Alkenyl -metal complexes

Coupling Reactions.—Acetylenes react with organoalanes, e.g. MesAl, and zirconocene dichloride to form the alkenyl metal complexes (38) in high yield. The exact structure of these complexes is not known but both aluminium and zirconium are essential. The stereoselectivity of the reaction is shown to be 98% c/s-addition, and once formed the complexes (38) undergo a variety of useful... 

The general mechanism of coupling reactions of aryl-alkenyl halides with organometallic reagents and nucleophiles is shown in Fig. 9.4. It contains (a) oxidative addition of aryl-alkenyl halides to zero-valent transition metal catalysts such as Pd(0), (b) transmetallation of organometallic reagents to transition metal complexes, and (c) reductive elimination of coupled product with the regeneration of the zero-valent transition metal catalyst. 

Transition metal complexes that are easy to handle and store are usually used for the reaction. The catalytically active species such as Pd(0) and Ni(0) can be generated in situ to enter the reaction cycle. The oxidative addition of aryl-alkenyl halides can occur to these species to generate Pd(II) or Ni(II) complexes. The relative reactivity for aryl-alkenyl halides is RI > ROTf > RBr > RC1 (R = aryl-alkenyl group). Electron-deficient substrates undergo oxidative addition more readily than those electron-rich ones because this step involves the oxidation of the metal and reduction of the organic aryl-alkenyl halides. Usually... 

During the last ten years, the chemistry of 1,1-boriozirconocene complexes has been studied. Both hydrozirconation and hydroboration reactions are well established, and are widely applicable to a wide variety of vinyl and acetylene derivatives [1], Alkenylboranes and alkenylzirconium compounds can also be readily prepared. Therefore, hydrometalla-tion of the corresponding alkenyl metals should offer a convenient method for preparing gem-boriozirconocenes [24]. 

Alkynes react with the bulky germanium hydride (MejSdjGeH to selectively yield (Z)-alkenes (Equation (105)).67 The hydrogermylation of alkynols or alkynes can be catalyzed by a rhodium complex (Equation (106), Table 18) and some of the intermediates were identified (Scheme 16).132 Similar rhodium species react with alkynes to yield alkenyl complexes,133 and other transition metal complexes have been employed as hydrogermylation catalysts including those containing palladium.134,135... 

The treatment of cyclopentadienyI-metal-2-alkenyl carbonyl complexes, such as 274 and 275, with SO2 leads to the formation of thietane 1,1-dioxide as ligand (Eqs. 73 and 74). Thietane and platinum(II) have also been found... 

Three transmetallation reactions are known. The reaction starts by the oxidative addition of halides to transition metal complexes to form 206. (In this scheme, all ligands are omitted.) (i) The C—C bonds 208 are formed by transmetallation of 206 with 207 and reductive elimination. Mainly Pd and Ni complexes are used as efficient catalysts. Aryl aryl, aryl alkenyl, alkenyl-alkenyl bonds, and some alkenyl alkyl and aryl-alkyl bonds, are formed by the cross-coupling, (ii) Metal hydrides 209 are another partner of the transmetallation, and hydrogenolysis of halides occurs to give 210. This reaction is discussed in Section 3.8. (iii) C—M bonds 212 are formed by the reaction of dimetallic compounds 211 with 206. These reactions are summarized in Schemes 3.3-3.6. 

Addition reactions of three kinds of main group metal compounds, namely R—M X (carbometallation, when R are alkyl, alkenyl, aryl or allyl groups), H—M X (hydrometallation with metal hydrides) and R—M —M"—R (dimetallation with dimetal compounds) to alkenes and alkynes, are important synthetic routes to useful organometallic compounds. Some reactions proceed without a catalyst, but many are catalysed by transition metal complexes. 

There is some ambiguity as to whether the alkenyl- and alkynylsilane-forming reactions are true dehydrocoupling reactions. According to the definition as embodied in Eq. (1), this would require the direct activation of a C-H bond, such as the = C—H bond of an olefin. The general consensus is that the alkenyl- and alkynylsilane-forming reactions are more likely to take place via an insertion of the olefin into an M-Si bond, followed by /3-hydride elimination [Eqs. (18) and (19)] in the case of electron-rich metal complexes,1616 or by bond metathesis of M-Si and H -C= in the case... 

After these pioneering studies, a number of other research groups reported on the cleavage of C-H bonds via the use of a stoichiometric amount of transition-metal complexes [7]. To date, several types of catalytic reactions involving C-H bond cleavage, for example, alkyl, alkenyl, aryl, formyl, and active methylene C-H bonds have been developed [8]. In many cases,for these types of catalytic reactions, ruthenium, rhodium, iridium, platinum, and palladium complexes all show catalytic activity. 

Additions of aromatic C-H bond to olefins and acetylenes result in the formation of aryl-alkyl and aryl-alkenyl bonds. This type of addition reaction is not applicable to aryl-aryl bond formation. Catellani and Chiusoli [52] reported the first example of this type of arylation in 1985. To date, several arylation reactions of aromatic rings have been developed. In almost all cases, C-H bond cleavage proceeds through electrophilic substitution with transition-metal complexes [53]. 

Relatively few examples of metal trifluorophosphine complexes of formula [MRx(PF3)y are known (see Table XVI). 

The reactions of alkenyl carbene complexes with isonitriles can produce a variety of products whose distribution is sensitive to the nature of the isonitrile. Reacting 3 equiv. of aryl isonitrile with the alkenyl complex (383) produces the dihydro- y-carbolinone (385). 74 The ketenimine in (384) is liberated with a second equivalent of the isonitrile, and the the third equivalent undergoes a formal [4 -i- 2] cycloaddition with the free ketenimine. With sterically encumbered isonitriles, the ketenimine is more rapidly liberated from the metal by a second isonitrile and 2 equiv. are required to drive the reaction to completion. Free ketenimines that are generated in this manner can be trapped via their [4 + 2] cycloadditions with maleic anhydride. 7 Primary and secondary alkyl isonitriles will react with alkenyl carbene complexes to give cycloadducts that can be converted to pyrroles or products that contain five-member ring carbo-cycles. 7 ... 

Oxidative addition of the carbon-halogen bond is a well-documented reaction for Group 10 transition metal complexes, but it is relatively limited for ruthenium. The example given here involves the reversible oxidative addition of allyl halide to RuCp(CO)2X to produce RuCp(p -allyl)X2 [78]. Oxidative addition of allyl halide to a Ru(0) complex Ru(l,5-COD)(l,3,5-COT) is also reported, but the product yield was poor [79]. Nevertheless, a catalytic Heck-type alkenylation of bromostyrene with methyl acrylate by Ru(l,5-COD)(l,3,5-COT) proceeded smoothly [80]. A cross-coupling reaction of alkenyl halide with Grignard reagents or alkyl lithium also pro-... 

The method was made considerably more general by inclusion of a catalytic amount of a palladium(O) complex during addition of the organometallic. Alkenylcopper reagents actually react relatively slowly with acid halides but, in a fashion analogous to other alkenyl metal species (see Section 1.13.4), they may be readily transmetallated to form an acylpalladium(II) complex which then undergoes reductive elimination to the product (Scheme 29)." A further discussion of acylation mediated by palladium complexes is included in Section 1.13.4. Interestingly, a,P-unsaturated acid chlorides react under these conditions to form divinyl ketones. 

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