A-Halo organometallic

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... 

Numerous examples of unwanted or deliberate Pd-mediated homocoupling of organometallic reagents have been reported (Scheme 8.14) [38, 122, 123]. Suitable oxidants include a-halo ketones [116, 124, 125] (which can, therefore, not be used as electrophilic component in cross-coupling reactions), oxygen [120, 124, 126], 1,2-diiodoethene [127], 2,3-dibromopropionic acid esters [119], and CuCl2 [128]. 

Grignard reagents add to the carbonyl group of a-halo ketones to give low yields of a-halo a/cof>o/s. The reaction is complicated by further action of the organometallic reagent with the halohydrin. 

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. 

Lithio-heterocycles have proved to be the most useful organometallic derivatives they react with the whole range of electrophiles in a manner exactly comparable to that of aryllithiums and can often be prepared by direct metallation (C-hydrogen deprotonation), as well as by halogen exchange between a halo-heterocycle and an alkyllithium. As well as reaction with carbon electrophiles, Uthiated species are often the most convenient source of heterocyclic derivatives of less electropositive metals, such as zinc, boron, silicon and tin, as will be seen in the following sections. 

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). 

The method is not restricted to secondary aryl alcohols and very good results were also obtained for secondary diols [39], a- and S-hydroxyalkylphosphonates [40], 2-hydroxyalkyl sulfones [41], allylic alcohols [42], S-halo alcohols [43], aromatic chlorohydrins [44], functionalized y-hydroxy amides [45], 1,2-diarylethanols [46], and primary amines [47]. Recently, the synthetic potential of this method was expanded by application of an air-stable and recyclable racemization catalyst that is applicable to alcohol DKR at room temperature [48]. The catalyst type is not limited to organometallic ruthenium compounds. Recent report indicates that the in situ racemization of amines with thiyl radicals can also be combined with enzymatic acylation of amines [49]. It is clear that, in the future, other types of catalytic racemization processes will be used together with enzymatic processes. 

The phosphorus atom of the cyclophosphazene ring can also be involved in interaction with transition metal atoms either by a coordinate or by a covalent linkage. The former occurs with hydridocyclo-phosphazenes by a tautomerization of the P-H to result in a P(III) center (222). The covalent mode of linkage occurs by the reaction of the appropriate organometallic or metal fragment with the halo-genocyclophosphazenes (223). 

Halides are ubiquitous co-ligands for cobalt(III), and are met throughout this review. Anation of (solvent)cobalt(III) complexes by halide has been examined from time to time. An example is substitution of coordinated acetonitrile in [Co(L)(MeCN)2]3+ (L = tetraaza-macrocycle) by Cl-and Br-.1096 A mechanism involving interchange from within tight ion pairs was proposed. Halo-bridged polymeric complexes are well known with both classical and organometallic complexes. 

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