Insertion of carbonyl compounds

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. 

To develop new methods for organic synthesis, Woerpel and coworkers exploited the inherent reactivity of di -fc/ f-butylsilacyclopropanes to create new carbon-carbon bonds in a stereoselective fashion (Scheme 7.7).62 They discovered that transition metal salts catalyze the insertion of carbonyl compounds into the strained carbon-silicon bond to form oxasilacyclopentanes. The regioselectivity of insertion could be controlled by the identity of the catalyst. Copper promoted the insertion of croto-naldehyde into the more substituted C-Si bond of 52 to afford oxasilacyclopentane 53,63 whereas zinc catalyzed the insertion of butyraldehyde into the less substituted bond of 52 to provide the complementary product, 54.64 Oxasilacyclopentanes (e.g., 55) could be transformed into useful synthetic intermediates through oxidation of the C-Si bond,65 66 which provided diol 56 with three contiguous stereocenters. 

Examples of the insertion of carbonyl compounds into metal-oxygen bonds are shown in Table... 

Although the insertion of carbonyl compounds into the A1—O bond may be important in the Tischenko reaction and in the polymerization of aldehydes by aluminium alkoxides , only the group 14 adducts are well characterized. The adducts usually dissociate readily unless the carbonyl group carries a strongly electron-withdrawing substituent. 

The chemistry of germylphosphines continues to develop. A full report of the insertion of carbonyl compounds and other unsaturated systems, e.g. acrylonitrile, into the Ge—P bond of germaphospholans (103) has now appeared. The Ge—P bond in (103) is cleaved by methoxide ion in methanol to form the secondary phosphine (104), which re-forms the germaphospholan during distillation under reduced... 

Tributyltin hydride reduction of carbonyl compounds. The reduction of carbonyl compounds with metal hydrides can also proceed via an electron-transfer activation in analogy to the metal hydride insertion into TCNE.188 Such a notion is further supported by the following observations (a) the reaction rates are enhanced by light as well as heat 189 (b) the rate of the reduction depends strongly on the reduction potentials of ketones. For example, trifluoroacetophenone ( re<1 = —1.38 V versus SCE) is quantitatively reduced by Bu3SnH in propionitrile within 5 min, whereas the reduction of cyclohexanone (Erea — 2.4 V versus SCE) to cyclohexanol (under identical... 

Insertion of carbon monoxide into Csp2—Zr bonds occurs readily at ambient temperatures or below to produce a,(5-unsaturated, reactive acyl zirconocene derivatives [27—29]. Early work by Schwartz demonstrated the potential of such intermediates in synthesis [5d], as they are highly susceptible to further conversions to a variety of carbonyl compounds depending upon manipulation. More recently, Huang has shown that HC1 converts 16 to an enal, that addition of a diaryl diselenide leads to selenoesters, and that exposure to a sulfenyl chloride gives thioesters (Scheme 4.11) [27,28]. All are obtained with (F)-stereochemistry, indicative of CO insertion with the expected retention of alkene geometry. 

In addition to reactions characteristic of carbonyl compounds, Fischer-type carbene complexes undergo a series of transformations which are unique to this class of compounds. These include olefin metathesis [206,265-267] (for the use as metathesis catalysts, see Section 3.2.5.3), alkyne insertion, benzannulation and other types of cyclization reaction. Generally, in most of these reactions electron-rich substrates (e.g. ynamines, enol ethers) react more readily than electron-poor compounds. Because many preparations with this type of complex take place under mild conditions, Fischer-type carbene complexes are being increasingly used for the synthesis [268-272] and modification [103,140,148,273] of sensitive natural products. 

Since many of the reactions of C atoms are extremely exothermic, it may be that they proceed without an enthalpic barrier. Thus, selectivities observed in C atom reactions may result from free energy barriers in which entropy considerations are the major factor. In discussions of C atom reactions, we shall see that carbenes are often intermediates. Two ways in which carbenes can be produced in C atom reactions are C—H insertion (Eq. 7) and deoxygenation of carbonyl compounds (Eq. 8). In several cases, the same carbene has been generated by both methods. When this comparison has been made, the reactions will be discussed together even though they represent different aspects of C atom reactivity. 

