C-O Insertion

Insertion of a ketocarbene moiety into a C—O bond of orthoesters is normally performed with catalysis by BF3 Et20. Copper(II) trifiouromethanesulfonate was found to be a similarly efficient catalyst also, at least in some cases, whereas Rh2(OAc)4 was much less suited to promote this transformation l60). Besides the C/O insertion product 343, the alcohol insertion product 344 and, in reactions with ethyl diazoacetate, the formal carbene dimers were obtained. In agreement with BF3 EtzO, Cu(OTf)2 did not bring about insertion into a C—O bond of trimethyl... 

Ab initio study of the insertion of SiH2 into the C—O and O—H bonds of methanol revealed that a complex is formed in which SiH2 is coordinated to O245. Formation of the complex was found to be exothermic by 19.9 kcalmol-1 at MP2/6-31G //HF/6-31G with barriers for rearrangements to the O—H and C—O insertion products of 0.6 and 20.5 kcalmol-1, respectively. 

Interaction of a metal carbene with an unshared electron pair of an ether oxygen atom leads to an oxonium ylide which is able to rearrange via a 1,2-carbon shift to a formal C—O insertion product. It seems, however, that this is a rather unfavorable pathway, since reports on this reaction mode are scarce >. One may expect... 

To probe the selective activation of the C-OAc bond instead of the weaker carbonyl C-O bond, Liu and coworkers [11] recently conducted DFT calculations on potential transition states during the oxidative addition step (Scheme 2.17). The authors found that Ni complex 97, which was formed during oxidative addition of the weaker carbonyl C-O bond, is indeed lower in energy in comparison to the transition state 95 originating from the oxidative insertion of the desired C-OAc bond. The resulting oxidative addition adduct of carbonyl C-O insertion 98, however, is a tricoordinated Ni(II) species that is higher in energy relative to the C-OAc... 

A reviewer has su ested that following the first C—O insertion, a second reduction step could occur generating another U(ll). Direct P-H elimination or C—O insertion followed by loss of ethylene from this U(II) species would generate another U(II) species. We feel that this step is unlikely, rather, the U(II) species generated from the second reduction would likely insert into another DME molecule. The U(IV) species so generated could subsequently produce ethylene and methyl vinyl ether by the pathways indicated. 

Fullerenes are a range of stable closed-shell carbon molecules and their derivatives, of which Qo is the archetype. The next highest stable member of the series is C70 which is found in small quantities with in arc electrode soot. C,o may be regarded as a Qo molecule with an extra belt of hexagons inserted at the... 

The proposed meehanism for this eyeloearbonylation was shown to involve the insertion of Pd(0) speeies into the C—O bond of the substrate followed by rearrangement to the allenylpalladium intermediate 29 (as proposed for alkyl systems). Insertion of CO and subsequent reduetive elimination may lead to the 2,3-dienoie aeid 30, whieh undergoes eyelization, eatalyzed by traee quantities of an aeid present in the solvent, to the 2(5//)-furanone 28 (Seheme 10) (93JOC1538). 

Recently, a variety of silylenes were generated and characterized by matrix isolation techniques. The observed loose donor adducts between silylenes and the matrix molecules (THF, CO) are only stable at very low temperatures. Melting of the matrix induces polymerization of the silylenes which proceeds through disilenes. However, 0->Si transfer reactions do not occur only in the case of 1-methyl-THF has an insertion of the silylene into the C —O bond been observed [155-158],... 

Initiation resulting from insertion of the monomer into the Al—Cl bond is followed by propagation involving insertion between the porphinato-aluminum and the alkoxide group of the growing polymer, coupled with P-scission of the C—O bond of the oxirane monomer (demonstrated by nmr results) it yields a polyether terminated by a CH2C1 end-group. 

A possible reaction mechanism shown in Scheme 7-10 includes (a) oxidative addition of the S-H bond to Pd(0), (b) insertion of the allene into the Pd-H bond to form the tt-allyl palladium 38, (c) reductive elimination of allyl sulfide, (d) oxidative addition of the I-aryl bond into the Pd(0), (e) insertion of CO into the Pd-C bond, (f) insertion of the tethered C=C into the Pd-C(O) bond, and (g) P-elimination to form 37 followed by the formation of [baseHjI and Pd(0). 

Basically, the cleavage of the C-O bond takes place through two mechanisms. The first mechanism (the hydrogenolytic cleavage) is a hydrogen-assisted bond-cleavage reaction, whereas the second mechanism is an ionic insertion of the metal into the C-O bond. The characteristic features of the two mechanisms are summarized in Table 4.1. 

On Pt and Pd, cleavage of the C-O bond results from a hydrogenolytic cleavage, whereas on Ni and Cu, an insertion mechanism occurs. The regiose-lectivity of the two mechanism is different. The less sterically hindered bond (b) is cleaved on Pt and Pd, whereas the more hindered bond (a) is cleaved on Ni and Cu (Scheme 4.62). 

The mechanism for the formation of complex hydrocarbons through fullerenes is loosely taken from Helden et al.119 and Hunter et al.,120 and is depicted in Figure 2. As in the work of Thaddeus,117 linear carbon clusters grow via carbon insertion and radiative association reactions, although in this case a large number of additional reactions involving neutral atoms such as C, O, and H and neutral molecules such as H2 are also included. Reactions with H and H2 serve to produce... 

Kinetic studies (31P- and H-NMR) both of the direct reaction of 14 with TMS and of the rearrangement of 24 resulted in rate constants which were inconsistent with our original picture of C-Si activation exclusively via C-H insertion. The separate 1st order rearrangement of 24 occurs so slowly that 24, were it the only intermediate in the reaction of TMS with the [(dtbpm)Pt(O)] fragment, would accumulate and lead to an experimentally observable concentration, which was never observed in the kinetic runs. 

One of the possible synthetic ways to obtain heterocyclic phosphines is the insertion of carbonyl compounds into the P—E (E = Si, Ge) bond of sila- and germa-phospholanes. Thus, the enlargement of the ring takes place and the P—C—O—E fragment is formed (9) [Eq. (7)] (74MI1 75JOMC35 77JOM35). The heterocyclic phosphepanes are obtained as a mixture of stereoisomers. 

---