Parent Triple-bonded Systems

Methylidynephosphine (HC=P), the parent member of this class of compounds, is isoelectronic with acetylene. It stands at the very beginning of the history of phosphaalkyne chemistry 5 two decades passed before the successful synthesis of the kinetically stabilized compound 9 (R = i-Bu, Scheme 2)19,20 that is employed most frequently for studies on this class of compounds. Figure 8.2 shows a survey of the general reactions of this highly reactive triple bond system. 

Diethynylallenes, i.e. derivatives of the parent system 8 in Scheme 5.1, are obtained when the propargylic substrate already contains an additional triple bond as shown in Scheme 5.14. Here the bispropargyl carbonate 107 - for example with R = n-hexyl - is coupled with tris(isopropyl)silylacetylene to provide the protected hydrocarbon 108 in excellent yield (94%) [39, 40]. 

These compounds are the parents of the most important class of complexes containing an Mo—Mo triple bond not only do they exhibit a rich coordination chemistry, but also a versatile reactivity.7 810164 Many of the compounds of this class have been characterized by X-ray crystallography and a selection of these is presented in Table 2, together with their Mo—Mo separation. The chemistry of these systems has been developed in parallel to that of their tungsten analogues and the latter has helped illuminate the former. 

The parent structure is the longest continuous chain that contains the triple bond, and the positions of both the substituents and the triple bond are indicated by numbers. According to this system the simplest alkyne should be named ethyne however, this compound commonly is called acetylene. The other alkynes are usually named according to the IUPAC system. 

There has been considerable interest during the past decade in the study of compoimds with multiple bonds to silicon. In particular Si=Si and Si=C double bonds have been studied extensively both experimentally and theoretically [1]. In contrast, relatively little is known about triple bonds to silicon and a stable compound of this family has not been prepared yet. However, there are three reports on the spectroscopic identification of transient compounds with Si-N triple bonds, the silanitrile, HSiN [2], the silaisonitrile, HNSi [3], and its phenyl substituted derivative, PhNSi [4] (the latter formally possess an Si=N double bond). Calculations for the parent silanitrile/silaisonitrile system [3d, 5] and for their phenyl-and methyl-substituted derivatives [6] predict that RSi=N, which formally contains a Si=N triple bond, is significantly less stable than the isomeric silaisonitrile, RN=Si, and that the rearrangement barriers of RSiN to RNSi are relatively small e.g., for R = H the barrier is only 8.3 kcal mol" at G2 [3d]. This stability order is opposite to that in the isovalent carbon analogs HCN/HNC [7]. However, despite the... 

In the lUPAC system, change the -ane ending of the parent alkane to the suffix -yne. Choose the longest carbon chain that contains both atoms of the triple bond and number the chain to give the triple bond the lower number. 

We form lUPAC names of alkynes by changing the -an- infix of the parent alkane to -yn- (Section 3.5). Thus, HC = CH is named efhyne, and CHsC=CH is named propyne. The lUPAC system retains the name acetylene therefore, there are two acceptable names for HC = CH ethynedcnd acetylene. Of these two names, acetylene s used much more frequently. For larger molecules, we number the longest carbon chain that contains the triple bond from the end that gives the triply bonded carbons the lower set of numbers. We indicate the location of the triple bond by the number of the first carbon of the triple bond. 

Once the parent molecules and their syntheses have been covered, we show the reader that these x-bond rich systems may be used as starting points for more extended cross-conjugated compounds, the extension originating from the insertion of unsaturated moieties - for example, triple bonds, aromatic rings, and so on - into the x-framework of the starting compounds. 

This nomenclature type uses substractive prefixes to indicate removal of atoms or groups of atoms from a trivially or systematically named parent structure. While additive nomenclature is used above all for hydrogenated cyclic systems, subtractive nomenclature focuses primarily on the opposite phenomenon, i. e. introduction of unsaturation. This has already been dealt with extensively in the discussions relating to hydrocarbon systems, where the terminal syllables. .. ene and. ..yne signify loss of two and four hydrogen atoms with concomitant formation of a double and triple bond, respectively. A more explicit symbolization of dehydrogenation in the guise of a subtractive prefix is particularly recommendable for certain natural products of the steroid and carbohydrate series as well as for dehy-droarenes (less precisely also named arynes) and dehydroannulenes. 

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