A-Halo organometallic compounds

Because most carbenes are so reactive, it is often difficult to prove that they are actually present in a given reaction. The lifetime of formylcarbene was measured by transient absorption and transient grating spectroscopy to be 0.15-0.73 ns in dichloromethane. In many instances where a carbene is apparently produced by an a elimination or by disintegration of a double-bond compound, there is evidence that no free carbene is actually involved. The neutral term carbenoid is used where it is known that a free carbene is not present or in cases where there is doubt. a-Halo organometallic compounds (R2CXM) are often called carbenoids because they readily give a elimination reactions (e.g., see 12-37). ° ... 

The addition of a-deprotonated alkyl halides to alkenes or carbonyl compounds can, because of the good leaving-group properties of halides, also lead to formation of cyclopropanes [292] or epoxides [187, 304, 306, 310], respectively. Because of the inherent instability of a-halo organometallic compounds, these intermediates should be handled carefully and on a small scale only. The ketone produced by the last reaction in Scheme 5.34 is probably formed by Oppenauer oxidation of the intermediate alcohol by the excess benzaldehyde [310],... 

The most widely used route to organoboranes is hydroboration, introduced in Section 4.5.1, which provides access to both alkyl- and alkenylboranes. Aryl-, methyl-, allylic, and benzylboranes cannot be prepared by hydroboration, and the most general route to these organoboranes is by reaction of an organometallic compound with a halo- or alkoxyboron derivative.1... 

A couple of years ago we have disclosed a new mode of alkyne activation towards isomerization as a detouring outcome of the Sonogashira coupling. As a result of coupling electron deficient (hetero)aryl halides (or a,p-unsaturated p-halo carbonyl compounds) 11 and aryl propargyl alcohols 12 a new access to 1,3-di (hetero)aryl propenones 13, i.e., chalcones, was established (Scheme 9) [77, 78]. The scope for electron deficient (hetero)aromatic halides 11 is fairly broad and even organometallic complexes like 13c can be synthesized by this sequence. 

In this reaction organometallic compounds incapable of existence in high concentration are formed and utilized immediately. When an aide hyde or ketone is condensed with a halo ester the product is a fi-bydroxy ester. Sometimes dehydration occurs to give olefinic esters directly (method 19). The use of an ester as the carbonyl compound leads to /3-keto esters (method 234). The halo esters most commonly employed are of three types XCHjCOjCjHj, RCHXC02CjHj, and R CXCO CjH,. Vinylogous halo esters, such as y-bromocrotonate, and certain benzyl halides have been used with variable success. 

Various other preparative methods, such as the trap of enolates formed by dissolving metal reduction of a.P-unsaturated ketones or a-halo ketones (eq (53)) [48], the 1,4-hydrosilation of a,P-unsaturated ketones (eq (54)) [49], and the addition of organometallic compounds to a-silyl ketones followed by the Brook rearrangement (eq (55)) [50], have been investigated. 

Alternatives which are more useful for /3-hydroxy carbonyl compounds are organometallic reagents. Organozinc reagents (67) react with aldehydes and ketones but not with esters so they can be made from a-halo esters. This, the Reformatsky reaction, has the advantage that the -halo esters are easily made (see Chapter 7). 

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