var sentence = "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua.".ToCharArray();
var buffer = new char[sentence.Length];
var reversedSentence = new char[sentence.Length];
var writeIndex = 0;
var bufferIndex = 0;

for (int i = sentence.Length -1; i > -2; i--)
{
    if(i == -1 & bufferIndex > 0)
    {
        /* beginning of sentence, write buffer if any and done */
        for (int x = bufferIndex - 1; x > -1; x--)
        {
            reversedSentence[writeIndex] = buffer[x];
            writeIndex++;
        }

        break;
    }

    switch (sentence[i])
    {
        case ' ':
        case ',':
        case '.':
            {
                if (bufferIndex > 0)
                {
                    for (int x = bufferIndex - 1; x > -1; x--)
                    {
                        reversedSentence[writeIndex] = buffer[x];
                        writeIndex++;
                    }

                    reversedSentence[writeIndex] = sentence[i];
                    writeIndex++;

                    bufferIndex = 0;
                }
                else
                {
                    reversedSentence[writeIndex] = sentence[i];
                    writeIndex++;
                }

                break;
            }

        default:
            {
                buffer[bufferIndex] = sentence[i];
                bufferIndex++;

                break;
            }
    }
}

Console.WriteLine($"sentence: {new string(sentence)}");
Console.WriteLine($"sentence: {new string(reversedSentence)}");

Console.ReadLine();