Inversion alkyl carbon

Diaziridines also show slow nitrogen inversion, and carbon-substituted compounds can be resolved into enantiomers, which typically racemize slowly at room temperature (when Af-substituted with alkyl and/or hydrogen). For example, l-methyl-3-benzyl-3-methyl-diaziridine in tetrachloroethylene showed a half-life at 70 °C of 431 min (69AG(E)212). Preparative resolution has been done both by classical methods, using chiral partners in salts (77DOK(232)108l), and by chromatography on triacetyl cellulose (Section 5.08.2.3.1). 

More recently, a preparation of iV-alkylated pyridotriazolones was developed using a Mitsunobu protocol, pointing out the acidity of the NH group. Compound 54 (Equation 3) displaced the hydroxyl group with classical inversion at carbon, leading to 55 <2005SC2939>. 

A. Inversion at a Primary Alkyl Carbon Bonded to Zinc. 217... 

TABLE 6. Attempts to measure secondary alkyl carbon-metal bond inversion by NMR"... 

An O-bonded sulfinate is a likely intermediate en route to the sulfinate-S product. An alternative radical chain mechanism has been proposed to explain SO2 insertion reactions in some alkylcobaloxime complexes that also proceed with inversion of configuration at the alkyl group. It is interesting to contrast the stereochemistry of SO2 (inversion at carbon) with that of the weak electrophile CO2 (retention at the alkyl carbon). 

A backside attack of C02> on the alkyl carbon, analogous to the SO2 insertion mechanism was ruled out by studies of the a-carbon stereochemistry upon insertion (25). The insertion of CO2 into the metal carbon bond of threo-cis-W(CO) (L)(CHD-CHD-Ph) (L = CO and PMc3) proceeds with retention of configuration at the a-carbon (Scheme 1) (26). This is in contrast to the inversion of configuration at the alpha carbon found in backside SO2 insertion reactions. 

With alkyl carbon-aluminum bonds, NMR studies have shown that configurational inversion occurs more readily than most vinylalanes, namely, between 50 and 100°C (49, 94). Accordingly, although syn-hydralumination of suitable olefins might lead to stereochemically defined carbon-aluminum bonds (86), configurational inversion might well occur at a rate ( 2) comparable to hydralumination (k ) [Eq. (43)]. 

In the case where XY is an alkyl halide, another mechanism is possible. This is the normal nucleophilic substitution with inversion at carbon (see Fig. 14c), followed by the addition of the halide ion in a second step. There is some evidence against this mechanism, but the critical test of determination of configuration at carbon remains undone. When Y is a poor leaving group such as H or CHi, of course the two-step mechanism is impossible, and concerted addition is the only plausible mechanism. 

Preparation.—Alcohols (or their trimethylsilyl ethers) can be converted into alkyl iodides, with inversion at carbon, by reaction with trimethylsilyl iodide [equation (12)]. 

It has been observed that this reaction proceeds with inversion at the alkyl carbon bonded to Fe, and the mechanism is believed to involve electrophilic attack of SOj at this C followed by rearrangement to the 0-bonded isomer. An analogous pathway has been found d for SOj insertion into W(CO)5(Y(CH3)3)", where Y= Si or Sn. 

The reduction of cyanocobalamin (b, R=CN) occurs in two one-electron steps, through Bj2r cobalt(II) species which exhibits radical character, to an anionic strongly nucleophilic species. Reduction of the model cobaloxime [Co(dmgH)2Clpy] proceeds similarly. The best route to alkyl derivatives is to react (Cb) with an alkyl halide or tosylate. These (Cb) species are very strong nucleophiles to saturated carbon centres. With tosylates substitution occurs with inversion at carbon, indicating an Sj, 2 mechanism. 

Cleavage of the Zr—C c-bond of [(rj-CBH5)aZr(Cl)(CHDCHDBu )] (1) with bromine and iodine proceeds with retention of configuration at carbon, in contrast to M—C bond cleavage in alkyl-iron and -cobalt complexes in which inversion at carbon is the norm (see Scheme 1). This difference is attributed to the fact that with... 

When an allyl group migrates, an additional stereochemical feature arises because of p-orbital of migrating carbon of alkyl group. In this case, the thermal [l,3]-suprafacial alkyl shift is an allowed process with inversion of configuration of migrating alkyl carbon and the thermal suprafacial [l,5]-alkyl shift is also allowed process with retention of configuration of alkyl carbon (Pig. 4.5) [1, 2],... 

Thermal [1,3]-suprafacial hydrogen shift is orbital symmetry forbidden process, but [1,3]-suprafacial alkyl shift is symmetry allowed process with inversion of configuration of migrating alkyl carbon. For example, the thermal rearrangement of bicyclo-[3.2.0]-heptene 1 to bicyclo-[2.2.1]-heptene 2 [4]. 

Sulfur dioxide is a strongly electrophilic species with a vacant orbital on sulfur, which it can use to attack even 18e metal complexes. Wojcicki and co-workers have studied these reactions in detail and find that the SO2 can give electrophilic attack at the a carbon of the alkyl from the side opposite the metal, which leads to the formation of an alkyl sulfinate ion (RS02 ) with inversion at carbon. Since the anion has much of its negative charge on the oxygens, it is not surprising that the kinetic product of ion recombination is the 0-bound sulfinato complex. On... 

Alkyl groups can be exchanged between metals with inversion at carbon. This transmetalation reaction provides a route for the racemization of a metal... 

A further useful preparative method of esterification is based on the reaction of carboxylic acids with acetals of A, A-dimethyl-formamide (21) [66]. The process occurs under mild conditions (1-2 hr, 20-80°) without acid or base catalyst and generally gives high yields (80-95%) of esters (methyl, ethyl, and benzyl). A two-step mechanism has been proposed (a, b—Fig, 5.10) and the second stage is envisaged as a normd Sn2 displacement proceeding with inversion at the alkyl carbon atom ( ). 

---