Additionsreaktionen von 3-Dimethylamino-2,2-dimethyl-2H-azirin an Phenylisocyanat und Diphenylketen

Addition Reaction of 3-Dimethylamino-2,2-dimethyl-2H-azirine with Phenylisocyanate and Diphenylketene 3-Dimethylamino-2,2-dimethyl-2H-azirine ( 1a ) reacts with carbon disulfide and isothiocyanates with splitting of the azirine N(1), C(3)-double bond to give dipolar, fivemembered heterocyclic 1:1 adducts. In some cases, these products can undergo secondary reactions to yield 1:2 and 1:3 adducts. In this paper it is shown that the reaction of 1a with phenylisocyanate also takes place by cleavage of the N(1), C(3)-bond, whereas with diphenylketene N(1), C(2)-splitting is observed. The reaction of 1a and phenylisocyanate in hexane at room temperature yields the 1:3 adduct 2 in addition to the trimeric isocyanate 3 (Scheme 1). A mechanism for the formation of 2 is given in Scheme 5. Hydrolysis experiments with the 1:3 adduct 2 , yielding the hydantoins 4–6 and the ureas 7 and 8 (Schemes 3 and 5), show that the formation of this adduct via the intermediates d , e and f is a reversible reaction. The aminoazirines 1a and 1b undergo an addition reaction with diphenylketene to give the 3-oxazolines 14 (Scheme 8), the structure of which has been established by spectral data and oxidative degradation of 14a to the 3-oxazolin-2-one 15 (R¹ ? R² ? CH₃, Scheme 9).     

Reaktionen der valenzpolaromeren Ketenform mesoionischer Heterocyclen mit 3-Dimethylamino-2H-azirinen

Reactions of valencepolaromeric ketenes of mesoionic heterocyles with 3-dimethylamino-2H-azirines Reactions of the 3-dimethylamino-2H-azirines 1a and 1b with the mesoionic oxazole 5 and the mesoionic dithiole 6 in acetonitrile at room temperature yield the 1:1 adducts 11 , 12 , 19 and 20 , respectively (Schemes 5 and 8). These products can be formulated as adducts of the aminoazirines and the ketenes 5a and 6a , which are valence polaromeric forms of the mesoionic heterocycles 5 and 6 (Scheme 2). The structure of the adducts has been elucidated by spectral data and their comparison with the data of (Z)- 11 , the structure of which has been established by X-ray [19]. Oxidation of the 1:1 adducts with KMnO₄ in a two-phase system yields 4-dimethylamino-3-oxazolin-2-ones (cf. Scheme 6) by clevage of the exocyclic C,C-double bond. A mechanism for the formation of the adducts is given in Scheme 9: Nucleophilic attack of 1 on the ketene leads to a primary adduct of type a , which undergoes clevage of the former N(1), C(2)-azirine bond to give adducts of type 11 or 19 . The N(1), C(2)-ring opening of 1a in the reaction with ketenes contrasts with the N(1), C(3)-opening of 1a in the addition with, for instance, isothiocyanates. These different ring openings are explained by the difference in nucleophilicity of the heteroatoms X and Y in a ′ (Scheme 10).     

Regioselectivity of the 1,3‐Oxathiolane Formation in the Lewis Acid‐Catalyzed Reaction of Thioketones with Asymmetrically Substituted Oxiranes

The reactions of the aromatic thioketone 4,4′‐dimethoxythiobenzophenone ( 1 ) with three monosubstituted oxiranes 3a – c in the presence of BF₃⋅Et₂O or SnCl₄ in dry CH₂Cl₂ led to the corresponding 1 : 1 adducts, i.e., 1,3‐oxathiolanes 4a – b with R at C(5) and 8c with Ph at C(4). In addition, 1,3‐dioxolanes 7a and 7c , and the unexpected 1 : 2 adducts 6a – b were obtained (Scheme 2 and Table 1). In the case of the aliphatic, nonenolizable thioketone 1,1,3,3‐tetramethylindane‐2‐thione ( 2 ) and 3a – c with BF₃⋅Et₂O as catalyst, only 1 : 1 adducts, i.e. 1,3‐oxathiolanes 10a – b with R at C(5) and 11a – c with R or Ph at C(4), were formed (Scheme 6 and Table 2). In control experiments, the 1 : 1 adducts 4a and 4b were treated with 2‐methyloxirane ( 3a ) in the presence of BF₃⋅Et₂O to yield the 1 : 2 adduct 6a and 1 : 1 : 1 adduct 9 , respectively (Scheme 5). The structures of 6a , 8c , 10a , 11a , and 11c were confirmed by X‐ray crystallography (Figs. 1 – 5). The results described in the present paper show that alkyl and aryl substituents have significant influence upon the regioselectivity in the process of the ring opening of the complexed oxirane by the nucleophilic attack of the thiocarbonyl S‐atom: the preferred nucleophilic attack occurs at C(3) of alkyl‐substituted oxiranes (O−C(3) cleavage) but at C(2) of phenyloxirane (O−C(2) cleavage).     

