From substituted metal carbonyls

With the norbornadiene complex, C H8Mo(CO)4, and aniline, small yields of anilineMo(CO)3 are formed [586]. 

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Deviating from the route via nucleophilic attack of the carbanion at the carbon atom of a CO ligand and then reaction of the acylmetallate with an electrophile are those methods which involve (a) addition of the carbanion to the carbon atom of a carbyne ligand, (b) displacement of halides from transition-metal carbonyl halides by cyclohepta-trienyllithium, or derivatives thereof, followed by hydride abstraction or (c) substitution of a coordinated solvent from a metal-carbonyl complex (see also reaction of LiR with carbene complexes). 

Clark and co-workers have shown that trifluorophosphine can replace all the CO groups from certain metal carbonyl complexes (56, 58, 59). Other fluorophosphines behave similarly, the reactions usually being thermally or photochemically induced (320, 57, 121, 284, 287, 61). In the case of HCo(CO)4, however, substitution by PFg occurs rapidly and spontaneously even at —20°C (320). 

Carbon-13 NMR parameters for all metal carbonyl complexes discussed in this section appear in Table XL. Additional carbonyl data for carbene substituted metal carbonyl complexes may be found in Tables XXXVI and XXXVII. Notably absent from this section is a discussion of polynuclear metal carbonyl complexes. These studies are considered in the section of fiuxional complexes. It is sufScient to note at this point that bridging carbonyl ligands always appear at lower fields than terminal carbonyl ligands. 

The use of both LIU and HIU has been shown to increase the efficiency of the P-K reaction, which involves the formation of cyclopentenone from the annulation of a cobalt alkynyl carbonyl complex and an alkene. The use of low-power ultrasound, as for example, from a cleaning bath, although capable of producing intramolecular P-K reactions, generated relatively low cyclization yields. The motivation for the use of high intensity came from its ability, as previously described, to effectively decarbonylate metal carbonyl and substituted metal carbonyl complexes. Indeed, HIU produced by a classic horn-type sonicator has been shown to be capable of facile annulation of norbornene and norbornadiene in under 10 min in the presence of a trimethylamine or trimethylamine N-oxidc dihydrate (TMANO) promoter, with the latter promoter producing cleaner product mixtures. This methodology also proved effective in the enhancement of the P-K reaction with less strained alkenes such as 2,5-dihydrofuran and cyclopentene, as well as the less reactive alkenes -fluorostyrene and cycloheptene. The mechanism has been postulated to involve decarbo-nylation of the cobalt carbonyl alkyne, followed by coordination by the amine to the vacant coordination sites on the cobalt. 

Several lines of evidence impute appreciable Jt-acidity to thioethers. Infrared studies of substituted metal carbonyl complexes show that trans thioethers increase the CO stretching frequency more than pyridine or an aliphatic amines, but less than phosphines [2, 19, 20, 21]. To the extent that such changes derive from differences in n-backbonding, these results indicate n-acidity intermediate between that of amines and that of phosphines. 

Because dissociation of CO from many metal-carbonyl complexes is slow, methods for ffie catalytic or stoichiometric assistance of ligand substitution reactions have received considerable attention. These methods are presented in ffie following sections. 

Metal carbonyls have been known for over 70 years. Within the last 35 years derivatives of the anions of the metal carbonyls have been synthesized. Within the last 10 years various derivatives of the metal carbonyl anions have acquired significance as intermediates in the preparation of interesting organometallic derivatives of the transition metals. In this chapter applications of anions derived from the metal carbonyls and the substituted metal carbonyls in syntheses of unusual organometallic derivatives of transition metals and the chemistry of the organometallic derivatives thus synthesized will be discussed. 

Atom abstraction reactions also occur with // -cyclopentadienyl substituted metal carbonyl complexes. Laser photolysis (460-490 nm) of [ / -cpM(CO)3]2 in the presence of organic halides (RX) leads to the formation of the halo compounds / -cpM(CO)3X (M = Mo, W). The reactivity trends RI > RBr > RCl, and benzyl > allyl > 3 > 2 > 1 > CH3, follow those expected for a free radical pathway. The reaction involves formation of the 17-electron intermediate / -cpM(CO)3, which then abstracts a halogen atom from the substrate (Scheme 6.9). The cpM(CO)3 radical is trapped by oxygen at a rate that is close to diffusion controlled. 

The hydroformylation reaction is carried out in the Hquid phase using a metal carbonyl catalyst such as HCo(CO)4 (36), HCo(CO)2[P( -C4H2)] (37), or HRh(CO)2[P(CgH3)2]2 (38,39). The phosphine-substituted rhodium compound is the catalyst of choice for new commercial plants that can operate at 353—383 K and 0.7—2 MPa (7—20 atm) (39). The differences among the catalysts are found in their intrinsic activity, their selectivity to straight-chain product, their abiHty to isomerize the olefin feedstock and hydrogenate the product aldehyde to alcohol, and the ease with which they are separated from the reaction medium (36). 

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