P C—H bonds

The transition state for disproportionation requires overlap of the p C—H bond undergoing scission and the p-orbital containing the unpaired electron.18 This requirement rationalizes the specificity observed in disproportionation of radicals 29 (Section 1.4,2) and provides an explanation for the persistency of the triisopropylmcthyl radical (33) and related species (Section 1.4.3.2).166 In the case of 33, the P-bydrogens are constrained to lie in the nodal plane of the p-orbital due to stcric buttressing between the methyls of the adjacent isopropyls. 

Recently, Rankin et al. [84] obtained the X-ray structure of the stable pnictinyl radical and observed a very unusual conformational arrangement, i.e., a syn-syn arrangement of the H-C-P-C-H bond sequence (Fig. 7). 

The detailed decomposition (P-H ehminahon) mechanism of the hydrido(alkoxo) complexes, mer-crs-[lr(H)(OR)Cl(PR 3)3] (R = Me, Et, Pr R = Me, Et H trans to Cl) (56, 58, 60), forming the dihydrides mer-cis-[lr H)2Cl PR )2] (57, 59) along with the corresponding aldehyde or ketone was examined (Scheme 6-8). The hydrido(ethoxo) as well as the hydrido(isopropoxo) complexes 60 could also be prepared by oxidative addition of ethanol and isopropanol to the phosphine complexes 39 [44]. In the initial stage of the P-H elimination, a pre-equiUbrium is assumed in which an unsaturated pentacoordinated product is generated by an alcohol-assisted dissociation of the chloride. From this intermediate the transition state is reached, and the rate-determining step is an irreversible scission of the P-C-H bond. This process has a low... 

The basic ethoxide ion begins to remove a proton from the P-carbon using its electron pair to form a bond to it. At the same tim, the electron pair of the P C-H bond begins to move in to become the 7t bond of a double bond, and the bromide begins to depart with the electrons that bonded it to the a carbon. 

In radical terminology, the opposite of transience is not stability but persistence.14 Persistent radicals do not react with themselves at diffusion-controlled rates however, they may still react readily with other radicals or with triplet oxygen. Thus, persistence is a kinetic property that is more often related to sterically hindered recombination than to electronic stabilization. Persistent radicals typically also lack P-C—H bonds and they cannot disproportionate. Several persistent radicals are illustrated in Scheme 4. Persistent radicals are rarely present in synthetic applications, but when they are, there will be important consequences. 

In addition to insertion into p-C—H bonds, cyclopropylidenes can undergo other reactions such as alkylation (c/. Section 4.7.3.2), dimerization, insertion into C—H bonds of the ether solvent (equation 60)183 or reaction with alkenes to afford spirocyclopropanes (equation 61).184 Addition of stoichiometric amounts of Bu OK has been shown to promote the reactions of lithium carbenoids, even at -83 C, with THF to give the insertion product (equation 62).185 Addition to alkenes is also promoted under these conditions. Intramolecular addition of the carbenoid to double bonds has been exploited in the synthesis of spirotricyclic compounds (equation 63).186... 

Insertions into O—H bonds, N—H bonds and C—H bonds adjacent to oxygen and nitrogen have found use in the synthesis of a number of heterocycles (see Section 4.7.3.9).187-189 The reaction of alcohol (25) with methyllithium in ether, leading to the ketone (26), has been interpreted190 as involving the insertion of an intermediate carbenoid into a P-C—H bond followed by ring opening (equation 64). 

The C-H bonds at the P position relative to R3M- (M = Si, Ge, Sn, Pb) substituents are activated towards attack by electrophilic reagents. Two types of electrophilic attack at a P C-H bond are considered in Scheme 3 Path 1 involves hydride abstraction by the electrophile, resulting in the formation of a carbenium ion intermediate, a process that is assisted by the metal P-effect. Such a pathway might be expected to be followed by strongly Lewis acidic reagents, such as carbenium ion reagents. 

Path 2 involves the insertion of an electrophilic carbene reagent into the P C-H bond. Although this pathway does not involve the development of a formal positive charge on the P carbon, it is well known that there is a significant buildup of positive charge on the carbon at the transition state for the insertion reaction.57 Both pathways should, then, display significant P-metal effects. In a hypothetical situation in which there is more than one C-H bond p to the metal substituent, the... 

Activation by silicon of a P-C-H bond to an intramolecular carbene insertion reaction is exemplified by the silicon-directed Bamford-Stevens reaction.68 For example, thermal decomposition of P-trimethylsilyl /V-aziridinyl imines 72 in toluene (Scheme 8) [with or without Rh2(OAc)4 catalyst] results in the formation of allylic silanes 73 as major or exclusive products by the preferential insertion of the carbene intermediate into the C-H bond P to the silicon substituent. 

P-Lactams,2 Decomposition of N-benzyl-N-r-butyldiazoacetoacetamide (1) with Rh2(OAc)4 in refluxing benzene results iti the P-lactam 2 in 98% yield. The acetyl group on the diazo carbon is necessary for this insertion into the p-C—H bond rather than reaction of the carbene with the aromatic ring. The /-butyl group... 

The electron pair in the p C-H bond forms the new n bond between the a and p carbons. 