Stereochemical Course of the Reaction between Thiocarbonyl Compounds and Oxiranes: Reaction with cis‐ and trans‐2,3‐Dimethyloxirane

The reactions of thiocarbonyl compounds with cis‐2,3‐dimethyloxirane ( 1a ) in CH₂Cl₂ in the presence of BF₃⋅Et₂O or SnCl₄ led to trans‐4,5‐dimethyl‐1,3‐oxathiolanes, whereas with trans‐2,3‐dimethyloxirane ( 1b ) cis‐4,5‐dimethyl‐1,3‐oxathiolanes were formed. With the stronger Lewis acid SnCl₄, the formation of side‐products was also observed. In the case of 1,3‐thiazole‐5(4H)‐thione 2 , these side‐products are the corresponding 1,3‐thiazol‐5(4H)‐one 5 and the 1 : 2 adduct 8 (Schemes 2 – 4). Their formation can be rationalized by the decomposition of the initially formed spirocyclic 1,3‐oxathiolane and by a second addition onto the C=N bond of the 1 : 1 adduct, respectively. The secondary epimerization by inversion of the configuration of the spiro‐C‐atom (Schemes 5 – 7) can be explained by a Lewis‐acid‐catalyzed ring opening of the 1,3‐oxathiolane ring and subsequent ring closure to the thermodynamically more stable isomer (Scheme 12). In the case of 2,2,4,4‐tetramethyl‐3‐thioxocyclobutanone ( 20 ), apart from the expected spirocyclic 1,3‐oxathiolanes 21 and 23 , dispirocyclic 1 : 2 adducts were formed by a secondary addition onto the C=O group of the four‐membered ring (Schemes 9 and 10).     

Studies on Reactions of Thioketones with Trimethyl(trifluoromethyl)silane Catalyzed by Fluoride Ions

Treatment of 2,2,4,4‐tetramethylcyclobutane‐1,3‐dione ( 6 ) in THF with CF₃SiMe₃ in the presence of tetrabutylammonium fluoride (TBAF) yielded the corresponding 3‐(trifluoromethyl)‐3‐[(trimethylsilyl)oxy]cyclobutanone 7 (Scheme 1) via nucleophilic addition of a CF anion at the CO group and subsequent silylation of the alcoholate. Under similar conditions, the ‘monothione' 1 reacted to give thietane derivative 8 (Scheme 2), whereas in the case of ‘dithione' 2 only the dispirodithietane 9 , the dimer of 2 , was formed (Scheme 3). A conceivable mechanism for the formation of 8 is the ring opening of the primarily formed CF₃ adduct A followed by ring closure via the S‐atom (Scheme 2). In the case of thiobenzophenones 4 , complex mixtures of products were obtained including diarylmethyl trifluoromethyl sulfide 10 and 1,1‐diaryl‐2,2‐difluoroethene 11 (Scheme 4). Obviously, competing thiophilic and carbophilic addition of the CF anion took place. The reaction with 9H‐fluorene‐9‐thione ( 5 ) yielded only 9,9′‐bifluorenylidene ( 14 ; Scheme 6); this product was also formed when 5 was treated with TBAF alone. Treatment of 4a with TBAF in THF gave dibenzhydryl disulfide ( 15 ; Scheme 7), whereas, under similar conditions, 1 yielded the 3‐oxopentanedithioate 17 (Scheme 9). The reaction of dithione 2 with TBAF led to the isomeric dithiolactone 16 (Scheme 8), and 3 was transformed into 1,2,4‐trithiolane 18 (Scheme 10).     

