Unstable organometallic reagents

Below are described five approaches to epoxide synthesis by way -.if haLohydrins. These halohydrina may be isol ble purifi ble intermediates or they may be transient, unstable species that undergo spontaneous ring closure under the conditions used to generate them. Jho former are typical of (1) addition of hypobalous acids to olefins 111 chemical reduction of a-h Iocarbonyl compounds, and (3) addition if organometallic reagents to o-halocarbonyl compounds the latter,... 

It is, therefore, to be expected that a compound such as trimethyl-methylene stiborane, (CHs SbCtE, should be relatively unstable and attempts to synthesize this species would have to be carried out at low temperature. Another difficulty arises from the experimental fact (104) that all conventional methods of synthesis for stibonium ylides lead to pentaalkylstiboranes instead of ylides (see Introduction). Thus mcthyl-ation of (CH3)4SbX or (CHY) sHbX2 halides by organometallic reagents of lithium, magnesium, aluminum, or zinc invariably yield (CHY) 5Sb as the sole product. 

Copper(II) chloride has often been employed in reactions with lithium and Grignard reagents to obtain organocopper(I) compounds. Two equivalents of the organometallic reagent are required, one of which is used to reduce Cu(II) to Cu(I). Unstable alkylcopper(II) compounds may be intermediates. The reaction is usually represented by the equation... 

Monoalkylation of primary amines. A two-step sequence for this reaction involves conversion of the amine to the N-(cyanomethyl)amine (2) by reaction with chloroaceto-nitrile or with formaldehyde and KCN. Reaction of 2 with an organolithium or a Grignard reagent generates an unstable formaldehyde imint (a) that reacts with a second equivalent of the organometallic reagent to form a secondary amine (3). 

The two perpendicular pi bonds of a triple bond can be considered separately, and thus this source becomes merely a subset of the double-bond sources. Alkynes are slightly poorer electron sources than alkenes because the vinyl cation formed upon electrophile attack is relatively unstable. Acetylide anions, RC=C M+, are treated as organometallic reagents (Section 5.3.1) and not as alkyne sources. 

There are no systematic studies on the reaction of sulfur dichloride with polar organometallic reagents. The high reactivity of sulfur dichloride allows the couplings to be carried out at very low temperatures, even at —100 °C the reactions proceed almost instantaneously. The use of an excess of SC12 should be avoided, since this reactive molecule may easily add to the hetero-aromatic system or form an (unstable) sulfonium compound with the sulfur between the two heterocyclic units. 

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