No abstract

Wenngleich elektronenarme Azide sich bevonugt an elektronenreiche Alkine anlagern, lassen sich aus Tosylazid mit Acetylendicarbonsilureester, Propiolsttureester, Phenylpropiolsiluter und Phenylacetylen die I-Tosyl-1.2.3-triazole erhalten. Phenylazid tritt mit Methyl-propiolat zu 88 % 1 -Phenyl-triazol-4carbonester und 12 % des isomeren 5-Carbonesters zusammen. 4-Nitro-phenylazid addiert sich rascher als 4-Methoxy-phenylazid an khoxy-acetylen zum 1 -Aryl-5-ilthoxy-triazol. Bern-in, das als gespanntes Cycloalkin betrachtet werden kann, nimmt glatt Phenylazid und 4-Methoxy-phenylazid auf.Die Bildung der 1.2.3-Triazole aus Aziden mit Acetylen und seinen Derivaten gehBrt zwar zu den Iangst-, nicht aber den bestbekannten 1.3-Dipolaren Cycloadditionens). Bei der Anlagerung an bindungsunsymmetrische Alkine fehlt hilufig der Nachweis der Additionsrichtung oder die Kenntnis des Produktverhaltnisses der stellungsisomeren Triazole.Kinetische Untersuchungep der A2-1.2.3-Triazolin-Bildung6) lehrten, daI3 elektronenreiche CC-Doppelbindungen besonders rasch mit elektronenarmen Aziden (also solchen mit elektronenanziehenden Substituenten) reagieren und vice versa. Die praparative Erfahrung legt ahnliche Verhaltnisse bei Additionen an Alkine nahe. A. ADDITIONEN DES TOSYLAZlDSHier ist das Azidsystem unter dem EinfluI3 des elektronenanziehenden Arylsulfonyl-Restes stark an Elektronen verarmt. Die rasche Addition des Tosylazids an die elektronenreiche Dreifachbindung des Athoxy-acetylens7) bestiitigt obige Regel. Lassen sich auch noch Additionen an elektronenarme Acetylenderivate erzielen? Die Angabe von Curtius und Kfavehns), daD Arylsulfonylazide bei der Addition an Acetylendicarbonsriureester versagen, konnen wir nicht bestatigen. Nach 8 Tagen bei

The empirical expression (1)J(CLi) = L[n(a + d)](-1) is proposed; it claims a reciprocal dependence of the NMR coupling constant (1)J((13)C, Li) in a C-Li compound on two factors: (i) the number n of lithium nuclei in bonding contact with the observed carbanion center and (ii) the sum (a + d) of the numbers a of anions and d of donor ligands coordinated at the Li nucleus that generates the observed (1)J(CLi) value. The expression was derived from integrations of separate NMR resonances of coordinated and free monodentate donor ligands (t-BuOMe, Et2O, or THF) in toluene solutions of dimeric and monomeric 2-(alpha-aryl-alpha-lithiomethylidene)-1,1,3,3-tetramethylindan at moderately low temperatures. This unusually slow ligand interchange is ascribed to steric congestion in these compounds, which is further characterized by measurements of nuclear Overhauser correlations and by solid-state structures of the dimers bearing only one donor per lithium atom (d = 1). Increasing microsolvation numbers d are also accompanied by typical changes of the NMR chemical shifts delta (positive for the carbanionic (13)C(alpha), negative for C(para) and p-H). The aforementioned empirical expression for (1)J(CLi) appears to be applicable to other cases of solvated monomeric, dimeric, or tetrameric C-Li compounds (alkyl, alkenyl, alkynyl, and aryl) and even to unsolvated (d approximately 0) trimeric, tetrameric, or hexameric organolithium aggregates, indicating that (1)J(CLi) might serve as a tool for assessing unknown microsolvation numbers. The importance of obtaining evidence about the (13)C NMR C-Li multiplet splitting of both the nonfluxional and fluxional aggregates is emphasized.

Improved preparations of 2,6-dimethylstyrene (5) and its abromo derivative (10) are described. The Br/Li exchange reaction of 10 provides single crystals of the title compounds 11 or 12, which were characterized as disolvated dimers by X-ray analyses. A similar dimer persists in diethyl ether, tertbutyl methyl ether, and toluene at all accessible temperatu-res, with significant lithiation NMR shifts (relative to 5) partially due to charge delocalization from the sp2-carbanionic center. Some NMR coupling constants are typical of the dimeric aggregate. The configurational (E,Z) lability is quantified in toluene solution.

The deceptively simple "cross-coupling" reactions Alk(2)C=CA-Cl + RLi --> Alk(2)C=CA-R + LiCl (A = H, D, or Cl) occur via an alkylidenecarbenoid chain mechanism in three steps without a transition metal catalyst. In the initiating step 1, the sterically shielded 2-(chloromethylidene)-1,1,3,3-tetramethylindans 2a-c (Alk(2)C=CA-Cl) generate a Cl,Li-alkylidenecarbenoid (Alk(2)C=CLi-Cl, 6) through the transfer of atom A to RLi (methyllithium, n-butyllithium, or aryllithium). The chain cycle consists of the following two steps: (i) A fast vinylic substitution reaction of these RLi at carbenoid 6 (step 2) with formation of the chain carrier Alk(2)C=CLi-R (8), and (ii) a rate-limiting transfer of atom A (step 3) from reagent 2 to the chain carrier 8 with formation of the product Alk(2)C=CA-R (4) and with regeneration of carbenoid 6. This chain propagation step 3 was sufficiently slow to allow steady-state concentrations of Alk(2)C=CLi-Aryl to be observed (by NMR) with RLi = C6H5Li (in Et2O) and with 4-(Me3Si)C6H4Li (in t-BuOMe), whereas these chain processes were much faster in THF solution. PhC[triple bond]CLi cannot perform step 1, but its carbenoid chain processes with reagents 2a and 2c may be started with MeLi, whereafter LiC[triple bond]CPh reacts faster than MeLi in the product-determining step 2 to generate the chain carrier Alk(2)C=CLi-C[triple bond]CPh (8g), which completes its chain cycle through the slower step 3. The sterically congested products were formed with surprising ease even with RLi as bulky as 2,6-dimethylphenyllithium and 2,4,6-tri-tert-butylphenyllithium.