Studies for a Variable Synthesis of Colchicinoids: Construction of Ring A on a Heptalene Moiety

It is shown that heptalene‐4,5‐dicarboxylates 2 , which react with lithiated methyl sulfones mainly in a Michael fashion at C(3) (cf. Scheme 2), so that the formation of 3‐sulfonylbenzo[a]heptalene‐2,4‐diols 5 is repressed or completely suppressed, can be transformed into corresponding pseudo‐esters 15 (Scheme 4). These pseudo‐esters, on treatment with lithiated methyl sulfones, followed by addition of BuLi, furnish the 3‐sulfonylbenzo[a]heptalene‐2,4‐diols 5 in excellent‐to‐moderate yields without formation of Michael adducts or their follow‐up products (cf. Scheme 5 and 6). The reaction of the pseudo‐ester 15a with Li[¹³C]H₂SO₂Ph, followed by treatment with non‐labeled LiCH₂SO₂Ph and then BuLi, led to the exclusive formation of 3‐(phenylsulfonyl)‐[1‐¹³C]benzo[a]heptalene‐2,4‐diol 5a* (Scheme 9). This experiment demonstrates that the (phenylsulfonyl)acetyl groups at C(4) and C(5) of the heptalene core retain their individual positions in the course of the benzo[a]heptalene‐2,4‐diol formation. These findings are only compatible with an intramolecular rearrangement mechanism as depicted in Scheme 10.     

New Products from the Heptalene‐Forming Reaction of Azulenes and Acetylenedicarboxylates in Polar Media

A number of azulenes 1 , in particular those with π‐substituents at C(6) such as phenyl, 3,5‐dimethylphenyl, and 4‐biphenyl, have been reacted with 3 mol‐equiv. of dimethyl acetylenedicarboxylate (ADM) in MeCN at 110° (cf. Scheme 1). Main products had been, in all cases, the corresponding heptalene‐4,5‐dicarboxylates 2 . However, a whole number of side products, mainly rearranged (1+2)‐adducts with two molecules of ADM, in amounts of 0.2–9% were also isolated and characterized (cf. Scheme 2). The 2a,8a‐dihydro‐3,4‐ethenoazulene‐1,2‐dicarboxylates 14 , formed by energetically favorable ring closure from the solvent‐stabilized zwitterions 15 , resulting from bond heterolysis in the primary cycloadducts 12 (cf. Scheme 3), have been mechanistically identified as the pivotal intermediates responsible for the formation of all side product (cf. Schemes 5, 9, 12, and 13). Deuterium‐labeling experiments were in agreement with the proposed mechanisms, indicating that sigmatropic [1,5s]‐H shifts in 14 (cf. Scheme 6) as well as isoconjugate [1,4s]‐H shifts in resonance‐stabilized zwitterions of type 21 (cf. Scheme 9) are the crucial steps for side‐product formation. It is postulated that a concluding antarafacial 8e‐dyotropic rearrangement is responsible for the appearance of the 2,4a‐dihydrophenanthrene‐tetracarboxylates of type trans‐ 6 (cf. Scheme 9) in the reaction mixtures, which further rearrange thermally by a not fully understood mechanism into the isomeric tetracarboxylates 7 (cf. Schemes 10 and 11). Most surprising is the presence of a small amount (0.3–1%) of the azulene‐4,5,7,8‐tetracarboxylate 9 in the reaction mixture of azulene 1a and ADM. It is proposed that the formation of 9 is the result of a [1,5s]‐C shift in the spiro‐linked intermediates 24 , which, after prototropic shift and take‐up of a third molecule of ADM, disintegrate by a retro‐Diels‐Alder reaction into 9 and the phthalic diesters 30 (cf. Scheme 12). The UV/VIS spectra of the π‐substituted heptalene‐4,5‐dicarboxylates 2d – 2f and their double‐bond shifted (DBS) forms 2d – 2f (cf. Table 4 and Figs. 9–12) exhibit in comparison with the heptalene‐dicarboxylates 2a and 2′a , carrying a t‐Bu group at C(8), only marginal differences, which are mainly found in the relative intensity and position of heptalene bands II and III .     

A 1,1‐Carboboration Route to Bora‐Nazarov Systems

Hydroboration of the conjugated enynes 1 a and 1 b with Piers’ borane [HB(C₆F₅)₂] gave the respective dienylboranes trans‐ 2 c and trans‐ 2 d . Their photolysis resulted in the formation of the dihydroborole products 3 c and 3 d . Both were converted to their pyridine adducts 5 c and 5 d , respectively. Compounds 3 c and 5 c,d were characterized by X‐ray diffraction. The reaction of the bis(enynyl)boranes 6 a and 6 b with B(C₆F₅)₃ resulted in the formation of the dihydroboroles 7 a and 7 b , respectively. This reaction is thought to proceed by 1,1‐carboboration of one of the enynyl substituents at boron to generate the dienylborane intermediates 8 a / 8 b , followed by thermally induced bora‐Nazarov ring‐closure and subsequent stabilizing 1,2‐pentafluorophenyl group migration from boron to carbon. Compound 7 a was characterized by X‐ray diffraction and solid‐state ¹¹B NMR spectroscopy.     

