Hydrido-allyl complexes

Similar mixtures of hydrido-allyl compounds are obtained from complexes of 1-propene and 1-butene (Scheme 28), the difference being that discrete Tp Ir (alkene)2 complexes have not been isolated, only their hydridovinyl isomers. It is interesting to note that the analogous reaction of Tp IrH(CH = CH2) (rj - 2 -4) (203) affords (kinetically) the tethered butene complex Tp IrH ((7-7T-CH2CH2CH = CH2) (216, 80%, vide supra), and a small quantity of the hydrido-crotyl complex Tp IrH(f/ -C3H4Me) (217), which becomes the sole product upon prolonged heating (100 °C, 20 h). A series of related hydrido-allyl complexes (328-331) are obtained from ambient temperature photolysis of the 1,3-butadiene complexes 232-234 (Section III-B.2). 

Reductive elimination also occurs in the case of unstable hydrido allyl complexes. 

The diene complexes [Tp Ir( 7 -2,3-RR C4H4)] [R, R = H R = Me, R = H R, R =Me] were chosen for comparative studies on photochemical C-H bond activation reactions which take place only at the C-R and C-R moieties. The C-H activation was found to depend on the nature of the R and R substituents. Relevant to this section, the dimethyl substituted diene ligand was readily activated at one of the methyl groups, yielding the hydrido-allyl complex [Tp Tr(H)( -CH2C C(Me)=CH2)CH2 ] 909. ... 

Reactions of the hydrido(hydroxo) complex 2 with several substrates were examined (Scheme 6-14) [6]. The reactions are fairly complicated and several different types of reachons are observed depending on the substrate. Methyl acrylate and small Lewis bases such as CO, P(OMe)3, BuNC coordinate to the five-coordinated complex 2 affording the corresponding six-coordinate complexes. In reactions with the unsaturated bonds in dimethylacetylenedicarboxylate, carbon dioxide, phenylisocyanate indications for the addition across the O-H bond but not across the Os-OH bond were obtained. In reactions with olefins such as methyl vinyl ketone or allyl alcohol, elimination of a water molecule was observed to afford a hydrido metalla-cyclic compound or a hydrido (ethyl) complex. No OH insertion product was obtained. 

Yasuda and coworkers51 reported a route to niobocene-allyl compounds by hydrometa-lation of conjugated dienes with niobium hydrido-olefin complexes, NbH(C5H5)2(olefin),... 

The coordinated ethylene is readily expelled at 25 °C by the attack of a conjugated diene and the hydride is transferred to the sterically less crowded diene terminus. Thus the niobium hydrido-olefin complexes serve as convenient reagents for the preparation of 1,2- or 1,3-dialkyl-substituted allylniobium compounds starting from butadiene, (E,E) and (/i,Z)-2,4-hcxadicnc, (E)- and (Z)-l,3-pentadiene (equation 1), 3-methyl-l,3-pentadiene and isoprene (equation 2). All the allyl niobium compounds synthesized were isolated as air-sensitive pale-yellow crystals by crystallization from hexane. 

The transition metal complex-catalyzed formation of 1,3-dioxepanes from vinyl ethers has also been described. For example, reaction of allyl vinyl ether 157 with a nonhydridic ruthenium complex at higher temperatures and without any solvent produced 1,3-dioxepane 159 whereas, the use of a hydridic ruthenium complex resulted in the formation of vinyl ether 158 by double-bond isomerization (Scheme 43). It was suggested that cyclic acetal formation proceeds via a 7i-allyl-hydrido transient complex, which undergoes nucleophilic attack of the OH group at the coordinated Jt-allyl <2004SL1203>. 

Thus reaction with alkyl halides such as allyl bromide or pro-pargyl bromide allow for the introduction of oleflnlc2. . or acetylenic side groups onto the phosphazene ring VI, while alcohol leads to the formation of hydrido-phosphazene complexes VII. The hydrogen in these compounds can be replaced with halogen to yield the first series of iodor-phosphazene compounds VIII. 