Figure G shows some insertion reactions of carbonyl compounds. In the isocyanate and ketene cases, the addition takes place, not to the carbonyl double bond, but to the carbon—nitrogen or the carbon—carbon double bond.

Dormond ef rrZ. have exploited compounds 30 and particularly 32 as synthetically useful reagents. The uranium-mediated methylenation of carbonyl compounds (cf.. the Wittig, Peterson or Tebbe reactions) is illustrated in Equation (5.1). The instantaneous formation under ambient conditions of the insertion product 33 was essentially quantitative (e.g./ =... 

Addition of hydrosilane to alkenes, dienes and alkynes is called hydrosilylation, or hydrosilation, and is a commercially important process for the production of many organosilicon compounds. As related reactions, silylformylation of alkynes is treated in Section 7.1.2, and the reduction of carbonyl compounds to alcohols by hydrosilylation is treated in Section 10.2. Compared with other hydrometallations discussed so far, hydrosilylation is sluggish and proceeds satisfactorily only in the presence of catalysts [214], Chloroplatinic acid is the most active catalyst and the hydrosilylation of alkenes catalysed by E PtCU is operated commercially [215]. Colloidal Pt is said to be an active catalytic species. Even the internal alkenes 558 can be hydrosilylated in the presence of a Pt catalyst with concomitant isomerization of the double bond from an internal to a terminal position to give terminal silylalkanes 559. The oxidative addition of hydrosilane to form R Si—Pt—H 560 is the first step of the hydrosilylation, and insertion of alkenes to the Pt—H bond gives 561, and the alkylsilane 562 is obtained by reductive elimination. 

ZnBr2 also is an effective catalyst for the carbonyl insertion (Equation (75)).290 The Zn-catalyzed reaction is applicable to various aldehydes and ketones including aliphatic compounds. In sharp contrast to the Cu-catalyzed reaction, the carbonyl insertion occurs on the less substituted side with high regioselectivity. ZnBr2 most likely serves as electrophilic activation of carbonyl compounds. 

The catalytic hydrosilation and hydrogenation of carbonyl compounds in the presence of chiral organometallic catalysts is an important method for the catalytic synthesis of chiral synthons. Again, the actual insertion of the carbonyl group into an M-H bond at the catalyst site is rarely observed. ... 

Many of the combinations of reactions illustrated earlier [11.5.1, reactions (c)-(j)] can be identified in the insertion reactions of carbonyl compounds. Polyaddition [11.5.1, reaction (f)] occurs when tributyltin(IV) methoxide is treated with an excess of chloral, but the reaction of chloral and of a weaker accepter gives instead an equilibrium mixture of the two possible adducts, perhaps through a six-centered process ... 

After oxidative addition of an unsymmetrical cr bond (E-H) to the metal, an unsaturated compound can insert into either the M-E or the M-H bond. In some cases, such as the hydrosilylation of carbonyl compounds, the 7r bond of the substrate inserts into the M-E bond, whereas, in others, insertion of the ir bond of the substrate into the M-H bond occurs faster. In any case, either pathway gives the same product after reductive elimination. 

Photolysis reactions of 9,10-disilaphenanthrene in the presence of ketones or formaldehyde have been studied in detail (Scheme 8) the product distribution obtained depends on the type of carbonyl compound used <87CL983>. Insertion reactions of this phenanthrene are also shown (Scheme 8) <87CL1075>. 

The Criegee mechanism of ozonolysis characterizes the interaction of dipolar, electronically deficient ozone with the Jt-electron-rich C=C bond forming a cyclic intermediate [16, 17]. It rearranges with insertion of an O atom into the C-C bond and finally hydrolyzes in two moles of carbonyl compound with the formation of one mole of hydrogen peroxide. This mole of peroxide acts in situ as an oxidant of aldehyde to the carboxylic group. Oxidation of aldehydes can be completed with a number of reagents two are cited in Scheme 5.56. 

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