And bridged compounds

Remind yourself (p. 000) of the difference between fused compounds (one bond in common), spiro compounds (one atom in common), and bridged compounds (rings joined at two non-adjacent atoms). Each of these three examples has two five-membered rings... 

The soft Pt" in [PtCU] " reacts witli 1,2-dithiooxalate through the S, as expected, to form stable [Pt(C202S2- 3)2] with five-membeied rings. Dithioace-tate and others also give chelates and bridged compounds. 

Various bicyclic and polycyclic compounds are produced by intramolecular reactions] 127]. In the syntheses of the decalin systems 157 [38] and 158 [128], cis ring Junctions are selectively generated. In the formation of 158, allyhc silyl ether remains intact. A bridged bicyclo[3.3. l]nonane ring 159 was constructed... 

Zinc oxide—eugenol impression pastes are used primarily as corrective washes over compound impressions, as veneer impressions, as temporary liners or stabilizers in base-plates and dentures, and as bite-registration pastes for recording occlusal relationships in inlay, crown, and bridge techniques. 

Oxidation of /U-cresol afforded a triphenol 22 which is approximately half the molecule. The central hydroxyl of the triphenol could be selectively methylated and then the compound was ort/ro-brominated and bridged using 1,3-dibromopropane to give 23. Metallation with butyllithium followed by iron catalyzed coupling afforded the macrocycle as indicated. 

The relation of rates of reduction with NaBH4 to variations in structure in a wide variety of monocyclic and bridged bicyclic compounds has also been discussed for example, a methyl a to a ketone slows the rate of reduction. Brown ° stated that reactions should not be discussed in terms of axial and equatorial attack, since the rates simply reflect differences in the energies of the possible transition states and not enough is known about the transition state to analyze it. He accepted th concepts of SAC and PDC, but preferred to call them steric strain contrpl and product stability control. ... 

From the addition reaetions of acrolein- to aldehyde-derived enamines, aminotetrahydropyrans have been obtained. On heating, these products rearranged to enaminoaldehydes in examples where the initial enamine was disubstituted (320,321). The addition of acrolein to ketone derived enamines has been applied to syntheses of heterocyclic and bridged bieyclic compounds (301,321-323). 

By far the most common CN of hydrogen is 1, as in HCl, H2S, PH3, CH4 and most other covalent hydrides and organic compounds. Bridging modes in which the H atom has a higher CN are shown schematically in the next column — in these structures M is typically a transition metal but, particularly in the Mi-tnode and to some extent in the x3-mode, one or more of the M can represent a main-group element such as B, Al C, Si N etc. Typical examples are in Table 3.3. Fuller discussion and references, when appropriate, will be found in later chapters dealing with the individual elements concerned. 

As with AI(BH4)3 and related compounds (p. 230), solutions of AbMe6 show only one proton ninr signal at room temperature due to the rapid interchange of bridging and terminal Me groups at —75° this process is sufficiently slow for separate resonances to be observed. 

InPhj mp 208°, TlPhj mp 170°. For Ga and In compounds the primary M-C bonds can be cleaved by HX, X2 or MX3 to give reactive halogen-bridged dimers (R2MX)2. This contrasts with the unreactive ionic compounds of T1 mentioned above, which can be prepared by suitable Grignard reactions ... 

The oxygen-bridging mode (v) is less common but occurs together with modes (iii) and (iv) in the following centrosymmetrieal trimeric Ni complex and related compounds. 

The structures are shown in Fig. 26.8c and d and differ in that, whereas the Ir compound consists of a tetrahedron of metal atoms held together solely by M-M bonds, the Rh and Co compounds each incorporate 3 bridging carbonyls. A similar difference was noted in the case of the trinuclear carbonyls of Fe, Ru and Os (p. 1104) and can be explained in a similar way. The M4 tetrahedra of Co and Rh are small enough to be accommodated in an icosahedral array of CO ligands whereas the larger Ir4 tetrahedron forces the adoption of the less dense cube octahedral array of ligands. 

The corresponding ethano and propano compounds only exist in their propellane forms. Apparently, the smaller bridge leads to higher strain in the azocine tautomer, so that the triene structure predominates. Longer chains (> hexano), however, exhibit the normal behavior of tetraenic compounds. 

---