Paolo Zanirato

Regioselective addition reactions of group IVB organometallic radicals with p-diones. An ESR study

Abstract The group IVB organometallic radical adducts of a number of substituted p -diones and related compounds have been investigated by ESR spectroscopy. In the case of substrates with unequivalent carbonyl groups two distinct isomeric adducts have been detected, and the factors affecting the regioselectivity of the addition reaction are discussed. The magnitude of the hyperfine splitting constants at the metal atoms and the trend of the g -values are consistent with a conformation for the adducts where the organometallic substituent eclipses the p π orbital on the radical centre.

Reactions of boranes with aromatic azides: III. A simple route to 1,2-dihydro-N-phenyl,thieno[b]- and thieno[c]-[1,2]azaborines

Some halogeno(organyl)boranes have been formed to promote fragmentation of organic azides, through decomposition of the Lewis acid-base adducts, to give products whose structures depend on the nature of the reagents. The reactions between cw,Sunsaturated orrho-thienylazides and phenylboron dichloride gave 1,2dihydro-1-phenyl-Zchloro, thieno[ b]-, or thieno[ cl-[1,2]azaborines in very good yield, via 1,6-cyclisation of the intermediate N, N-phenyldichloroborylamine resulting from a 1,Zshift of the phenyl group. New l-phenyl-2-oxybis-(dithieno)-, and -(thieno)[1,2]-azaborines were prepared by hydrolysis of the labile [1,2]azaborinechloro derivatives; cyclisation to give these systems involves an intramolecular electrophilic substitution which occurs regiospecificity at the cu-position in the thiophene ring. Similar reactions with 2azidobiphenyl using wider example of chloroboranes, i.e. diphenylboron chloride, phenylboron dichloride, or boron trichloride, led to [l,l’-biphenyl]-2-amine-IV-phenyl or carbazole in different ratios (9.8/0.2; &O/2.0; O/10). The results can be interpreted in terms of the degree of concertedness of the three steps of the rearrangement.

One-pot preparation of isomeric acetyl- and 2,2,2-trifluoroacetyl-azidothiophenes by selective bifunctionalization of dibromothiophenes via halogen–lithio exchange

A series of isomeric acetyl-(1) and 2,2,2-trifluoroacetyl-azidothiophenes (2) are prepared from 2,3-, 3,4- and 2,5-dibromothiophenes via a one-pot procedure entailing stepwise halogen–lithium exchange and successive reaction of the resulting thienyllithium derivatives with N,N-dimethylacetamide (or N,N-diethyl-2,2,2-trifluoroacetamide) and tosyl azide.

Reactivity of aryl and heteroaryl azides with vinylsilane and alkynylsilane. Formation of C-silylated 1,2,3-triazolines and 1,2,3-triazoles

The reactions of the silylated dipolarophiles trimethyl(vinyl)silane, trimethoxy(vinyl)silane and (trimethylsilyl)acetylene, with some para-substituted phenyl azides, 2-azidobenzo[b]thiophene (2-BTA) and 3-azidobenzo[b]thiophene (3-BTA) have been examined. At room temperature these reactions give the respective 1-substituted 4-silylated-1,2,3-triazoline. However, only the reactions of 4-nitrophenyl and 4-cyanophenyl azide occur smoothly, furnishing triazolines in high yields; similar reactions with phenyl, 4-tolyl, 4-methoxyphenyl and 4-chlorophenyl azide are slower, affording mixtures of a 3-(N-arylamino)methyl-3,5-bis(trimethylsilyl)-1-pyrazoline, arising from the primary triazoline adduct through ring-opening to diazo compounds 2, and the corresponding 1-aryl-2-(trimethylsilyl)aziridine, resulting from nitrogen extrusion from the same elusive triazoline. On the other hand reactions of 2-BTA and 3-BTA involve rapid extrusion of nitrogen to afford exclusively the corresponding 1-(benzo(b)thienyl)-2-silylated aziridine.Results suggest that all these azides generally undergo 1,3-dipolar cycloadditions to terminally silylated alkenes to give triazoline adducts with the same geometrical orientation and whose thermal behaviour was found to be strongly dependent on the activating or deactivating effects exerted by N-1 substituents.Suitable support for electronic and steric factors acting in the same direction in the orientations of the former cyclo-ANAE adducts has been provided by the observation that the same aryl azides, 2-BTA and 3-BTA exhibit analogous 1,3-dipolar cycloadditions with (trimethylsilyl) acetylene from which stable 1-aryl(or heteroaryl)-4-trimethylsilyl-1,2,3-triazoles are obtained in quantitative yields.

Thermal decomposition of o-azidobithienyls

Thermal decomposition of the six possible o-azidobithienyls (1)—(6) has been investigated as a possible source of the hitherto unknown dithienopyrrole ring system. It was found that the nature and yield of products and the temperature required for decomposition are strongly dependent on the structure of the starting azides. 3-Azido-2,2′-bithienyl (1) and 3-azido-2,3′-bithienyl (2) gave 4H-dithieno[3,2-b;2′,3′-d]pyrrole (7) and 4H-dithieno[2,3-b;2′,3′-d]pyrrole (8), respectively, in very good yields. 4-Azido-3,3′-bithienyl (3) and 4-azido-2′,3-bithienyl (4) were relatively stable under the same conditions, but polymeric materials were obtained when the thermal decomposition was performed under more vigorous conditions. By contrast, 2-azido-3,3′-bithienyl (5) and 2-azido-2′,3-bithienyl (6) extrude nitrogen at room temperature resulting in ring-opening fragmentation. Thermolyses of 3-azido-2-cyclohex-1-enylthiophen (11) and 4-azido-3-cyclohex-1-enylthiophen (12) were also investigated to elucidate the different behavioural patterns exhibited by the 3-azidothienyl derivatives.

