. The B-homo-6α, 7α-epoxide VIIIc on cleavage with aqueous perchloric acid have solely the cyclic ether XVc and by treatment with hydrobromic acid VIIIc afforded the mixture of XVc, as the main product, and of the bromohydrin XVIc. Discussed is the similarity of the bromonium ion cleavage with the fission of the corresponding epoxides, the mechanism of these reactions and the difference in the behaviour of the isomeric olefins Ia-c, IIIa-c, Va-c and VIIa-c and epoxides IIb,c, IVb,c, VIb,c and VIIIb,c. The competition between ambident neighbouring group participation and external nucleophile attack is discussed as well as the dependence of the products ratio on the nucleophilicity of the attacking species.

Reaction Mechanism of a Nonheme Iron Enzyme Catalyzed Oxidative Cyclization via C–C Bond Formation

2018 ◽  
Vol 21 (1) ◽  
pp. 228-232 ◽  
Author(s):  
Wei-chen Chang ◽  
Zhi-Jie Yang ◽  
Yueh-Hua Tu ◽  
Tun-Cheng Chien
Keyword(s):  
Reaction Mechanism ◽  
Bond Formation ◽  
Oxidative Cyclization ◽  
Nonheme Iron

Selectivity in carbon–carbon coupling reactions at palladium(IV) and platinum(IV)

2019 ◽  
Vol 97 (7) ◽  
pp. 529-537 ◽  
Author(s):  
Richard J. Puddephatt
Keyword(s):  
Activation Energy ◽  
Transition State ◽  
Computational Study ◽  
Bond Formation ◽  
Reductive Elimination ◽  
Organometallic Complexes ◽  
Axial Position ◽  
Coupling Reactions ◽  
Methyl Group ◽  
Trigonal Bipyramidal

The isomerization and reductive elimination reactions from octahedral organometallic complexes of palladium(IV) and platinum(IV) usually occur through five-coordinate intermediates that cannot be directly detected. This paper reports a computational study of five-coordinate complexes of formulae [PtMe3(bipy)]+, [PtMe2Ph(bipy)]+, and [PtMe(CH2CMe2C6H4)(bipy)]+ (M = Pd or Pt, bipy = 2,2′-bipyridine), particularly with respect to reactivity and selectivity in reductive elimination. All of the complexes are predicted to have square pyramidal structures with the bipy and two R groups in the equatorial positions and one R group in the axial position, and axial–equatorial exchange occurs by a pairwise mechanism, with the transition state having a pinched trigonal bipyramidal (PTBP) stereochemistry, with one nitrogen and two R groups in the trigonal plane. The activation energy for isomerization is lower than that for reductive elimination in all cases. For the complexes [MMe2Ph(bipy)]+, the activation energies for reductive elimination with Me–Me or Me–Ph coupling are similar. For the complexes [MMe(CH2CMe2C6H4)(bipy)]+, the reductive elimination with Me–C6H4 bond formation from the isomer with the methyl group in the axial position is predicted and is attributed to it having the best conformation of the Me and C6H4 groups for C–C bond formation. In all cases, the selectivity for reductive elimination is similar for M = Pd or Pt, but reactivity is higher for M = Pd. The relevance of this work to selectivity in catalysis is discussed.

Oxidant-Free C(sp2)–H Functionalization/C–O Bond Formation: A Kolbe Oxidative Cyclization Process

2018 ◽  
Vol 83 (6) ◽  
pp. 3200-3207 ◽  
Author(s):  
Lei Zhang ◽  
Zhenxing Zhang ◽  
Junting Hong ◽  
Jian Yu ◽  
Jianning Zhang ◽  
...  
Keyword(s):  
Bond Formation ◽  
Oxidative Cyclization ◽  
Cyclization Process
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