Insertion O-H bonds

Triplet diphenylcarbene, detected spectroscopically on excimer laser flash photolysis of diphenyldiazomethane, readily abstracts hydrogen from cyclohexane to give the CHPh2 radical.Analogous hydrogen abstraction also occurs from alcohols but at low temperatures and in a polycrystalline matrix in contrast to reaction in solution in which O-H bond insertion is preferred. In a similar fashion, irradiation of diphenyldiazomethane in polycrystalline (S)-butan-2-ol at — gave amongst other products an enantiomerically pure... 

Ethyl alkoxyaceUUes. Various platinum(II/rV) salts and complexes catalyze the O-H bond insertion of alcohols, phenols, and acetic acid hy carbenoids generated from ethyl diazoacetate. PtClj is slightly more efficient than PtCl. ... 

The catalyst Cu/(S S,S)-23a was also efficient for the insertion of O—H bonds of phenols (Scheme 44) [109], Under similar conditions as that for N—H insertion reactions, a wide range of phenol derivatives underwent O—H bond insertion with a-diazopropionates, providing a-aryloxypropionates with excellent enantioselectivities (95-99.6% ee). The Pd/(S, S,5 )-23a-catalyzed asymmetric O—H bond insertion reaction between a-aryl-a-diazoacetates and phenols provided the first enantioselective method for the preparation of chiral a-aryloxy-a-arylacetates, which are ubiquitous in biologically active molecules (Scheme 45) [110]. 

Rhodium carboxylates. 13, 266-269 15, 278-280 16, 289-292 17, 298-302 a-Alkoxy esters. Rhodium carbenoids derived from a-diazo esters undergo O-H bond insertion in the reaction with alcohols or phenols. Low to moderate asymmetric induction from chiral esters is observed. ... 

Since the strategies above to construct oxa-bridge before IMDA were failed, new reaction systems should be built up via literature survey. a-Diazo carbonyl compounds could be catalyzed by Rh (11) to form carbene species, which would trigger the intramolecular O-H bond insertion reaction [4-6]. The background of the reaction will be introduced in the following part. 

In 2009, Prof. Yu s group from Peking University concluded the generally accepted O-H bond insertion mechanism of metal carbene [8], as shown in Fig. 3.25. 

Prof. Yu summarized that the metal carbene O-H bond insertion mechanism includes three steps. Step a is that transition metal catalyst decomposes the a-diazo carbonyl compound A and releases nitrogen to form the metal carbene product B Step b is that metal carbene product B and the substrate R2OH form metal-associated oxonium ylide C Step c is that metal-associated oxonium ylide (C or... 

Fig. 3.25 The general O-H bond insertion mechanism (Reprinted with the permission from Ref. [8]. Copyright 2009 American Chemical Society)...

D) or free oxonium ylide E triggers [1,2]-hydrogen shift to form the product F with regeneration of the metal catalyst. They calculated that, for the Rh (Il)-catalyzed O-H bond insertion reaction, the product F was formed through the metal-free oxonium ylide E. For Cu (I)-catalyzed O-H bond insertion reaction, product F tended to be formed via oxonium ylide coordinated by monovalent copper ion. 

Professor Zhou s group from Nankai University reported that chiral spiro ligands developed in their laboratory cooperated with monovalent copper can catalyze intramolecular asymmetric O-H bond insertion, which could be used to construct various ring systems, including five-, six- and seven-membered ring [10]. Catalyzed by copper(I), the a-diazo carbonyl compounds containing primary... 

The general route of total synthesis of natural product Maoecrystal V is shown in Fig. 3.35. Starting from the known compound 3.13, utilizing the rhodium-catalyzed intramolecular O-H bond insertion reaction, a high-tension seven-membered... 

These carbene (or alkylidene) complexes are used for various transformations. Known reactions of these complexes are (a) alkene metathesis, (b) alkene cyclopropanation, (c) carbonyl alkenation, (d) insertion into C-H, N-H and O-H bonds, (e) ylide formation and (f) dimerization. The reactivity of these complexes can be tuned by varying the metal, oxidation state or ligands. Nowadays carbene complexes with cumulated double bonds have also been synthesized and investigated [45-49] as well as carbene cluster compounds, which will not be discussed here [50]. 

Rh(Por)l (Por = OEP. TPP, TMP) also acts as a catalyst for the insertion of carbene fragments into the O—H bonds of alcohols, again using ethyl diazoacetate as the carbene source. A rhodium porphyrin carbene intermediate was proposed in the reaction, which is more effective for primary than secondary or tertiary alcohols, and with the bulky TMP ligand providing the most selectivity. ... 

Pd, or Ni (Scheme 5-3). First, P-H oxidative addition of PH3 or hydroxymethyl-substituted derivatives gives a phosphido hydride complex. P-C bond formation was then suggested to occur in two possible pathways. In one, formaldehyde insertion into the M-H bond gives a hydroxymethyl complex, which undergoes P-C reductive elimination to give the product. Alternatively, nucleophilic attack of the phosphido group on formaldehyde gives a zwitterionic species, followed by proton transfer to form the O-H bond [7]. 