Hazardous N-containing system: thermochemical and computational evaluation of the intrinsic molecular reactivity of some aryl azides and diazides

The exothermic decompositions of the tosyl azide 1, five substituted aryl monoazides 2––6 and two diazides: 1-azido-4-(4-azidophenoxy)benzene 7 and 1-azido-4-[(4-azidophenyl)sulfanyl]benzene 8 were studied experimentally using DSC, weight loss TGA-FTIR and C80-FTIR techniques, and theoretically using the CHETAH software. Numerical modelling and electron impact mass spectrometry (EI-MS) were also performed to investigate the nature of the intrinsic molecular reactivity of azides 1––8, and the possible early stage rate-controlling of an oxidative self-heating process. Significant data were obtained in the instances of 4-methylbenzenesulfonyl azide 1, 4-azido-1,1′-biphenyl-2,2-azido-1,1′-biphenyl 3 and 1-azido-2-(trifluoromethyl)benzene 6. The most likely decomposition pathways of the azides are proposed to explain the observed thermal behavior.

High pressure assisted 1,3-dipolar cycloadditions: formation of 1,2,3-triazoles from aryl azides and (trimethylsilyl)acetylene

The cycloaddition of aryl azides to the corresponding 1,2,3-triazoles has been studied by means of FTIR spectroscopy in a diamond anvil cell at pressures up to 1 GPa. The rate of reaction increases logarithmically with pressure and the yield is close to quantitative.

Preparation, reactivity, NMR properties and semiempirical MNDO/PM3 structural calculations of 2-azido- and 3-azido-selenophene

The preparation, by azido-transfer reaction, of 2-azido-1 and 3-azido-selenophene 2 using the appropriate heteroarylithium derivatives and tosyl azide, followed by fragmentation of the intermediate triazene lithium salts is reported here. Different chemical reactivity and kinetic behaviour were observed for azides 1 and 2 in either 1,3-cycloaddition reactions, with (trimethylsilyl)acetylene and trimethyl(vinyl)silane, or thermal decomposition. Compound 1 gives cyclo-ANAE adducts (silylated triazole or triazoline)ca. three times faster than compound 2. Both the elusive triazoline adducts undergo rapid ring-contraction, with extrusion of nitrogen, to give rise to the corresponding 1-(selenophenyl)-2-(trimethylsilyl)aziridine (1b, 2b). Kinetic measurements of the unimolecular thermal decompositions afford distinct activation parameters: Ea= 21.5 and 30.4 kcal mol–1, ΔS‡=–10.7 and –0.9 cal mol–1 K–1 for 1 and 2. respectively. Experimental data, as a result of geometric and electronic disturbances exerted by the azido-group located at α- or β-positions of the selenophene ring, are qualitatively supported by measurement of 1H, 13C and 77Se NMR chemical shifts. The present experimental evidence and those previously obtained with related 2-azido- and 3-azido-thiophenes are corroborated with the determination of the structures and comparison between 13C substituent chemical shift (SCS) and charge distributions by using a semiempirical computational MNDO/PM3 method.

Aliphatic CH, N-insertion versus aromatic N-substitution in the reaction of arylnitrenium-boron trifluoride complexes with methylated benzenes

Abstract The boron trifluoride promoted decomposition of a number of substituted phenyl azides in toluene, para -xylene, meta -xylene or mesitylene at 60°C, leads preferentially to N -benzylamines or diarylamines depending on both ring substituent effect and nucleophilic character of the solvent.

Synthesis and thermal decomposition of azidovinylbenzo ( b) thiophenes

Condensation of 3-azidobenzo[b]thiophene-2-carbaldehyde 1 with acetone, diethyl malonate, ethyl acetoacetate and pentane-2,4-dione furnished the 3-azido-2-vinylic derivatives 4a–d in fairly good yields. In refluxing toluene the azides 4a, b, d smoothly gave solely fused pyrrole products, 5a, b, d, whereas the azide 4c furnished an isomeric mixture of the cyclized pyrroles 5c and 6. The observed 1H-pyrroles 5a–d and 6 are assumed to occur through aromatization of initially formed 2H-pyrroles 7a–d. Attempted condensation of 2-azidobenzo[b]thiophene-3-carbaldehyde 2 with acetone or pentane-2,4-dione gave no vinylic product, but instead resulted in reduction of the azide 2 to the amine 8 or in its conversion into the 1,2,3-triazole adduct 9, respectively.Azido group transfer of 3-styrylbenzo[b]thiophene with tosyl azide yielded the ortho-vinyl-substituted α-azide 10, but in poor yield. Thermolysis of the azide 10 gave quantitatively the benzothiopyran 12 clearly ascribable to an initial ring-opening reaction. It is therefore inferred that, in the presence of ortho-vinyl substituent, an α-azidobenzo[b]thiophene can exhibit ‘normal’ ring-cleavage fragmentation, in contrast with previous deoxygenations of 2-nitro-3-vinylbenzo[b]thiophenes reported to yield cyclized benzothieno[2,3-b]pyrroles. Evidence is also presented that, upon thermolysis, the o-azidoaldehyde 1 cleanly affords a benzothienoisoxazole product, whereas the isomeric azide 2 essentially leads to intractable material.