Alkynes three-bond

Fluorine NMR data for a number of heptafluoro-n-propyl compounds are given in Scheme 6.19. Little vicinal three-bond F—F coupling is observed for most n-C3F7 compounds (alkyne exception below) rather more prominent is the four-bond coupling (probably significantly through space). 

Reactions involving the [4 + 1 + 1] principle, an example of which is shown in equation (136), are rather uncommon and of strictly limited utility [3 + 2 + 1] and [2 + 2 + 2] processes, on th,e other hand, are well known. Representative [3 + 2+1] three-bond formation processes are given in equations (137)—(141), from which it can be seen that the common situation is where ammonia, a substituted amine or formamide constitutes the one-atom fragment. Many [2 + 2 + 2] atom fragment syntheses are known and some are familiar reactions. Thus, the cobalt(I)-catalyzed condensation of nitriles and isocyanates with alkynes gives pyridines and 2-pyridones, often in excellent yield (e.g. equation 142), while the cyclotrimerizations of nitriles, imidates, isocyanates, etc., are well established procedures for the synthesis of 1,3,5-triazine derivatives (e.g. equation 143). Further representative examples are given in equations (144)-(147), and the reader is referred to the monograph chapters for full discussion of these and other [2 + 2 + 2] processes. Examination of the... 

Both double and triple bonds are multiple bonds. Therefore alkynes are unsaturated hydrocarbons, just as alkenes are. To name alkynes and draw their structures, you follow the same rules that you used for alkenes. The only difference is the suffix -yne, which you need to use when naming alkyne compounds. Also, remember to count the number of bonds for each carbon. An alkyne bond counts as three bonds. 

New, Unsaturated Three- and Four-Membered Rings Formal Addition of CH2, SiH2, GeH2, or SnH2 to Phospha-alkyne Triple Bonds. ... 

You have also seen the same argument used in Chapter 8 to justify the unusual acidity of C-H protons on triple bonds (such as all nes and HCN), and alluded to in Chapter 3 to explain the stretching frequency of the same C-H bonds. Like alkynes, three-membered rings are also unusually easy to deprotonate in base. H LI... 

Unsaturated hydrocarbons that contain one or more triple bonds between carbon atoms in a chain are called alkynes. Triple bonds involve the sharing of three pairs of electrons. The simplest and most commonly used alkyne is ethyne (C2H2), which is widely known by its common name acetylene. Study the models of ethyne in the following diagram. 

In general, metathesis occurs only with disubstituted alkynes. Monosubstituted alkynes do not undergo metathesis instead they form cyclic products or polymerize. In the latter reaction two out of three bonds are broken and a new double bond... 

In linear acetylenes the two n-MOs of the triple bonds are degenerate. This degeneracy is lifted when the triple bond is incorporated in a ring system. Thus, in cyclic alkynes, three different sides of the triple bond, which correspond to three different modes of reactivity, can be distinguished (see Fig. 8-1) ... 

The metathesis reactions of alkynes provide some surprises. They fall into two distinct groups (i) metathesis involving complete cleavage of the triple bond and (ii) polymerization involving cleavage of two of the three bonds. 

Alkynes are unsaturated hydrocarbons that contain a carbon-carbon triple bond, i.e., two adjacent carbon atoms are joined by three bonds. Alkynes have the general formula C H2 2 where n is an integer greater than 1. The simplest member of the alkyne family is C2H2 (lUPAC name ethyne common name acetylene) ... 

Yes, if two carbon atoms form one o and two n bonds (for a total of three bonds, known as a triple bond), the group is called an alkyne. The simplest alkyne is acetylene (C2H2). Welding torches use a combination of oxygen and acetylene to reach temperatures of over 6000 °F (3300 °C). 

The three bonding interactions of metal-alkyne complexes. 

Fragments 2 and 3 are not real because each carbon (marked in red) has only three bonds. As noted before, there must be a protocol that converts these fragments into real molecules. Disconnection of the indicated bond in 4 can be correlated with two real molecules 5 and 6. A reaction in Chapter 18 prepared the indicated bond of an alkyne-alcohol (4) from fragments 5 and 6 (Section 18.3.2). Note the similarity of 5 to 2 and of 6 to 3. In terms of a disconnection, this simply means that the bond in 4 is disconnected because that bond can be formed in a synthesis by an acyl addition of the anion of 5 to 6. Acyl addition... 

Three-component coupling with vinylstannane. norbornene (80). and bro-mobenzene affords the product 91 via oxidative addition, insertion, transme-tallation, and reductive elimination[85]. Asymmetric multipoint control in the formation of 94 and 95 in a ratio of 10 1 was achieved by diastereo-differ-entiative assembly of norbornene (80), the (5 )-(Z)-3-siloxyvinyl iodide 92 and the alkyne 93, showing that the control of four chiralities in 94 is possible by use of the single chirality of the iodide 92. The double bond in 92 should be Z no selectivity was observed with E form[86]. 

Aliphatic hydrocarbons include three major groups alkanes alkenes and alkynes Alkanes are hydrocarbons m which all the bonds are single bonds alkenes contain at least one carbon-carbon double bond and alkynes contain at least one carbon-carbon... 

Diynes and triynes refer to alkynes containing two or three triple bonds poly-ynes contain multiple triple bonds. A conjugated triyne is a straight-chain hydrocarbon with triple bonds alternating with single bonds. An examples is... 

Strategy Compare the product with the starting material, and catalog the differences. In this case, we need to add three carbons to the chain and reduce the triple bond. Since the starling material is a terminal alkyne that can be alkylated, we might first prepare the acetylide anion ol 1-pentyne, let it react with 1-bromopropane, and then reduce the product using catalytic hydrogenation. 

For the cyclotrimerization of alkynes, several mechanisms have been proposed. The most plausible ones are a concerted fusion of three ir-bonded alkyne molecules, and stepwise processes involving a cyclobutadiene complex or a five-membered metallocyclic intermediate (98). In the case of the cyclotrimerization of a-alkynes it is possible to discriminate between a reaction pathway via a cyclobutadiene complex and the other reaction pathways, by analysis of the products. If cyclotrimerization proceeds via a cyclobutadiene complex and if steric factors do not affect the reaction,... 

The subsequent insertion of the alkyne into the metal-carbene bond affords the (r]1 r]3)-vinylcarbene complex D, which may exist either as a (Z)- or an ( )-metallatriene. This intermediate maybe considered as a branching point in the benzannulation reaction as three diverging routes starting from this point have been explored. 

The four structures with three double bonds (third row) and the one with four double bonds are the most plausible Lewis structures, (b) The structure with four double bonds fits these observations best, (c) +7 the structure with all single bonds fits this criterion best, (d) Approaches (a) and (b) are consistent but approach (c) is not. This result is reasonable because oxidation numbers are assigned by assuming ionic bonding. 2.109 The alkyne group has the stiffer C—H bond because a large force constant, k, results in a higher-frequency absorption. 

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