Rearrangement into metal allyls

Osborn and Green s elegant results are instructive, but their relevance to metathesis must be qualified. Until actual catalytic activity with the respective complexes is demonstrated, it remains uncertain whether this chemistry indeed relates to olefin metathesis. With this qualification in mind, their work in concert is pioneering as it provides the initial experimental backing for a basic reaction wherein an olefin and a metal exclusively may produce the initiating carbene-metal complex by a simple sequence of 7r-complexation followed by a hydride shift, thus forming a 77-allyl-metal hydride entity which then rearranges into a metallocyclobutane via a nucleophilic attack of the hydride on the central atom of the 7r-allyl species ... 

The transition-metal induced rearrangement of strained cyclopropanes is mostly caused by inserting metal atoms into a three-membered ring, thus producing metallacycles and/or rf- allyl metal complexes. Tipper reported the first example of the metallacycles obtained from [Pt(C2H4)Cl2]2 [3]. The stereospecific addition of cyclopropanes has been investigated from both mechanistic and synthetic view points [4],... 

Dehydrobromination of bromotrifluoropropene affords the more expensive trifluoropropyne [237], which was metallated in situ and trapped with an aldehyde in the TIT group s [238]synthesis of 2,6-dideoxy-6,6,6-trifluorosugars (Eq. 77). Allylic alcohols derived from adducts of this type have been transformed into trifluoromethyl lactones via [3,3] -Claisen rearrangements and subsequent iodolactonisation [239]. Relatively weak bases such as hydroxide anion can be used to perform the dehydrobromination and when the alkyne is generated in the presence of nucleophilic species, addition usually follows. Trifluoromethyl enol ethers were prepared (stereoselectively) in this way (Eq. 78) the key intermediate is presumably a transient vinyl carbanion which protonates before defluorination can occur [240]. Palladium(II)-catalysed alkenylation or aryla-tion then proceeds [241]. 

The insertion of 1,3-dienes into a ir-allylpalladium complex is believed to proceed via an intermediate in which the metal is complexed to the less hindered double bond of an unsymmetrical diene, followed by an electrocyclic rearrangement which links the more substituted allyl terminus with the more substituted alkene (equation 77).246-251 Electron-withdrawing substituents on the ir-allyl fragment generally increase the rate of insertion,248 whereas substituents on the diene generally slow the rate.268... 

Another allyl compound which reacts stoichiometrically with carbon dioxide is (fj5-C5H5)2Ti(l-methylallyl) (120). The titanium acetate complex which is formed is interesting in that the carbon dioxide carbon atom is attached to the substituted end of the allyl. It seems unlikely, then, that the product is the result of C02 insertion into the -methylallyltitanium bond in view of the fact that methyl-substituted allyls tend to form fj -complexes in which the metal is bonded to the least substituted end of the allyl. One possible explanation offered by the authors is that the allyl is bonded to titanium at the methylene carbon, but that rearrangement occurs subsequent to adduct formation [Eq. (49)]. 

This reaction based on the petrochemical crude material isobutylene makes the synthetic route to P-ionone (36) substantially shorter and cheaper, especially since the isomeric double bond proves to be advantageous in the subsequent reactions. In addition, i-methylheptenone (37 a) can be converted into methylheptenone (37) by noble metal-catalyzed isomerization. The reaction steps ethynylation (C2 addition), Carroll reaction (C3 addition), ethynylation and partial hydrogenation (C2 addition) lead from methylheptenone (37) via dehydrolinalool (42), pseudoionone (43) and p-ionone (36) to the C15 alcohol p-vinylionol (44). With triphenylphosphine (15), the desired C15 phosphonium salt (13), which is the second important synthetic building block for vitamin A and carotenoids16), is obtained directly from p-vinylionol, by allyl rearrangement. 

The synthesis and properties of 7r-allyl transition metal complexes have been discussed elsewhere (127-129). We shall draw attention to the most interesting properties of n-allyl complexes such as structure, coordinative unsaturation of the metal, and the ability of the 77-allyl ligand to enter into intermolecular rearrangements. 

Insertion of the monomer, bonded to the metal in rf-cis fashion, into the metal-polymer bond forms a new 7t-allyl polymer end with the substituent at the anti-position. Successive insertion of the new monomer with if-cis coordination would produce cis- 1,4-polybutadiene. Insertion of if-trans-co-ordinated monomer into the metal-polymer bond leads to trans-1,4-polybu-tadiene via syn zr-allyl intermediates. The above anti Tt-allyl polymer end is often equilibrated with the thermodynamically more favorable syn zr-allyl structure via n-a-n rearrangement. Thus, the ratio of cis-1,4 and trans-1,4 repeating units of the polymer produced depends on the relative rates of the two reactions C-C bond formation between the monomer and the polymer end, and anti to syn isomerization of the zr-allyl end of the growing polymer. If the anti-syn isomerization of the anti zr-allyl polymer end occurs more rapidly than the insertion of a new monomer, the polymer with trans-1,4 units is formed even from 7j4-ds-coordinated monomer. The polymerization catalyzed by Ti, Co, or Ni complexes shows high cis-1,4 selectivity, while that with low monomer concentration results in increase of the trans content of... 

The 1,2-polymerization of diene may be accounted for by assuming rearrangement of the 7r-allyl polymer into a a-allyl structure having a single bond between the metal and a CH allyl carbon of the polymer, and insertion of the diene monomer into the M-C o bond. A more convincing and generally accepted rationalization for the smooth 1,2-polymerization of 1,3-dienes is shown in Scheme 3. The diene monomer coordinates to the metal at the... 

Prior chapters have covered the use of transition metals in asymmetric hydrogenations ( 6.2 and 7.1), hydroborations ( 7.3), hydrosilylations and hydro-cyanations ( 6.3, 6.4, 7.4 and 7.5), cyclopropanations ( 7.19), aldol reactions ( 6.11), allylations of carbanions ( 5.3.2), and some sigmatropic rearrangements ( 10.3). This chapter covers other reactions catalyzed by transition metal complexes including coupling of organometallic reagents with vinyl, aryl or allyl derivatives, Heck reactions allylamine isomerizations, some allylation reactions, car-bene insertions into C-H bonds and Pauson-Khand reactions. 

On the other hand a coordinatively unsaturated paramagnetic d metal center could oxidatively add the allyl radical at the metal center to form a labile d° n-aWyl oxo species as organometallic intermediate (step B). The latter may rearrange via a ff-allyl intermediate to a d allyloxy species (step D). The latter is believed to convert to acrolein and a reduced metal center via an /0-hydrogen abstraction by the metal center. In an attempt to gain insight into fundamental reactions involved in this catalytic cycle, some authentic d° allyl 0X0 complexes of rhenium [23] and tungsten... 

Because organoboranes and organoalanes form relatively stable intermediate complexes with various substrates as a prelude to final product formation, it seems permissible to try to extend the scope of the Woodward-Hoffmann principle to the reorganization pathways of such complexes. Thus, it would be useful, for example, to consider whether the chemical behavior of an allylic aluminum system complexed with a ketone (3) might resemble the thermally allowed [3, 3] sigmatropic rearrangement (4). The value of viewing the collapse of such complexes as potential pericyclic processes will become evident in Section IV,C, where the interplay of kinetic versus thermodynamic control on ketone insertions into carbon-metal bonds is discussed. 

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