1,3-Dipolare Cycloadditionen eines Carbonyl-ylids mit 1,3-Thiazol-5(4H)-thionen und Thioketonen

1,3-Dipolar Cycloadditions of a Carhonyl-ylide with 1,3-Thiazole-5(4H)-thiones and Thioketones Inp-xylene at 150°, 3-phenyloxirane-2,2-dicarbonitrile ( 4b ) and 2-phenyl-3-thia-1-azaspiro[4.4]non-1-ene-4-thione ( 1a ) gave the three 1:1 adduets trans- 3a , cis- 3a , and 13a in 61, 21, and 3% yield, respectively (Scheme 3). The stereoisomers trans- 3a and cis- 3a are the products of a regioselective 1,3-dipolar cycloaddition of carbonylylide 2b , generated thermally by an electrocyclic ring opening of 4b (Scheme 6), and the C?S group of 1a . Surprisingly, 13a proved not to be a regioisomeric cycloadduct of 1a and 2b , but an isomer formed via cleavage of the O? C(3) bond of the oxirane 4b . A reaction mechanism rationalizing the formation of 13a is proposed in Scheme 6. Analogous results were obtained from the reaction of 4b and 4,4-dimethyl-2-phenyl-1,3-thiazole-5 (4H)-thione ( 1b , Scheme 3). The thermolysis of 4b in p-xylene at 130° in the presence of adamantine–thione ( 10 ) led to two isomeric 1:1 adducts 15 and 16 in a ratio of ca. 2:1, however, in low yield (Scheme 4). Most likely the products are again formed viathe two competing reaction mechanisms depicted in Scheme 6. The analogous reactions of 4b with 2,2,4,4-tetramethylcyclobutane-1,3-thione ( 11 ) and 9H-xanthene-9-thione ( 12 ) yielded a single 1:1 adduct in each case (Schemes). In the former case, spirocyclic 1,3-oxathiolane 17 , the product of the 1,3-dipolar cycloaddition with 2a corresponding to 3a , was isolated in only 11 % yield. It is remarkable that no 2:1 adduct was formed even in the presence of an excess of 4b. In contrast, 4b and 12 reacted smoothly to give 18 in 81 % yield; no cycloadduct of the carbonylylide 2a could be detected. The structures of cis- 3a , 13a , 15 , and 18 , as well as the structure of 14 , which is a derivative of trans- 3a , have been established by X-ray crystallography (Figs. 1–3, Table).     

Umsetzung von Diazoessigsäure-ethylester mit 1,3-Thiazol-5(4H)-thionen

Reaction of Ethyl Diazoacetate with 1,3-Thiazole-5(4H)-thiones Reaction of ethyl diazoacetate ( 2a ) and 1,3-thiazole-5(4H)-thiones 1a,b in Et₂O at room temperature leads to a complex mixture of the products 5–9 (Scheme 2). Without solvent, 1a and 2a react to give 10a in addition to 5a–9a . In Et₂O in the presence of aniline, reaction of 1a,b with 2a affords the ethyl 1,3,4-thiadiazole-2-carboxylate 10a and 10b , respectively, as major products. The structures of the unexpected products 6a, 7a , and 10a have been established by X-ray crystallography. Ethyl 4H-1,3-thiazine-carboxylate 8b was transformed into ethyl 7H-thieno[2,3-e][1,3]thiazine-carboxylate 11 (Scheme 3) by treatment with aqueous NaOH or during chromatography. The structure of the latter has also been established by X-ray crystallography. In the presence of thiols and alcohols, the reaction of 1a and 2a yields mainly adducts of type 12 (Scheme 4), compounds 5a,7a , and 9a being by-products (Table 1). Reaction mechanisms for the formation of the isolated products are delineated in Schemes 4–7: the primary cycloadduct 3 of the diazo compound and the C?S bond of 1 undergoes a base-catalyzed ring opening of the 1,3-thiazole-ring to give 10 . In the absence of a base, elimination of N₂ yields the thiocarbonyl ylide A ′, which is trapped by nucleophiles to give 12 . Trapping of A ′, by H₂O yields 1,3-thiazole-5(4H)-one 9 and ethyl mercaptoacetate, which is also a trapping agent for A ′, yielding the diester 7 . The formation of products 6 and 8 can be explained again via trapping of thiocarbonyl ylide A ′, either by thiirane C (Scheme 6) or by 2a (Scheme 7). The latter adduct F yields 8 via a Demjanoff-Tiffeneau-type ring expansion of a 1,3-thiazole to give the 1,3-thiazine.     

