Lewis bases, metal carbonyl derivatives

Keto Derivatives of Group IV Organometalloids, 7, 95 Lewis Base-Metal Carbonyl Complexes, 3, 181 Ligand Substitution in Transition Metal ir-Complexes, 10, 347 Literature of Organo-Transition Metal Chemistry 1950-1970, 10, 273 Literature of Organo-Transition Metal Chemistry 1971,11, 447 Literature of Organo-Transition Metal Chemistry 1972, 12, 379 Mass Spectra of Metallocenes and Related Compounds, 8, 211 Mass Spectra of Organometallic Compounds, 6, 273... 

Nitrile derivatives of the metal carbonyls have been discussed together with other nitrogen donor molecules in a number of contexts. Much of the early work has been reviewed by Manuel 337) in his article on Lewis base-metal carbonyl complexes in Volume 3 of this series, and by Stroh-meier 436) in his review of photochemical substitution reactions. In general, nitriles are weaker Lewis donors than phosphorus and nitrogen bases 436), but compared to carbon monoxide, better electron donors but poorer acceptors 427). Force constants and assignments for a series of complexes [(MeCN)jjM(CO)g J (M = Cr, Mo, W) were studied 165, 228, 296) and... 

Transition metal carbonyl derivatives of magnesium can be prepared readily by the reductive cleavage of the metal-metal bond in numerous dimeric transition metal carbonyl complexes or by the removal of halogen from transition metal carbonyl halide complexes with an excess of 1 % magnesium amalgam in the presence of a Lewis base. These preparations may be represented by the general equations... 

The related reactions between Lewis bases, especially amines, and various metal carbonyl derivatives have been utilized largely by Hieber and coworkers to prepare a variety of metal carbonyl anions. Examples of reactions of this type are illustrated by the following equations ... 

This method of generating anionic metal carbonyl derivatives by reaction of metal carbonyl derivatives with Lewis bases has the following disadvantages ... 

Interference by Letms base. The presence of a Lewis base in the system may interfere with the use of the anionic metal carbonyl derivative for reaction with another reagent. For example, the presence of pyridine would interfere with the reaction of a metal carbonyl with methyl iodide or acetyl chloride even the presence of methanol would interfere with the reaction between an anionic metal carbonyl derivative and acetyl chloride. If the salt of the metal carbonyl anion is isolated in the pure state, the presence of relatively weakly bonded, coordinated Lewis-base molecules in a cation, such as [Mn(base)j] + or [Fe(base)g] +, may interfere in a similar manner. 

For these reasons the reactions between metal carbonyls and Lewis bases are not frequently used when the resulting carbonyl anion is desired for a further reaction. Therefore, the reactions between metal carbonyl derivatives and Lewis bases to prepare anionic metal carbonyls are emphasized less in this article than other preparative methods. 

FeCo3(CO)i2] is the only anion presently known which cannot be prepared without using a Lewis base. Sometimes, as in the case of [Co(CO)3(PR3)2]", the cation formed in the reaction between a metal carbonyl derivative and a Lewis base is of interest, for there is no other method of preparation. 

The preparation of metal carbonyl anions from Lewis bases and certain neutral metal carbonyl derivatives was described above. In general, such reactions involve disproportionation of the zero-valent metal atom in the metal carbonyl to a cation, with coordination of the base to the metal atom, and to the metal carbonyl anion. A different t3rpe of reaction between Lewis bases and metal carbonyls involves displacement of carbonyl groups in the metal carbonyl by the Lewis base without change in the oxidation state of the central metal atom, e.g.,... 

For the activation of a substrate such as 19a via coordination of the two carbonyl oxygen atoms to the metal, one should expect that a hard Lewis acid would be more suitable, since the carbonyl oxygens are hard Lewis bases. Nevertheless, Fu-rukawa et al. succeeded in applying the relative soft metal palladium as catalyst for the 1,3-dipolar cycloaddition reaction between 1 and 19a (Scheme 6.36) [79, 80]. They applied the dicationic Pd-BINAP 54 as the catalyst, and whereas this type of catalytic reactions is often carried out at rt or at 0°C, the reactions catalyzed by 54 required heating at 40 °C in order to proceed. In most cases mixtures of endo-21 and exo-21 were obtained, however, high enantioselectivity of up to 93% were obtained for reactions of some derivatives of 1. 

Several methods promoted by a stoichiometric amount of chiral Lewis acid 38 [51] or chiral Lewis bases 39 [52, 53] and 40 [53] have been developed for enantioselective indium-mediated allylation of aldehydes and ketones by the Loh group. A combination of a chiral trimethylsilyl ether derived from norpseu-doephedrine and allyltrimethylsilane is also convenient for synthesis of enan-tiopure homoallylic alcohols from ketones [54,55]. Asymmetric carbonyl addition by chirally modified allylic metal reagents, to which chiral auxiliaries are covalently bonded, is also an efficient method to obtain enantiomerically enriched homoallylic alcohols and various excellent chiral allylating agents have been developed for example, (lS,2S)-pseudoephedrine- and (lF,2F)-cyclohex-ane-1,2-diamine-derived allylsilanes [56], polymer-supported chiral allylboron reagents [57], and a bisoxazoline-modified chiral allylzinc reagent [58]. An al-lyl transfer reaction from a chiral crotyl donor opened a way to highly enantioselective and a-selective crotylation of aldehydes [59-62]. Enzymatic routes to enantioselective allylation of carbonyl compounds have still not appeared. 

Diorganylmetallenes have both electron-donating (nucleophilic) and weak electron-accepting (electrophilic) properties. They are good r-acceptors. Diorganylmetallenes exhibit Lewis base properties, because they can coordinate with unoccupied orbitals of electrophilic molecules, most often with those of transition metal derivatives . In this case weaker ligands can be displaced by diorganylmetallenes from their coordination compounds (for example, carbonyls). 

Tebbe found that titanocene complexes promoted olefin metathesis in addition to carbonyl olefination. Despite the fact that these complexes have low activity, they proved to be excellent model systems. For example, the Tebbe complex exchanges methylene units with a labeled terminal methylene at a slow rate that can be easily monitored (Eq. 4.6) [54]. This exchange is the essential transformation of olefin metathesis. When reactions with olefins are performed in the presence of a Lewis base, the intermediate titanium metallacycle can be isolated and even structurally characterized (Eq. 4.7) [61] These derivatives were not only the first metathesis-active metallacyclobutane complexes ever isolated, but they were also the first metallacyclobutanes isolated from the cycloaddition of a metal-carbene complex with an olefin. These metallacycles participate in all the reactions expected of olefin metathesis catalysts, especially exchange with olefins... 

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