Importance of frontier orbital interactions in addition reaction of water to disilene

Abstract

Addition mechanisms of water to disilene were investigated in detail using the high level ab initio molecular orbital (MO) theory. We have found that the search for transition states by ab initio MO calculations combined with the frontier MO theory provides a good perspective view of the mechanisms of the reaction of disilenes and water. The initial electrophilic and nucleophilic interactions of the water with disilene give the corresponding weak complexes, C E and C N , which leads to the next nucleophilic and electrophilic attack of the water part, respectively. The latter pathway requires an antarafacial approach, while the former pathway proceeds via suprafacial addition. Whereas in the model disilene-water addition reaction, the pathway from C E is more favorable than the pathway from C N , the stereochemical ambiguity of the addition reactions found experimentally may be explained by the competition of the two pathways.

1,2-Addition Reaction of Monosubstituted Disilenes: An Ab Initio Study

“…The activation free energy is low (12.6 kcal/mol by the CBS-Q method), which makes the reaction feasible (Table ). A similar mechanism occurs in the reaction of H 2 SiSiH 2 with acidic alcohols such as CF 3 OH. ,9a

6 Free energy diagram at 298 K by the CBS-Q method in kcal/mol for disilene + HF ( R6 ). Imaginary frequency modes of the transition structures are indicated by arrows.

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“…The Si−O distance is also shorter than in the case of T E , indicating the enhanced interaction. The rotating and strongly pyramidal silicon carries both electrons of the original π-bond in a lone electron pair …”

“…In our subsequent work, we observed the sensitivity of the reaction mechanism to the acidity of the reagents. In case of acidic alcohol (CF 3 OH) ,9a or hydrogen halides 9b only electrophilic channels could be found. This mechanism is, however, different from that of the usual addition reactions to CC bond in a few steps.…”

“…In the reaction mechanisms of disilenes with alcohols, a four-centered cyclic mechanism, including a nucleophilic attack of the alcoholic oxygen at the unsaturated silicon, has been first proposed on the basis of the theoretical and experimental studies. , However, Apeloig and Nakash found experimentally that an electrophilic attack prompts the reaction of disilene with phenols having electron-withdrawing substituents . In our previous paper, we reinvestigated the water addition reaction to disilene by ab initio quantum-mechanical calculations and have found both an electrophilic and a nucleophilic channel, as shown in Scheme . The first step of both channels is the appearance of the initial complexes C E and C N .…”