Allenes carbon-metal bonds

Benzeneselenol as a representative selenol is a colorless liquid of greater acidity than benzenethiol (p a = 5.9 (PhSeH) 6.5 (PhSH)). The bond energy of Se-H is 73 kcal/mol, is smaller than S-H (87 kcal/mol) [82]. These properties may contribute to the efficiency in the oxidative addition of selenols to low-valent transition metals, ligand-exchange reaction between high-valent transition metal complexes and selenols, and protonation process of carbon-metal bonds. Indeed, several transition metal complexes catalyze the highly selective hydrothiolation of alkynes and allenes. 

A significant part of the examples of transition metal catalyzed formation of five membered heterocycles utilizes a carbon-heteroatom bond forming reaction as the concluding step. The palladium or copper promoted addition of amines or alcohols onto unsaturated bonds (acetylene, olefin, allene or allyl moieties) is a prime example. This chapter summarises all those catalytic transformations, where the five membered ring is formed in the intramolecular connection of a carbon atom and a heteroatom, except for annulation reactions, involving the formation of a carbon-heteroatom bond, which are discussed in Chapter 3.4. 

Hydrothiolation of Allenes The first example of a gold-catalyzed carbon-sulfur bond formation was published by Kraus et al. who synthesized 2,5-dihydrothiophenes by allenyl thiocarbinols [38]. The best results were obtained with AuCI in CH2C12, providing an impressive diastereoselectivity. In the same study, other coin-metal precatalysts were tested but only gold afforded the cydization product. 

The formal addition of a C-H bond at activated methylenes and methynes (pronucleophiles) to activated alkenes in the presence of a base is well known as the Michael reaction (Scheme 1, Type A) [1]. In modem organic syntheses, the use of transition metal (TM) catalysts enables the C-H addition of activated methylenes and methynes to activated alkenes perfectly under neutral conditions (Scheme 1, Type B) [2]. In general, the nonfunctionalized carbon-carbon multiple bonds (for example, EWG2 = H in Scheme 1) are unreactive toward carbon nucleophiles because of their electron rich Jt-orbitals. The pioneering efforts by various research groups resulted in the development of transition metal-catalyzed addition of a C-H bond at active alkanes to such unactivated C-C multiple bonds. This reaction consists of the formal addition of a C-H bond across the C-C multiple bonds and is called a hydrocarbonation reaction. As a milestone in this hydro-carbonation area, early in the 1970s, Takahashi et al. reported the Pd-catalyzed addition of the C-H bond of pronucleophiles to 1,3-dienes [3], The first Pd-catalyzed reaction of activated methylenes with unsubstituted allenes was apparently reported by Coulson [4]. The synthetic applications of this reaction were very limited. In the last decade, the Pd-catalyzed addition of C-H bonds to various unacti-... 

Active methylene compounds can be added to polar double bonds such as those in acrylate esters and methyl vinyl ketone as has been described in the previous section. Active methylene compounds can also be added to carbon-carbon multiple bonds in allenes and alkynes with the aid of the transition metal complexes as the catalyst. The addition of methylmalononitrile to 3-phenyl-l,2-butadiene takes place in the presence of Pd2(dba)3-CHCl3 to give the corresponding addition product with E-stereochemistry (Eq. 67) [137 a]. The C-C bond formation occurs exclusively at the terminal position of the allenes. Trost et al. independently reported the similar results with respect to palladium-catalyzed addition of C-H bonds in active methylene compounds to allenes [137 b. ... 

Structures of these compounds are analogous to those of olefin and alkyne complexes. In square-planar compounds, the allene molecule is approximately perpendicular to the ML3 plane, while in the remaining types of compounds, the allene molecule lies almost in the ML2 plane. The angle between this plane and the plane containing the allene carbon atoms equals about 10°. The character of the metal-allene bond may be described by means of the Dewar-Chatt-Duncanson model as in the case of alkene and acetylene complexes. The formation of the a(d -n) and n d -n ) bonds causes an increase in the carbon-carbon distance and tilting of the allene substituents away from... 

This situation may be explained by stronger interaction between the middle carbon atom and the metal atom. Such interaction is caused by the presence of two mutually perpendicular bonding and antibonding n orbitals. The terminal carbon atoms have only n and tt orbitals. This is equivalent to the interaction of the metal with the uncoordinated double bond of the allene which also leads to the change of distance d between carbon atoms. The smaller distance is a result of a smaller atomic radii of the middle allene carbon atom which forms two double bonds. The distance and angles in the allene-metal complexes are given in Table 6.25. Structures of some allene complexes are depicted in Figure 6.24. 

Thus, the optimized carbometallation reaction of propargylic alcohols with aromatic Grignard reagents proceeds without an additional metal catalyst and leads to the formation of an alkenyl fluoride. The stereoselective defluorometallation proceeds to form predominantly the fluorinated (Z)-carbon-carbon double bond in the final allene 276. Two reasons were proposed by the authors for such a high level of stereoselectivity the defluorometallation is favored from the intermediate... 

Other bonds involved in [2-1-2] cycloaddition reactions with allenes include carbon/metal double bonds and N=N and S=0 bonds. 

Since allenes have higher coordination ability and are reactive compared with alkenes, the transition-metal-catalyzed addition of thiols to the carbon-carbon double bonds is expected to proceed. Indeed, several transition metal complexes exhibit catalytic activity toward the addition of thiols to allenes. Of great importance in the addition of thiols to allenes is controlling the selectivity. Formally, four regioisomers are considered in the addition of thiols to terminal allenes (Scheme 22). In the radical addition of benzenethiol to terminal allenes initiated by molecular oxygen, the formed thio radical adds to both the inner and terminal carbon of allenes, affording a mixture of vinyl and aUyl sulfides [73]. 

Transition metals 172 a-bonded to cyclopropanes, substituted on the a-carbon with a halogen atom, are interesting intermediates for cyclopropylidene complexes 173 or allene ones 174 [88]. The former complexes are also supposed to be precursors of the above-mentioned nickel enolates. (Scheme 65)... 

Not only heteroatom-H bonds but also activated C-H bonds can add to the jr-system of an allene. Since carbon lacks a free electron pair, the transition metal catalyst must first activate the C-H bond the new species formed will then react with the C=C double bond. For efficient activation of that kind, two acceptors (typically esters, nitriles and/or sulfones) are necessary. In accord with this mechanistic picture is the fact that the reaction does not benefit from an additional base (which would deproto-nate the pronucleophile). Hence neutral conditions are even better. 

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