Stereochemistry at Carbon

Many experiments have shown that CO insertion typically occurs with retention of configuration at the a-carbon. Racemization has been observed in a few cases that occur by radical paths, but inversion has never been observed. The first imequivocal demonstration of the stereochemistry of the a-carbon during inserfiorf is illustrated in Equation 9.21. The threo alkyl complex afforts the threo acyl complex. 

The stereochemistry at the a-carbon during carbonylation is often deduced from a two-step sequence, such as oxidative addition followed by carbonylation. Because oxidative addition of nonpolar substrates occurs with retention of configuration, as presented in Chapter 6, the insertion step is considered to proceed with retention of configuration when the overall process occurs with retention of configuration. 

CO insertion into alkenyl-metal bonds also occurs stereospecifically in many cases. An example of insertion of CO into an alkenyl complex is shown in Equation 9.23. Complexes containing either cis- or frflns-CH=CHPh groups insert CO with retention of configuration. Similar stereochemical results have been observed for decarbonylation. For example, the reaction of RhCl(PPh3)3 with E-PhCH=C(Et)CHO induces decarbonylation with retention of the double-bond stereochemistry. ° 

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The mechanism of the Sfs2 reaction. The reaction takes place in a single step when the incoming nucleophile approaches from a direction 180° away from the leaving halide ion, thereby inverting the stereochemistry at carbon. 

Single Electron Transfer A single electron transfer (SET) mechanism is often difficult to distinguish from an SN2 reaction because the principal product of these two pathways is the same, apart from the stereochemistry at carbon (race-mization instead of inversion). The radicals formed can recombine rapidly in a solvent cage (inner-sphere ET) [2, 193, 194]. The [HFe(CO)5] -catalyzed deiodina-tion of iodobenzene may serve as an example [179] (Eq. (13)). 

Schwartz and coworkers, in their development of hydrozirconation, studied the stereochemistry at carbon during the alkoxide forming reaction (equation 35).114 115... 

Radical cyclization with iodine atom transfer of a highly functionalized propiolic ester 103 using dibenzoyl peroxide as an initiator gave the a-methylene-y-butyrolactone 104 in good yield [95T11257]. The relative stereochemistry at carbon atoms 4 and 5 are established during the reaction. The intermediate 104 has been converted to the anti-tumor agent (-)-methylenolactocin 105. 

An alkyl halide reacts with a nucleophile to give a substitution product by a mechanism that involves inversion of stereochemistry at carbon ... 

The stereochemistry at carbon is inverted as the C—OH bond forms fully and the bromide ion departs with the electron pair from the former C-Br bond. 

Mechanism Type Species Adding to ML Number of steps Stereochemistry at carbon Stereochemistry of new ligands on the metal... 

The evidence thus available is consistent with the structure XLIX for macrosalhine the relative stereochemistry at carbon atoms 3, 4, 5, 15, 16, and 20 follows from the particular nature of the ring system involved and the only centers of uncertain configuration are C-19 and C-21. The structure XLIX for macrosalhine has been confirmed by the X-ray crystal structure analysis of macrosalhine bromide this work has also defined the relative stereochemistry at C-19 and C-21, as shown in XLIX (34). 

Cleavage of metal-alkyl bonds in d° metal complexes, therefore, occur by an 5 2 mechanism involving direct attack on the metal-alkyl bond. The product of this process is usually formed with retention of stereochemistry at carbon. This stereochemistry implies that the electrophile reacts by frontside attack at the M-C bond, rather than backside attack at the a carbon. Equation 12.21 shows an example of electrophilic attack on a (P metal complex that occurs with retention of configuration at the metal-carbon bond. The coordina-tive unsaturation of the 16-electron Zr(IV) complex may facilitate reaction with retention of configuration because it allows coordination of the incipient Br during the reaction, as depicted in Figure 12.1. 

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