Some Unusual Products from the Thermal Reaction of Azulenes with Dimethyl Acetylenedicarboxylate

The thermal reaction of azulene-1-carbaldehydes 5 and 6 with excess dimethyl acetylenedicarboxylate (ADM) in decalin leads mainly to the formation of (1 + 1) and (1 + 2) adducts arising from the addition of ADM at the seven-membered ring of the azulenes (cf. Schemes 2 and 4). The (1 + 2) adducts are formed in a homo-Diels-Alder reaction of ADM and isomeric tricyclic carbaldehydes which are derived from the primary tricyclic carbaldehydes by reversible [1s5s]-C shifts (cf. Schemes 3 and 5). The thus formed pentacyclic carbaldehydes seem to undergo deep-seated skeletal rearrangements (cf. Scheme 7) which result finally in the formation of the formyl-tetrahydrocyclopenta[bc]acenaphthylene-tetraesters 12 and 19 , respectively. In other cases, e.g., azulene-1-carbaldehydes 7 and 8 (cf. Scheme 8), the thermal reaction with excess ADM furnishes only the already known tetracycfic (1 + 2) adducts of type anti- 26 to ‘anti’- 29 . The thermal reaction of 1,3,4,8-tetramethylazulene ( 9 ) with excess ADM in decalin resulted in the formation of two (1 + 2) and one (1 + 3) adduct in low yields (cf. Scheme 9). The latter turned out to be the 2,6-bridged barrelene derivative 32 . There are structural evidences that 32 is formed by similar pathways as the formyl-tetrahydrocyclopenta[bc]acenaphthylene-tetraesters (cf. Schemes 7 and 11). [²H₃]Me-Labelling experiments are in agreement with the proposed mechanisms (cf. Scheme 13).     

1,3-Dipole mit zentralem S-Atom aus der Umsetzung von Aziden mit Thiocarbonyl-Verbindungen: Eine unerwartete MeS-Wanderung im Abfangprodukt eines ‘Thiocarbonyl-aminids’ mit Dithiobenzoesäure-methylester

1,3-Dipoles with a Central S-Atom from the Reaction of Azides and Thiocarbonyl Compounds: An Unexpected MeS Migration in the Trapping Product of a ‘Thiocarbonyl-aminide’ with Methyl Dithiobenzoate Reaction of PhN₃ with O-methyl thiobenzoate ( 11a ) and thioacetate ( 11c ) as well as with the dithio esters 11b,d at 80° yields the corresponding imidates and thioimidates 12 (Scheme 3). The formation of 12 is rationalized by a 1,3-dipolar cycloaddition of the azide and the C?S group followed by successive elimination of N₂ and S. In the three-component reaction of 11b , PhN₃, and the sterically crowded thioketone 1a , 1,2,4-trithiolane 13a and 1,4,2-dithiazolidine 3a are formed in addition to 12b (Scheme 4). The heterocycles 13a and 3a are trapping products of 1a and ‘thiocarbonyl-thiolate’ 5a and ‘thiocarbonyl-aminide’ 2a (Ar?Ph), respectively (Scheme 6). These 1,3-dipoles are formed as reactive intermediates. Surprisingly, in the presence of catalytic amounts of acids, the major product is the (methyldithio)cyclobutyl thioimidate of type 14 (Scheme 5), formed by an acid-catalyzed MeS migration in dithiazolidine 17 . A reaction mechanism is proposed in Scheme 7.     

Intramolecular Photocyclization of 2‐Acylphenyl Methacrylates: a Convenient Access to 4,5‐Dihydro‐1,4‐epoxy‐2‐benzoxepin‐3(1H)‐ones (= Benzo[c]‐6,8‐dioxabicyclo[3.2.1]octan‐7‐ones