The proposed mechanism, based on detailed studies [S.-I. Inoue, H. Takaya, K. Tani, S. Otsuka, T. Sato and R. Noyori, J. Am. Chem. Soc., 1990,112, 4897] is schematically represented in Figure 22. Dissociation of one of the BINAP ligands gives the solvated species 1 which coordinates an allylamine molecule affording 11. Complex 11 undergoes (3-elimination to form the transient imi-nium-rhodium hydrido n complex 111. This complex isomerizes to the aza-allyl... 

Often a base such as NaOAc is necessary to make the reaction go smoothly (281), Basic solvents such as A, A -dimethylformamide (DMF) also aid in the formation of 77-allyl compounds from 1-olefins (200) in this case [(DMF)2H][Pd2Cle] is also formed. In a recent study of the formation of 77-allylic complexes from 1-olefins in DMF under mild conditions, it was suggested that bases may not only shift the equilibrium to the right by neutralizing the acid but may also aid in removal of the proton from the allylic position (40). One possible mode of promotion might be the stabilization of intermediate Pd(IV) hydrido species. However, more work is required before the role of basic agents in Pd(II) catalysis is finally understood. 

In the first stage Lewis acids are absent and further hydroeyanation of the monoolefm products 3-PN 40 and 2M3BN 41 does not readily oeeur. The monoeyanation of butadiene is similar to HCN addition to olefins. An individual feature of hydrocyanation of conjugated dienes is the intermediate appearance of TT-allylic complexes 43, which participate in the successive carbon-carbon coupling. Equations (12) and (13) demonstrate the reaction of butadiene with the hydrido-nickel complex 42 leading to formation of nitrile 40 (a) and explain the generation of byproducts, i.e., the branched nitrile 41 via an alternative pathway (b) [68-70]. 

The metal inserts into the C—H bond and the reaction may be considered as a concerted oxidative addition. This is a rare example of an allyl-hydrido-metal complex being isolated from allylic C—H bond cleavage by the metal. Other examples are the reaction of Mo(0) and Ru(0) complexes with olefins where isolable hydrido-7t-allyl-metal(II) complexes are characterized (vide infra). 

The mechanism proposed in the original paper, [2] however, proved to be inconsistent with further mechanistic studies by Stanley et al. The original catalytic cycle was based on neutral dinuclear rhodium intermediates, the first of which, the neutral hydrido-carbonyl complex rac-[Rh2(CO)2( / 7 -L)], was believed to form from the dicationic rac-1 under syngas (CO/H2) conditions with liberation of and norbomadiene. In order to model the intermediacy of this neutral species, Stanley et al. designed the neutral allyl (C3H5) analogue rac-[Rh2(7/ -C3H5)2( / / -L)] (rac-... 

In some cases a coordinatively unsaturated species and then a product of oxidative addition can be formed via a rearrangement of the complex molecule without elimination of any particles (e.g., olefin complex IV-5 may be converted into the t-allyl hydride derivative IV-6 in Scheme IV. 11 [20c]). Analogously, photolysis of butadiene indium complex IV-7 gives rise to the formation of a hydrido-allyl isomer IV-8 [20d]. 

Allyl rhodium complexes deposited on silica gel activate methane. The products of the reaction are allyl and hydrido rhodium complexes, as well as propylene and small amounts of butene and butane. 

Isotope H/D exchange between a coordinated olefin molecule and Dj or rJ 2n may take place as a result of formation of allyl complexes (via hydrogen elimination) or G carbyl complexes, if hydrido compounds are present and allow formation of the metal-alkyl bond by migration of the hydrogen atom to the olefin. 

Conjugated dienes, as a result of the addition to the hydrido complex, form allyl complexes which subsequently undergo hydrogenation. 

The field of both a- and w-allyl complexes was brought into focus following studies by Jonassen and co-workers on the complex C4H7Co(CO)3, which they prepared by treating hydrido-cobalt tetracarbonyl with butadiene. An early structural proposal for this complex was shown in (V) (59). 

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