Suprafacial carbon shift

These are suprafacial sigmatropic shifts of order [1,5] and should occur with retention of configuration at the migrating carbon. This stereochemical course has been established for the 1,5-alkyl shift that converts 16 to 17. The product which is isolated, 18, results from a subsequent 1,5-hydrogen shift, but this does not alter the stereochemistry at the migrating... 

Thus the conversion of cis-6, 9 dimethylspiro [4, 4] nona-1, 3 diene to dimethyl bicyclo nona-diene is an example. The first product is a suprafacial [1, 5] carbon shift with retention of configuration at the migrating carbon. This is then followed by a [1, 5] hydrogen shift which becomes the major product. 

Sandmeyer reaction, 306 Sandwich compounds, 275 Sawhorse projections, 7 Saytzev elimination, 249, 256 Schiff bases, 221 Schmidt rearrangement, 122 Selectivity, 156, 169, 326 a, 362 a, 370 a+, 372 ct , 385 a bonds, 6 a complexes, 41,131 Sigmatropic rearrangements, 352-357 antarafacial, 353 carbon shifts, 354 hydrogen shifts, 352 orbital symmetry in, 352 photochemical, 354 suprafacial, 353 thermal, 353... 

Figure 12.28 The 1,3 carbon shift. HOMO-LUMO requires suprafacial inversion (si) not supra retention (sr).

There are three principal possibilities (i) The suprafacial sigmatropic [1,5] carbon shift 45a -> 45b is controlled by orbital symmetry (4) proceeding with retention at the migrating carbon C-7 (sr process). This requires the substituents X and Y to interchange places in 45a with respect to the six-membered ring, (ii) The intermediate formation of the diradical 46 (61). In this case the potential... 

When an allyl group migrates, an additional stereochemical feature arises because of p-orbital of migrating carbon of alkyl group. In this case, the thermal [l,3]-suprafacial alkyl shift is an allowed process with inversion of configuration of migrating alkyl carbon and the thermal suprafacial [l,5]-alkyl shift is also allowed process with retention of configuration of alkyl carbon (Pig. 4.5) [1, 2],... 

Thermal [1,3]-suprafacial hydrogen shift is orbital symmetry forbidden process, but [1,3]-suprafacial alkyl shift is symmetry allowed process with inversion of configuration of migrating alkyl carbon. For example, the thermal rearrangement of bicyclo-[3.2.0]-heptene 1 to bicyclo-[2.2.1]-heptene 2 [4]. 

Suprafacial 1,3-shifts with inversion of configuration at the migrating carbon have also been observed in the thermal conversion of bicyclo[2.1.1]hexenes to bicyclo[3.1.0]-hexenes. ... 

Thermal 1,5-hydrogen shifts are thus allowed and, because of the symmetry of the T.S. (39), the H atom in the product (37, x = 1) will be on the same side of the common plane of the polyene s carbon atoms as it was in the starting material (36, x = 1) this is described as a suprafacial shift. This latter point would not be experimentally verifiable in the above example, but that thermal 1,5-shifts (which are quite common) do involve strictly suprafacial migration has been demonstrated in the compound (40). This is found, on heating, to yield a mixture of (41) and (42), which are produced by suprafacial shifts in the alternative conformations (40a) and (406), respectively ... 

The suprafacial shift along the carbon framework is not restricted to cyclic systems but may also prevail in acyclic cases. In the example given in Scheme 11, minimization of dipolar repulsion between the two C-0 bonds mandates a preferred conformation of the initial radical, leading to a stereo-chemically defined alkene radical cation and, ultimately, to a single diastere-omer of the product [119]. 

Depending on whether the starred carbon of 11 pivots so as to bond backside or frontside, there will be a 60° (82) or 120° (83) periodicity in the motion around the remaining carbons. The 60° pattern allows for raeemization in two ways if the starred carbon passes over the marker substituent M, the species develops a plane of symmetry if the starred carbon never passes over M, it still leads to dZ-pairs of nocaradiene or tropilidene. The 120° pattern does not allow for raeemization so that optical activity in the isomers of 11 would be preserved. According to the orbital symmetry rules (Fig. 20a), the [1,5] suprafacial shift is allowed the 120° pattern with retention should be the one observed, if the proposed mechanism is correct. 

When the migrating group in a [l,w]-shift is a carbon atom, two more possibilities arise. In addition to moving either suprafacially or antarafacially, the migrating group can migrate with retention of configuration or with inversion of... 

The simplest sigmatropic reaction, 1,2-shift (2-electron system), in carbocations is the well-known 1,2-alkyl shift (Schemes 2.9 and 2.10). This shift can be concerted Wagner-Meerwein rearrangement (see section 2.1.3) and suprafacial in carbocations. The 1,2-methyl shift involves three carbons held together by a three-centre two-electron bond at the transition state, representing the smallest and simple system (Scheme 8.14). 

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