2-Oxaallyl

Use orbital interaction theory to develop the orbitals of the 2-oxaallyl system, R2C— —CR2, also known as a carbonyl ylide. Show why 2-oxaallyl readily reacts with alkenes and alkyne in a 4 + 2 cycloaddition reaction (an example may be found in El-Saidi, M. Kassam, K. Pole, D. L. Tadey, T. Warkentin, J., J. Am. Chem. Soc., 1992, 114, 8751-8752). 

Molybdenum and tungsten C-enolates can be generated by reaction of complexes with a-chlorocarbo-nyls (Scheme 67). These 2-oxaallyl(T) -C-enolate) complexes react with aldehydes in a photoreaction to produce aldol products, by way of the ii -enolate. 

Overlap, effect of, 44-45 general principles, 52 integrals, 37 matrix, 230 sigma versus pi, 45 2-Oxaallyl, 309 2-Oxacyclohexanol, 310-311 Oxaloacetic acid, 12 2-Oxanol, 310-311 Oxetane... 

Radicals containing heteroatoms (e.g. N, O) as it-centers are included when, in terms of valence bond resonance structures, the unpaired electron is not located at the heteroatom (e.g. 2-azaallyl, 2-oxaallyl). 

The catalytic cycle for the Rh-catalyzed 1,4-addition of phenylboronic acid to an a,[5-unsaturated ketone could be nicely described by in situ P NMR (see Figure 1.4) [19]. The three S -Binap species RhPh(PPh3)(Binap), 11, Rh(oxaallyl)(Binap), 12 and [Rh(OH)(Binap)]2, 13, have all been detected. Complex 11 affords a modestly complicated spectrum (see spectrum A), due to the ABMX spin system. The oxa-allyl complex, 12, in spectrum B exists in two diastereomeric forms (with overlapping signals between 48 and 49 ppm). The resonances for the bridging hydro-... 

Tanaka has recently reviewed the hydrogenation of ketones with an emphasis on the mechanistic aspects of the reaction.233 Numerous references related to this subject can be found in his article. Deuteration of cyclohexanones and an application of NMR spectroscopy to the analysis of deuterated products have revealed that on ruthenium, osmium, iridium, and platinum, deuterium is simply added to adsorbed ketones to give the corresponding alcohols deuterated on the Cl carbon, without any deuterium atom at the C2 and C6 positions, while over palladium and rhodium the C2 and C6 positions are also deuterated.234 A distinct difference between rhodium and palladium is that on rhodium deuterium is incorporated beyond the C2 and C6 positions whereas on palladium the distribution of deuterium is limited to the C2 and C6 carbons.234,235 From these results, together with those on the deuteration of adamantanone,236 it has been concluded that a Tt-oxaallyl species is formed on palladium while deuterium may be propagated by an a, 3 process237 on rhodium via a staggered a, 3-diadsorbed species. 

Excess hydridocobaltcarbonyl reduces a, -unsaturated ketones and aldehydes in moderate yield and good regioselectivity. The reaction involves complexation of the double bond to cobalt, followed by migratory insertion of hydride into the enone, forming an oxaallyl cobalt complex. Poor chemoselectivity is one of the major drawbacks of this reaction, as simple alkenes are rapidly hydroformylated to the corresponding aldehyde under the reaction conditions (25 °C, 1 atm of CO). 

As in hydrogen transfer between alcohols and saturated ketones, the rate-determining step in the reaction with a,p-unsaturated ketones is hydrogen abstraction from the a-carbon atom. It has been suggested that the hydrogen atom is transferred directly to the P-carbon of the enone, yielding an in -oxaallyl complex which following protonation yields the saturated ketone. ... 

This chapter focuses the attention on the reactions of nonstabilized carbanionic compounds such as alkyl, vinyl, aryl, alkynyl metals, etc., and the chemistry of the stabilized system, i.e. allylic, propargylic or oxaallylic carbanions is presented in Volume 2 of this series. Electrophiles with C=X bonite which are discussed include aldehydes, ketones, epoxides, aziridines, acetals, orthoesters and imines, all of which turn into highly reactive electrophiles in the presence of Lewis acids. 

An example of an oxaallylic carbanion has been described by Oppolzer et alP In the alkylation of 3-triethylsilyloxypentadienyllithium (5 X = H) the y-alkylated product predominated. With the anion (5 X = SMe) alkylation occurs exclusively at the y-carbon atom. The synthetic potential of the anions (5) was demonstrated by the synthesis of bicyclic compounds via intramolecular Diels-Alder reactions of the alkylation products (Scheme 5). 

Hoppe has investigated the regioselectivity of the reaction of oxaallylic anions generated from the N,N-diisopropyl carbamates of a variety of allylic alcohols. With carbonyl compounds reaction was found to take place predominantly at the y-carbon. The directing influence of the carbamoyloxy group was unfortunately diminished in alkylation reactions. 

The anions generated from the adducts of a, 3-unsaturated aldehydes and (8) are alkylated exclusively at the y-position. Evans et al. have also reported the alkylation of similar oxaallylic anions and the alkylation chemistry of diethyl (2-trimethylsilyl)ethoxymethylphosphonate has been reported. ... 

The enolates (48) and (49) of the transition metals tungsten, rhenium and molybdenum can be successfully prepared by the nucleophilic displacement of a-chloro ketones and a-chloro esters with the appropriate transition metal anion (Scheme 6). They are isolated as C-bound enolate derivatives and, except for the rhenium enolate (49), do not undergo thermal aldol additions to benzaldehyde. However, Bergman and Heathcock et al. have found that an aldol reaction of complex (48) with benzaldehyde can occur on irradiation via the rearranged q -oxaallyl derivative (50), where the metal aldolate (51) can then be... 

The relatively low-temperature conversion of MSD to MBD can reasonably be attributed to the formation of a trimethylenemethane biradical, presumably in its singlet state, which is further stabilized by a cyclopentadienyl moiety (Scheme 9.29). Of interest is the extent to which this species might be stabilized by electron transfer to form a species reminiscent of the oxaallyl zwitterion from cyclopropa-none (see Chapter 5, Section 2). Of further interest is the fact that the dimers formed are not those from a trimethylenemethane triplet state. 

The complex obtained from the reaction of phenacyl bromide with penta-cyanocobaltate(II) is assigned structure(II), a Tt-oxaallyl type (11), on the basis of absence of carbonyl absorption (IR) and vinylidene hydrogen (NMR) acidification releases acetophenone. Evidence is being sought for the possible intermediacy of a ir -homoallyl complex (structure III) in the reduction of bicyclo[2,2, l]heptadiene. 

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