The photochemical reactions of 2‐acylphenyl methacrylates (= 2‐acylphenyl 2‐methylprop‐2‐enoates) 1 were investigated. Irradiation of 2‐acylphenyl methacrylates 1a – d in MeCN gave the tricyclic lactones 2a – d in good yields, together with a small amount of O CO bond cleavage product, the 2‐acylphenols 3a – d (Scheme 2, Table). The formation of the tricyclic lactones 2 probably follows a mechanism involving a 1,7‐diradical through ζ‐H abstraction (1,8‐H transfer) by the excited carbonyl O‐atom (Scheme 3). Irradiation of 2‐acylphenyl tiglate (= 2‐acylphenyl (2E)‐2‐methylbut‐2‐enoate) 1e and 2‐acylphenyl methacrylates 1g – i , substituted by a MeO group (δ‐H) at the 3,5‐positions of the phenyl group, also gave the tricyclic lactones 2e and 2g – i , but in low yields. On the other hand, no H‐abstraction products were observed on irridation of 2‐(ethoxycarbonyl)phenyl methacrylate 1f , of 2‐acylphenyl methacrylate 1j which is substituted by a Me group (γ‐H) at the 3,5‐positions of the phenyl group, and of 1k with an OH group at the 3‐position of the phenyl group.     

Conjugate Addition of Lithiated (S)‐4‐Isopropyl‐3‐ [(methylthio)methyl]‐5,5‐diphenyloxazolidin‐2‐one to Cinnamoyl Derivatives: Preparation of Enantiomerically Pure 1,4‐Diols

The Li derivative of (S)‐4‐isopropyl‐3‐[(methylthio)methyl]‐5,5‐diphenyloxazolidin‐2‐one (Li‐ 2 ; synthetically equivalent to a chiral formyl anion) adds to enones and enoates in a 1,4‐fashion. Best results are obtained with 1,3‐diarylpropenones (chalcones; Scheme 2), trityl enones, and 2,6‐di(tert‐butyl)‐4‐methoxyphenyl cinnamates (Scheme 3), with yields up to 80% and diastereoselectivities up to and above 99 : 1 of the products ( 5a – f and 8a , b , e ) containing three stereogenic centers! X‐Ray crystal‐structure analysis reveals that the C,C‐bond formation occurs preferentially with relative topicity ul (Re/Si; Fig. 2). The MeS group of the 1,4‐adducts can be replaced by RO groups in Hg²⁺‐assisted substitutions, with subsequent removal and facile recovery of the chiral auxiliary (Schemes 4–6). 4‐Hydroxycarbonyl derivatives (‘homoaldols') and mono‐, di‐, and trisubstituted 1,4‐diols are, thus, accessible in enantiomerically pure forms (cf. 15, 16 , and 18 – 20 ).     

Cycloadditions of `Thiocarbonyl Ylides' with Tetracyanoethylene (=Ethenetetracarbonitrile): Interception of Intermediates

Thiocarbonyl ylides (=sulfonium ylides) belong to the most nucleophilic 1,3‐dipoles (high HO energy). In their reactions with tetracyanoethylene (TCNE=ethenetetracarbonitrile; low LU energy), a borderline crossing from the concerted mechanism to a two‐step pathway via a 1,5‐zwitterion was observed. Steric hindrance at one or both termini of the 1,3‐dipole is an additional requirement. The ylides 3 and 13 , set free by N₂ elimination of dihydro‐1,3,4‐thiadiazoles, underwent electrocyclization or 1,4‐H shift. Ylides 3 and 13 are bases and afforded MeOH adducts of different regiochemistry. Whereas 3 and TCNE in abs. THF at 45° furnished the (3+2) cycloadduct 20 , a MeOH content of 0.5 – 5 vol‐% in THF gave rise to a seven‐membered lactim ether 22 and thiolane 20 in a 65: 35 ratio (Scheme 4). Water (0.5 – 1 vol‐%) in THF led to lactam 24 and adduct 20 in the same ratio. The zwitterion 26 , assumed to be the first intermediate, enters competing reactions: the irreversible ring closure to thiolane 20 and the reversible formation of a strained, cyclic seven‐membered `ketene imine' 28 , which is intercepted by MeOH or H<sub>2</sub>O. The gauche‐conformation 32 of an analogous zwitterion, produced from the tetrasubstituted `thiocarbonyl ylide' 13 with TCNE (Scheme 5), led to the thiolane derivative 35 , while the anti‐conformation 33 afforded the thioxo compound 5 and cyclopropane derivative 36 by intramolecular nucleophilic substitution.     