In conformation A (equatorial Ci group), a p C-H bond and a C-CI bond are never anti periplanar therefore, no E2 elimination can occur. 

Two bonds are broken and two bonds are formed in a single step the base (pyridine) removes a proton from the p carbon the electron pair in the p C- H bond forms the new n bond the leaving group ( OPOCy comes off with the electron pair from the C-O bond. 

Most of the reactions where a side bond of bicyclobutane is formed involve the insertion of a carbenoid generated from a geminate dibromocyclopropane into a P-C-H bond. In some cases the yield of these reactions is almost quantitative (equation 8) . ... 

The synthesis of model compounds from l,l,l,3,3-pentachloro-3-oxo-lA., 3A. -diphosphaz-l-ene (Fig. 7) confirmed that this type of rearrangement occurs during anion exchange of the peralkylated polyaminophosphonium chloride oligomers derived from secondary amines containing p C-H bonds. 

Whilst the mechanism has not been proven, it is proposed that this rearrangement is similar to Hoffman rearrangement reactions, which occur with quaternary ammonium compounds containing P C-H bonds [16]. With the peralkylated polyaminophosphonium hydroxides, this rearrangement reaction occurs readily at room temperature, suggesting that in these molecules nitrogen has sufficient electropositive character to facilitate the rearrangement reaction. 

It could be expected that if a simple a-diazo ketone with P-C—H bonds were exposed to the rhodium catalyst, metallocarbene formation would proceed as usual, but that P-hydride elimination would com-... 

It must be pointed out that there are other complications regarding the addition of alkyl C-H bonds to a metal. The resulting metal alkyl complex could undergo either reductive elimination back to starting material (Section 7-3) or, if a p C-H bond is present, rapid... 

A common motif in organometallic chemistry is the agostic interaction, which can act to stabilize low-coordination low-e-count complexes. The requirement is an alkyl group with a P- or a 7-C—H bond attached to the metal within reach of (i.e., cis to) an empty coordination site. An attractive interaction occurs with the C—H bond acting as a 2e donor into the low-lying metal valence orbital that occupies that site. In the case of a P-C—H bond, hydride transfer may occur with little activation, resulting in an M—H sigma bond and a n complex with an alkene as discussed above. 

Multiple exchange proceeds most readily in the presence of P-C-H bonds, which can be explained by p-elimination leading to an olefin-hydrido complex either with a single metal atom or with the participation of a neighboring metal atom. The importance of intermediate complex formation with an olefin is confirmed by the absence of exchange via a quaternary carbon atom. Cyclopentane in the isotope exchange with molecular deuterium catalyzed by metals of... 

VIII-8 and of the S p -C-H bond of the methyl group of VIII-9. Additional heating of complexes VIII-8 and VIII-9 led to the palladium mediated cleavage of an sp -C-sp -C bond between two non-activated carbons to produce derivative VIII-10 [38] (Scheme VIII.6). 

As the rest of this section shows, any haloalkanc with a p C-H bond cun undergo E2 elimination. In the case of 1° and 2° halides, E2 and Sn2 reactions compete. However, as the following sections in the text and below will show, it is very easy to predict the favored products in these cases. 

In all cases, the quantum yield of the molecular elimination of either methane or hydrogen is considered to be smaller than O.OS. Thus, the main primary processes involve either the a(C-C) or the P(C-H) bond ruptures. This observation differs from that made for ethylene, where at least 40% of the fragmentation involves the molecular elimination of hydrogen. May this behavior be linked to the differences observed in the absorption spectra At least, it may be said that well-defined absorption bands, one of which is probably Rydberg in nature, are observed in the ethylene spectrum. Conversely, the spectra of methyl substituted ethylenes are rather unstructured (1). The UV absorption spectrum of 1-butene is shown in Figure 4. We shall come back lat to diis point. 

Section 5.17 A P C—D bond is broken more slowly in the E2 dehydrohalogenation of alkyl halides than a p C—H bond. The ratio of the rate constants / h/ d is a measure of the deuterium isotope effect and has a value in the range 3-8 when a carbon-hydrogen bond breaks in the rate-determining step of a reaction. 

It has been found that this oxidative cyclization does not exhibit kinetic isotope effect ( h/ d = 10), thus indicating that the cleavage of the p-C-H bond in the amidine adduct is not involved into the rate-determining step. Although the exact mechanistic details are not clear yet, it is assumed that the I species are initially coordinated with the amidine nitrogen atom, followed by ring closure and elimination of iodobenzene and acetic acid from the azoline intermediate 66 (Scheme 42). It is worth mentioning that the reaction proceeds under mild conditions and is free from acid or metal catalyst. 

In addition, the imine-directed Rh-catalyzed olefinic C—H bond functionalization reaction has also been employed in the asymmetric synthesis of ( )-incarvillateine by the groups of Bergman and Ellman. The chiral o,p-un-saturated imine substrate 121 was subjected to slightly modified reaction conditions for the p-C—H bond functionalization and was further transformed to the desired chiral y-lactam 124, the key building block for the synthesis of ( )-incarvillateine via subsequent reduction/condensation sequence (Scheme 5.43). 

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