Unexpected cleavage of ether bonds of 1,3-dimethoxypropane in Grignard–Wurtz synthesis of a MgCl₂–donor adduct

Highlights

• 1,3-Dimethoxypropane was cleaved in Grignard–Wurtz synthesis of MgCl₂–donor adduct.

• A Grignard reagent, BuMgCl most likely causes the cleavage reaction.

• Final product of the reaction is methoxymagnesium chloride.

• There seem to be at least two reaction paths occurring in parallel.

• 1-Heptene and propene are formed as by-products in the cleavage reaction.

Abstract

Diethers are an important group of electron donors in Ziegler–Natta catalysts. A simple diether, 1,3-dimethoxypropane was studied as an electron donor in Grignard–Wurtz synthesis of a MgCl₂–donor adduct. 1,3-Dimethoxypropane was unexpectedly found to undergo a cleavage reaction during the synthesis producing methoxy groups (OCH₃). Each mole of 1,3-dimethoxypropane produced approximately 2 moles of methoxy groups, which are probably bound to magnesium chloride as methoxymagnesium chloride. A Grignard reagent, BuMgCl formed in the Grignard–Wurtz reaction most likely causes the cleavage of the ether bonds in 1,3-dimethoxypropane and there seem to be at least two parallel reaction paths taking place and producing at least two different by-products. The first step in the cleavage of 1,3-dimethoxypropane is a Grignard reagent (BuMgCl) induced elimination of OCH₃, which gives 3-methoxy-1-propene. This intermediate product reacts further in a substitution reaction caused by the Grignard reagent producing 1-heptene as the by-product. The cleavage of the ether bond in 3-methoxy-1-propene and formation of OCH₃ can also occur through another reaction path, which produces propene as the by-product.

Introduction

MgCl₂ has become a widely studied and commercially important compound after it was discovered to be a suitable support material for Ziegler–Natta catalysts [1]. In order to have useful properties in the catalysis, MgCl₂ has to be highly disordered. This activated MgCl₂ (or δ-MgCl₂) can be prepared in several ways. Methods include mechanical and chemical activation or their combination [2], [3]. In mechanical activation, ordered α-MgCl₂ is grinded in a ball mill [4]. Chemical activation methods enable better control over the morphology, the size and size distribution of particles and have thus widely replaced traditional mechanical activation methods [2]. In the most common chemical method, adducts of MgCl₂ and Lewis bases are first formed. Afterwards, the Lewis base is removed and δ-MgCl₂ is obtained [5], [6]. Activated MgCl₂ can also be prepared with so called Grignard–Wurtz reaction, which is an easy one-step chemical method [3], [7], [8]. In Grignard–Wurtz reaction, magnesium metal reacts with n-alkyl chloride producing δ-MgCl₂. The reaction can be described using four reaction equations:

$$\text{Mg + RCl}\to \text{RMgCl}$$

$$\text{2 RMgCl}\rightleftharpoons {\text{MgCl}}_2+{\text{MgR}}_2$$

$$\text{Mg + 2 RCl}\rightleftharpoons {\text{MgCl}}_2{\text{+ R}}_2$$

$$\text{RMgCl + RCl}\rightleftharpoons {\text{MgCl}}_2{\text{+ R}}_2$$

The first step of the reaction is formation of Grignard reagent (Eq. (1)) [9], [10]. In solutions of Grignard reagents, an equilibrium (Schlenk equilibrium) between alkyl magnesium chloride and the products, magnesium dialkyl and magnesium chloride, is reached (Eq. (2)) [9], [11]. Alkyl chloride can also react with Mg or RMgCl forming magnesium chloride in Wurtz coupling reactions (Eq. (3) and (4)) [12].

Lewis bases acting as electron donors can coordinatively bind to unsaturated magnesium atoms of MgCl₂. Two types of electron donors (internal and external) are used in Ziegler–Natta catalysts. Internal electron donors are added during the preparation of precatalyst, whereas external electron donors are added to the precatalyst with the co-catalyst in the polymerization [2]. Internal electron donors have a significant role in formation of δ-MgCl₂ [13], [14]. It has been shown that these donors affect relative stabilities of different catalytically relevant surfaces (namely (104) and (110)) and thus the structure of MgCl₂ [15]. Both internal and external electron donors also have a great influence on the properties of Ziegler–Natta catalysts and polymers produced. The productivity and hydrogen response of the catalyst as well as the stereoregularity, molar mass and molar mass distribution of the polymers formed are affected by the electron donors [2], [15]. Electron donors regulate the content and distribution of active species by competing from the same surface sites and also affect the stereospesificity of active sites due to steric effects [13], [16], [17]. Typical electron donors in Ziegler–Natta catalysts are e.g. aromatic esters, phthalates, diethers and succinates [18]. Diethers are perhaps the most interesting group of electron donors due to their ability to produce highly isotactic polypropylene with a very high productivity without the presence of an external electron donor [19].

In our studies, MgCl₂ supports have been synthesized with Grignard–Wurtz reaction in the presence of different ethers as electron donors [20], [21], [22]. In the case of diethers, we have observed a cleavage of the CO bond during the Grignard–Wurtz synthesis. The cleavage reaction is undesired if the aim is the synthesis of MgCl₂ supports containing diethers as electron donors. The cleavage reaction can also be adverse if it takes place during the other steps of catalyst preparation (e.g. addition of TiCl₄) or even during polymerization. The aim of this study is to gain insight into the cleavage reaction of diethers, especially 1,3-dimethoxypropane, which is the simplest of 1,3-diethers. The cleavage reactions of simple ethers have been studied [23], [24], [25] in the past decades, but according to our knowledge no reports on the cleavage of diethers in the Grignard–Wurtz synthesis have been published.