Metal salts, addition carbonyls

The carbonyl addition reactions of benzylmetals, compared to the allylic counterparts, have found few applications in stereoselective synthesis, apparently for the following reasons The carbonyl addition of alkali metal salts (M = Li, Na, K, Cs) of benzyl anions, with few exceptions, usually proceeds with low levels of simple diastereoselectivity affording mixtures of syn- or <7 / -diastereomers (see Section 1.3.2.3.1.). 

When a carbonyl group is bonded to a substituent group that can potentially depart as a Lewis base, addition of a nucleophile to the carbonyl carbon leads to elimination and the regeneration of a carbon-oxygen double bond. Esters undergo hydrolysis with alkali hydroxides to form alkali metal salts of carboxylic acids and alcohols. Amides undergo hydrolysis with mineral acids to form carboxylic acids and amine salts. Carbamates undergo alkaline hydrolysis to form amines, carbon dioxide, and alcohols. 

Since 1985, several thousands of publications have appeared on complexes that are active as catalysts in the addition of carbon monoxide in reactions such as carbonylation of alcohols, hydroformylation, isocyanate formation, polyketone formation, etc. It will therefore be impossible within the scope of this chapter to review all these reports. In many instances we will refer to recent review articles and discuss only the results of the last few years. Second, we will focus on those reports that have made use explicitly of coordination complexes, rather than in situ prepared catalysts. Work not containing identified complexes but related to publications discussing well-defined complexes is often mentioned by their reference only. Metal salts used as precursors on inorganic supports are often less well defined and most reports on these will not be mentioned. 

Application of metal salts and well-defined metal complexes in ROP has enabled the exploitation of a three-step coordination-insertion mechanism, first formulated in 1971 by Dittrich and Schulz [17]. This proceeds through coordination of lactide by the carbonyl oxygen to the Lewis acidic metal center, leading to the initiation and subsequent propagation by a metal alkoxide species. This species can be either isolated or generated in situ by addition of an alcohol to a suitable metal precursor to result in the formation of a new chain-extended metal alkoxide, as shown in Scheme 3 [16]. 

Nickel carbonyl charged, or formed in the carbonylation reaction mixture, can catalyze the carbonylation of methanol (11). To maintain the activity of the nickel carbonyl catalyst high temperature and pressure are required (12-14). However, certain promoters can maintain an active, soluble, nickel carbonyl species under much milder conditions. The most reactive promoters are phosphines, alkali metal salts, tin compounds, and 2-hydroxypyridine. Reaction rates of 2 to 7 X 10-3(mol/1.sec) can be achieved without the use of high concentration of iodine (Table II). in addition, high reaction rates... 

There are three important routes to the formation of the mercury-transition metal bond (a) displacement of halogen or pseudohalogen from mercury(II) salts with carbonyl metallate anions (b) reaction of a halo-phenylmercury compound with a transition metal hydride and (c) oxidative addition of a mercury halide to neutral zero valent metals.1 We report here the syntheses of three compounds containing three-centre, two-electron, mercury-ruthenium bonds utilizing trinuclear cluster anions and mercury(II) halides.2-4... 

In a general sense, the Reformatsky reaction can be taken as subsuming all enolate formations by oxidative addition of a metal or a low-valent metal salt into a carbon-halogen bond activated by a vicinal carbonyl group, followed by reaction of the enolates thus formed with an appropriate electrophile (Scheme 14.1).1-3 The insertion of metallic zinc into a-haloesters is the historically first and still most widely used form of this process,4 to which this chapter is confined. It is the mode of enolate formation that distinguishes the Reformatsky reaction from other fields of metal enolate chemistry. 

Titanium compounds are frequently investigated as Lewis acids in radical reactions [677-680]. When addition of an alkyl radical to a chiral vinylsulfoxide was conducted in the absence or presence of Ti(0-/-Pr)2Cl2, the stereochemistry of the product was reversed, very high diastereoselectivity being observed in the presence of the titanium salt (Eq. 302) [681,682]. The stereochemistry and high selectivity in the presence of the titanium salt were readily rationalized on the basis of a chelation intermediate between the titanium metal and the carbonyl and sulfoxide oxygens, as shown in Eq. (302). 

Similar photoinduced dimerizations and ligand substitutions in the presence of additives such as triphenylphosphine are observed with ion-pairs salts of Mn(CO)s and V(CO)6" with cobaltocenium or other cationic acceptors such as Ph2Cr", pyr-idinium, quinolinium, etc [118]. Most importantly, all photochemical transformations of the various carbonyl metallate salts are initiated by actinic light that solely excites the charge-transfer absorption bands of the contact ion pairs whereas local excitation of the separate ions is deliberately excluded. 

In the nickel(ll)-catalyzed NHK reaction, the first step is the reduction of Ni " to Ni that inserts into the halogen-carbon bond via an oxidative addition. The organonickel species transmetallates with Cr " to form the organochromium(lll) nucleophile, which then reacts with the carbonyl compound. To make the process environmentally benign, a chromium-catalyzed version was developed where a chlorosilane was used as an additive to silylate the chromium alkoxide species in order to release the metal salt from the product. The released Cr " is reduced to Cr " with manganese powder. 

One of the many important differences between phosphorus and nitrogen chemistry is the relative strengths of their bonds to hydrogen. The relatively weak P—H bond means that this functionality can be added across a wide variety of unsaturated molecules (alkenes, alkynes, carbonyls) and hence this represents an excellent method for preparing tertiary phosphines. The addition of P 11 compounds to C=0 and C=N has been described in detail by Gilheany and Mitchell.2 The reaction can be catalyzed by base (potassium hydroxide, butyllithium), acid (HC1, carboxylic acids, sulfonic acids, boron trifluoride), free radical (uv, organic peroxides, AIBN) or metal (simple metal salts, late transition-metal complexes). In some circumstances no catalyst is required at all for P 11 additions to proceed.60... 

The Michaelis-Arbuzov reaction works well for the less complex aroyl and alkanoyl chlorides, where purification by distillation is possible. Moreover, there has been less success in the preparation of oc,P-unsaturated acylphosphonates, where multiple addition products are often observed. Recourse has been found in an attractive route that entails oxidation of dialkyl 1-hydroxyalkyl- or 1-hydroxyaryIphosphonates produced by nucleophilic addition of dialkyl phosphites to carbonyl compounds under basic conditions (Pudovik reaction). Alkali metal salts of dialkyl phosphites are currently used in the Pudovik reaction, and the more common procedure of generating the anion involves the addition of a small amount of alcoholic alkoxide ion to the reaction mixture. Neutral amines represent an alternative to the use of anionic bases. In recent years, the use of solid-phase materials as basic catalysts has been successfully developed (Scheme 7.7). One system involves the addition of basic alumina to the carbonyl compound and dialkyl phosphite the other involves the addition of KF or CsF to the mixture of carbonyl compound and dialkyl phosphite. - Such a process for generating dialkyl 1-hydroxyalkyIphosphonates is very flexible and accommodates a large variety of carbonyl compounds. 

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