Reaction of Isonitriles with Dibenzoylacetylene Leading to Furan and Difuropyran Derivatives

A mixture of an isocyanide and dibenzoylacetylene in dry CH₂Cl₂ undergoes a smooth addition reaction at ambient temperature to furnish 3‐[5‐(alkylimino)‐3,4‐dibenzoyl‐2‐phenylfuran‐2(2H)‐yl]‐ 1‐phenylprop‐2‐yn‐1‐ones (1 : 2 adduct) and {2,5‐bis(alkylimino)‐4,7,8a‐triphenyl‐5H‐difuro[2,3‐b:3′,4′‐e]pyran‐3(8aH)‐yl}(phenyl)methanones (2 : 2 adduct). Single‐crystal X‐ray analyses conclusively confirmed the structures of the adducts.     

Über Umlagerungen bei der Cyclialkylierung von Arylpentanolen zu 2,3‐Dihydro‐1H‐inden‐Derivaten, 2. Mitteilung

On Rearrangements by Cyclialkylations of Arylpentanols to 2,3‐Dihydro‐1 H ‐indene Derivatives. Part 2. An Unexpected Rearrangement by the Acid‐Catalyzed Cyclialkylation of 2,4‐Dimethyl‐2‐phenylpentan‐3‐ol under Formation of trans ‐2,3‐Dihydro‐1,1,2,3‐tetramethyl‐1 H ‐indene The acid catalyzed‐cyclialkylation of 4‐(2‐chloro‐phenyl)‐2,4‐dimethylpentan‐2‐ol ( 1 ) gave two products: 4‐chloro‐2,3‐dihydro‐1,1,3,3‐tetramethyl‐1H‐indene ( 2 ) and also trans‐4‐chloro‐2,3‐dihydro‐1,1,2,3‐tetramethyl‐1H‐indene ( 3 ). A mechanism was proposed in Part 1 (cf. Scheme 1) for this unexpected rearrangement. This mechanism would mainly be supported by the result of the cyclialkylation of 2,4‐dimethyl‐2‐phenylpentan‐3‐ol ( 4 ), which, with respect to the similarity of ion II in Scheme 1 and ion V in Scheme 2, should give only product 5 . This was indeed the experimental result of this cyclialkylation. But the result of the cyclialkylation of 1,1,1,2′,2′,2′‐hexadeuterated isomer [²H₆]‐ 4 of 4 (cf. Scheme 3) requires a different mechanism as for the cyclialkylation of 1 . Such a mechanism is proposed in Schemes 5 and 6. It gives a satisfactory explanation of the experimental results and is supported by the result of the cyclialkylation of 2,4‐dimethyl‐3‐phenylpentan‐3‐ol ( 9 ; Scheme 7). The alternative migration of a Ph or of an i‐Pr group (cf. Scheme 6) is under further investigation.     

Hexakis‐Adducts of [60]Fullerene with Different Addition Patterns: Templated Synthesis,Physical Properties,and Chemical Reactivity

Representatives of two classes of hexakis‐adducts of C₆₀ were prepared by templated synthesis strategies. Compound 8 with a dipyridylmethano addend in a pseudo‐octahedral addition pattern was obtained by DMA‐templated addition (DMA=9,10‐dimethylanthracene; Scheme 1) and served as the starting material for the first supramolecular fullerene dimer 2 . Hexakis‐adduct 12 also possesses a pseudo‐octahedral addition pattern and was obtained by a sequence of tether‐directed remote functionalization, tether removal, and regioselective bis‐functionalization (Scheme 2). With its two diethynylmethano addends in trans‐1 position, it is a precursor for fascinating new oligomers and polymers that feature C₆₀ moieties as part of the polymeric backbone (Fig. 1). With the residual fullerene π‐electron chromophore reduced to a `cubic cyclophane'‐type sub‐structure (Fig. 4), and for steric reasons, 8 and 12 no longer display electrophilic reactivity. As a representative of the second class of hexakis‐adducts, (±)‐ 1 , which features six addends in a distinct helical array along an equatorial belt, was prepared by a route that involved two sequential tether‐directed remote functionalization steps (Schemes 3 and 5). In compound (±)‐ 1 , π‐electron conjugation between the two unsubstituted poles of the carbon sphere is maintained via two (E)‐stilbene‐like bridges (Fig. 4). As a result, (±)‐ 1 features very different chemical reactivity and physical properties when compared to hexakis‐adducts with a pseudo‐octahedral addition pattern. Its reduction under cyclic voltammetric conditions is greatly facilitated (by 570 mV), and it readily undergoes additional, electronically favored Bingel additions at the two sterically well‐accessible central polar 6‐6 bonds under formation of heptakis‐ and octakis‐adducts, (±)‐ 30 and (±)‐ 31 , respectively (Scheme 6). The different extent of the residual π‐electron delocalization in the fullerene sphere is also reflected in the optical properties of the two types of hexakis‐adducts. Whereas 8 and 12 are bright‐yellow (end‐absorption around 450 nm), compound (±)‐ 1 is shiny‐red, with an end‐absorption around 600 nm. This study once more demonstrates the power of templated functionalization strategies in fullerene chemistry, providing addition patterns that are not accessible by stepwise synthetic approaches.     