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. 

The mechanism involves a metal atom insertion into the O—H bond, thus resulting in the formation of an adsorbed metal—OH species (at the same or similar binding site) and a new metal—H bond. This is a classic bond activation process, which involves a significant stretch of the O—H bond in order to lower the antibonding ooh orbital to enable it to accept electron density from the metal. The reaction has been calculated by DFT to be endothermic by +90 kJ/mol over Pt(lll) surfaces with an activation barrier of +130 kJ/mol [Desai et al., 2003b]. 

The equilibrium interconversion between an ethylene phosphite and a bicyclic spirophosphorane is shown to proceed by the insertion of the phosphite into the labile O-H bond of the hydroxyethyl ester. The mechanism is similar to the insertion of carbenes or nitrenes. Energy relationships of reaction intermediates were studied by MO RHF, MP2(full), MP4SDTQ, and DFT calculations. In most cases, they predicted that hydroxyethyl ethylene phosphates were more stable than the strained spirophosphoranes, which is not supported by the experimental evidence. The best correspondence to experimental data was obtained by DFT calculations with Perdew-Wang correlation functions <2003JST35>. 

Olefinic alcohols other than allyl alcohol display a preference for O/H insertion which is quite similar to that of the latter and rather independent of the particular compound 162). Relative reactivity studies show, however, that an allylic O—H bond reacts faster than a non-allylic one, and that steric hindrance slows... 

Diazo compounds and oxonium salts are the most efficient alkylating agents in the synthesis of alkyl nitronates. It is assumed that diazo compounds are inserted into the O-H bond in the aci forms of the corresponding AN, whereas oxonium salts generally react with AN anions. 

The insertion reaction chemistry of the well-known germylene Ge [CH(Si M03)3)2 50 is very diverse.154 323-328 This species inserts readily into the C-H bonds of mono- and dinitrile substrates (Scheme 47),154 the C-H bonds of ethers and alkanes in the presence of Phi (Scheme 48),323 the C-H bonds of ketones in the presence of MgCE (Scheme 49) and the O-H bonds of the tautomer enols in the absence of MgCl2 (Scheme 50),324 and the C-H bonds of diketones resulting in cyclic products (Scheme 51). 

Electrophilic carbene complexes can react with amines, alcohols or thiols to yield the products of a formal X-H bond insertion (X N, O, S). Unlike the insertion of carbene complexes into aliphatic C-H bonds, insertion into X-H bonds can proceed via intermediate formation of ylides (Figure 4.7). 

Electrochemical reductions of CO2 at a number of metal electrodes have been reported [12, 65, 66]. CO has been identified as the principal product for Ag and Au electrodes in aqueous bicarbonate solutions at current densities of 5.5 mA cm [67]. Different mechanisms for the formation of CO on metal electrodes have been proposed. It has been demonstrated for Au electrodes that the rate of CO production is proportional to the partial pressure of CO2. This is similar to the results observed for the formation of CO2 adducts of homogeneous catalysts discussed earlier. There are also a number of spectroscopic studies of CO2 bound to metal surfaces [68-70], and the formation of strongly bound CO from CO2 on Pt electrodes [71]. These results are consistent with the mechanism proposed for the reduction of CO2 to CO by homogeneous complexes described earlier and shown in Sch. 2. Alternative mechanistic pathways for the formation of CO on metal electrodes have proposed the formation of M—COOH species by (1) insertion of CO2 into M—H bonds on the surface or (2) by outer-sphere electron transfer to CO2 followed by protonation to form a COOH radical and then adsorption of the neutral radical [12]. Certainly, protonation of adsorbed CO2 by a proton on the surface or in solution would be reasonable. However, insertion of CO2 into a surface hydride would seem unlikely based on precedents in homogeneous catalysis. CO2 insertion into transition metal hydrides complexes invariably leads to formation of formate complexes in which C—H bonds rather than O—H bonds have been formed, as discussed in the next section. 

The quenching of a triplet carbene reaction with methanol is frequently used as the standard means of probing the singlet-triplet gap. It is widely believed that singlet carbenes insert readily into the O—H bonds of methanol, while the triplet states undergo hydrogen abstraction from the C—H bonds." The behavior of diarylcar-... 

The LFP of diphenyldiazomethane ( DDM ) in a variety of solvents produces triplet diphenylcarbene ( DPC, 14a), whose transient absorption is readily monitored. The optical absorption spectrum of DPC is quenched by methanol and yields the product of O—H insertion, suggesting that DPC is quenched by the O—H bond of methanol. The quenching rate constant (fex) is determined to be 6.8 X 10 M s in benzene. ... 

The reaction with 4-hydroxy-TEMPO leading to the ether is particularly interesting as the reaction could be spin allowed (i.e., doublet - - triplet —> doublet) if sufficient interaction between the O—H and nitroxide centers takes place. However, the EPR parameters for 4-hydroxy-TEMPO suggest that the interaction between the two sites is small.The magnitude of the relaxation of spin conservation rules seems unclear, but the kinetic results show virtually no effect. The rate constant for insertion at the O—H bond is 2 x 10 s, which is essentially the... 

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