Organic molecule bonding alkynes

The potential synthetic utility of titanium-based olefin metathesis and related reactions is evident from the extensive documentation outlined above. Titanium carbene complexes react with organic molecules possessing a carbon—carbon or carbon—oxygen double bond to produce, as metathesis products, a variety of acyclic and cyclic unsaturated compounds. Furthermore, the four-membered titanacydes formed by the reactions of the carbene complexes with alkynes or nitriles serve as useful reagents for the preparation of functionalized compounds. Since various types of titanium carbene complexes and their equivalents are now readily available, these reactions constitute convenient tools available to synthetic chemists. 

In comparison with the hydroboration and diborafion reactions, thioboration reactions are relatively limited. In 1993, Suzuki and co-workers reported the Pd(0)-catalyzed addition of 9-(alkylthio)-9-BBN (BBN = borabicyclo [3.3.1] nonane) derivatives to terminal alkynes to produce (alkylthio)boranes, which are known as versatile reagents to introduce alkylthio groups into organic molecules [21], Experimental results indicate that the thioboration reactions, specific to terminal alkynes, are preferentially catalyzed by Pd(0) complexes, e.g. Pd(PPh3)4, producing (thioboryl)alkene products, in which the Z-isomers are dominant. A mechanism proposed by Suzuki and co-workers for the reactions involves an oxidative addition of the B-S bond to the Pd(0) complex, the insertion of an alkyne into the Pd-B or Pd-S bond, and the reductive elimination of the (thioboryl)alkene product. 

Unsaturated organic molecules such as alkenes, alkynes, dienes, polyenes and arenes can also stabilize low oxidation states in metal complexes, being both o donors (filled bonding jt orbitals) and jt acceptors (empty antibonding jt orbitals). In these so-called Jt complexes, only jt orbitals are involved in the metal-to-ligand bonds. This latter type of complex is beyond the scope of this chapter and only a few examples will be given. 

In Chapter 11 we continue our focus on organic molecules with electron-rich functional groups by examining alkynes, compounds that contain a carbon-carbon triple bond. Like alkenes, alkynes are nucleophiles with easily broken n bonds, and as such, they undergo addition reactions with electrophilic reagents. 

The geometry will be linear, i.e. the bond angle is 180°. Even though this is rather rare in organic molecules, there are important classes of compounds that have this geometry, e.g. alkynes and... 

One of the many important differences between phosphorus and nitrogen chemistry is the relative strengths of their bonds to hydrogen. The relatively weak P—H bond means that this functionality can be added across a wide variety of unsaturated molecules (alkenes, alkynes, carbonyls) and hence this represents an excellent method for preparing tertiary phosphines. The addition of P 11 compounds to C=0 and C=N has been described in detail by Gilheany and Mitchell.2 The reaction can be catalyzed by base (potassium hydroxide, butyllithium), acid (HC1, carboxylic acids, sulfonic acids, boron trifluoride), free radical (uv, organic peroxides, AIBN) or metal (simple metal salts, late transition-metal complexes). In some circumstances no catalyst is required at all for P 11 additions to proceed.60... 

Alkene, alkyne, and polyene complexes D. M. P. Mingos, Bonding of Unsaturated Organic Molecules to Transition Metals, In Comprehensive Organometallic Chemistry, G. Wilkinson, F. G. A. Stone, and E. W. Abel, Eds., Pergamon Press Oxford, 1982, Vol. 3, pp. 47-67. 

The study of the reactivity of transition metal hydride compounds towards unsaturated organic molecules, mainly olefines and alkynes, has been traditionally centered in monohydrides. In general, the reactions lead to the insertion products, alkyl or alkenyl, which have a limited chemistry. Furthermore, the generation of subsequent carbon-carbon or carbon-heteroatom bonds requires the presence of other active ligands, such as halogen or carbon monoxide, in the coordination sphere of the metallic center of the alkyl or alkenyl intermediates. 

Alkyne An organic molecule whoso functional group is a carbon-carbon triple bond. 

Catalysis of the [2+2+2] cycloaddition of alkynes by transition metal complexes has been extensively exploited for the synthesis of complex organic molecules [30-34]. The accepted mechanism for this transformation, shown in Scheme 10, involves coordination of two alkyne molecules to the metal centre followed by oxidative coupling to form the coordinatively unsaturated metallocyclo-pentadiene 49, which can coordinate a third molecule of alkyne to afford 50. Insertion of the alkyne in a metal-carbon bond of this complex leads to met-allocycloheptadiene 51, and reductive elimination then affords cyclotrimer 52 and regenerates the catalytic species. Alternatively, the transformation of 49 into 52 might involve a Diels-Alder reaction giving intermediate 53, followed by reductive elimination [35]. 

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