Hans Nehl

Structure, thermal stability and decomposition of bis-allyl-zinc compounds

Abstract The structure, thermal stability and decomposition of solutions of diallylzinc (I), bis(2-methylallyl)zinc (II), bis(3-methylallyl)zinc (III) and bis(3,3-dimethylallyl)zinc (IV) in deuterated solvents, have been investigated by1H NMR and by kinetic measurements at temperatures between −125 and +180°C. At room temperature I, II, III and IV are dynamic systems and are best described as being rapidly equilibrating mixtures of all isomeric σ-allyl forms; the NMR spectra are averages weighted according to the relative concentrations of the respective forms. I displays a1H NMR spectrum of a static σ-allyl system only below −125°C and II only below −115°C. At temperatures above 100°C the thermal decomposition of I–IV results in coupling of the allyl groups, decomposition via radicals being the major process. The coupled products exhibit CIDNP, in which the multiplet polarisations confirm a decomposition via randomly diffusing allyl radicals. In the allyl radicals CH2CR1CR2R3 an alternating spin density was proved experimentally. The thermal stability decreases in the order I > II > III > IV.

Additionen von 2-alkenylzink an ungesättigte kohlen-wasserstoffe

Abstract Dicrotylzinc (I) adds to CC double bonds in olefins even at temperatures between 20 and 60°. With ethylene bis(3-methyl-4-pentenyl)zinc (II) is formed selectively, and with 1-octene the 1 1 adduct as which is derived from the 1-methyl-2-propenyl form of (I) is formed almost exclusively. In the reactions with styrene and butadiene metal-to-C(1) addition competes with metal-to-C(2) addition: with butadiene, products which are derived from the 2-butenyl structure of (I) are formed in only 3–13% yield. In comparison with crotylmagnesium and crotylaluminium, (I) reacts more selectively to give products derived from the 1-methyl-2-propenyl form.

Bis(alk-2-enyl)zink aus organomagnesiumhalogeniden

The Dialk-2-enyl zinc compounds II, XI–XIV can be prepared via reaction of ZnCl2 with the corresponding alk-2-enylmagnesium halides I, VII–X at 20–35°C in ether in 68–94% yield. Diprop-2-enylzinc (II) oligomerizes into 2-allylpropane-1,3-diylzinc (III) during attempted sublimation at 30–50°C. The yield of II decreases therefore to <15%. The oligomerization can be prevented by complexation of II with dioxane. In this case II is obtained in 86% yield by cleavage of the dioxane complex.

Bis(alk-4-enyl)zink-verbindungen durch addition von alk-2-enylzink an olefine

Abstract The dialk-2-enylzinc compounds I–IV add to ethylene, oct-1-ene and 3,3-dimethylcyclopropene to give the corresponding di-alk-4-enylzinc compounds VI–IX (88–98% yield), XV and XVI (90–94% yield), XVII (7%) and XXIIIa–XXVIa (91–98% yield). Additions to XIII are regioselective with Zn → C(1) while stereoselective cis-addition to XXII is observed.

Gemischte Organozinkverbindungen durch Austauschreaktionen von Diorganozinkverbindungen

Abstract The exchange of organyl groups between various diorganozinc compounds, R21Zn (R1 = CHMe2 (1), CH2(CH2)2Me (2), CH2C(Me)CH2 (3), CH2CHCHMe (4)) and R22Zn (R2 = Et (5), CH(Me)Et (6), CH2(CH2)3Me (7), CMe3 (8), Menthyl (9), Ph (10), Cyclohexyl (11)), has been followed by mass spectroscopy: molecular ions for the mixed compounds R1R2Zn were observed. The exchange equilibrium constants for 1–5 and 7–11 in toluene were shown calorimetrically to lie between 108 and 0.2. In contrast, the rates of exchange are comparable.

Steuerung der regioselektivität bei der addition von but-2-enylzink-verbindunger an styrol

Abstract The addition of the butenyl—zinc bond in CH3CHCHCH2ZnX derivatives VI–XII (X  n-C4H9, t-C4H9, Ph, cyclohexyl, Cl, OCH3, OPh) to styrene (XIII) affords a mixture of Zn → Cprim and Zn → Csec products. The regioselectivity can be controlled by the inductive effects of the substituents X. The log (Zn → Cprim/Zn → Csec) depends linearly on the polar Taft constants of the substituents X.

Thermische stabilität von Bis(alk-2-enyl)zink-Verbindungen

Abstract The dialk-2-enylzinc compounds I–III react slowly at 20 to 50°C by addition of the ZnC bond to the CC bond of an alk-2-enyl group to give oligomers from which the alkenes XIII–XV are released on hydrolysis. For I–III homolytic cleavage of the ZnCallyl bond, followed by coupling of the allyl radicals to give the alkadienes V–VII, IX and XI predominates above 50°C. IV decomposes mainly homolytically even at 20°C.