Multinuclear NMR studies and ab initio structure calculations of hindered phencyclone diels‐alder adducts from symmetrical cyclic dienophiles: Cyclohexene,vinylene carbonate,vinylene trithiocarbonate and two N‐aryl maleimides

A series of hindered Diels‐Alder adducts have been prepared from phencyclone, 1 , with various unusual symmetrical cyclic dienophiles, including cyclohexene, 2a ; vinylene carbonate, 2b ; vinylene trithiocarbonate, 2c ; and the N‐aryl maleimides: N‐(4‐dimethylamino‐3,5‐dinitrophenyl)maleimide (“Tuppy's maleimide”), 2d ; and N‐[3,5‐bis(trifluoromethyl)phenyl]maleimide, 2e . The highly hindered adducts, 3a‐e , respectively, were extensively characterized by one‐ and two‐dimensional NMR methods, observing proton, carbon‐13 and fluorine‐19. High resolution COSY45 spectra permitted rigorous proton NMR assignments. The 2D heteronuclear C‐H chemical shift correlation spectra (HETCOR, XHCORR) were obtained for adducts 3a‐d , allowing specific assignments for protonated carbons. Corrections to earlier proton NMR assignments for the vinylene carbonate adduct are given; results of the gated decoupling ¹³C NMR experiment for this adduct supported endo adduct stereochemistry. Relative proton chemical shifts for bridgehead phenyls of adduct 3c appeared anomalous relative to other adducts, suggesting possible special anisotropic interactions (with endocyclic sulfur or other anisotropic groups in the product) due to the unusual calculated orientation of the phenyls. The unsubstituted bridgehead phenyls in all adducts were shown to exhibit slow exchange limit (SEL) ¹H and ¹³C spectra on the NMR timescales at ambient temperatures (7 tesla) showing slow rotations about the C(sp³)‐C(aryl sp²) bonds. The rapid rotation of the N‐aryl rings of the maleimide adducts was indicated by fast exchange limit spectra, suggesting that ortho substitution of the N‐aryl ring may be necessary to slow this rotation to the SEL regime. Ab initio geometry optimizations at the Hartree‐Fock level were carried out for each adduct, with the 6‐31G* basis sets. Appreciable geometry differences were seen in calculated structures, and significant NMR chemical shift differences were experimentally observed, depending on the nature of the groups attached to the (Z)‐HC=CH moiety of the dienophiles.     

Borohydrides from Organic Hydrides: Reactions of Hantzsch’s Esters with B(C6F5)3

We report herein that the reaction between a series of Hantzsch’s ester analogues 1 a – d with the Lewis acidic species B(C₆F₅)₃ results in facile transfer of hydride to boron. The main products of this reaction are pyridinium borohydride salts 2 a – d , which are obtained in high to moderate yields. The N‐substituted substrates (N‐Me, N‐Ph) reacted in high yield 90–98 % and the connectivity of the products were confirmed by an X‐ray crystallographic analysis of the N‐Me borohydride salt 2 a . Unsubstituted Hanztsch’s ester 1 a reacted less effectively generating only 60 % of the corresponding borohydride salt, with the balance of the material sequestered as the ester‐bound Lewis acid–base adduct 3 a . Formation of the Lewis acid–base adduct could be minimized by increasing the steric bulk about the ester groups as in 1 d . The connectivity of the carbonyl‐bound adduct was confirmed by an X‐ray crystallographic analysis of 3 e the product of the reaction of methyl ketone 1 e with B(C₆F₅)₃. We also explored the generation of these pyridinium salts by employing frustrated Lewis pair methodology. However, the reaction of mixtures of the corresponding pyridine and B(C₆F₅)₃ with hydrogen gas only resulted in formation of trace amounts of the pyridinium borohydride, along with the Lewis acid–base adduct of the starting material and B(C₆F₅)₃. The 1,2‐dihydropyridine adduct was the final product of this reaction. This was ascribed to the low basicity of the pyridine nitrogen and the complicating formation of an ester bound Lewis acid–base adduct.